HK1097661B

HK1097661B - Electrical device, power supply for electrical device, and method for supplying power to electrical device

Info

Publication number: HK1097661B
Application number: HK07103291.3A
Authority: HK
Inventors: 润牧野; 王忠方
Original assignee: 创新科技有限公司
Priority date: 2005-03-17
Filing date: 2007-03-28
Publication date: 2010-12-03

Description

Electronic device, power supply for electronic device and method for supplying power to electronic device

Technical Field

The invention relates to a power supply of an electronic device and a method for supplying power to the electronic device. In particular, the present invention relates to a power supply with low power consumption.

Background

It has become more and more important to save energy, reduce power consumption, and a power supply with low power consumption is also becoming more and more important. Such a power supply can be applied in a variety of situations, for example as: standby power in electronic devices (e.g., televisions, washing machines); a standby power supply in the external power supply for supplying power to detect whether the electronic device is connected to the external power supply and turning on the main power supply (for example, in a portable phone charger in which a phone is put into a slot for charging); or a separate power source for low power electronics (e.g., a night light that plugs into an AC wall outlet to provide dim light).

In a first known arrangement, a power supply (for a variety of applications) comprises a transformer having a primary winding directly connected to an AC power source and a secondary winding providing an output voltage for an electronic device. To achieve low power consumption in this arrangement, the current through the primary winding of the transformer (which is directly connected to the AC power supply) must be small. To obtain a small current, the impedance of the primary winding as seen from the AC power source must be large. At typical AC power frequencies (50 or 60Hz), a large inductance would be required to obtain a large primary winding impedance. To obtain such a large primary winding inductance, more turns are required, which would make the transformer impractically large. Alternatively, to avoid large transformers, thinner wires can be used in the windings, but this means higher resistance, i.e. greater losses. In summary, to achieve very low power consumption in this setup we need a perfect inductor with high inductance, which is not feasible.

A second known arrangement is known as a Switched Mode Power Supply (SMPS), which has a number of different implementations. Although SMPS has certain advantages over the first arrangement, the fast switching in this arrangement creates a lot of noise. Further, SMPS are more complex in design and more costly.

At present, the standby power supply generally has power consumption of hundreds of milliwatts or even up to several watts. However, typical power requirements for control circuitry used to "wake up" a device from standby may be as low as a few milliwatts. Thus, there is a significant mismatch between the actual power required and the power consumed by the standby mode device.

Disclosure of Invention

According to a first embodiment of the present invention, there is provided a power supply for an electronic device, the power supply including:

a) a transformer comprising a primary winding on a primary side connectable to an AC voltage source and a secondary winding on a secondary side, circuitry on the secondary side being arranged to provide a DC output voltage for the electronic device;

b) a switch between the primary winding of the transformer and the AC voltage source;

c) a switch timer for controlling a switching timing of the switch; and

d) a rectifier for rectifying an AC voltage of the AC voltage source; wherein the switch is arranged to switch on at some point in time when the rectified AC voltage increases from zero to a maximum value and when the rectified AC voltage has increased to a predetermined value not equal to zero, under control of the switching timer, to provide current through the primary winding and thus through the secondary winding, and wherein the switch is arranged to switch off under control of the switching timer, before the rectified AC voltage starts to increase again.

In this arrangement, there is no current drain through the transformer winding until the switch is turned on. Once the switch is turned on, current flows through the primary winding and energy is stored in the primary winding. The predetermined value reached by the rectified AC voltage is preferably a large part of the peak voltage, even preferably the peak voltage.

The circuit may also include a current limiter. The current limiter limits the current flowing through the primary winding so that the energy consumption can be controlled.

The AC power source is typically a mains power source, such as 110VAC, 120VAC, 230VAC or 240VAC at 50 or 60 Hz.

In a preferred embodiment, the switch timer for switching the switch on and off may be arranged to switch the switch on at a certain moment when the rectified AC voltage increases from zero to a maximum. The switch timer may also be arranged to switch off the switch before the rectified AC voltage starts to rise again.

The switching timer may be an RC timer comprising a resistor and a capacitor between a node, the voltage of which matches the rectified AC voltage, and ground. In this case, the capacitor may be arranged to charge as the rectified AC voltage increases from zero to a maximum. When the switch is turned on, the capacitor may be discharged and the energy stored in the capacitor is transferred to the primary winding.

The switch is preferably arranged to conduct near each peak of the rectified AC voltage. If the apparatus includes an RC switch timer, the values of the resistor and capacitor may be selected such that the switch conducts near each peak of the rectified AC signal. This maximizes the current through the primary winding of the transformer by providing a maximized voltage across the primary winding of the transformer when the switch is on.

The switch timer may be coupled to the switch controller. In one embodiment, the switch comprises a MOSFET. In a preferred embodiment, the switch timer is coupled to a switch controller, the switch comprising a MOSFET, the switch controller comprising a thyristor device for turning the MOSFET on and off.

In one embodiment, the current limiter comprises at least one charge storage device. In a preferred embodiment, the current limiter comprises two charge storage devices. Each charge storage device may be a capacitor. The value of the one or more capacitors may be suitably selected to limit the current through the primary winding to a desired current level.

The power supply may be arranged such that current no longer flows through the primary winding once the at least one charge storage device of the current limiter has been almost fully charged, i.e. the switch may be arranged to turn off once the one or more charge storage devices of the current limiter have been almost fully charged. If the arrangement comprises a switching timer, the switching timer may be arranged to switch off the switch once the charge storage device or devices of the current limiter have been fully charged, and this preferably occurs at some point when the rectified AC signal falls from its peak value towards zero. Once the switch is turned off, no current flows from the windings of the transformer, and therefore, as described above, the amount of current flowing through the windings can be set by appropriately setting the value of the one or more charge storage devices of the current limiter.

In one embodiment, the switch timer may work with a switch timer reset that is set to reset the switch timer after the switch has been turned off (i.e., once current has stopped flowing through the transformer windings). Resetting the switch allows the switch to turn on again as the rectified AC signal rises from zero to its next maximum. If the switching timer is an RC timer, the switching timer may be reset when the capacitor is fully discharged, which may occur when the rectified AC voltage drops from its maximum value towards zero.

In another embodiment, the current limiter may be omitted from the power supply, as the current may be controlled by a thyristor device.

Thus, in a preferred embodiment, the operation is such that as the rectified AC voltage increases from zero to a maximum, the capacitor of the RC switch timer is charged, and as soon as it charges to a certain amount (which preferably coincides with the peak of the rectified AC voltage), the switch timer turns on the switch, thereby providing a current through the winding that corresponds to the current flowing from the AC power source and also to the discharge of the RC switch timer capacitor, but the latter is less important. This occurs when the rectified AC signal again falls from its maximum value towards zero, so that the switch is ready to conduct when the rectified AC voltage rises again.

