HK1232376B - A phase control dimmer circuit with short-circuit protection - Google Patents

Description

Phase control dimmer circuit with short-circuit protection

Technical Field

The present invention relates to a phase control dimmer circuit with short circuit protection for controlling alternating current (AC) power to a load.

In particular, but not exclusively, the present invention relates to a trailing edge phase control dimmer circuit having a MOSFET switching circuit with short circuit protection for controlling a capacitive load, such as a driver for an LED lamp.

Background Art

Dimmer circuits are commonly used to control power to a load, such as a light source, particularly alternating current (AC) mains power. In one existing approach, phase-controlled dimming can be used to dim the light source, thereby controlling the power supplied to the load by varying the amount of time a switch connecting the load to the mains power is on during an AC cycle (i.e., varying the duty cycle). Specifically, the AC power to the load is switched on and off during each half-cycle of the AC, and the amount of dimming provided to the load is determined by the amount of on-time relative to the off-time during each half-cycle.

Phase control dimmer circuits are typically operated as trailing edge or leading edge dimmer circuits, and the two circuits are suitable for different applications. In a leading edge circuit, power is cut off at the beginning of each half cycle. In a trailing edge circuit, power is cut off at the rear of each half cycle (e.g., towards the end of each half cycle). Leading edge dimmer circuits are generally better suited for controlling power to inductive loads, such as small fan motors and iron core low-voltage lighting transformers. On the other hand, trailing edge dimmer circuits are generally better suited for controlling power to capacitive loads, such as drivers for light emitting diode (LED) lamps.

Phase-control dimmer circuits typically use a high voltage to switch AC power on and off to a load. Typically, the dimmer circuit and the load are connected in series with the AC power. Therefore, if a defect occurs in the load circuit or in the load itself, the dimmer circuit interprets the short circuit as a load fault, potentially causing a sudden surge of high current that can damage the load and/or the dimmer circuit. Accordingly, exemplary prior art phase-control dimmer circuits employ various techniques to provide short-circuit protection against load faults, such as improper wiring of the load circuit.

More specifically, in existing examples of MOSFET switching dimmer circuits, a short circuit event or overcurrent condition can be determined by monitoring the voltage drop across a series current sensing resistor element, monitoring the voltage drop across the MOSFET's intrinsic diode (in the AC half-cycle polarity when the intrinsic diode is forward biased), or monitoring the voltage drop across the MOSFET's channel resistance (in the AC half-cycle polarity when the intrinsic diode is reverse biased). In examples that monitor the voltage drop across the MOSFET's channel resistance, an additional comparator circuit is required. Those skilled in the art will understand that the MOSFET channel resistance is a component of the on-state resistance of the MOSFET switching dimmer circuit. For example, the on-state resistance of the MOSFET at maximum operating temperature is 1Ω.

In this example, the dimmer circuit is a trailing-edge phase-controlled dimmer circuit having a MOSFET switching circuit for controlling the delivery of AC power to a load and a switch control circuit for controlling the switching of the MOSFET. The MOSFETs are configured so that they alternately control the delivery of power to the load during different polarity half-cycles of the AC power. That is, the MOSFETs switch the switching circuit on and off during each cycle of the AC power, respectively, causing the load (e.g., a driver for an LED downlight) to dim in proportion to the amount of time the switching circuit is off during each cycle. The MOSFETs of the example switching circuit have an ON-state resistance composed of several resistance components, including: MOSFET source diffusion resistance, channel resistance, accumulation resistance, "JFET" component resistance, drift region resistance, and substrate resistance. In the example, an additional comparator circuit is used to compare the MOSFET on-state voltage drop with a reference voltage to determine whether a short circuit condition has occurred in the example dimmer circuit and/or the load. If the MOSFET on-state voltage drop is greater than the reference voltage, the comparator circuit activates a cut-out circuit to remove gate drive from the MOSFET.

Those skilled in the art will appreciate that the MOSFET on-state voltage drop indicates the load current. An increase in the magnitude of the load current indicates a short-circuit condition, and the MOSFET gate voltage is subsequently modified to shut off the MOSFET. However, when the MOSFET is shut off in the non-conducting state, the comparator circuit must have components selected to withstand the high voltage across the MOSFET, which adds additional complexity and cost to the exemplary dimmer circuit.

