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Old 2nd February 2011, 10:48 PM   #11
CBS240 is offline CBS240  United States
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from the switching noise of the rectifier diodes. Diodes make noise when they change state between conduction and off. Shottkys produce less, but it is still there. This is a large problem in SMPS because of the higher switching frequency but it is still present in 60Hz rectifiers, though not as significant. It is also helpful to place a small film cap, 1uf or so, in paralell with the electrolytic filter caps to help filter and source current to the load for the higher frequecies.
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Old 2nd February 2011, 11:53 PM   #12
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the noise comes from the tank circuit formed by the leakage inductance of the transformer, the junction capacitance of the diode (changes with voltage) and parasitic capacitance and inductance -- for torroid transformers the leakage inductance is low, so the frequency is pretty high. I've used EI transformers from HP amplifiers and the leakage inductance can be in the milli-Henries which will put the resonant frequency into the audio band.
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Old 3rd February 2011, 01:09 AM   #13
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So, without measuring these unknowns, can snubbers be implemented?
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Old 3rd February 2011, 01:14 AM   #14
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Default Question please, anyone.

Quote:
Originally Posted by Neutrality View Post
Im making a PSU PCB with onboard rectifiers.
I noticed in the picture (schematic) of the power supply that it uses two bridge rectifiers. What is the difference or advantage when using two bridge rectifiers as opposed to one?
Thank you
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File Type: png Simple PSU PCB Rectifiers.png (32.7 KB, 264 views)

Last edited by AEIOU; 3rd February 2011 at 01:19 AM.
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Old 3rd February 2011, 01:40 AM   #15
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Default I would say yes, but. . . .

Quote:
Originally Posted by ra7 View Post
So, without measuring these unknowns, can snubbers be implemented?
How do you know it is having the desired effect if any effect at all unless you can measure the noise/ripple?
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Old 3rd February 2011, 09:27 AM   #16
Telstar is offline Telstar  Italy
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Quote:
Originally Posted by CBS240 View Post
from the switching noise of the rectifier diodes. Diodes make noise when they change state between conduction and off. Shottkys produce less, but it is still there.
Exactly.

Quote:
It is also helpful to place a small film cap, 1uf or so, in paralell with the electrolytic filter caps to help filter and source current to the load for the higher frequecies.
Even better two a bigger and a smaller film to avoid resonances, like 47uF and 0,1uF.
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Old 3rd February 2011, 09:29 AM   #17
Telstar is offline Telstar  Italy
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Quote:
Originally Posted by jackinnj View Post
the noise comes from the tank circuit formed by the leakage inductance of the transformer...
There are many source of noises...
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Old 3rd February 2011, 02:38 PM   #18
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Quote:
Originally Posted by Telstar View Post
There are many source of noises...
The noise we are talking about is that arising from the switching action of the diode -- if you;ve a scope with a delayed trigger you can see it and measure the frequency.

Let's take for example a 2x42V 500VA toroid from Antek. Leakage inductance measured on a GR 1658 was about 580uH. If you're using MUR860 diodes, the junction capacitance is around 80pF -- the frequency at which the tank resonates is 1/f=2PI SQRT (LC) =~ 740kHz

Values are R=2.7k, C=500pF per Hagerman "Calculating Optimum Snubbers".

Here's a Bode plot of before and after:
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Old 3rd February 2011, 04:23 PM   #19
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It works, but is it audible? To be honest I doubt it.
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Old 6th February 2011, 06:45 AM   #20
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Quote:
Originally Posted by jackinnj View Post
The noise we are talking about is that arising from the switching action of the diode -- if you;ve a scope with a delayed trigger you can see it and measure the frequency.

Let's take for example a 2x42V 500VA toroid from Antek. Leakage inductance measured on a GR 1658 was about 580uH. If you're using MUR860 diodes, the junction capacitance is around 80pF -- the frequency at which the tank resonates is 1/f=2PI SQRT (LC) =~ 740kHz

Values are R=2.7k, C=500pF per Hagerman "Calculating Optimum Snubbers".

Here's a Bode plot of before and after:
What's the effect of changing the frequency of V1 and V2 from 1 kHz to that of full wave rectified mains (i.e. 100 Hz or 120 Hz)?
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