In a preferred embodiment, the switch is arranged to use positive feedback to achieve fast switching from off to on. The fast switching from off to on means that the time for the voltage to drop through the switch is minimized and this reduces the power loss in the switch itself.

In this embodiment, the switch may include a first transistor and a second transistor, the collector of the first transistor being coupled to the base of the second transistor. Furthermore, the collector of the second transistor may be coupled to the base of the first transistor, which may be achieved via a feedback capacitor. This arrangement may provide positive feedback because as the collector voltage of the second transistor increases, the base voltage of the first transistor also increases, which further increases the voltage of the second transistor collector, and so on.

The power supply may also include a voltage limiter to prevent breakdown of the device at high voltages. The voltage limiter may comprise a charge storage device arranged to charge up when the rectified AC voltage increases from zero to a maximum value. The charge storage device may be a high voltage capacitor.

In a preferred embodiment of the invention, the rectifier is arranged to full-wave rectify the AC voltage. This means that the rectified AC voltage rises from zero to a peak twice in each cycle of the original AC signal.

Although the rectifier is described as full-wave rectifying the AC voltage in the preferred embodiment, of course, the rectifier may only half-wave rectify the AC voltage. In this case, the rectified AC voltage rises from zero to a maximum value only once per AC cycle.

The secondary side circuitry may provide a DC output voltage for the electronic device via a charge storage device (e.g., a capacitor) that is charged in each AC cycle. In this arrangement, the capacitor is preferably located between ground and the output node so that as the capacitor charges up in each AC cycle, the voltage at the output node rises towards the steady state DC voltage.

The power supply may also include circuitry for reducing electromagnetic radiation caused by the switch switching between the on and off modes. Electromagnetic radiation, known as ringing (ringing), can be caused by fast on-off switching and can be reduced by using appropriate circuitry. In one embodiment, the circuit includes a capacitor and a resistor suitably arranged between the secondary winding and the output node.

The power supply may further include a voltage regulator for stabilizing the DC output voltage. This is useful when the load requires a very stable DC voltage. The voltage regulator may be located between the output voltage and the switch and may be arranged to switch off the switch when the output voltage exceeds a selected threshold. The voltage regulator may include a zener diode.

According to the invention, an electronic device comprising the power supply is also provided.

According to a first aspect of the present invention, there is also provided a power supply for an electronic device, the power supply comprising:

c) a rectifier for full-wave rectifying the AC voltage; and

d) a current limiter including a first capacitor and a second capacitor; wherein the switch is arranged to switch on at some point when the rectified AC voltage rises from zero to a maximum within each half cycle of the rectified AC voltage and when the rectified AC voltage has risen to a non-zero value to provide current through the primary winding and thereby through the secondary winding, wherein the current limiter is arranged to limit the amount of current flowing through the primary winding by preventing current flow through the primary winding when the first and second capacitors are fully charged, and wherein the current limiter is arranged to limit the amount of current flowing through the primary winding and wherein the current limiter is arranged to limit the amount of current flowing through the secondary winding when the first and second capacitors are fully charged

Wherein the switch is arranged to switch off as the rectified AC voltage decreases from a maximum value towards zero in each half cycle of the rectified AC voltage.

According to a second aspect of the present invention, there is provided a method for supplying power to an electronic device, the method comprising the steps of:

a) providing a transformer having a primary winding and a secondary winding, the primary winding being connected to an AC voltage source via a switch;

b) providing a rectifier for rectifying an AC voltage of an AC voltage source;

c) when the rectified AC voltage increases from zero to a maximum value, and when the rectified AC voltage has increased to a predetermined value not equal to zero, the switch is turned on under control of the switch timer to provide current through the primary winding and thus through the secondary winding, the amount of current flowing through the primary winding being limited by a current limiter connected between the AC voltage source and the rectifier;

d) converting the current through the secondary winding to a DC output voltage for the electronic device; and

e) under the control of the switching timer, the switch is turned off before the rectified AC voltage starts to rise again.

In this method, there is no current drain through the transformer winding until the switch is turned on. Once the switch is on, current from the AC power source is transferred to the primary winding, which provides a large enough voltage drop to provide a DC output voltage for the electronic device. However, the current flowing through the primary winding is limited by a current limiter, so that power consumption can be controlled.

The AC power source is typically a mains power supply, such as 110VAC, 120VAC, 230VAC or 240VAC at 50 or 60 Hz.

The step c) of turning on the switch may include turning on the switch by a switch timer. The step e) of turning off the switch may include turning off the switch by a switch timer. The switch timer may be arranged to switch the switch on at some point when the rectified AC voltage increases from zero to a maximum value. The switch timer may be arranged to switch off the switch before the rectified AC voltage starts to rise again.

The switch timer may be an RC timer comprising a resistor and a capacitor between a node and ground, wherein the voltage of the node matches the rectified AC voltage. In this case, the capacitor may be arranged to charge as the rectified AC voltage increases from zero to a maximum. When the switch is turned on, the capacitor may be discharged and the energy stored in the capacitor is transferred to the primary winding.

Preferably, the step c) of turning on the switch comprises turning on near each peak of the rectified AC voltage. If the step of turning on the switch comprises turning on the switch by an RC timer, the values of the resistor and capacitor may be selected such that the switch conducts at the peak of the rectified AC signal. This maximizes the current through the primary winding by providing a maximized voltage across the transformer primary winding when the switch is on.

In one embodiment, the current limiter comprises at least one charge storage device. In a preferred embodiment, the current limiter comprises two charge storage devices. The or each charge storage device may be a capacitor.

In one embodiment, the step e) of turning off the switch may comprise turning off the switch when the one or more charge storage devices of the current limiter have become almost full. If the arrangement includes a switch timer, the switch timer may be arranged to switch off the switch when one or more charge storage devices of the current limiter are fully charged. In this arrangement, when the switch is off, no current flows through the winding. Thus, the amount of current drawn through the windings can be controlled by appropriately sizing the charge storage devices.

The method also includes the step of charging a charge storage device as the rectified AC voltage increases from zero to a maximum, the charge storage device acting as a voltage limiter to prevent breakdown of the device at high voltages.

In one embodiment, the switch timer may work with a switch timer reset for resetting the switch timer after the switch has been turned off. If the switching timer is an RC switching timer, the switching timer reset may be arranged to reset the switch when the capacitor of the RC timer has been completely discharged. Resetting the switch allows the switch to conduct when the rectified AC signal again rises from zero to a maximum value.