Summary of the Invention

Accordingly, in one aspect, the present invention provides a trailing-edge phase control dimmer circuit for controlling alternating current (AC) power to a load with short-circuit protection, the circuit comprising: a switching circuit for controlling the delivery of AC power to the load by conducting power to the load in an on-state and not conducting power to the load in an off-state, wherein the on-state is a conducting period and the off-state is a non-conducting period; a switch control circuit for controlling the disconnection and connection of the switching circuit in each half cycle of the AC to control the switching between the on-state and the off-state of the switching circuit; and a rectifier for switching the AC power to the load in the non-conducting period. The C power is rectified to generate a rectified dimmer voltage to be provided to the dimmer circuit, wherein the switch control circuit includes a zero-crossing detection circuit, which is configured to: detect the zero crossing of AC and detect the crossing of a first threshold and a second threshold of the rectified dimmer voltage, wherein the zero-crossing detection circuit is further configured to: start the switch circuit to start one of the conduction periods when the rectified dimmer voltage is lower than the first threshold, and start the switch circuit to terminate one of the conduction periods early when the rectified dimmer voltage is higher than the second threshold, for providing short-circuit protection for the trailing-edge phase-controlled dimmer circuit.

In one embodiment, the switching circuit includes two MOSFETs for controlling the switching between the off and on states, respectively, during each half cycle of the AC current. The switch control circuit thus provides gate control of the MOSFETs to control the switching between the off and on states. In other embodiments, the switching circuit includes other switching devices, such as other field-effect transistors, to control the switching between the off and on states during each half cycle of the AC current. Alternatively, the switching circuit includes an IGBT switching device in place of a FET.

In one embodiment, the MOSFET has a gate drive latch that latches the MOSFET into an on-state during each half cycle of AC when a zero-crossing detection circuit detects that the rectified dimmer voltage is below a first threshold. Furthermore, the MOSFET gate drive latch unlocks the MOSFET into an off-state during each half cycle of AC at the end of the on-period, and the MOSFET gate drive latch unlocks the MOSFET into an off-state when the zero-crossing detection circuit detects that the rectified dimmer voltage is above a second threshold. In other words, the dimmer circuit is configured to effectively latch into the on-period at each half-cycle zero crossing and subsequently unlock at the end of a predetermined half-cycle on-period, or to unlock earlier than the predetermined half-cycle on-period if a short-circuit condition is detected.

Furthermore, this embodiment utilizes the MOSFET on-state resistance as the current sensing element for short-circuit shutoff. The MOSFET is configured as part of a latching function, latching into the on-state upon a lower-threshold zero-crossing event. The MOSFET transitions from the off-state to the on-state, maintaining the zero-crossing detector output in an active state, which conducts to maintain the MOSFET in the on-state, thereby achieving a latched on-state condition. Under short-circuit load conditions during the half-cycle on-period, the MOSFET on-state voltage rises proportionally to the product of the on-state resistance and the short-circuit current magnitude.

Preferably, the second threshold of the rectified dimmer voltage is selected based on the worst-case condition of the MOSFET on-state resistance (i.e., when hot) and the desired current threshold. For example, for a MOSFET with a maximum on-state resistance of 1Ω and a target current cutoff threshold of 15A, the second threshold selected for the zero-crossing detection circuit is 15V. It should be understood that it is expected that lower performance MOSFETs (e.g., MOSFETs with higher on-state resistance) will need to be operated using a lower second (e.g., power-off) threshold to avoid excessive temperature rise under sustained short-circuit load conditions. In addition, the second power-off threshold needs to have sufficient margin above the peak current experienced under normal operating conditions of the dimmer circuit. For example, for a 2A rated dimmer, this may be approximately 3A for a resistive load, but may exceed 10A when considering issues such as load inrush current for, for example, capacitive loads.

