Hi Roman,

> I'm curious, why fuse the output and not the input?
> Also, they are two big and expensive Zeners! Couldn't
> you use a smaller zener clamped with a larger capacitor?
> It's not as though you need 10W of zeners to handle 4W
> of surge for a few mS.

I have fused the output to protect the circuit from applying an external
power source to the output. Since this DC-DC converter is a stand-alone
unit, it might be wired visa-versa. Applying an external DC source with
correct polarity doesn't harm the unit because of a blocking diode. But when
the polarity is reversed, the output fuse would blow.
But indeed, when protecting for external power sources on the output is no
issue, I could move the fuse to the input.

The reason why I use these two 5W zeners in series can indeed not be fully
justified. I had these 10V parts lying around and needed 20V. I am sure just
one 5W zener could do the job too. However, since I am using a slow blow
200mA fuse (fast types cannot be used due to relatively high capacitive
loads), even these two 5W series zeners get pretty hot when shorting out
180VDC.

I know TVS's are able to absorb more energy than a simple zener. But these
parts are twice as expensive than my zener too.


> You can even reduce the Zener surge current even more by
> adding a resistor between the cap and zener, like this,
> (if your load can tolerate a slight overvoltage)

Yes, I have thought of that too. But that will cost me an extra resistor. Of
course the zener could be smaller too then, however the smaller the zener
the larger the series resistor value has to be. And that will only increase
the overvoltage level. But perhaps, some tweaking might be interesting since
I can handle some overvoltage (larger than 20V). :-)

Daniel...

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