In a preferred embodiment, the switch is arranged to use positive feedback to achieve fast switching from off to on. The fast switching from off to on means that the fall time of the voltage on the switch is minimized, which reduces the power loss in the switch itself.

Preferably, the rectifier is arranged to full-wave rectify the AC voltage. This means that the rectified AC voltage rises from zero to a peak twice in each cycle of the original AC signal.

In one embodiment, the step d) of converting the voltage peak in the secondary winding to a DC output voltage for the electronic device comprises charging a capacitor during each AC cycle, the voltage across the capacitor being the DC output voltage. In this arrangement, the capacitor is preferably located between ground and the output node so that as the capacitor charges in each AC cycle, the voltage at the output node rises towards a steady state DC voltage.

The method may further comprise the step of stabilizing the DC output voltage. This is useful when the load requires a very stable DC voltage supply. The voltage regulator may be located between the output voltage and the switch and may be arranged to turn off the switch when the output voltage exceeds a selected threshold.

In one embodiment, steps c), d) and e) of the method are repeated until a steady state DC output voltage for the electronic device is obtained.

According to a second aspect of the present invention, there is also provided a method for supplying power to an electronic device, the method comprising the steps of:

b) providing a rectifier for full-wave rectifying the AC voltage;

c) when the rectified AC voltage has reached a non-zero value as the rectified AC voltage increases from zero to a maximum value within each half cycle of the rectified AC voltage, switching on the switch to provide current through the primary winding and thereby through the secondary winding, the amount of current flowing through the primary winding being limited by a current limiter comprising two capacitors;

e) the switch is turned off as the rectified AC voltage decreases from a maximum value towards zero during each half cycle of the rectified AC voltage.

According to a second aspect of the present invention, there is also provided a method for powering an electronic device, the method comprising the steps of:

b) providing a rectifier for rectifying an AC voltage;

c) performing the following steps at least once per AC cycle:

i) turning on the switch to provide current through the primary winding and thereby through the secondary winding when the rectified AC voltage has risen to a non-zero value as the rectified AC voltage rises from zero to a maximum value, the amount of current flowing through the primary winding being limited by a current limiter;

ii) converting the current through the secondary winding to a DC output voltage for the electronic device by charging an output charge storage device, the voltage across the charge storage device being the DC output voltage; and

iii) turning off the switch before the rectified AC voltage starts to rise again.

Wherein the output charge storage device is stably charged, such that after a plurality of AC cycles, the output charge storage device is nearly fully charged, thereby providing a steady state DC output voltage for the electronic device.

According to a third aspect of the present invention, there is provided an electronic device connectable to an AC voltage source and operable in each of a normal mode and a standby mode, the electronic device comprising:

a main power supply for supplying power during a normal mode;

control means for switching on and off of the main power supply; and

a standby power supply for supplying power for turning on a main power supply to the control apparatus during a standby mode, the standby power supply comprising:

a) a transformer comprising a primary winding on a primary side connectable to an AC voltage source and a secondary winding on a secondary side, circuitry on the secondary side arranged to provide a DC output voltage for an electronic device;

c) a switch timer for controlling a switching timing of the switch; and

d) a rectifier for rectifying an AC voltage of the AC voltage source;

wherein the switch is arranged to switch on at some point in time when the rectified AC voltage rises from zero to a maximum value and when the rectified AC voltage has risen to a predetermined value not equal to zero, under control of a switch timer, to provide a current through the primary winding and thus a current through the secondary winding, and

wherein the switch is arranged to switch off before the rectified AC voltage starts to rise again under control of a switch timer.

The main power supply provides power (to the apparatus itself and to the control means) during the normal mode, while the standby power supply provides power to the control means during the standby mode, so that the control means has the power required to switch on the main power supply when the apparatus enters the normal mode from the standby mode.

In standby power, there is no current drain through the transformer winding until the switch is turned on. Once the switch is on, current from the AC power source is transferred to the primary winding, which provides a large enough voltage drop to provide a DC output voltage for the electronic device. The current limiter limits the current through the primary winding so that power consumption in standby mode can be controlled.

In the first embodiment, the control means may be a receiver for receiving an instruction to turn on and off the main power supply. This may be the case if the electronic device is a device that requires power for its own operation during the normal mode, and during the standby mode it is required that the control means is switchable from the standby mode back to the normal mode. Examples of this type of appliance are washing machines, radios or microwave ovens. The receiver may be a remote control receiver for receiving remote instructions to switch the main power supply on and off. This may be the case if the electronic device is operable in a normal mode and a standby mode and can be switched between the two by using a remote control, for example a television, DVD player or radio or other type of electronic device having a normal and standby mode.

In a second embodiment, the electronic device is an external power supply for the appliance, the device operating in a normal mode when the appliance is electrically connected to the device and in a standby mode when the appliance is not electrically connected to the device.

If the electronic device is an external power source, the control means may be a sensor for sensing when the appliance is electrically connected to the device. Thus, when the sensor senses that the appliance is electrically connected to the device (e.g. when the phone is placed in an outlet for charging), it may be arranged to switch on the main power supply using the power provided by the standby power supply. When the sensor senses that the appliance is no longer electrically connected (e.g. the phone has been removed from the charging socket), it may be arranged to switch off the main power supply, at which point power will be provided by the standby power supply.

According to a third aspect of the present invention, there is also provided an external power supply for an appliance, the external power supply being connectable to an AC voltage source, the device operating in a normal mode when the appliance is electrically connected to the external power supply and in a standby mode when the appliance is not electrically connected to the external power supply, the external power supply comprising:

a main power supply for supplying power during a normal mode;

a sensor for sensing when the electric appliance is electrically connected to an external power source and switching on and off of the main power source; and

a standby power supply for supplying power to the sensor for turning on a main power supply during a standby mode, the standby power supply comprising:

a) a transformer comprising a primary winding on a primary side connectable to an AC voltage source and a secondary winding on a secondary side, circuitry on the secondary side arranged to provide a DC output voltage for the electronic device;

c) a rectifier for rectifying the AC voltage; and

d) a current limiter;

wherein the switch is arranged to switch on at some point when the rectified AC voltage increases from zero to a maximum and when the rectified AC voltage has increased to a non-zero value to provide current through the primary winding and thus through the secondary winding,

wherein the current limiter is arranged to limit the amount of current flowing through the primary winding, and wherein the switch is arranged to switch off before the rectified AC voltage starts to rise again.

Some examples of electronic devices have been given, but it will be appreciated by those skilled in the art that the invention is applicable to many different devices and is not limited to those listed. Furthermore, it should be understood that features described in connection with one aspect of the invention may also be applied to another aspect of the invention.

