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# Analysis and Introduction of Using Simple Latch Circuit to Protect Power Supply

Have you ever needed a simple, low-cost latch circuit? Figure 1 shows such a circuit. It can provide power failure protection with components of only a few yuan. It is basically a thyristor rectifier (SCR) combined with some discrete components. The two transistors are normally off. To turn on the latch, you need to drive the PNP base to low level or the NPN base to high level until one of the transistors is turned on. This creates a collector current that turns on the other transistor, further turning on the initial transistor. The circuit performs the latch operation in a regenerative manner. The current is limited only by the power supply impedance and transistor characteristics, allowing the circuit to discharge the capacitor quickly.

An interesting feature of this circuit is that you can establish the holding current of the SCR by selecting the resistor value. In order to keep the latch circuit open after triggering, the two base emitter nodes must have sufficient voltage ( 0.7 V) to keep it open. This means that if the current supplied to it is above VBE / R1 VBE / R2, the circuit latches. If a low current capacitor is connected to the latch circuit, the latch circuit discharges the capacitor. Once the current of the circuit decreases below the holding current, it turns off.

Figure 1 uses discrete components to build an SCR with controlled holding current

Figure 2 shows a good way to use this circuit. The figure shows a high-voltage input, 48-v output reverse converter, which uses SCR to turn off the power supply in case of output overvoltage caused by control circuit failure. When the input voltage is first applied to the circuit, the current flowing through R3 and R4 charges the high-capacity capacitor C3. When the voltage of C3 reaches high enough, the control IC starts to work, switches the power FET Q3 and transmits energy to the output. By controlling the current of U1, the output voltage is adjusted, so as to control the energy transmitted through the transformer. This circuit also provides isolated overvoltage protection through U3. We chose to use zener diodes d5 and D6, which are not conductive during normal operation. In the case of overvoltage, they begin to conduct electricity and suppress the current of optocoupler U3. U3 triggers a latch circuit composed of Q4 and Q5. The latch circuit discharges the bias capacitor C3, and U2 stops working when the VDD voltage reaches the undervoltage stop point of U2.

The latch circuit continuously discharges the bias capacitor until the voltage approaches 1 volt. In this way, the values of R3, R4, R14 and R16 become important. R3 and R4 limit the effective current of the input line, while R14 and R16 determine the required holding current in the latch circuit. If the value of R14 and R16 is small, the latch circuit is closed, the bias capacitor is charged, and the power supply attempts to provide output power again.

In case of failure, this method can provide the function of continuous retry. If the value of the resistor is large enough, the latch remains open and needs to be reset by restarting the power supply. In this case, there are no consecutive retries. Another important component in this circuit is R5, which limits the bias power supply after the latch circuit is turned on. Normally, this component is required to prevent peak bias from being detected.

There are many ways to use this circuit, especially when you use the lifting edge to trigger it. For example, by connecting a zener diode between Q5 bias and base, overvoltage protection can be realized on the primary side. You can use a negative conversion temperature sensor to drive the base of Q4. Alternatively, you can use a comparator on the secondary side to achieve a very accurate overcurrent closing function through an optocoupler very similar to that shown.

In short, this latch circuit composed of \$0.03 transistors is very general. It can be triggered by negative or positive conversion, and can be latched or not, depending on your resistor value. Next time, we will compare the transient response of discontinuous and continuous power supplies to show that efficiency is not the only reason to use synchronous rectifiers.

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