# Transient voltage suppression diode not doing its job

I have a circuit which works on 21V DC and draws 3A maximum current. I need to get it tested to IEC 61000-4-4 and 61000-4-5 standards which are surge immunity tests.

I use a UL certified switch mode power supply to power my unit. The supply actually emits surges which is more than 21V (approximately +/-150V).

So I put in a TVS diode SMLJ22CABCT-ND in my circuit to protect against surges. This diode has a response time of about 5 picoseconds. But the pulses which are given in the surge tests are 8/20 μs pulses of 2KV. which in turn makes the power supply to give out surges as shown in the image. As you can see in the image, the output varies for a few microseconds.

My question is why hasn't the TVS diode suppressed the high voltage. My circuit is damaged by this over voltage.

The schematic of the power circuit is given below.

There is not a lot of protection in the circuit. The 21V is taken for a different interface. I need to find a fix for this rather than designing a new circuit board.

The testing circuit for the IEC 61000-4-5 is standard.

1KV across Live and Neutral; 2KV across Live and Earth, Neutral & Earth (detailed description)

• Do you have a circuit of the test setup? – Spoon Aug 1 '14 at 12:46
• Let's see your circuit, as Spoon says, but that doesn't sound like a very good power supply- or something else is going on. – Spehro Pefhany Aug 1 '14 at 12:54
• Yea, we are going to need a circuit, or a schematic preferably – Funkyguy Aug 1 '14 at 13:43
• Show us your complete power-conditioning circuit, including decoupling capacitors and any series inductors/beads. If all you have is the TVS, no wonder it isn't very effective. – Dave Tweed Aug 1 '14 at 13:55
• A tvs alone probably won't work. There needs to be either resistance or inductance or both to allow it to do its job properly. – Erik Friesen Aug 2 '14 at 19:09

Very often the parasitic impedance between the lines and diode is some nanohenries. This caused by the length of diode leads.

This impedance slowing the response of TVS diode by nanoseconds, thus the transient pulse can pass.
You can use a TVS diode with so called “flow through design” in which you have to pass rail and ground through the same package.

Of course this needs a proper design of PCB.

• This is the TVS diode which i'm using now. link I have soldered the TVS diode on to the pins directly. I am working with an existing PCB. I'm trying to apply a fix. Will adding a capacitor in parallel to the diode solve the issue here? .( counteract the inductance) – mic Aug 3 '14 at 5:18
• I just read it in a similar post here. I've also found an article about overcoming the parasittic inductance issues here. – mic Aug 3 '14 at 6:39
• @mic may be not a capacitor but it will be interesting to add an inductor 100 ~ 470μH (at the rated current) after the diode, in series with power supply rail of your under protection device, just to slow the rise time of transient pulse and finaly to absorbed by shunt. I haven't test this suggestion – GR Tech Aug 3 '14 at 7:10
• @ GR Tech should it be before or after? According to the doc, shouldn't it be before the diode? – mic Aug 3 '14 at 7:20

To bypass the transient, you need to use a capacitor of $$1.6 \mu f \text{ }(or\text{ }larger)$$ in parallel with the diode.

• There is a capacitor in the circuit already. the 470uf capacitor is in parallel with the tvs diode. Also is there any calculation involved in determining the value of the capacitor – mic Aug 11 '14 at 8:43
• This isn't always a possibility. You cannot just chuck capacitance down to slew the transient, what about on a communication bus? silicon TVS's are fine as long as they are correctly sized, placed correctly with appropriate steering diodes (if needed) – JonRB Jun 22 '15 at 11:40
• @mic: Even though you already have a 470uf across the diode, for some reason they are not good at suppressing high frequency noise. Try a ceramic of around 2 uf, of the appropriate voltage. – Guill Apr 16 '17 at 3:11