Drawings

The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a block diagram of a first embodiment of the present invention;

FIG. 2 shows a circuit implementation of the first embodiment of the invention shown in FIG. 1;

FIG. 3 is a graph of voltage versus time for node 200 of FIG. 2;

FIG. 4a is a graph of voltage across the primary winding of transformer X1 of FIG. 2 versus time;

FIG. 4b is an enlarged view of one cycle of FIG. 4 a;

FIG. 5a is a graph of current consumption versus time through the primary winding of transformer X1 of FIG. 2;

FIG. 5b is an enlarged view of one cycle of FIG. 5 a;

FIG. 6a is a graph of voltage across the secondary winding of transformer X1 of FIG. 2 versus time;

FIG. 6b is an enlarged view of one cycle of FIG. 6 a;

FIG. 7 is a graph of voltage versus time for the output node 206 of FIG. 2;

FIG. 8 is a block diagram of a second embodiment of the present invention;

FIG. 9 shows a first circuit implementation of the second embodiment of the invention shown in FIG. 8;

FIG. 10 shows a second circuit implementation of the second embodiment of the invention shown in FIG. 8;

FIG. 11 is a block diagram of a third embodiment of the present invention;

FIG. 12 shows a circuit implementation of the third embodiment of the invention shown in FIG. 11;

FIG. 13 is a block diagram of a fourth embodiment of the present invention;

FIG. 14 shows a circuit implementation of the fourth embodiment of the invention shown in FIG. 13;

FIG. 15 is a graph of voltage versus time at node 200 of FIG. 14;

FIG. 16 is a graph of voltage versus time at node 201 of FIG. 14; FIG. 17 is a graph of voltage versus time at node 202 of FIG. 14;

FIG. 18 is a graph of voltage versus time at node 203 of FIG. 14;

FIG. 19 is a graph of voltage across the primary winding of transformer X1 of FIG. 14 versus time;

FIG. 20 is a graph of voltage across the secondary winding of transformer X1 of FIG. 14 versus time;

fig. 21 shows a standby power supply of the present invention used in a first application;

FIG. 22 shows the first application shown in FIG. 21 including the second embodiment of the invention shown in FIG. 8; and

fig. 23 shows a standby power supply of the present invention used in the second application.

Detailed Description

Fig. 1 is a block diagram of a first embodiment of the present invention, and fig. 2 shows a circuit implementation of this embodiment.

Referring to fig. 1 and 2, the input is AC power V1. The AC power source may be any AC voltage of any frequency, such as 110VAC, 120VAC, 230VAC, or 240VAC at 50 or 60 Hz. The AC power source V1 is connected to a current limiter 101 including two capacitors C1 and C2. As described below, power consumption can be controlled by varying the value of the capacitor. Then, the AC signal is rectified by the rectifier 103 formed of 4 diodes D1, D2, D3, and D4. Note that the rectifier is a full wave rectifier that provides a DC output with two maxima per AC cycle. Capacitor C3 acts as a voltage limiter 105 to limit the voltage at node 200 to prevent damage to the device from excessive voltages. Capacitor C3 may be omitted if the circuit element has a very high breakdown voltage, i.e., greater than the maximum value of the AC supply peak voltage. Capacitor C3 will be discussed further below.

Setting 111 is a switch between the AC power source and the primary winding of transformer X1, so when the switch is on there is current consumption through the primary winding, and when the switch is off there is no current consumption through the primary winding. Resistor R1 and capacitor C4 together form an RC timer 107 that controls the switching timing of switch 111, as described below. In addition, a large resistor R1 and a small capacitor C4 are selected so that current consumption is small and loss is avoided. Diode D5 acts as timer reset 109 for RC timer 107, providing a discharge path for capacitor C4 when the AC signal at node 200 is low after switch 111 is turned on.

The switch 111 is formed of two transistors Q1 and Q2, two resistors R2 and R3, and a capacitor C5, and is connected to a transformer X1. The switch 111 is set to turn on very quickly by using positive feedback. The advantages of fast turn-on will be discussed later.

On the secondary side of the transformer X1, the diode D6 acts as a rectifier and the capacitor C7 is a filter capacitor. Capacitor C7 charges, thereby providing load R at output node 206_loadProviding a steady state DC voltage.

The operation of the arrangement of fig. 2 is as follows. During the first half of operation, as the voltage of the rectified AC signal at node 200 rises, capacitors C1 and C2 discharge (from the previous half cycle), while capacitor C4 (and capacitor C3, if present) is charged.

When the voltage at node 202 (which is also the base voltage of transistor Q1) is sufficiently high due to the charging of capacitor C4 (which occurs near the peak of the AC signal), the base-emitter of transistor Q1 is forward biased, causing transistor Q1 to turn on. When Q1 turns on, the voltage at node 203 (also the base voltage of transistor Q2) drops. This turns on transistor Q2, resulting in rapid current consumption through the primary side of transformer X1 and resistor R3, which means the voltage at node 204 rises. This voltage rise is transferred back to node 202 via feedback capacitor C5. This means that the voltage at node 202, and therefore the base-emitter voltage of transistor Q1, rises faster, causing more current to flow through the collector-emitter of transistor Q1, resulting in more current being dissipated through the emitter-collector of transistor Q2, and the voltage at node 204 rises further. That is, this arrangement provides a positive feedback system that can produce extremely fast turn-on.

Fast switching is advantageous in that losses in the switch itself are reduced. When the switch 111 is turned on, current flows through the switch. The voltage across the switch (in this case specifically the emitter-collector voltage of transistor Q2) will cause losses. Ideally, the switch should be instantaneously conductive so that the time required to drop the voltage on the switch to ground is instantaneous. (the voltage across the switch is indicated at node 207.) however, instantaneous switching is not possible in practice, but fast switching will shorten the time required to reduce the voltage across the switch, thereby reducing losses. Thus, using positive feedback to increase switching speed reduces losses in the switch itself.

As described above, the switch 111 is turned on once the rectified signal voltage reaches or approaches its peak value. This closes the circuit and causes current to flow rapidly through the primary side of transformer X1 and C1, C2, charging C1 and C2. When this occurs, the voltage at node 200 drops rapidly to ground because node 200 is shorted to ground through the primary winding of transformer X1 when switch 111 is turned on, and because capacitors C1 and C2 in the AC input line act as impedances, there is a voltage drop across C1 and C2 when node 200 drops to ground. Once C1 and C2 are fully charged, current stops flowing (i.e., the switch is effectively turned off). This limits the amount of current flowing to the primary winding of the transformer in each cycle.

