HK1118968A - Switch power supply unit and ic for overload and saturation protection - Google Patents

Description

Switching power supply device and IC for preventing overload and saturation

The present application is a divisional application of chinese patent application having an application date of 27/1/2003, an application number of 03808179.2, and a name of "green switching power supply with standby function and IC thereof".

Technical Field

The invention belongs to the technical field of switching power supply design, and particularly relates to a method for preventing current overload and saturation of a switching power supply, the switching power supply and an IC (integrated circuit).

Background

As for the switching power supply, there are classifications or definitions such as DC-DC (which means that one direct current power supply is converted into another direct current power supply or more than one direct current power supply) or AC-DC (which means that an alternating current power supply is converted into one direct current power supply or more than one direct current power supply), isolated or non-isolated conversion circuits, Boost (Boost type) or Buck (Buck type) conversion circuits, CCM (continuous current mode) or DCM (discontinuous current mode), and the like; the isolated conversion circuit has a single-end type (flyback type, forward type) and double-end type (push-pull type, half-bridge type and full-bridge type) structure; the transformation techniques include hard switching transformation, soft switching (or resonant switching, or zero switching) transformation; the control technology comprises a PFM (pulse frequency modulation) mode, a PWM (pulse width modulation) mode, a voltage mode and a current mode; and so on. Regardless of the classification and definition of the above known technologies, various switching power supply circuits can be simplified to: the conversion circuit comprises a power tube, a transformer or an inductor and one or more than one rectifying and filtering output circuit, wherein the power tube has one single-end power tube and a plurality of double-end power tubes, the soft switch conversion is adopted and one or more than one auxiliary power tube and the inductor are used for simple non-isolated DC-DC conversion, and the determination of the conversion circuit means that the switching power supply adopts the inductor or the single-end or double-end, hard switch or soft switch and the like; the feedback circuit (voltage or current, etc.) comprises a sampling circuit, an error amplifier and sometimes a feedback isolation circuit, wherein the sampling circuit samples signals of the voltage or current, etc. of the output circuit and sends the signals to the error amplifier for comparison and amplification, and the error amplifier outputs error signals; the control circuit comprises a pulse modulation circuit and a driving circuit, wherein the pulse modulation circuit has a PFM mode (comprising a PFM circuit and a pulse width shaping circuit which are not commonly used), a PWM mode (comprising a PWM circuit and an oscillator which are commonly used) and other modes, generates basic pulses according to error signals, adopts a double-end mode and a frequency division complementary double-pulse generation circuit and adopts soft switching conversion and a soft switching multi-pulse generation circuit, the basic pulses or the double pulses or the multi-pulse are transmitted to the driving circuit to drive a power tube, generally, the larger the error signals are, the larger the pulse duty ratio is, the higher the peak current of the power tube is, and the easier the transformer is saturated; and the auxiliary circuit is selected from auxiliary circuits such as a starting circuit, a protection circuit, a voltage reference circuit, an EMC circuit, an alternating current rectification filter circuit and the like according to needs, wherein the protection circuit comprises protection circuits such as low voltage, high voltage, upper limit current and the like. When the switching power supply is started or overloaded, the transformer or the inductor is easily saturated, and the power tube is easily subjected to overcurrent; therefore, in the prior art, the switching power supply IC (e.g., UCx842, TOP210, etc.) adopts an upper-limit current protection circuit, i.e., the power tube is immediately turned off when the upper-limit current is reached, and the control circuit and the power tube are required to have a fast response capability, otherwise, the potential safety hazards such as damage to the transformer or the power tube still exist. As the start-up circuit, a resistance start-up circuit and a turn-off constant current source start-up circuit are known.

Disclosure of Invention

The invention aims to overcome the defects of the prior art and provides a method for preventing the current overload and saturation of a switching power supply, the switching power supply and an IC (integrated circuit).

The invention provides a method for preventing current overload and saturation of a switching power supply, which is characterized by comprising the following steps of:

1) detecting whether the current of a primary or an inductor or a power tube of the transformer exceeds an upper limit current or not;

2) if the current exceeds the upper limit current, an adjusting signal adjusting or an error signal is generated to adjust, so that the duty ratio is reduced and the peak current of the primary side of the transformer or the inductor or the power tube is reduced in the next pulse modulation period or a plurality of pulse modulation periods. The error signal is an output signal of the error amplifier or an input signal of the pulse modulation circuit; the adjustment error signal is a direct adjustment error signal; the indirect adjustment of the error signal is achieved by adjusting an input signal of the error amplifier or adjusting an output signal of the sampling circuit.

