Main feedback output short-circuit protection circuit of switching power supply

Abstract

A switching power supply main feedback output short circuit protection circuit comprises an optocoupler PC1 and a triode TR2. The positive electrode of a light emitting end of the optocoupler PC1 receives starting voltage, the negative electrode of the optocoupler PC1 is grounded through a voltage-regulator tube TR1, a collector electrode of the optocoupler PC1 is used for being connected with a first port of a chip and is further used for being connected with a second port of the chip through a resistor Rpull1, an emitting electrode of the optocoupler PC1 is connected with a base electrode of the triode TR2, the emitting electrode of the triode TR2 is grounded, and the emitting electrode of the triode TR2 is grounded. A collector electrode is connected with the resistor Rpull1 through a resistor R1, and the collector electrode is also used for being connected with a third port of the chip; and the resistor R1 is grounded through a diode D1 and a capacitor C3 in sequence.

Description

Technical Field

[0001] The utility model relates to the field of switching power supply circuits, in particular to a switching power supply main feedback output short-circuit protection circuit. Background Art

[0002] Traditionally, short-circuit protection for switching power supplies is based on comparing the voltage of the primary sampling resistor with the output voltage of the error amplifier of the power control chip. Taking UC2844 as an example, there are two ways to turn off the PWM output: pull ISENSE up to more than 1V, or pull COMP down to a voltage lower than two diode drops.

[0003] However, in engineering applications, short circuit protection cannot be achieved by relying solely on the peak current detection of UC2844 for power supply short circuits of 3.3V, 5V and single output secondary output windings, especially when short circuit occurs under high voltage input and light load output. Since UC2844 is based on periodic modulation, and the inductance and leakage inductance values ​​of different windings are different, the current change rate is inconsistent, and the change in the primary current is naturally inconsistent. For example, when 5V changes from light load to short circuit, the error amplifier of UC2844 outputs a high state of 6.2V, which is clamped to 1V by the inverting voltage regulator of the current sampling comparator. The positive end has not risen to 1V, and the PWM latch is still working. The switching power supply is now working in a 5V output overload state. At this time, the auxiliary power supply winding voltage increases instead, and the switching power supply continues to work. If the sampling resistor value is increased, the switching power supply will be overcurrent protected before it is fully loaded. Utility Model Content

[0004] In order to solve the above problem, the present technical solution provides a switching power supply main feedback output short-circuit protection circuit.

[0005] To achieve the above purpose, the technical solution is as follows:

[0006] A switching power supply main feedback output short-circuit protection circuit comprises an optical coupler PC1 and a transistor TR2;

[0007] The positive electrode of the light-emitting end of the optical coupler PC1 receives the starting voltage, and the negative electrode is grounded through the voltage regulator tube TR1. The collector of the optical coupler PC1 is used to connect to the first port of the chip, and the collector is also used to connect to the second port of the chip through the resistor Rpull1. The emitter of the optical coupler PC1 is connected to the base of the transistor TR2, and the emitter of the transistor TR2 is grounded. The collector is connected to the resistor Rpull1 through the resistor R1, and the collector is also used to connect to the third port of the chip;

[0008] The resistor R1 is connected to the ground via the diode D1 and the capacitor C3 in sequence.

[0009] In some embodiments, a resistor R3 is connected in parallel with the capacitor C3.

[0010] In some embodiments, the emitter of the optocoupler PC1 is grounded through a resistor R2.

[0011] In some embodiments, the collector of the optocoupler PC1 is grounded through a capacitor C1, and the resistor Rpull1 is grounded through a capacitor C2.

[0012] In some embodiments, a resistor is connected between the positive and negative electrodes of the light-emitting end of the optocoupler PC1.

[0013] The beneficial effects of this application are as follows:

[0014] For switch-mode power supply products, short-circuit protection is essential. If short-circuit protection cannot be achieved quickly and reliably, it will lead to product failure or system malfunction. The short-circuit protection of this application uses the current comparison unit and error amplifier comparison unit of the power control chip to block the drive output, causing the switch-mode power supply to enter the hiccup state and protecting the power devices from damage. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments.

[0016] Figure 1 is a schematic diagram of the working process of the embodiments of the present invention;

[0017] Figure 2 is a schematic diagram of the circuit structure of the chip UC2844 in the embodiments of the present invention;

[0018] Figure 3 is a schematic diagram of the circuit structure of the embodiments of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0019] In order to make the technical problems, technical solutions, and beneficial effects solved by the present invention more clearly understood, the following further details the present invention with reference to the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0020] Aiming at the problem that the prior art cannot protect through the current sampling pin of the power control chip when the secondary winding is lower than 5V and the power supply is single-output, this circuit controls the input of the error comparator for current sampling by adjusting the input of the error amplifier of the power control chip.

[0021] Please refer to Figures 1 - 3 As shown, a short-circuit protection circuit for the main feedback output of a switch-mode power supply includes an optocoupler PC1 and a triode TR2;

[0022] The positive electrode of the light-coupler PC1 receives the startup voltage, and the negative electrode is grounded through the voltage-regulator diode TR1. The collector of the light-coupler PC1 is used to connect to the first port of the chip (specifically, the CMOP port of UC2844), and this collector is also used to connect to the second port of the chip (specifically, the Vref port of UC2844) through the resistor Rpull1. The emitter of the light-coupler PC1 is connected to the base of the triode TR2. The emitter of the triode TR2 is grounded, and the collector is connected to the resistor Rpull1 through the resistor R1, and this collector is also used to connect to the third port of the chip (specifically, the Vfb port of UC2844);

[0023] The resistor R1 is grounded through the diode D1 and the capacitor C3 in sequence.