When the switch 111 is turned on, the capacitors C3 and C4 are discharged through the primary winding of the transformer X1. Once capacitor C4 has been discharged, RC timer 107 is reset and RC timer 107 and switch 111 wait for the next peak from the rectified AC signal at node 200 in the next half-cycle. In the next half-cycle, when the rectified AC signal rises from 0 to a maximum, the capacitors C1 and C2, which have now been charged, may discharge. As described above, the large resistor R1 is selected so that the current consumed through it is negligible. Therefore, all current will be dissipated through the primary side of transformer X1, which keeps losses at a minimum. It should be appreciated that the direction of the voltage across C1 and C2 alternates every half cycle due to the voltage of the original AC signal.

The short pulses of current consumed on the primary side of transformer X1 cause corresponding current pulses through the secondary side of transformer X1. On the secondary side of the transformer X1, the diode D6 acts as a rectifier and the capacitor C7 is a filter capacitor. Each half cycle of operation, there are current pulses through the secondary side of the transformer X1, and as a result of these current pulses, the capacitor C7 is gradually charged until a steady state DC voltage is reached at the output node 206. The DC voltage is supplied to a load R_load(R_loadFor example, may be a remote control receiver that requires power in a standby mode). The output node 206 provides the necessary output voltage. The value of capacitor C7 is suitably selected to ensure R_loadAnd the device works normally under the required voltage.

As described above, once capacitors C1 and C2 are fully charged, current no longer flows through the windings therefore, the values of capacitors C1 and C2 may be selected to set the current through the windings to a desired level.

Note that in this embodiment it is important that diode D5 be present because it causes the switch to be reset every cycle. Without diode D5, switch 111 would never be reset and the arrangement would not work because after first switching on it would not be able to switch off, the arrangement would only work as the prior art arrangement with constant current consumption through the transformer winding and insufficient voltage across the winding to provide a DC output voltage.

It is further noted that the switch 111 is preferably turned on as close to the peak of the AC signal as possible. This creates a maximum voltage peak across the winding when switch 111 is turned on. If the switch 111 is turned on at the beginning of the AC signal (i.e., when the AC voltage is 0), the arrangement will not work, just as there would be no switch, and there would be no sudden flow of current from the AC power source, nor would there be time for the capacitor C4 (and C3, if present) to charge. That is, the switch must conduct after the rectified AC signal has risen a little bit, and the switch preferably conducts near the peak of the rectified AC signal, as this maximizes the voltage peak.

As described above, C3 acts as a voltage limiter and may be omitted in some cases. However, if C3 is present, it will charge up with C4 as the rectified AC signal rises toward its peak. Thus, when switch 111 is turned on, the energy stored in C4 and C3 will be transferred to the transformer windings. In fact, the contribution of C4 (and C3, if present) to the voltage peak is small; the voltage peak is mainly provided by the current flow directly from the AC power source.

The capacitor C6 and the resistor R4 together form a hysteretic circuit (snubber circuit) 117. The function of the hysteresis circuit 117 is to reduce ringing caused by switching transients. This is added in practical applications to reduce the electromagnetic radiation from the circuit caused by ringing, but the arrangement works without the hysteresis circuit 117.

Figures 3, 4a, 4b, 5a, 6b, 6a, 6b and 7 illustrate various properties of points on the circuit of figure 2 with respect to time. These figures illustrate the processes that occur in each AC cycle as the voltage at the output node increases toward a steady state voltage.

Fig. 3 is a graph of the voltage-time relationship of node 200. During each cycle, the voltage at node 200 rises to a peak value. Then, when switch 111 is turned on, resulting in current being drawn through the primary side of transformer X1, the voltage at node 200 drops to ground. In this example, it can be seen that each period is 10ms, i.e. the frequency of the rectified AC signal is 100Hz, so the AC supply operates at 50 Hz.

Fig. 4a is a graph of voltage across the primary winding of transformer X1 versus time. In each cycle, when the switch 111 is on, there is a voltage peak corresponding to the current consumption through the primary side of the transformer X1. The voltage peak shown in fig. 4a is sharp. Of course, these voltage peaks are not instantaneous, and fig. 4b shows an enlarged view of one cycle of fig. 4 a. Note that with this arrangement, the voltage peaks are large (much larger than they would be in a prior art arrangement where there is no switch between the transformer primary winding and the AC supply) and therefore the DC output voltage can be supplied to the load.

Fig. 5a is a graph of current through the primary winding of transformer X1 versus time. In each cycle, there is a sharp current drain when the switch 111 is on. The consumption current peak corresponds to the voltage peak of fig. 4 a. Of course, the consumption current is not instantaneous, and fig. 5b shows an enlarged view of one cycle of fig. 5 a. The length of time that current is consumed through the primary winding is determined by the supply voltage, the inductance in the transformer winding, and the voltage limiter capacitance C3 (if present).

With each current drain through the primary winding of transformer X1, there is a corresponding current pulse through the secondary winding of transformer X1. Fig. 6a is a graph of voltage across the secondary winding of transformer X1 versus time. It can be seen that in each cycle there is a voltage peak corresponding to the current pulse. Fig. 6b shows an enlarged view of one cycle of fig. 6 a.

As described above, with each current pulse through the secondary winding of transformer X1, capacitor C7 is slightly charged, i.e., the voltage at node 206 rises a little bit.

Fig. 8 is a block diagram of a second embodiment of the invention, and fig. 9 shows a first circuit implementation of this embodiment.

It can be seen that the second embodiment shown in fig. 8 is the same as the first embodiment except for the addition of the voltage regulator 119. That is, in summary, the arrangement includes an AC power source V1, a current limiter 101 (implemented by two capacitors C1 and C2), a rectifier 103 (implemented by 4 diodes D1, D2, D3 and D4), a voltage limiter 105 (capacitor C3), an RC timer 107 (implemented by resistors R1 and C4) for a switch 111 (implemented by transistors Q1 and Q2, resistors R2 and R3 and capacitor C5) and a timer reset 109 (diode D5), a transformer X1, a rectifier 113 (diode D6), a filter 115 (capacitor C7) and an optional hysteretic circuit 117 (implemented by a capacitor C6 and a resistor R3).

The arrangement preferably includes a voltage regulator 119. The function of the voltage regulator 119 is to reduce fluctuations in the DC voltage output (node 206) to the load. This is important for loads that require good supply voltage stability.

Fig. 9 shows a first circuit implementation of the embodiment of fig. 8. In this embodiment, the voltage regulator 119 includes a transistor Q3, a resistor R6, and a zener diode D7. If the output voltage at node 206 (see fig. 7) becomes too high, zener diode D7 will break down. This forward biases the base emitter of transistor Q3, causing transistor Q3 to turn on. By turning on Q3, the charging of C4 will stop because current flows from resistor R1 and transistor Q3 to ground. In fact, RC timer 107 is turned off, and thus switch 111 is turned off. This causes the transfer of energy from the primary side to the secondary side of the transformer X1 to temporarily stop, thereby causing the charging of the capacitor C7 to stop until the output voltage at node 206 falls below the breakdown voltage of the zener diode D7.