The specific method of the step 2) can be; when the over-limit current is detected, the error signal is adjusted once, and the adjustment amount is a fixed value.

The specific method of the step 2) can also be used; when the current exceeding the upper limit is detected, continuously adjusting the error signal in the current and the following pulse modulation periods, wherein the adjustment amount is from large to zero; in the continuous adjustment process, if the over-limit current is detected again, the adjustment process is restarted, and the adjustment amount is increased to zero again.

The specific method of step 2) is preferably to adjust the error signal, and the output of the error amplifier is preferably an open-circuit pull-up output and has a load capacitor, and is forcibly adjusted to discharge capacitor charges.

The invention provides a switching power supply adopting the method, which is an overload-proof and saturation-proof switching power supply and comprises the following steps: a conversion circuit (including a power tube, a transformer or an inductor, one or more than one rectifying and filtering output circuits, and sometimes a soft switch circuit, etc., which are known various conversion circuits), a feedback circuit (including a sampling circuit, an error amplifier, and sometimes a feedback isolation circuit), the control circuit (including pulse modulation circuit and drive circuit, the pulse modulation circuit has PFM way and PWM way or other ways, adopt PFM way have pulse frequency regulating circuit and pulse width shaping circuit, adopt PWM way have pulse width regulating circuit and oscillator, adopt the double-ended type and also frequency division complementary double pulse generating circuit, adopt the soft switch technology and also soft switch multipulse generating circuit) and auxiliary circuit (choose according to the needs of the switching power supply in starting circuit, protective circuit, voltage reference circuit, EMC circuit, AC rectification filter circuit, etc.; the protection circuit of the auxiliary circuit at least comprises a current sampling circuit of a primary transformer or an inductor or a power tube, an upper limit current detection circuit of the primary transformer or the inductor or the power tube and an adjusting circuit for adjusting or indirectly adjusting an error signal according to an output signal of the detection circuit, wherein the current sampling circuit, the upper limit current detection circuit and the adjusting circuit are sequentially connected. The adjusting circuit can adopt a D trigger with high level presetting triggered by a falling edge, a clock signal of the D trigger is a basic pulse generated by a pulse modulation circuit of the control circuit according to an error signal from a feedback circuit (when the basic pulse is high level, the only power tube or one power tube of the converting circuit is opened), a data port of the D trigger inputs low level, a preset input port of the D trigger inputs an output signal of the detecting circuit (when the detecting circuit detects that the upper limit current is exceeded, the high level is output), and when the D trigger is high level, an open circuit output executes adjustment or indirectly adjusts the error signal; therefore, when the over-limit current is detected, the adjusting circuit adjusts the error signal once, and the adjusting amount is a fixed value.

The switching power supply adopts a single-end type conversion circuit as a conversion circuit, a triode is adopted as a power tube, and the driving circuit has at least two paths of output signals, wherein one path of output signals is connected with the base electrode of the triode, and the other path of output signals is connected with the emitting electrode of the triode; the base of the triode is connected with a high-voltage power supply through a resistor with high resistance. The conversion circuit is matched with a corresponding circuit, a resistor with a high resistance value and a triode can be used as a part of a power-on starting circuit of the auxiliary circuit, and the withstand voltage of the triode is improved.

The switching power supply adopts a switching power supply IC, and the IC at least integrates part of a control circuit and part of a protection circuit.

The invention has the characteristics and good effects that: the overload protection and saturation prevention are adopted, the method has outstanding technical advantages and high quality, and can effectively reduce the cost and improve the quality.

Drawings

Fig. 1 is a schematic diagram of an overload protection and saturation protection unlimited PWM switching power supply with a starting circuit.

Fig. 2 is a schematic diagram of another overload protection and saturation prevention non-limiting PWM switching power supply with a starting circuit.

Fig. 3 is a schematic diagram of an embodiment of a preferred overload and saturation prevention non-limiting PWM main power supply.

The voltage values and the resistance values marked in all the figures are unlimited values and can be set to other values according to design targets and requirements; the MOS tube or the triode can change the drive to be exchanged with the triode or the MOS tube.

Detailed Description

The following is a description of non-limiting embodiments of the invention and the accompanying drawings.

Fig. 1 and 2, the Q1 is a preferred economical power transistor (e.g. 13003, BUX87, etc.) as an independent switch power supply (e.g. a charger, a green switch power supply IC standby power supply unit, or a universal switch power supply); qd is a preferable internal power tube or an external power tube; the dotted line frame is an IC part, however, Rb and Qa can be integrated in the IC or externally arranged according to the semiconductor process, Rb can be integrated in the IC according to the optimized resistance value of the general smaller output power, and the external parallel resistor can meet the requirement when the output power is larger than the internal output power. Fig. 3 can be used as the main power source of a green switching power supply IC, the dashed box is the IC circuit part, and the high-power tube Q2 can be externally arranged or integrated in the IC. Ia and Ib current sources.