[0024] Working principle:

[0025] The technical problem to be solved by this circuit is to provide a reliable short-circuit protection circuit for light load of a switching power supply in view of the existing technical problems, and to design a circuit that can close-loop control the output of the error amplifier in UC3844 to control the flip of the PWM latch using basic electronic components.

[0026] In view of the technical problems to be solved, this circuit provides the following technical solutions:

[0027] The 5V voltage on the secondary side is input to the primary side of the light-coupler. The TR1 voltage-regulator diode stabilizes the cathode pin of the light-coupler at a certain fixed voltage, and the PC1 light-coupler isolates the high-voltage and low-voltage signals between the primary and secondary sides.

[0028] The collector of the secondary side of the light-coupler is connected to the COMP pin of UC2844, the emitter is connected to the pull-down resistor R2 and simultaneously connected to the base of the NPN-type triode TR2; the collector of the triode TR2 is connected to the Vfb pin of UC2844; Rpull1 pulls up the COMP and connects it to Vref.

[0029] R1, D1, C3, and R3 form a charging (discharging) loop; when the switching power supply is working normally, the light-coupler is turned on, COMP provides a positive voltage, the triode is turned on to connect Vfb to PN, and at this time the loop feedback works in a closed-loop state; when a short circuit occurs, the current on the primary side of the light-coupler is not enough to support the continuous operation of the light-coupler and it is turned off, and the TR2 triode is then turned off, and the external loop feedback works in an open-loop state. At this time, the output of COMP is at a high level of 6.2V, and the internal comparator of UC2844 can be used to control the level flip of the COMP pin. Vref charges C3 through R1 and D1, causing the voltage of Vfb to rise (the duration depends on R1 and C3). After exceeding 2.5V, the output of the error amplifier immediately drops to zero, and then the PWM latch flips, the chip is protected, VCC enters the "hiccup state", Vref has no output, and C3 discharges through R3, waiting for the short circuit to be removed.

[0030] In this embodiment, a resistor R3 is connected in parallel with the capacitor C3.

[0031] In this embodiment, the emitter of the optocoupler PC1 is grounded through the resistor R2.

[0032] In this embodiment, the collector of the optocoupler PC1 is grounded through the capacitor C1, and the resistor Rpull1 is grounded through the capacitor C2.

[0033] In this embodiment, a resistor is connected between the positive and negative electrodes of the light-emitting end of the optocoupler PC1.

[0034] 1. The circuit controls the output of the error amplifier by controlling its input stage, thereby reducing the input terminal of the current sampling error comparator to below 1V, thus avoiding the problem that it is difficult for ISENSE to rise above 1V during light-load short circuit.

[0035] 2. The on-off of the triode is controlled by the on-off of the optocoupler during short circuit to achieve self-recovery of Vfb.

[0036] 3. The charging circuit and the discharging circuit are decoupled by a diode. The charging time should be > 20ms (related to the starting characteristics of UC2844), and discharging. The discharging time should not be too long, otherwise, in the case of high-voltage input, even if the short-circuit state is exited, the internal comparator output of 2844 will always be at a low level and cannot self-recover to the normal working state.

[0037] The above are only the preferred embodiments of the present application, and are not used to limit the scope of implementation of the present application. All other embodiments with the same or similar principles and basic structures as the present application are within the protection scope of the present application.

Claims

1. A switching power supply main feedback output short-circuit protection circuit, characterized in that: Including optocoupler PC1 and transistor TR2; The positive electrode of the light-emitting end of the optical coupler PC1 receives the starting voltage, and the negative electrode is grounded through the voltage regulator tube TR1. The collector of the optical coupler PC1 is used to connect to the first port of the chip, and the collector is also used to connect to the second port of the chip through the resistor Rpull1. The emitter of the optical coupler PC1 is connected to the base of the transistor TR2, and the emitter of the transistor TR2 is grounded. The collector is connected to the resistor Rpull1 through the resistor R1, and the collector is also used to connect to the third port of the chip; The resistor R1 is connected to the ground via the diode D1 and the capacitor C3 in sequence.

2. A switching power supply main feedback output short-circuit protection circuit according to claim 1, characterized in that: The capacitor C3 is connected in parallel with a resistor R3.

3. A switching power supply main feedback output short-circuit protection circuit according to claim 2, characterized in that: The emitter of the optical coupler PC1 is grounded via the resistor R2.

4. A switching power supply main feedback output short-circuit protection circuit according to claim 1, characterized in that: The collector of the optical coupler PC1 is grounded through the capacitor C1, and the resistor Rpull1 is grounded through the capacitor C2.

5. The switching power supply main feedback output short-circuit protection circuit according to claim 1, characterized in that: A resistor is connected between the positive electrode and the negative electrode of the light-emitting end of the optical coupler PC1.

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