In addition, the switch control circuit controls the MOSFET's cut-off transition between the on-state and the off-state, extending to a selected cut-off transition time, and controls the MOSFET's switch-on transition between the on-state and the off-state, extending to a selected cut-off transition time. The cut-off transition time is proportional to the discharge time of the MOSFET gate capacitance of the MOSFET. Those skilled in the art will appreciate that MOSFET switches use gate voltage to control drain current. However, MOSFETs have input and output capacitances that affect the switching time of the MOSFET. Thus, for example, when switching to the off-state, the MOSFET transitions through the cut-off transition, while the MOSFET capacitance (particularly the MOSFET gate capacitance) is discharged, which occurs over the cut-off transition time. During a short-circuit condition, the MOSFET short-circuit cut-off transition time should preferably occur as quickly as possible to avoid excessive energy dissipation by the MOSFET due to the abnormally high currents that occur during the short-circuit condition.

In one embodiment, the switch control circuit further includes a fast cutoff circuit for controlling a short-circuit cutoff transition of the MOSFET to an off state, latching the MOSFET to an off state for a selected short-circuit cutoff transition time, when the zero-crossing detection circuit detects that the rectified dimmer voltage is above a second threshold. The fast cutoff circuit is a circuit for controlling the cutoff transition during the conduction period, in addition to the normal operating condition cutoff circuit of the switch control circuit. Those skilled in the art will appreciate that when power is turned on and off to a load, the cutoff transition affects the generation of conducted harmonics that contribute to electromagnetic interference (EMI) emissions. Accordingly, existing exemplary trailing-edge phase-controlled dimmer circuits have been configured to generate smoother transitions between the conducting and non-conducting states of the switch circuit to minimize these EMI emissions. For example, in existing trailing-edge dimmer circuits, the cutoff transition time of the switch during each half-cycle is increased, resulting in a smoother cutoff of power to the load, reducing the amplitude of the associated radio frequency (RF) harmonics that contribute to EMI emissions and thereby minimizing line-conducted EMI emissions. In the trailing edge dimmer circuit, since the switching on is performed at the zero crossing of AC, the switching off of the switching circuit provides greater EMI emission than the switching on.

Nevertheless, in an embodiment, the fast shutoff circuit for controlling the short-circuit shutoff transition includes a transistor Q13 that is configured to be pulled low to discharge the MOSFET gate capacitance via a resistor R20 having a resistance selected to select a discharge time for the MOSFET gate capacitance. For example, resistor R20 is a 1KΩ resistor that provides a fast discharge time for the MOSFET gate capacitance.

Preferably, it is desirable to have a short-circuit-induced shutdown that is faster than the shutdown transition time during normal half-cycle operation. In particular, the short-circuit shutdown transition time should be significantly faster than the normal shutdown transition, for example, to limit the associated energy absorption of the dimmer circuit. As described above, the switch control circuit includes circuitry for controlling the shutdown of the MOSFET during each cycle of the AC current to control the switching of the MOSFET between the on and off states. The switch control circuit provides gate drive control of the MOSFET to control the shutdown transition of the MOSFET between the on and off states, extending for a selected shutdown transition time. In an embodiment, the switch control circuit includes a transistor Q12 configured to be pulled low to discharge the MOSFET gate capacitance via a resistor R16 having a resistance selected to select the discharge time of the MOSFET gate capacitance during the normal shutdown transition. For example, R16 is a 56KΩ resistor, which provides a slower discharge time for the MOSFET gate capacitance compared to the 1KΩ resistor R20 of the fast shutdown circuit.

In an embodiment, the zero-crossing detection circuit includes a differential transistor pair Q1 and Q2 to implement a comparator function that determines whether the rectified dimmer voltage is below a first threshold and above a second threshold. The zero-crossing detection circuit then outputs the determination of the comparator function to an on-period timing circuit of the switch control circuit, which is configured to determine the on-period, and wherein the on-period timing circuit is configured to vary the on-period based on the output of the zero-crossing detection circuit.

In another embodiment, the zero-crossing detection circuit incorporates a comparator reference threshold hysteresis to establish a second (power-off) threshold when turn-off of the MOSFET occurs.

