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Old 27th March 2013, 07:22 PM   #21
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Default Snubber for center tap

Do you think that this is the right way of implementing a snubber network for a full wave rectifier using a secondary with centre tap? I am referring to Hagerman's article as far as "calculated" is concerned:

center tap snubber.JPG

And I say one diode because he mentions that for a bridge rectifier, we should take into consideration two diodes in series, so half the diode capacitance in the equations.

Is there a way of installing only one snubber between across the whole secondary?

Thanks in advance.
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Old 27th March 2013, 10:18 PM   #22
DF96 is offline DF96  England
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As the two halves of the secondary are fairly strongly coupled together I would use only one snubber. Remember, many circuits work quite happily with no snubber so perfection is not required.
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Old 28th March 2013, 03:22 AM   #23
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I grounded the center tap and put the RC snubber across the entire secondary and the cap across entire secondary.

Last edited by SpreadSpectrum; 28th March 2013 at 03:25 AM.
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Old 28th March 2013, 07:03 AM   #24
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Quote:
Originally Posted by DF96 View Post
As the two halves of the secondary are fairly strongly coupled together I would use only one snubber. Remember, many circuits work quite happily with no snubber so perfection is not required.
While I have never seen any evidence that diodes need snubbering, the snubber should be placed close to the device being snubbered, try simulating it and then put a load of stray inductance in series with the snubber and observe what happens to the snubbering action.

Same goes for the assertion the the two halves of the secondary are closely coupled, at frequencies close to the transformer design frequency the coupling is strong, coupling falls off with frequency, then stray inductance and capacitance play all sorts of games.

Even a well designed audio output transformer has trouble maintaining good coupling over 3 decades of frequency, the design frequency for the snubber mentioned here is 4 decades above the transformer design frequency.
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Old 28th March 2013, 09:18 AM   #25
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Thanks for your replies.

I am mainly asking since I will order the transformer, and I don't already have an estimate of the winding capacitance. Should it be low, the individual diode capacitance could matter more, even though as you say (DF96) we don't have to get mad about perfection - I agree on that.

So, for two snubbers, I calculate one diode for each, and for one across the whole secondary, I calculate two? Curious about what gets into the math.
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Old 28th March 2013, 10:38 AM   #26
DF96 is offline DF96  England
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Just to avoid any confusion, we need to distinguish between transformer snubbering and diode snubbering. I assume that post 21 was about transformer snubbering.

Transformer snubbering is about what happens when the diodes switch off 'normally' - the issue is stored magnetic energy. A fairly low frequency issue.

Diode snubbering is about what happens when the diodes switch off 'suddenly' (i.e. with stored charge). A fairly high frequency issue.

In addition, in equipment containing RF oscillators there is the separate issue of modulation hum.
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Old 28th March 2013, 11:01 AM   #27
FoMoCo is offline FoMoCo  United States
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In a switching power supply one would normally place a series RC in parallel with each diode, if a snubber is used. I can't see a real benefit from placing it across the winding.

Personally I do not see the purpose of a snubber at line frequency. If your diodes are that noisy, you've used the wrong diode. There's nothing wrong with good ol' slow general purpose diodes here.
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Old 28th March 2013, 11:57 AM   #28
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Well I am actually referring to Hagerman's article - so, I mean diode snubbering, to kill the oscillation while switching off.

I have not built it yet, I am planning to do so. Which means I am designing it on the first place - should I ever need snubbering, I would like to know how to get it. And since I will use a toroidal transformer, I may get weird readings compared to the article, so I must really know what I am doing.
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Old 28th March 2013, 01:23 PM   #29
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To damp any overshoot, from diode turnoff delay the snubber should be in parallel with the diode(s) being snubbered, using short leads. Leads introduce series inductance which reduces the effectiveness of snubbers and provides a means to radiate the energy that the snubber is designed to absorb. This is general advice which applies to all RC snubber circuits.
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Old 28th March 2013, 01:42 PM   #30
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Quote:
Originally Posted by metalsculptor View Post
To damp any overshoot, from diode turnoff delay the snubber should be in parallel with the diode(s) being snubbered, using short leads. Leads introduce series inductance which reduces the effectiveness of snubbers and provides a means to radiate the energy that the snubber is designed to absorb. This is general advice which applies to all RC snubber circuits.
Could we say that this is what equally Hagerman does, placing it across the secondary? In terms of thinking that the reservoir capacitor will appear to be a short circuit during the high frequency oscillation, and will somewhat connect the diode to the snubber?

I may just be wrong.
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