Fig. 10 shows a second circuit implementation of the embodiment of fig. 8. In this implementation, the transistor Q3 in the voltage regulator 119 is replaced by an optocoupler IC1, and a resistor R6 and a zener diode D7 are connected as appropriate. The advantage of using an optocoupler is that there is no physical connection between the primary and secondary sides of the circuit. The optocoupler functions as a switch in the circuit, like transistor Q3 in fig. 9. When the voltage at the output node 206 is high enough to cause the zener diode D7 to break down, the Light Emitting Diode (LED) in the optocoupler emits light and the phototransistor therein is turned on. This causes current to flow through resistor R1 and the phototransistor of the optocoupler to ground.

As described above, the photocoupler means that the primary side and the secondary side of the circuit are not physically connected because the switching function is realized by light. For safety reasons (because of the high voltage on the primary side of the circuit), the use of optocouplers may be more acceptable, because both sides of the circuit are then isolated.

Fig. 11 is a block diagram of a third embodiment of the present invention, and fig. 12 shows a circuit implementation of this embodiment.

It can be seen that the third embodiment shown in fig. 11 is the same as the first embodiment, except that the timer reset 109 is no longer required and the switch controller 610 and MOSFET switch 611 replace the switch 111. That is, in summary, the arrangement includes an AC power source V1, a current limiter 101 (implemented by two capacitors C1 and C2), a rectifier 103 (implemented by 4 diodes D1, D2, D3 and D4), a voltage limiter 105 (capacitor C3), an RC timer 107 (implemented by a resistor R1 and a capacitor C4), a transformer X1, a rectifier 113 (diode D6), a filter 115 (capacitor C7) and an optional hysteretic circuit 117 (implemented by a capacitor C6 and a resistor R4). The arrangement also includes a MOSFET switching device 611 between the AC power supply and the primary winding and the switch controller 610. The same parts of the arrangement as in fig. 1 and 2 will not be described in detail below.

Arrangement 611 is a MOSFET switching device located between the rectified AC voltage and the primary winding of transformer X1 such that when the switch is on there is current consumption through the primary winding and when the switch is off there is no current consumption through the primary winding, resistor R1 and capacitor C4 together form an RC timer 607 which controls the switching timing of switch 611 by switch controller 610, as described below, in addition, large resistor R1 and small capacitor C4 are selected to minimize current consumption and thereby prevent losses, switch controller 610 (formed by two transistors Q11 and Q12, zener diode D11 and two resistors R12, R13) is connected to transformer X1., transistors Q11 and Q12 forming thyristor devices.

The operation of the arrangement of figures 11 and 12 is as follows. In the first half-cycle of operation, as the voltage of the rectified AC signal at node 200 rises, capacitors C1 and C2 are discharged (starting from the previous half-cycle), while capacitor C4 (and capacitor C3, if present) is charged. When the voltage at node 702 is sufficiently high due to the charging of capacitor C4 (which occurs near the peak of the AC signal), the thyristor device will turn on because the voltage across C4 rises sufficiently above the breakdown voltage of zener diode D11 to forward bias the base of Q12. After the thyristor device is turned on, the switch 611 is turned on. Switch 611 remains on until C4 has discharged through the thyristor device and the voltage at node 703 falls below the gate threshold voltage of switch 611. This cycle is then repeated continuously.

As described above, the switch 611 conducts once the rectified signal voltage reaches or approaches its peak value. This closes the circuit and causes current to flow rapidly through the primary side of transformer X1 and C1, C2, charging C1 and C2. When this occurs, the voltage at node 200 drops rapidly to ground because node 200 is shorted to ground through the primary winding of transformer X1 when switch 611 is turned on, and because capacitors C1 and C2 in the AC input line act as a high impedance. When the node 200 falls to ground, there is a voltage drop across C1 and C2. Once C1 and C2 are fully charged, current stops flowing (i.e., the switch is effectively turned off). This limits the amount of current flowing to the primary winding of the transformer in each cycle.

When switch 611 is conductive, capacitor C3 discharges through the primary winding of transformer X1. Once capacitor C4 has been discharged through the thyristor device, RC timer 607 is reset and RC timer 607 and switch 611 wait for the next peak from the rectified AC signal at node 200 in the next half-cycle. In the next half-cycle, when the rectified AC signal rises from 0 to a maximum, the capacitors C1 and C2, which have now been charged, may discharge. As described above, the large resistor R1 is selected so that the current consumed through it is negligible. Therefore, all current will be dissipated through the primary side of transformer X1, which keeps losses at a minimum. It should be appreciated that the direction of the voltage across C1 and C2 alternates every half cycle due to the voltage of the original AC signal.

As with the other embodiments, the short pulses of current consumed on the primary side of transformer X1 cause corresponding current pulses through the secondary side of transformer X1, so that capacitor C7 is gradually charged until a steady state DC voltage is reached at output node 206. The DC voltage is supplied to a load R_load。

As described above, once the capacitors C1 and C2 are fully charged, current no longer flows through the windings. Thus, as in the other embodiments, the values of the capacitances C1 and C2 may be selected to set the current through the windings to a desired level. This controls the amount of power consumption.

As before, note that switch 611 preferably conducts as close as possible to the AC signal peak. This creates a maximum voltage peak across the winding when switch 611 is turned on. If the switch 611 is turned on at the beginning of the AC signal (i.e., when the AC voltage is 0), the arrangement will not work because this is just as if there were no switch 611, and there would be no sudden flow of current from the AC power source, nor would there be time for the capacitor C4 (and C3, if present) to charge. That is, the switch 611 must be turned on after the rectified AC signal has risen a little bit, and the switch preferably turns on near the peak of the rectified AC signal because this maximizes the voltage peak.

As described above, C3 acts as a voltage limiter and may be omitted in some cases. However, if C3 is present, it will charge up with C4 as the rectified AC signal rises toward its peak. Thus, when switch 611 is conducting, the energy stored in C4 and C3 will be transferred to the transformer windings.

Fig. 13 is a block diagram of a fourth embodiment of the present invention, and fig. 14 shows a circuit implementation of this embodiment.