In another embodiment, the on-period timing circuit further includes timer output transistors Q7 and Q8 that are non-conductive during the on-period to enable the self-biasing transistor Q9 of the on-period timing circuit to provide base drive current to the gate transistor Q14 of the fast cutoff circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

1 is a block diagram showing some circuits of a trailing edge phase control dimmer circuit according to an embodiment of the present invention;

FIG2 shows a trailing edge phase control dimmer circuit according to an embodiment of the present invention;

3A shows operating waveforms during normal operating conditions of a trailing edge phase control dimmer circuit according to an embodiment of the present invention; and

3B illustrates operating waveforms during a short-circuit operating condition of a trailing-edge phase-controlled dimmer circuit according to an embodiment of the present invention.

DETAILED DESCRIPTION

FIG1 illustrates in block diagram form some circuits of a 2-wire trailing-edge phase-controlled dimmer circuit 10 with short-circuit protection according to an embodiment of the present invention, which is configured to control power to a load. Those skilled in the art will appreciate that many circuits of the dimmer circuit 10 do not affect the provision of short-circuit protection and are therefore not discussed in detail herein.

The dimmer circuit 10 shown in the embodiment of FIG. 1 includes an AC switching circuit 12 for controlling the delivery of AC power to a load by conducting power to the load in an on-state and not conducting power to the load in an off-state, as described above. Those skilled in the art will appreciate that the on-state is a conducting period, while the off-state is a non-conducting period, configured to have a duration to control dimming of, for example, an LED lamp, when the load is an LED lamp driver. The AC switching circuit 12 is connected to a switch control circuit 14, which includes multiple circuits for controlling the switching of the AC switching circuit 12 between the on and off states during each half-cycle of the AC current, thereby controlling the switching of the switching circuit 12 between the on and off states. The switch control circuit 14 also controls the short-circuit protection function of the dimmer circuit 10, as described above. Furthermore, the dimmer circuit 10 includes a rectifier 16 for rectifying the AC power during the non-conducting period to generate a rectified dimmer voltage to be provided to the dimmer circuit 10.

The switch control circuit 14 has a zero-crossing detection circuit 18 that is configured to detect zero crossings of the AC and to detect the crossing of a first threshold and a second threshold of the rectified dimmer voltage. The zero-crossing detection circuit 18 is connected to an on-period timing circuit 20 that is configured to determine and change the on-period based on the output of the zero-crossing detection circuit 18. Furthermore, the on-period timing circuit 20 is connected to a gate drive circuit 22 that switches the AC switch circuit 12 on and off, thereby providing conducting and non-conducting periods. In this embodiment, the gate drive circuit 24 includes a fast cutoff circuit 26 that controls the short-circuit cutoff transition of the AC switch circuit 12 when a short-circuit condition is detected, and a normal operation gate drive circuit 26 that switches the AC switch circuit 12 on and off during normal operating conditions.

As described above, the zero-crossing detection circuit 16 is configured to detect AC zero crossings and the crossing of first and second thresholds of the rectified dimmer voltage. An AC zero crossing occurs when the AC line voltage is equal to zero between the polarities of a half-cycle. As described above, in practice, the crossing between the conduction and non-conduction periods typically does not occur exactly at the AC line voltage zero crossing. For example, in an embodiment, the first threshold is selected as a low threshold voltage of 5V, while the second threshold is a high threshold voltage of 10V. The selection of the first threshold is based on two considerations: First, since this is the start of the line voltage zero crossing for the conduction period of the half-cycle, it is selected in conjunction with the sum of the gate drive circuit propagation delay and the MOSFET turn-on delay (the latter being dominant); second, the first threshold must have sufficient margin above the MOSFET on-state voltage drop under normal operating conditions (e.g., under maximum load). Therefore, for example, for a total delay of 50µs, 5V is selected as the first threshold. As discussed above, the high threshold of 10V is selected based on the worst-case conditions of the MOSFET on-state resistance (1Ω) and the desired current threshold (10A).

Furthermore, the zero-crossing detection circuit 16 is configured to activate the AC switch circuit 12 to begin the conduction period when the rectified dimmer voltage is below a first threshold, and to activate the AC switch circuit 12 to prematurely terminate the conduction period when the rectified dimmer voltage is above a second threshold, thereby providing short-circuit protection for the trailing-edge phase-controlled dimmer circuit. That is, the zero-crossing detection circuit 16 activates control of the gate drive circuit 22 to cut off the AC switch circuit 12 to provide conduction and non-conduction periods, and to provide short-circuit protection if a short-circuit condition is detected.

