Electronics Forums > Need help wih snubber

# Need help wih snubber

Senior Member
Join Date: Dec 2011
Posts: 122

 02-26-2012, 02:54 PM
There is some ringing on the primary side of my DC to DC converter that happens when the switch closes. I thought I understood how to make the snubber, but no luck yet. All I am doing is burning resistors. Here are my calculations for the RC value. For L, I used the leakage inductance of the transformer, and for C I used the interwinding capacitance of the transformer. I dont know if this is what I need or not. Also, I think I need a RCD snubber so the snubber is not conducting white the mosfet/switch is conducting. Here are my values:

.144uH and 76pF transformer:

R = (L/C)^(1/2) = (.000000144/.000000000076)^(1/2) = 1894.73^(1/2) = 12.37 Ohm

C = 2Pi(LC)^(1/2)/R = 2 x 3.14 x (.000000144 x .000000000076)^(1/2) / 12.37 = 1.13uF

I attached a pic of the snubber I soldered together to test. The anode of the uf4007 is placed where the drain and "low" side of the transformer primary meet. The resistor and cap go to the "high" side where the 12V comes into the primary. So, I am pulsing 12v through the transformer primary and need a snubber for this. So confusing...
Attached Thumbnails

Last edited by bonedoc; 02-26-2012 at 02:57 PM..

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 02-26-2012, 03:36 PM
How much current are you switching?

Too often, the ringing is blamed on the Inductive load, when the truth is that the ringing
is actually caused by the Inductance of the DC Link at turn off.

(DC Link refers to the input side DC voltage to the switch.)

Think of it this way, when you turn off the current, the commutating Diode of the device sends the power back to the input capacitors.

If you have high Inductance, both in the wiring and capacitors themselves, you will see
large spikes at turn off.

No snubber in the world is going to help that!

I switch 600VDC through IGBT H-Bridges at over 2000 Amps. It took me a year to discover that the spikes weren't from the transformer. I tried every snubber combination you could imagine, but still had spikes reaching 1200V or more.

When I got rid of the Litz Wires, and went to a laminated Buss, the spikes practically dissapeared! And finding capacitors with much lower inductance removed the problem altogether.

Can you provide photos of the input capacitors, and the wiring to the device?

I would bet I can provide some solutions to solve the problem.

Senior Member
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 02-26-2012, 03:48 PM
Hmmm.....I dont believe I have an input capacitor. Never heard of one ;-)

Here is a schematic of the primary side of my transformer. Driver and mosfet on the left. You can see where the 12V goes into the transformer and then through the switch. The snubber is on the right. I missing something.
Attached Thumbnails

Senior Member
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 02-26-2012, 04:23 PM
OK.....now that I see what you are trying to do........

First of all, remove the capacitor and resistor that are in series with the diode.

Then connect the Cathode of the diode to the 12V input.

Where are you measuring the spike, referenced to what?

Measure the spike across the FET at turn off. how much is it!

The diode is designed to route any voltage more than 12V back to the 12V source.

You need a low inductance/low esr capacitor connected to the juncion of the diode and the 12V. The wires also neet to be short.

Let me know how that works.

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 02-26-2012, 04:58 PM
Ok. What value and rating of cap should I use? I am not at my desk right now but can post a pic later. The noise only occurs during switching, and I can see it anywhere in the circuit :-( I figured it was from spikes when switching closes. It is less than a volt at the drain with reference ti ground, but enough to cause some erratic behavior. Once I get this primary side dampened and I see the effects, I will probably have questions about the high voltage secondary side.

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 02-26-2012, 05:03 PM
Is a small ceramic disc cap fine? Are you saying the diode goes anode the drain and cathode to 12v, and then connect the cap from cathode to 12v but not in series?

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 02-26-2012, 08:14 PM
Ok, here are a couple of pics. They are at 1us/cm and .1v/cm. The probe is on the drain and the ground clip is at the source. I tried the uf4007 alone, with no cap and things got hot. See the second picture. It is crappy pic, but that is what happens with the diode anode at drain and cathode at the 12v going into the transformer.

I need to add the cap, but I am a little confused. Do you mean circuit A or B below? Seems like B would be sending electricity to the cap when the switch was on or off.
Attached Thumbnails

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 02-27-2012, 12:00 AM
no you need to remove the cap from between the diode and supply as gonzo said
I suspect he meant the cap to go between + and - rails of the 12V supply and probably should be at least a 100uF electrolytic of ~ 25V.

Your circuit A stops the diode from its back EMF protection
your circuit B just puts a short cct across the capacitor and so it doesnt do anything

The inductor... what is it ? it isnt the coil of a relay is it ?

Dave

No Mathematical model can ever predict my wife's mood swings

When all else fails.... Read the instructions!

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 02-27-2012, 12:30 AM
Thanks! The inductor is the primary side of a transformer in a dc to dc converter.

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 02-27-2012, 02:42 AM
B has no capacitor in it at all (it is shorted), It is also the circuit I would recommend trying first.

edit: but perhaps not for a DC-DC converter.

Last edited by (*steve*); 02-27-2012 at 02:46 AM..

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