Switching power supply short circuit protection circuit with soft start

Abstract

A kind of short-circuit protection circuit of switching power supply with slow start, comprising;Optocoupler PC2, the light-emitting end positive pole of the optocoupler PC2 is connected with feedback voltage, negative pole is grounded by switching tube TR1, the collector of the optocoupler PC2 is connected with end point COMP, this end point is also connected with voltage Vref, the emitter of the optocoupler PC2 is connected with end point PN by resistance R4;Still including switching tube TR3, the emitter of the switching tube TR3 is connected with the end point COMP, collector is connected with resistance R4, base is connected with the end point Vref;Still including switching tube TR2, the base of the switching tube TR2 is connected with the resistance R4, emitter is grounded, collector is connected with end point VFB.

Description

Technical Field

[0001] This utility model relates to the field of power device circuits in switching power supplies, and in particular to a short-circuit protection circuit for a switching power supply with soft start. Background Technology

[0002] During normal operation or transient conditions, switching power supply devices need to withstand voltage and current stresses. Excessive voltage or current stress can lead to device breakdown and overheating damage, directly affecting the reliability and lifespan of the power supply. In steady-state operation, the voltage, current, and control loop in the circuit are all in a stable and predictable state. However, at the moment of power-on, the control loop has not yet been established, causing the power controller to be in an open-loop or extreme state during the initial power-on phase, outputting the maximum duty cycle (Dmax). An excessively large duty cycle can cause a sharp rise in current in the transformer, forming a current spike. During startup, the MOSFET conducts for an extremely long time at maximum duty cycle, and the energy stored in the primary inductor (1 / 2*Lp*Ipk) is utilized. 2 The energy absorbed by the RCD clamping circuit is extremely large. The RCD clamping circuit cannot absorb such a large amount of energy instantaneously, causing the voltage across the absorption capacitor to spike very high. This results in the source-drain voltage Vds of the MOSFET far exceeding the steady-state value, leading to MOSFET breakdown. Consequently, the MOSFET's withstand voltage must also be selected far exceeding the steady-state value. The traditional solution is to maximize the energy absorbed by the RCD clamping circuit, but this consumes more energy in steady state, affecting steady-state efficiency and increasing the temperature of the absorption circuit, thus shortening its lifespan. Most importantly, the RCD clamping circuit does not fundamentally solve the problem of transient voltage and current stress; it passively absorbs the stress after it occurs. This circuit can fundamentally alleviate the current and voltage stress during transients (including power-on transients and short-circuit restart phases), protecting power devices, while also reducing the margin for device selection. Utility Model Content

[0003] To address the aforementioned issues, this technical solution provides a short-circuit protection circuit for a switching power supply with soft start.

[0004] To achieve the above objectives, the technical solution is as follows:

[0005] A short-circuit protection circuit for a switching power supply with soft start, comprising:

[0006] Optocoupler PC2, the positive terminal of the light-emitting terminal of the optocoupler PC2 is connected to the feedback voltage, the negative terminal is grounded through the switching transistor TR1, the collector of the optocoupler PC2 is connected to the terminal COMP, which is also connected to the voltage Vref, and the emitter of the optocoupler PC2 is connected to the terminal PN through the resistor R4.

[0007] It also includes a switching transistor TR3, whose emitter is connected to the terminal COMP, collector is connected to the resistor R4, and base is connected to the terminal Vref;

[0008] It also includes a switching transistor TR2, the base of which is connected to the resistor R4, the emitter is grounded, and the collector is connected to the terminal VFB.

[0009] In some embodiments, the endpoint Vref is connected to one end of resistor R1, the other end of resistor R1 is connected to one end of diode D1, and the other end of diode D1 is grounded through resistor R2 and capacitor C2 respectively.

[0010] In some embodiments, the endpoint Vref is connected to the resistor R4 in sequence through resistor R3 and capacitor C3, and the common junction of resistor R3 and capacitor C3 is connected to the base of the switching transistor TR3.

[0011] In some embodiments, the switching transistors TR2 and TR3 are transistors.

[0012] The beneficial effects of this application are:

[0013] This short-circuit protection circuit for switching power supplies with soft start can be used to solve various current and voltage stress problems generated during the transient process of switching power supplies. It can suppress input inrush current, effectively reduce output voltage overshoot, and reduce stress on power devices. This circuit consists of a short-circuit protection circuit and a soft-start circuit. The short-circuit protection circuit controls the input of the error comparator for current sampling by adjusting the input of the error amplifier of the power control chip (taking UC2844 as an example) to achieve short-circuit protection; the soft-start circuit introduces a voltage that gradually increases over time into the COMP pin of the power control chip to limit the maximum duty cycle during power-on and short-circuit startup. The two circuits work together to effectively protect the power devices in the circuit. Attached Figure Description

[0014] To more clearly illustrate the technical solutions in the embodiments of this utility model, the accompanying drawings used in the description of the embodiments will be briefly introduced below.