FIG2 illustrates in greater detail an embodiment of the functional implementation of the dimmer circuit 10, generally shown as a block diagram in FIG1 . In this embodiment, the dimmer circuit 10 is also a trailing-edge phase-controlled dimmer circuit having an AC switching circuit 12 and a switch control circuit 14 . As described above, the AC switching circuit 12 in this embodiment is a MOSFET switching circuit and includes MOSFET switching elements Q15 and Q16 (e.g., high-voltage (600V) N-channel MOSFETs such as FCPF11N60) that control the amount of AC power delivered to a load. As described above, the MOSFETs Q15 and Q16 are configured such that they alternately control the delivery of power to the load during different polarity half-cycles of the AC power. That is, the MOSFETs Q15 and Q16, respectively, switch the AC switching circuit 12 on and off during each cycle of the AC power, causing the load (e.g., a driver for an LED downlight) to conduct no power to the load in proportion to the amount of time the switching circuit 12 is off during each cycle. In this embodiment, the load is, for example, a capacitive load in the form of a driver for an LED lamp.

The switch control circuit 14 of the embodiment shown in FIG2 utilizes a gate drive circuit 26 under normal operating conditions and a fast cutoff circuit 24 to control the short-circuit cutoff transition under short-circuit conditions, utilizing inputs from the zero-crossing detection circuit 18 and the on-period timing circuit 20 to implement MOSFET gate drive control. Specifically, under normal operating conditions, the gate drive circuit 26 utilizes transistors Q11 and Q12 to implement MOSFET gate drive control. Here, transistors Q1 and Q2 are BC856 PNP transistors. The base drive of transistor Q11, which receives input from the zero-crossing circuit 16, is pulled high to charge the MOSFET gate capacitance of MOSFETs Q15 and Q16 via resistor R15, thereby maintaining MOSFETs Q15 and Q16 in an on-state condition. In the embodiment, R15 is a 1KΩ resistor. Diode D2 and Zener diode ZD3 are also used to clamp the on-state gate voltage of MOSFETs Q15 and Q16 to an appropriate level for proper biasing. D2 is also a 4148 high-speed diode, and ZD3 is a 7V5 Zener diode. The transistor Q12 base drive is also pulled low to discharge the MOSFET gate capacitance via resistor R16, the value of which is selected to provide the desired turn-off transition time for MOSFETs Q15 and Q16 of the AC switching circuit 12. Here, R16 is selected as a 56KΩ resistor.

During short-circuit operating conditions, the switch control circuit 14 uses a fast-cut switch circuit 24 to control the short-circuit turn-off transition, rather than the normal gate drive circuit 26. The on-period timing circuit 20 includes a self-biased transistor Q9 that provides base drive to a gate transistor Q14 of the fast-cut circuit 24 during the on-period to enable operation of the fast-cut circuit 24. The fast-cut circuit 24 includes a transistor Q13 that is configured to be pulled low when a short circuit is detected to discharge the MOSFET gate capacitance via resistor R20. In an embodiment, resistor R20 is selected as a 1KΩ resistor to provide a faster turn-off transition time relative to the normal turn-off time determined by a 56KΩ resistor.

As described above, the zero-crossing detection circuit 18 is configured to detect a short-circuit condition by detecting the crossing of a first (e.g., 5V) and a second (e.g., 10V) threshold value of the rectified dimmer voltage. Specifically, in an embodiment, the zero-crossing detection circuit 18 detects when the rectified dimmer voltage rises above the second threshold value of 10V, which indicates a short-circuit condition. During normal operating conditions, the zero-crossing detection circuit 18 detects when the rectified dimmer voltage falls below the first threshold value of 5V.