S0, Schmitt comparator, IC power voltage monitor circuit (or part of the start-up circuit), which is activated if S0 is low, and is activated if S0 is high. Fig. 1 shows a startup state, where Qa is off, pcl.qc is high-resistance (or output is controlled), and high-voltage high-resistance R1 provides a base micro-current to turn on a power tube Q1 with a smaller collector current, and charges an IC power capacitor C0 through a diode Da to form a startup circuit, and for the startup safety of Q1, the startup circuit can detect a charging current, control pcl.qc output, change Q1 base current, and make Q1 current a safety value (e.g., 3 mA); in a normal state, PCL, QC and Qa are normal outputs, and R1 is out of action; therefore, considering the amplification of Q1, the loss sustained by the start-up circuit in the normal state is at least an order of magnitude smaller than that of the start-up circuit limited by resistance. FIG. 2, in the start-up state, the high voltage current source is turned on to charge the capacitor C0, forming a PWMs start-up circuit; and in the normal state, the PWMs restore to the normal state, and the high-voltage current source is closed. In fig. 3, since the main power supply and the standby power supply can share the IC power supply voltage monitoring circuit, S0 can also be made to act on PWM2 (not shown), and in the on state, PWM2 is turned off.

FIG. 1, in normal state, PCL.QC and PCL.Q outputs are the same, e.g. output high, Q1 and Qa are turned on, Rb detects Q1 instantaneous current; if the output is changed from high level to low level, Qa is cut off, but due to the storage effect, Q1 is not cut off immediately, diode Da freewheels, or a delay circuit is designed to delay Qa to cut off Q1 and then turn off, or Qa clamps the emitter of Q1 to about 1.5V (the value can improve the withstand voltage of Q1 and lower the power consumption of IC), and the base voltage of Q1 is 0V, so that the collector withstand voltage of Q1 is improved. FIG. 2, in normal state, when PCLS. Q outputs high level, Qd is turned on, and Rb detects Qd instantaneous current; if the output is low, Qd is turned off. FIG. 3 shows that in normal state, if PCL2.Q outputs high level, Q2 is turned on, and R2 detects Q2 transient current; if the output is low, Q2 is off.

S2, the PWM comparator (or PWM circuit, shown in current mode or voltage mode) starts to conduct on the power tube at the rising edge of the oscillator Q, the primary current of the transformer increases, the voltage drop of Rb or R2 also increases, when the voltage drop is equal to or greater than the error signal (the voltage UC1 or UC2 reflected on C1 or C2), S2 outputs low level, and the power tube is turned off; however, the oscillator determines the maximum duty ratio, and the principle is that S2 still outputs high level, the oscillator Q becomes low level, the power tube is turned off S1, and the schmitt comparator (or as a main power supply inhibiting circuit) is based on the principle that the PWM period of the power tube is forcibly turned off when the error signal is lower than the set value (threshold), and the PWM period of the power tube is turned on when the error signal is higher than the set value, so that the conversion efficiency of the switching power supply under light load is improved.

S3, upper limit current comparator (or upper limit current detection circuit), if the transformer primary or power tube reaches the upper limit current, S3 turns on the overload protection and saturation protection control logic S5 (adjustment circuit) and turns off the power tube (optionally, the reason is that turning off the power tube is responsible for S2).There are many solutions to S5, and the simplest solution is considered in the present invention to be that S5 is enabled once, and S4 is turned on for one oscillator cycle; however, the condition is satisfied that the average current (referred to as I4) of S4 is greater than the current source Ia (fig. 1 and 2) or the main voltage feedback current reduction source Ib (fig. 3, difference Ic) during a PWM (or oscillator) cycle; i4 and Ia or Ic can jointly contribute to UC1 or UC2 within one PWM cycle, and can be selected at 2.8V^*Within (-10%), and the maximum output current should be above 95%, such as Ia contributing 2.8V to UC1^*3.3%, then I4 is preferably 3-4 times Ia; therefore, the error signal is reduced (i.e. the error signal is forcibly adjusted), the duty ratio becomes smaller in the next PWM period or periods, and the peak current of the transformer primary or the power tube is reduced; for a fast power tube, a transformer with sufficient capacity and a control circuit with fast response, when overload occurs, an error signal is near the maximum value; for a slower power tube, or a transformer with insufficient capacity (once the transformer is saturated, the primary current of the transformer rises rapidly and reaches or exceeds the upper limit current) or a control circuit with slow response, when the overload occurs, the error signal is smaller than the maximum value of theoretical calculation, and the control circuit switches off the power tube in advance; although the power tube exceeds the upper limit current or the transformer is saturated, the time is extremely short, the safety of the power tube and the transformer can be ensured, and the reliability is improved.

