Power Supply Design Tip 38: Use a Simple Latch Circuit to Protect Your Power Supply

Have you ever needed a simple, low-cost latch circuit?picture1 Shown is a circuit that provides power-failure protection for a few bucks in components, basically a Silicon Controlled Rectifier (SCR) combined with a few discrete components. Both transistors are normally off. To turn on the latch, you need to drive the PNP base low, or the NPN base high until one of the transistors turns on. This creates a collector current that turns on the other transistor, which turns on the original transistor further. The circuit performs the latch operation in a regenerative manner. The current is limited only by the source impedance and transistor characteristics, allowing the circuit to quickly discharge the capacitor.

An interesting feature of this circuit is that you can establish the holding current of the SCR by choosing the resistor value. In order for the latch circuit to remain on after triggering, the two base-emitter junctions must have sufficient voltage (~0.7 V) to keep them on. This means that if the current supplied to it is more than Vbe/R1 + Vbe/R2, the circuit latches. If the latch circuit is connected to a small current capacitor, the latch circuit discharges the capacitor. Once the circuit’s current decreases below the holding current, it turns off.

picture1Use discrete components to build a controlled holding currentSCR

picture2 A good use of this circuit is shown. Shown here is a high-voltage input, 48-V output flyback converter that uses SCRs to shut down power in the event of an output overvoltage condition caused by a control circuit failure. When the input voltage is first applied to the circuit, the current through R3 and R4 charges the bulk capacitor C3. When the voltage of C3 is high enough, the control IC starts to work, switches the power FET Q3, and transfers the energy to the output. By controlling the current of U1, the regulation of the output voltage is achieved, thereby controlling 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 do not conduct during normal operation. In the event of an overvoltage, they begin to conduct, suppressing the current of optocoupler U3. U3 triggers the 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 continues to discharge the bias capacitor until the voltage approaches 1 volt. Thus, 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 how much current is required to be held in the latch circuit. If the value of R14 and R16 is small, the latch circuit closes, the bias capacitor charges, and the power supply tries to provide output power again.

In the event of a failure, this approach provides the ability to retry continuously. If the value of the resistor is large enough, the latch remains on and a power cycle is required to reset it. In this case, there is no continuous retry. Another important component in this circuit is R5, which limits the bias supply after the latch circuit is turned on. Normally, this component is required to prevent peak bias detection.

picture2 programmingSCR latch control

There are many ways to use this circuit, especially if you use rising and falling edges to trigger it. For example, connecting a Zener diode between Q5’s bias and base provides overvoltage protection on the primary side. You can use a negative inversion temperature sensor to drive the base of Q4.Alternatively, you can also use a comparator on the secondary side with apicture2 A very similar optocoupler is shown that implements a very precise overcurrent shutdown.

All in all, this latch circuit, consisting of $0.03 transistors, is very versatile. It can be triggered on negative or positive transitions, latched or not, depending on your resistor values. Next time, we will compare the transient response of discontinuous and continuous power supplies, showing that efficiency is not the only reason to use a synchronous rectifier.

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The Links:   MG50Q6ES41 LM64183P