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Old 11th June 2011, 07:29 AM   #1
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Default RC snubbers for diode recovery noise

In an conventional, bridge rectified capacitor input supply, where is the best place to RC snub the resonant circuit formed by secondary distributed capacitance and leakage inductance of the winding? I have seen it suggested to place RC snubbers across each diode. Wouldn't RC across the secondary be sufficient? Are there any advantages to doing it one way vs. the other?
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Old 11th June 2011, 04:01 PM   #2
DF96 is offline DF96  England
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I would put a snubber on the secondary. If necessary (e.g. for RF equipment) a capacitor across each diode too.
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Old 11th June 2011, 05:26 PM   #3
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Jim Hagerman has written the best audio related treatise on the subject to which I'm aware.

http://hagtech.com/pdf/snubber.pdf
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Old 11th June 2011, 06:07 PM   #4
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I had read the Hagerman paper, which is what prompted the question. In the opening of the paper he states that the problem is usually dealt with using RC snubbers across the diodes. At the end of the paper, the simulated circuit shows RC across the secondary.

I have followed Hagerman's method with good results. I made some regulated supplies that didn't attenuate the switching noise that much. Snubbers made the output very quiet.

I was mostly just wondering if there was a compelling reason to put RC snubbers across the diodes. It would require more parts so is there an advantage?
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Old 11th June 2011, 08:27 PM   #5
DF96 is offline DF96  England
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Hagerman's article is interesting, but I think it contains an error. He correctly calculates the required resistance to damp an LC circuit. He then assumes that he can simply connect this resistor with a big capacitor in series with it. The snag with this is that the capacitor will lower the resonant frequency and the characteristic resistance of the tuned circuit. The result is that while significant damping will take place it will not be the critical damping he was aiming at. The result can be seen in Fig 10 - still a little ringing but at a much lower frequency.

He needs to do the full maths of the original LC with the CR snubber across it. I haven't done the sums, but I would guess you need a C about twice that of the LC (i.e. about a third of his value), and an R of about half his calculated value.

He also says that is important to use non-inductive resistors. Fine for damping UHF sproggies in a transmitter, but for damping sub-MHz ringing in a 50Hz power transformer any resistor will do. Film resistors are essentially resistive up to about 100MHz, so no need for carbon composition. Wirewound would do in most cases!
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Old 12th June 2011, 12:21 AM   #6
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In the paper, he states that he is shooting for a slightly under-damped response with a damping coefficient of 0.5, but your points are well taken.
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Old 13th June 2011, 02:40 PM   #7
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Quote:
Originally Posted by SpreadSpectrum View Post
I had read the Hagerman paper, which is what prompted the question. In the opening of the paper he states that the problem is usually dealt with using RC snubbers across the diodes. At the end of the paper, the simulated circuit shows RC across the secondary.

I have followed Hagerman's method with good results. I made some regulated supplies that didn't attenuate the switching noise that much. Snubbers made the output very quiet.

I was mostly just wondering if there was a compelling reason to put RC snubbers across the diodes. It would require more parts so is there an advantage?
My recollection is that Hagerman includes all first order reactive parasitics in his analysis, including rectifier diode capacitive reactance. They are treated as part of a single reactive network. So, my take is that rectifier diode commutation acts like the 'hammer' striking the resonant 'bell' formed by the reactive parasitics. I view rectifier diode commutation as providing an impulse to this resonant system, similar to what a hammer does every time it strikes a bell.

It would seem, although I have not done the analysis to confirm, that one could either damp the reasonant 'bell', or soften the impulse produced by the 'hammer' strike, as it were. So, my conclusion is that damping the effective tank circuit (per Hagerman), or reducing the effective dV/dT (or dI/dT) of the rectifier diode commutation would both work towards reducing high-frequency ringing by this network. Perhaps, the greatest effect would be had by doing both.
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Last edited by Ken Newton; 13th June 2011 at 03:02 PM.
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Old 13th June 2011, 03:15 PM   #8
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Maybe it's two separate issues, but I thought RF was generated at the diode when it switches and needs to be bypassed right at the diode because lead length would make it useless anywhere else. 0.01 uF is about right, no R needed.
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Old 13th June 2011, 04:11 PM   #9
DF96 is offline DF96  England
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There could be two issues here. Diode switch-off can be very sharp - it was even used as a microwave source although using diodes developed for the purpose. That is why in RF situations there are sometimes small caps directly across the diodes. As an audio amp should not be sensitive to RF (although they sometimes are) there is less need. The transformer ringing then becomes the issue, and a snubber across the secondary deals with this. Note that a cap across the diodes simply shifts the frequency of the transfomer resonance but does not damp it.

So to stop transformer ringing when the diodes switch off put a snubber across the transformer. To stop RF radiation (or modulation hum) from the diode transition put a cap across each diode - but this might not be necessary in audio equipment. A snubber across each diode is an expensive way of stopping transformer ringing (4 snubbers instead of 1), and probably won't be so good at stopping RF as a simple cap.
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Old 13th June 2011, 05:15 PM   #10
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Originally Posted by DF96 View Post
The transformer ringing then becomes the issue, and a snubber across the secondary deals with this. Note that a cap across the diodes simply shifts the frequency of the transfomer resonance but does not damp it.

So to stop transformer ringing when the diodes switch off put a snubber across the transformer. To stop RF radiation (or modulation hum) from the diode transition put a cap across each diode - but this might not be necessary in audio equipment. A snubber across each diode is an expensive way of stopping transformer ringing (4 snubbers instead of 1), and probably won't be so good at stopping RF as a simple cap.
I agree with the snubber across the secondary. Don't quite understand the use of caps across each diode. It would seem that they would bypass the diodes at RF, coupling any RF in the supply directly into the power entry to the amp. Could you elaborate on their use for stopping modulation hum?
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