It can be seen that the fourth embodiment shown in fig. 13 is identical to the third embodiment, except that the current limiter 101 and the voltage limiter 105 have been eliminated from the circuit. Broadly speaking, the arrangement comprises an AC supply V1, a rectifier 103 (implemented by 4 diodes D1, D2, D3 and D4), an RC timer 107 (implemented by a resistor R1 and a capacitor C4), a transformer X1, a rectifier 113 (diode D6), a filter 115 (capacitor C7) and an optional hysteretic circuit 117 (implemented by a capacitor and a resistor). In this embodiment, the rectifier 103 may be a half-wave rectifier including only one diode. The fourth embodiment also includes the MOSFET switch device 611 and the switch controller 610 of the third embodiment. The various parts of the circuit that have been described previously will not be described in detail below.

Fig. 15, 16, 17, 18, 19 and 20 show various circuit characteristics at points on the circuit of fig. 14 versus time. These figures show the circuit characteristics during each AC cycle as the voltage at the output node increases towards a steady state voltage. The operation of the fourth embodiment is described below with reference to fig. 15, 16, 17, 18, 19, and 20.

Fig. 15 is a graph of the voltage-time relationship of node 200. When the voltage at node 200 begins to rise from zero, capacitor C4 of RC timer 107 charges through resistor R1. When the voltage across C4 (the voltage at node 201 shown in fig. 16) reaches the breakdown voltage of zener diode D5, the thyristor device of Q1 and Q2 is turned on, and then switch Q3 is also turned on. To maximize the available power output, switch Q3 is preferably turned on when the voltage at node 200 reaches its peak. This synchronization may be achieved by changing the time constant of RC timer 107.

Once the thyristor device is turned on, it constantly consumes current from C4 and R1. Since R1 is a high resistance and C4 is a low capacitance, the voltage across C4 (at node 201) drops very quickly, as shown in fig. 16.

In order for C4 to charge in the next cycle, the thyristor device must be turned off before the next cycle begins. When the voltage at node 200 drops to a value that does not provide enough current through R1 to keep the thyristor device operational, the thyristor device turns off.

Referring to fig. 17, it can be seen that when the thyristor device is turned on, the gate voltage of MOSFET Q3 (node 202) rises rapidly, turning onQ3. When Q3 is "on," current flows through the primary winding of transformer X1. Then, the energy is expressed as E-1/2 LI²Is stored, where E is the stored energy, L is the inductance of the primary winding of the transformer, and I is the current flowing through the primary winding.

As the current flowing in the primary winding increases, the voltage across R4 (node 203) assumes a ramp-like pattern (fig. 18). In operation, the gate voltage of Q3 falls while its source voltage rises. When the difference between the gate-source voltages falls below its threshold voltage, Q3 turns off. Once Q3 is turned off, the energy stored in the primary winding is transferred to the secondary winding. Fig. 19 and 20 show the voltage across the primary and secondary windings of transformer X1, respectively.

It should be noted that Q3 only turns on and off once during a half AC cycle. Fig. 15 shows the switching operation at a frequency of about 120 Hz. Thus, this and the above embodiments of the invention have lower switching losses compared to conventional switched mode power supplies operating at higher frequencies.

Fig. 21 shows a power supply of the invention used in a first application and fig. 22 shows an application comprising a second embodiment of the invention (as shown in fig. 8). Fig. 21 and 22 show a power supply used as a standby power supply in an electric appliance such as a television or a washing machine. The appliance is directly connected to an AC power source to provide power for its own operation during normal use. (FIG. 23 shows a power supply for use in an external power supply, such as a mobile cellular telephone charger, which will be discussed below.)

Fig. 21 shows an appliance 1101 connected to an AC power source, e.g. mains power, which when in an operating mode operates on mains power, but which can be switched from operating mode to standby mode and vice versa the appliance 1101 generally has mains power and some form of control in this case the control function is implemented using a remote control receiver 1105 in the appliance, which can have external control means, e.g. a remote control, and internal control means, e.g. automatic standby after a period of idle time, the appliance also comprises a power supply 1107 and a control circuit 1109 according to the invention for providing power during standby mode.

The operation of this arrangement will now be described generally. During normal operation of the appliance 1101, the main power supply 1103 provides power to the remote control receiver 1105 and other functions of the appliance. When an instruction is given to the remote control receiver 1105 to put the system into a standby mode, the main power supply 1103 can be turned off, and the remote control receiver 1105 can control the main power supply 1103 to be turned off via the control circuit 1109. The provision of power to remote control receiver 1105 in standby mode will then be taken care of by standby power supply 1107 so that remote control receiver 1105 can wait for instructions to turn on the system. The standby power supply 1107 may also supply power for turning on the main power supply 1103 via the control circuit 1109 when an instruction to turn on the system is transmitted from the remote control receiver 1105, and the remote control receiver 1105 may control the main power supply 1103 to be turned on via the control circuit 1109.

Fig. 22 shows the arrangement of fig. 21 using the power supply of fig. 8 (i.e., the already described second embodiment) as the standby power supply 1107, and a more specific operation will now be described.

Fig. 22 shows AC power connected to the main power supply 1103 and the standby power supply 1107. The main power supply 1103 is connected to the output for the main functions of the appliance during normal operation. The main power supply is also connected to a control circuit 1109, the control circuit 1109 being connected to the output node 206 of the standby power supply 1107 and to a remote control receiver 1105 acting as a load for the standby power supply 1107. The main power supply 1103 also provides power to a microcontroller in the remote control device.

When the main power supply 1103 is on (i.e., during normal operation), the voltage provided to the remote control receiver 1105 at the output node 206 is set to be slightly higher than the breakdown voltage of the zener diode D7 of the voltage regulator 119. This will cause the base-emitter of transistor Q3 in regulator 119 to be forward biased, causing transistor Q3 to turn on. This means that current will flow to ground via resistor R1 and transistor Q3, thereby preventing capacitor C4 from being charged. Accordingly, the RC timer 107 is turned off, and thus the switch 111 is turned off. This means that during normal operation when the main power supply 1103 is on, the standby power supply 1107 is turned off.

When the main power supply 1103 is off (i.e., during standby operation), the voltage at the output node 206 will drop below the breakdown voltage of the zener diode D7. When transistor Q3 is turned off, RC timer 107 will turn on and switch 111 will activate, i.e., turn on and off twice per AC cycle according to RC timer 107 and timer reset 109, to provide a pulsed drain current through the secondary winding to stably charge capacitor C7. This means that when the main power supply 1103 is switched off, the standby power supply 1107 will be switched on to supply power (DC voltage at the output node 206) to the remote control receiver 1105 during standby mode. As described above, the standby power supply 1107 may also provide power for turning on the main power supply 1103 via the control circuit 1109 when there is an instruction from the remote control receiver 1105 to turn on the system.