Zero-crossing detection circuit 18 includes an input stage with a differential transistor pair Q1 and Q2 to implement this comparator function, and its output appears at the collector of transistor Q3, which is buffered by transistors Q4 and Q5. Voltage divider resistors R1 and R2 scale down the rectified dimmer voltage to be suitable for inverting the input at the base of transistor Q1 of zero-crossing detection circuit 18. A first threshold voltage (e.g., 5V) is a reference voltage used for the start of the half-cycle zero-crossing conduction period (when V+≤5V) and is primarily determined by resistors R4 and R5. In this embodiment, resistors R4 and R5 are 100KΩ and 3.9KΩ resistors, respectively. Furthermore, R1 is a 100KΩ resistor, while R2 is an 11KΩ resistor. Therefore, when the detected rectified dimmer voltage drops below the lower threshold of 5V, zero-crossing detector circuit 18 initiates the start of the half-cycle conduction period.

The output of the comparator function of the zero-crossing detection circuit 18 also includes positive feedback to a reference voltage first threshold (e.g., 5V) at the resistor R4/R5 junction via resistor R8, which generates a programmable hysteresis level. The hysteresis level allows selection of a desired second threshold, which is a short-circuit condition for shutting off (e.g., de-energizing) the MOSFETs. Thus, the zero-crossing detection circuit 18 incorporates comparator reference threshold hysteresis, such that when the MOSFET on-state voltage rises to a magnitude exceeding the original on-threshold, a short-circuit condition is detected, and the zero-crossing detection circuit 18 initiates shutoff of the MOSFETs Q16 and Q16. Thus, for example, the first and second thresholds of 5V and 10V for the rectified dimmer voltage (shown as reference voltage (V+) in the figure) are derived from the zero-crossing detection circuit 18 comparator hysteresis reference voltages of 0.5V and 1.0V. That is, the rectified dimmer voltage (V+) is greater than the MOSFET on-state voltage by an amount equal to the forward bias voltage of the rectifier diode (-0.5V in this example). The first and second threshold values are thus chosen to be sufficiently larger than the rectifier diode forward voltage drop so that the latter can be neglected in the calculation.

Detection of a short-circuit condition by zero-crossing detection circuit 18 initiates a fast-off output to on-period timing circuit 20, which activates the MOSFET gate drive and begins on-period timing, while also enabling fast-off circuit 24. If the rectified dimmer voltage rises above the upper threshold of the zero-crossing detector due to a short-circuit condition during the half-cycle on-period, MOSFETs Q15 and Q16 are quickly shut off to protect circuit 10 from high-energy conditions. During normal half-cycle operation, a slower shutoff transition occurs via gate drive circuit 26 due to expiration of the on-period timer. Specifically, at the crossing of the first threshold, diode D1 of on-period timing circuit 18 becomes reverse biased, enabling the on-period timing capacitor C1 to begin charging. Transistor Q6 (with a reference voltage based on Zener diode ZD1) acts as a constant current source for timing capacitor C1. For example, the timing capacitor is a 10nF capacitor. FIG3A shows the timing capacitor voltage, the zero crossing at the first threshold of 5V, the rectified dimmer voltage, and the AC line voltage waveforms.

As described above, during the half-cycle on-period, timer output transistors Q7 and Q8 are non-conductive, so self-biasing transistor Q9 provides base drive to fast-turn-off gate transistor Q14. During half-cycle operation, when faced with driving a short-circuited load, the rising voltage across MOSFETs Q15 and Q16 will be detected by zero-crossing detection circuit 18 as crossing a second threshold (e.g., a higher zero-crossing threshold, V+, ≥ 10V), causing the zero-crossing detection circuit 18 output to be pulled low. This provides base drive to transistor Q13 via resistor R17 and, therefore, enables faster turn-off due to the relatively low value of the associated gate discharge resistor R20. FIG3B illustrates the rectified dimmer voltage and zero-crossing waveforms during a short-circuited load. During short-circuited load half-cycle operation, the rectified dimmer voltage rises (e.g., in less than 0.1ms) above the second zero-crossing threshold, 10V, resulting in premature termination of the on-period. In normal half cycle operation with no short circuit load, the on-period timer output transistor Q8 collector is pulled low which has the dual effect of disabling the fast cutoff circuit 24 via transistors Q9 and Q14 and activating the normal slow cutoff via transistor Q12.

That is, immediately following detection of a short circuit condition by the zero-crossing detection circuit 18, during the normal on-period, the state of the on-period timing circuit 20 is, for example, to enable the fast cut-off circuit 24. The fast cut-off circuit 24 is normally enabled during the on-period, but is disabled at the end of the half-cycle on-period to allow for a slow cut-off transition for EMI limiting purposes.

