How do I add an external OVP (latch cutoff type) to a DC-DC converter that does not have the overvoltage protection (OVP) function?
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A. *Applicable conditions
Products without OVP function or when setting advanced OVP below the standard value of products with OVP function

1. Example circuit
Excess voltage is detected on the output side. The detection value is determined by the voltage of the Zener diode (ZD1) and the light-emitting side
forward voltage VF of the photocoupler (PC1).
The circuit in Figure 1 uses a phototriac coupler and the coupler itself has a latch function (breaker retention). However, since it is difficult to obtain such
components recently, an example circuit using a general-purpose photocoupler is also presented in Figure 2 as an alternative circuit.

Figure 1、Figure 2

2. Explanation of OVP action
When the input is turned on or in the steady state, Q1 is ON and the RC terminal is short-circuited to -Vin. This means that the DC-DC converter is also in
the ON state and the rated output voltage is generated. When, for some reason, an excess voltage condition occurs in the output and the output reaches
a voltage equal to the VZ (ZD1) + VF (PC1) voltage, PC1 turns ON and the RC pin of DC-DC turns OFF. Since this OFF state is retained (latched),
it is necessary to temporarily drop the input voltage in order to release the latch.

3. Important points when setting the constant number of products
1) An accurate OVP detection level can be obtained by setting the Zener diode (ZD1) current during OVP operation to near the ZD1 rated value
(example: 5mA).
2) If chattering occurs when turning on the input, the OVP may malfunction. Therefore, insert a bypass capacitor (Cin) in the input line as necessary.
3) The input capacitor (C1) of the thyristor circuit prevents malfunction when the input is turned on. Therefore, please be sure to attach an input capacitor.
4) Please optimize the power rating of the resistor being used and the pressure resistance of the capacitor according to the applied circuit.

  *A portion of the circuit described above is configured via theoretical review. Therefore, in order to confirm that no problems exist, please be sure to
evaluate the actual device before applying it to the machine.