Overvoltage protection

[meastwood98]

Hi,

Im using a DC-DC convertor and the datasheet suggests the following over-voltage protection (see attachment). The voltage in is 28v and the output is 3.3v. The max voltage my component it is powering can take is 3.6v.

I have selected a 2.4 zener diode and a 1.2v drop opto coupler, therefore once the output voltage becomes >3.6v, current will flow through the opto-coupler and resistor. My problem is calculating the value for the resistor.

If I design it for a voltage around 4v, if the output voltage error is 28v then the current through the opto coupler will be huge and probably damage it. However if I design it for a voltage of 28v, if the over voltage is around 4v, then there will not be enough current to switch on the transistor side of the opto coupler.

I hope this is clear,

Thanks,
Mark

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[crutschow]

The zener diode and opt coupler voltages have a loose voltage tolerance and are temperature sensitive, so are not a good choice to do your over-voltage circuit. You should add a precision shunt **broken link removed**(which acts like a precision zener) and a comparator, such as one of **broken link removed**, to drive the opto coupler.

 

[mneary]

If I design it for a voltage around 4v, if the output voltage error is 28v then the current through the opto coupler will be huge and probably damage it.

The output voltage will have to go through 4V on the way to any higher voltage. It must operate fast if it is going to save your circuit.

If your opto ever receives 28V it is too late already, your circuit is already destroyed and it doesn't matter if the opto is killed too.

 

[Boncuk]

Why not operate the entire circuit off an adjustable voltage regulator connected to the DC/DC converter?

BTW, there are regulated DC/DC converters also available.

I got some numbers wrong but you might adjust values for yours.

At 28V input voltage the output is 3.55V and If of the optocoupler will be 19.1mA (assuming Vf=1.2V and Ifmax=20mA).

At 4V input voltage the output is 2.36V and If is still 9.41mA, enough to switch the transistor on.

See schematic and results on the screenshots.

Boncuk

 

[indulis]

Who is the manufacturer & what is the part # of your DC-DC?