Fig. 23 shows the power supply of the present invention used in the second application. Fig. 23 shows a power supply used in an external power supply. An external power source is a power source that takes input from an AC power source and provides power to its load, typically in the form of a DC voltage. One example of such an external power source is a telephone charger.

Fig. 23 shows an external power supply 1301 for supplying external power, which is connected to an AC power supply (e.g., a mains power supply). During normal operation, the main power supply 1303 will provide power to the load at the output. The sensor 1305 may be a current sensor that turns on the main power supply 1303 when a load is present (e.g., when a device to be charged is connected to a charger) and turns off the main power supply when the load is removed. During standby mode, power is provided by standby power supply 1307.

This arrangement works in a similar manner to the arrangement described with reference to figures 21 and 22. When the main power supply 1303 is on (i.e., during normal operation), the standby power supply 1307 is turned off. When the load is removed (i.e., during standby mode), the sensor turns off the main power supply 1303 and the standby power supply 1307 is turned on. When a load is connected to the external power supply 1301, the standby power supply 1307 supplies power to the sensor 1305 to turn on the main power supply 1303 from the standby mode to the normal mode.

As can be seen from the above description, the present invention provides a power supply with low power consumption. The power supply can be used in many applications where low power consumption is important. Some examples are as follows: standby power in electronic devices (e.g., televisions, washing machines, microwave ovens, stereos, and other devices operating in normal and standby modes); a standby power supply in the external power supply for supplying power to detect whether the main power supply is connected and turned on to the electronic apparatus (for example, in a portable telephone charger); or a stand-alone power supply for electronic devices that require low power consumption, such as a night light that plugs into an AC wall outlet to provide dim light, including low power external power supplies. Other application examples are also contemplated.

The power consumption of the above power supply can be very low and certainly can be as low as a few milliwatts, which, as mentioned above, is typical of the power required to "wake up" a device from standby. This is in contrast to the typical power consumption using conventional methods, which typically range from a few hundred milliwatts to a few watts. The actual power provided can be set as desired by changing the values of the circuit components.

Claims

1. A power supply for an electronic device, the power supply comprising:

a transformer comprising a primary winding on a primary side connectable to an alternating voltage source and a secondary winding on a secondary side, circuitry on the secondary side being arranged to provide a direct current output voltage for the electronic device;

a switch between the primary winding of the transformer and the AC voltage source;

a switch timer for controlling a switching timing of the switch; and

a rectifier for rectifying an alternating voltage of the alternating voltage source;

wherein the switch is arranged to switch on at some point in time when the rectified AC voltage rises from zero to a maximum and when the rectified AC voltage has risen to a predetermined value not equal to zero, under control of the switch timer, to provide a current through the primary winding and thus a current through the secondary winding, and

wherein the switch is arranged to switch off before the rectified AC voltage starts to rise again under control of the switch timer.

2. The power supply of claim 1, wherein the switch is arranged to conduct near each peak of the rectified ac voltage.

3. The power supply of claim 1, wherein the switch timer is coupled to and controls the switching timing of the switch by a switch controller.

4. The power supply of claim 1, further comprising a current limiter connected between the ac voltage source and the rectifier for limiting the amount of current flowing through the primary winding.

5. The power supply of claim 4, wherein the current limiter comprises at least one charge storage device.

6. A power supply as claimed in claim 5, wherein the power supply is arranged to stop current flow through the primary winding once at least one charge storage device of the current limiter has been fully charged.

7. The power supply of claim 1, wherein the switch timer is operable with a switch timer reset, the switch timer reset being arranged to reset the switch timer after the switch has been turned off.

8. The power supply of claim 1, wherein the switch is arranged to use positive feedback to achieve fast switching from off to on.

9. The power supply of claim 1, further comprising a voltage limiter connected to the primary winding for preventing breakdown of the device at high voltages.

10. The power supply of claim 9, wherein the voltage limiter comprises a charge storage device configured to charge as the rectified ac voltage increases from zero to a maximum.

11. The power supply of claim 1, wherein the rectifier is configured to full-wave rectify an alternating voltage of the alternating voltage source.

12. The power supply of claim 1, wherein circuitry on the secondary side provides the output voltage to the electronic device via a charge storage device that is charged every ac cycle.

13. The power supply of claim 1, further comprising circuitry connected to the secondary winding for reducing electromagnetic radiation caused by switching of the switch.

14. The power supply of claim 1, further comprising a voltage regulator for stabilizing the dc output voltage.

15. The power supply of claim 1, wherein the current flows through the secondary winding when the switch is off.

16. An electronic device comprising the power supply of claim 1.

17. A method of providing power to an electronic device, the method comprising:

providing a transformer having a primary winding and a secondary winding, the primary winding being connected to an alternating voltage source via a switch;

providing a rectifier for rectifying an alternating voltage of the alternating voltage source;

turning on the switch under control of a switching timer to provide current through the primary winding and thus through the secondary winding when the rectified ac voltage has risen from zero to a maximum value and when the rectified ac voltage has risen to a predetermined value not equal to zero;

converting current through the secondary winding to a direct current output voltage for the electronic device; and

under control of the switch timer, the switch is turned off before the rectified AC voltage starts to rise again.

18. The method of claim 17, wherein the step of turning on the switch comprises turning on the switch near each peak of the rectified ac voltage.

19. The method of claim 17, wherein current flowing through the primary winding is limited by a current limiter connected between the ac voltage source and the rectifier.

20. The method of claim 19, wherein the current limiter comprises at least one charge storage device.

21. The method of claim 20, wherein the step of turning off the switch comprises turning off the switch once one or more charge storage devices of the current limiter have been fully charged.

22. The method of claim 17, further comprising the step of charging a charge storage device that is part of the switching timer as the rectified ac voltage increases from zero to a maximum value.

23. The method of claim 17, further comprising the step of charging a charge storage device as the rectified ac voltage increases from zero to a maximum, wherein the charge storage device acts as a voltage limiter to prevent breakdown of the device at high voltages.

24. The method of claim 17, wherein the current flows through the secondary winding when the switch is off.

25. An electronic device connectable to an alternating voltage source and operable in each of a normal mode and a standby mode, the electronic device comprising:

a main power supply for supplying power during a normal mode;

control means for switching on and off the main power supply; and

a standby power supply for supplying power for turning on the main power supply to the control apparatus during a standby mode, the standby power supply comprising:

a switch timer for controlling a switching timing of the switch; and

26. The electronic device defined in claim 25 wherein the standby power supply further comprises a current limiter connected between the ac voltage source and the rectifier for limiting the amount of current flowing through the primary winding.

27. The electronic device of claim 25, wherein the current flows through the secondary winding when the switch is off.