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Half wave rectifier snubber
Half wave rectifier snubber
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Old 31st December 2018, 09:25 AM   #1
JAY X is offline JAY X  Spain
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Default Half wave rectifier snubber

Hi!

In advance, Happy New year!!

I have designed a dual rail +/-17v voltage doubler psu, with LM317/337 regulators, powered by a 18v AC wall wart transformer.(no center tap).

The fact is that i still have too much ripple: vp-p is 5,7mv. 926uv RMS. for a current draw of aprox 45ma per rail.

I think one way to reduce the ripple is to add a cap in parallel with the rectifier diodes.(snubber) with 10 or 100nf 100v.

Another idea is to add a 10nf x2 cap at the AC input, with one cap leg to gnd. but i'm not sure about this.

Thank you for your advise!

Jay x

Last edited by JAY X; 31st December 2018 at 09:28 AM.
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Old 31st December 2018, 09:49 AM   #2
trobbins is offline trobbins  Australia
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Perhaps show your layout and schematic.
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Old 31st December 2018, 09:56 AM   #3
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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DO NOT add capacitors between the AC input and ground unless they are class X and class Y respectively. That will have no effect though.
I presume you have one rec for +ve and one for -ve, Ground being the transformer winding that is not connected to the diodes.
Have you used the correct value smoothing capacitors and taken the common to ground where the transformer is connected? That is very important.
A diagramme would be helpful.
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Old 31st December 2018, 10:00 AM   #4
JAY X is offline JAY X  Spain
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Hi!

Yes, X2 of course!!

attached is a layout of the psu. Right now i have not the schematic at hand... I will post it asap.
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File Type: jpg VOLTAGE DOUBLER PSU.jpg (226.9 KB, 315 views)
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Old 31st December 2018, 10:08 AM   #5
JAY X is offline JAY X  Spain
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The psu schematic!
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File Type: jpg voltage doubler psu schematic.jpg (438.1 KB, 315 views)
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Old 31st December 2018, 11:13 AM   #6
analog_sa is offline analog_sa  Europe
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Half wave rectifier snubber
Quote:
Originally Posted by JAY X View Post
I think one way to reduce the ripple is to add a cap in parallel with the rectifier diodes.(snubber) with 10 or 100nf 100v.

This is certainly not a way to reduce the ripple. You can either up the main smoothing caps, or add an LC/RC section, which is probably impossible.

Why are you unhappy with the amount of ripple?
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Old 31st December 2018, 11:28 AM   #7
trobbins is offline trobbins  Australia
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My suggestion would be to use UF4007 diodes.

Also the CT to rectifier filter caps connection has high current pulses, so better to restrict that connection to thick traces side by side with the + and - raw supply rails. Use the 0V end of the regulator adjust circuits as the star node to then flood out to everywhere, or even better to take C276/275 terminals back directly to that star node (rather than use the flood). There are many app notes for voltage regs that show the star aspect of that 0V node.

If the transformer secondary has significant leakage inductance, then also aim to use a CRC snubber across the winding as soon as it connects to the pcb - see Mark Johnson's quasimodo threads.

Last edited by trobbins; 31st December 2018 at 11:31 AM.
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Old 31st December 2018, 11:59 AM   #8
analog_sa is offline analog_sa  Europe
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Half wave rectifier snubber
It seems a lot of people do not make a distinction between a primary effect such as PS ripple and a remote tertiary such as transformer ringing. Bloody internet
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Old 31st December 2018, 12:12 PM   #9
JAY X is offline JAY X  Spain
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Hi!

5,7mv ripple is too much for so low current draw. The ripple noise level is about -40dbu, and is modulating slightly the audio band, altering the frequency response...

Jay x
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Old 31st December 2018, 12:49 PM   #10
trobbins is offline trobbins  Australia
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How are you measuring the ripple and pk-pk?
You are using half-wave.
Have you measured the raw input ripple and confirmed that the line regulation wrt ripple is as per datasheet nominal performance.
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