[0015] Figure 1 This is a structural schematic diagram of an embodiment of the present utility model. Detailed Implementation

[0016] To make the technical problems solved, technical solutions, and beneficial effects of this utility model clearer, the present utility model will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the present utility model and are not intended to limit the present utility model.

[0017] Please refer to Figure 1 As shown, a short-circuit protection circuit for a switching power supply with soft start includes:

[0018] Optocoupler PC2, the positive terminal of the light-emitting terminal of the optocoupler PC2 is connected to the feedback voltage, the negative terminal is grounded through the switching transistor TR1, the collector of the optocoupler PC2 is connected to the terminal COMP, which is also connected to the voltage Vref, and the emitter of the optocoupler PC2 is connected to the terminal PN through the resistor R4.

[0019] It also includes a switching transistor TR3, whose emitter is connected to the terminal COMP, collector is connected to the resistor R4, and base is connected to the terminal Vref;

[0020] It also includes a switching transistor TR2, the base of which is connected to the resistor R4, the emitter is grounded, and the collector is connected to the terminal VFB.

[0021] In this embodiment, the endpoint Vref is connected to one end of resistor R1, the other end of resistor R1 is connected to one end of diode D1, and the other end of diode D1 is grounded through resistor R2 and capacitor C2 respectively.

[0022] In this embodiment, the endpoint Vref is connected to the resistor R4 in sequence through resistor R3 and capacitor C3, and the common junction of resistor R3 and capacitor C3 is connected to the base of the switching transistor TR3.

[0023] In this embodiment, the switching transistors TR2 and TR3 are bipolar transistors.

[0024] The primary-side feedback circuit of the optocoupler is a loop feedback circuit based on TL431.

[0025] The collector of the optocoupler secondary side is connected to the COMP pin of the UC2844, and the emitter is connected to the pull-down resistor R4 and simultaneously connected to the base of the NPN transistor TR2; the collector of the transistor TR2 is connected to the Vfb pin of the UC2844; R_pull connects the COMP pull-up to Vref.

[0026] R1, D1, C2, and R2 form a charging (discharging) circuit. When the switching power supply is working normally, the optocoupler is on, COMP provides positive voltage, the transistor is on and connects Vfb to PN, and the error amplifier outputs a high level. When a short circuit occurs, the optocoupler is off, R4 pulls down the base of the transistor to turn it off, COMP outputs a high state of 6.2V, Vref charges C2 through R1 and D1, causing the Vfb voltage to rise (the duration depends on R1 and C2). After exceeding 2.5V, the error amplifier output immediately drops to zero, then the PWM latch flips, the chip is protected, Vout enters a "hiccup state", Vref has no output, C2 discharges through R2, waiting for the short circuit to be removed.

[0027] TR3, R3, and C3 form a soft-start circuit. The emitter of the PNP transistor TR3 is directly connected to COMP, its collector is pulled down to ground via R4, and its base is connected to Vref via resistor R3. C3 is connected in parallel between the collector and base of TR3. Initially, during startup, the COMP voltage is higher than Vb (0.7V), TR3 conducts, and COMP equals Vce. As capacitor C3 charges until Vb is greater than Ve, TR3 is turned off. During the slow rise of COMP, the duty cycle D gradually increases, thus avoiding the extreme current and voltage stress generated during startup.

[0028] The above description is only a preferred embodiment of this application and is not intended to limit the scope of implementation of this application. Any other embodiments whose principles and basic structures are the same as or similar to those of this application are within the protection scope of this application.

Claims

1. A short-circuit protection circuit for a switching power supply with soft start, characterized in that, include; Optocoupler PC2, the positive terminal of the light-emitting terminal of the optocoupler PC2 is connected to the feedback voltage, the negative terminal is grounded through the switching transistor TR1, the collector of the optocoupler PC2 is connected to the terminal COMP, which is also connected to the voltage Vref, and the emitter of the optocoupler PC2 is connected to the terminal PN through the resistor R4. It also includes a switching transistor TR3, whose emitter is connected to the terminal COMP, collector is connected to the resistor R4, and base is connected to the terminal Vref; It also includes a switching transistor TR2, the base of which is connected to the resistor R4, the emitter is grounded, and the collector is connected to the terminal VFB.

2. The short-circuit protection circuit for a switching power supply with soft start according to claim 1, characterized in that: The endpoint Vref is connected to one end of resistor R1, the other end of resistor R1 is connected to one end of diode D1, and the other end of diode D1 is grounded through resistor R2 and capacitor C2 respectively.

3. A short-circuit protection circuit for a switching power supply with soft start according to claim 2, characterized in that: The endpoint Vref is connected to the resistor R4 in sequence through resistor R3 and capacitor C3, and the common junction of resistor R3 and capacitor C3 is connected to the base of the switching transistor TR3.

4. The short circuit protection circuit for switching power supply with soft start according to claim 1, characterized in that: The switching transistors TR2 and TR3 are either transistors or MOSFETs.

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