Alternatively to S5, S5 is activated once, I4 ═ Ia (or Ic)^*1.2; subsequent PWM cycle S5 if not enabled, I4 ═ Ia (or Ic)^*0.8, and then stops S5; the above-mentioned multiples 1.2 and 0.8 may be other values greater than 1 and less than 1, but the transient response of the switching power supply should be taken into account; the scheme can further improve the protection of the power tube and the transformer and increase the maximum output current. The S5 may also adopt a digital processing logic scheme to adaptively converge the I4 when overloaded. To facilitate monitoring during use, S5 preferably outputs an overload monitoring signal (the principle is that S5 is activated once to produce an output, which can be directly indicated by an LED, not shown).

In the single-ended continuous current mode shown in fig. 1, 2 and 3, delay circuits should be designed for PCL, PCLs, PCL2 and S5 to prevent the power tube from being turned off by mistake or turned on by mistake by S5 due to turn-on spike.

The PWM control technology of the overload and saturation prevention switching power supply is also suitable for structures such as a push-pull type structure, a half-bridge type structure and a full-bridge type structure, if the overload and saturation prevention protection circuit is added to detect that the primary current of the power tube or the transformer exceeds the upper limit current, the error signal is forcibly adjusted (for example, S3, S5 and S5 are added to TL494 to forcibly adjust the level of 3 pins or 4 pins), so that in the next PWM period or a plurality of PWM periods, the duty ratio is reduced, the primary peak current of the power tube or the transformer is reduced, the power tube and the transformer are protected, and the safety and the reliability of the switching power supply are improved.

The above discussion of FIG. 1 may be partly modified as follows: a single economical switch power triode is adopted, and the PWM control circuit (or IC) is suitable for a single-end type. When the power supply is started, one path of high-resistance state (or controlled output) connected with the base electrode is provided, the high-voltage high-resistance resistor provides base electrode micro current of the power tube, and the power tube emits the micro current to charge (or limit current charging) the filter capacitor of the IC power supply through the diode, so that the starting is finished. In a normal state, the PWM is in a positive period, one path of the PWM makes the base electrode of the power tube biased positively, the other path of the PWM pulls down the emitting electrode of the power tube, and the power tube is conducted; in the PWM negative period, one path of the base electrode of the power tube is pulled down, the power tube cannot be immediately cut off due to the storage effect, the diode can be used for enabling the emitter of the power tube to continue current, or the pull-down emitter is turned off when the power tube is cut off in a time delay mode, or the emitter of the power tube is clamped, but after the power tube is cut off, the base electrode of the power tube is reversely biased, and therefore the collector withstand voltage of the power tube is improved.

Claims (4)

1. A method of preventing switching power supply current overload and saturation, comprising the steps of:

1) detecting whether the current of a primary or an inductor or a power tube of the transformer exceeds an upper limit current or not;

2) if the current exceeds the upper limit current, an adjusting signal adjusting or an error signal is generated to adjust, so that the duty ratio is reduced and the peak current of the primary side of the transformer or the inductor or the power tube is reduced in the next pulse modulation period or a plurality of pulse modulation periods.

2. A switching power supply employing the method of claim 1, comprising: the circuit comprises a conversion circuit, a feedback circuit, a control circuit and an auxiliary circuit; the protection circuit of the auxiliary circuit at least comprises a current sampling circuit of a primary transformer or an inductor or a power tube, an upper limit current detection circuit of the primary transformer or the inductor or the power tube and an adjusting circuit for adjusting or indirectly adjusting an error signal according to an output signal of the detection circuit, wherein the current sampling circuit, the upper limit current detection circuit and the adjusting circuit are sequentially connected.

3. A switching power supply IC using the method according to claim 1, characterized in that at least a control circuit and a protection circuit are integrated; the protection circuit at least comprises an upper limit current detection circuit of a transformer primary or an inductor or a power tube and an adjusting circuit which executes adjustment or indirectly adjusts an error signal according to an output signal of the detection circuit, wherein the upper limit current detection circuit and the adjusting circuit are sequentially connected.

4. The switching power supply IC of claim 3 wherein the control circuit includes a PWM circuit, an oscillator, and a driver circuit, the PWM circuit outputting pulses to the driver circuit, the driver circuit having two outputs, one of which drives the base of the power transistor and the other of which drives the emitter of the power transistor.