# Limiting voltage transients with a TVS diode

My goal is to suppress transient voltage spikes from damaging the 5V, 600mA Step-Down Voltage Regulator D36V6F5. I believe the transients are a result of high inductance in the wires between the power supply and the circuit.

There are a couple of solutions listed in Pololu's guide to transients, but only adding a series resistor or a large electrolytic cap is possible in my case.

The resistor would result in power loss and a cap is just physically very large.

So my idea was to use a TVS diode -- SMF48A-TP. Now ignoring the fact that the maximum breakdown voltage Vbr is 58.9V, which is already too much for the D36V6F5, whose limit is 50V, I'm still seeing spikes of up to 65V.

Why is that?

I don't have the circuit for the D36V6F5, but I do know that it has 5.3 uF and 100 nF capacitors on the input.

• Where are these transients coming from? Also, realize that the voltage drop across a TVS diode in breakdown varies with current. More current = more voltage drop even if it's clamping. Sep 5, 2019 at 1:19
• Experiment with capacitors. You may not need a resistor. An electrolytic capacitor acts as a damper for the transient. Just try whatever size you can fit comfortably and see if it works. Sep 5, 2019 at 6:25
• Also, if the input voltage is 50V, and the output is 5V, then the input current will only be around 70-75 mA. You can put a 10 Ohm resistor in series with 75 mA no problem. That is only 750 mV of drop in a 50V input. 10 Ohms plus the existing 5.3uF already on the D36V6F5 will kill any inrush transients. If you aren't comfortable with 10 Ohms at least try 4.7 or 2.2 Ohms. There has to be some amount of voltage you are willing to drop to get rid of this problem. Sep 5, 2019 at 6:29