Electronic – How to protect LM317 from output short


In the below circuit, the diodes protect the LM317 against both of the caps discharging into the IC when input or output is shorted. LM317 is itself capable of having it's output shorted without damage. But the input-output voltage difference must not exceed 40V. In my circuit, I'm applying 45V to the input, and getting 30V from the output. In normal conditions this means a 15V difference over the IC and everything is fine, but if the output gets shorted somehow (maybe because of the initial charging of large capacitors connected to the output), I get a full 45V above the IC and this is enough to damage it according to datasheet (it's above the 40V maximum).

What can I do to prevent this, without using another IC like LM317HV?


Best Answer

One simple option : replace D1 with a 30-ish volt zener rated for several amps, and a fuse (upstream of Vin) rated to blow before the zener does.

Normal operation : the zener should not normally conduct.

Power-off operation: the zener is forward biassed providing the same protection D1 does.

Short circuit operation : the 317 goes into current limit and the output voltage drops. The zener conducts before I/O voltage exceeds 40V. The fuse blows. This does depend on the rating of the zener diode. It must be chosen carefully, or a more complex arrangement adopted (essentially the zener diode supplies an emitter follower) as in the other answer.

Power-on operation : vith 45V in, the zener will supply current to the load as long as the LM317 is supplying less than 15V. After that, the LM317 takes over.