It should be understood that other variations and modifications of the configurations described herein are possible and fall within the scope of the present invention.

Claims (11)

1. A trailing-edge phase-controlled dimmer circuit with short-circuit protection for controlling alternating current (AC) power to a load, the circuit comprising: A switching circuit is used to control the delivery of AC power to the load by conducting power to the load in a conducting state and not conducting power to the load in a turning-off state, wherein the conducting state is a conducting period and the turning-off state is a non-conducting period. A switch control circuit for controlling the switching of the switch circuit during each half-cycle of the AC, thereby controlling the switching between the on and off states of the switch circuit; and A rectifier, used to rectify the AC power during the non-conducting period to generate a rectified dimmer voltage to be supplied to the dimmer circuit. The switching control circuit includes a zero-crossing detection circuit, which is configured to detect the zero-crossing of the AC and the intersection of a first threshold and a second threshold of the rectified dimmer voltage. The zero-crossing detection circuit is further configured to: activate the switching circuit to begin a conduction period when the rectified dimmer voltage is lower than the first threshold, and activate the switching circuit to prematurely terminate a conduction period when the rectified dimmer voltage is higher than the second threshold, for providing short-circuit protection for the trailing edge phase control dimmer circuit; Furthermore, the second threshold can change based on a programmable hysteresis level generated from positive feedback to the first threshold. 2. The trailing edge phase control dimmer circuit according to claim 1, wherein the switching circuit includes two MOSFETs for controlling the off and on states respectively in each half cycle of the AC. 3. The trailing edge phase control dimmer circuit according to claim 2, wherein the MOSFET has a gate drive latch, and the gate drive latch latches the MOSFET into the on state in every half cycle of the AC when the zero-crossing detection circuit detects that the rectified dimmer voltage is lower than the first threshold. 4. The trailing-edge phase-controlled dimmer circuit according to claim 3, wherein the MOSFET gate drive latch unlocks the MOSFET to the off state at the end of the conduction period and during each half-cycle of the AC, and the MOSFET gate drive latch unlocks the MOSFET to the off state when the zero-crossing detection circuit detects that the rectified dimmer voltage is higher than the second threshold. 5. The trailing edge phase-controlled dimmer circuit according to claim 4, wherein the switch control circuit further includes a fast cut-off circuit, the fast cut-off circuit being used to control the short-circuit cut-off transition of the MOSFET when the zero-crossing detection circuit detects that the rectified dimmer voltage is higher than the second threshold, unlocking to the off state, and extending to the selected short-circuit cut-off transition time. 6. The trailing edge phase control dimmer circuit according to claim 5, wherein the short-circuit cut-off transition time is proportional to the discharge time of the MOSFET gate capacitance. 7. The trailing-edge phase-controlled dimmer circuit of claim 6, wherein the fast cut-off circuit for controlling the short-circuit cut-off transition includes a transistor Q13 configured to be pulled low such that the MOSFET gate capacitance discharges via a resistor R20, the resistor R20 having a resistance selected for selecting the discharge time of the MOSFET gate capacitance. 8. The trailing edge phase control dimmer circuit according to claim 7, wherein the resistor R20 is a 1KΩ resistor. 9. The trailing-edge phase-controlled dimmer circuit according to any one of claims 1 to 8, wherein the zero-crossing detection circuit includes a differential transistor pair Q1 and Q2 for implementing a comparator function to determine whether the rectified dimmer voltage is below the first threshold and above the second threshold. 10. The trailing edge phase control dimmer circuit of claim 9, wherein the zero-crossing detection circuit outputs a determination of the comparator function to an on-time timing circuit of the switch control circuit, the on-time timing circuit being configured to determine the on-time, and wherein the on-time timing circuit is configured to change the on-time based on the output. 11. The trailing-edge phase-controlled dimmer circuit of claim 10 when attached to claim 7, wherein the on-time timing circuit further includes timer output transistors Q7 and Q8, which are not turned on during the on-time, such that the self-biased transistor Q9 of the on-time timing circuit can provide a base drive current to the gating transistor Q14 of the fast-off circuit.