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| Film cap across bridge rectifier and new "soft" diodes |
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| David Hess:
--- Quote from: coppercone2 on December 10, 2018, 10:39:32 pm ---is there some manufacturer of diodes that is known to keep a more consistent product out, or does additional documentation? --- End quote --- Other than various soft recovery rectifiers, I am not aware of any. If such a standard recovery part existed, it would probably cost more than adding snubbers or using a faster diode. --- Quote ---This is kind of a compliance problem if suddenly your product line that was working fine 20 years ago starts doing something funny because a process improved. --- End quote --- It sure is but that is what happens if you rely on unspecified behavior. --- Quote ---Also, whats the model of a great/good diode for audio rectifier power supplies, to be used with preamplifiers and a high power variant for power amplifiers? --- End quote --- I think fast recovery rectifiers like the 1N4933 series are the best compromise. Schottky rectifiers should avoid the issue entirely. Faster "soft" recovery rectifiers might still snap off too quickly but try them and find out. The last time I tested this, every diode type that I had available except for some standard recovery parts eliminated the problem entirely. --- Quote from: 001 on December 10, 2018, 11:19:53 pm ---Modern fast diodes like HER108 produce swithching noise at common 50Hz rectifier too isnt`it? Is capasitor still eliminate it? --- End quote --- I have not noticed the problem when using any fast recovery rectifier. --- Quote ---The second question is 40 years ago I shunt EVERY diode with cap New diodes own capasitance became low Is it mean that I must increase shunt cap value? :-// --- End quote --- The values are not that critical and diodes with equal current ratings tend to have about the same capacitance. --- Quote from: chris_leyson on December 10, 2018, 11:21:31 pm ---Some manufacturers give figures for trr for standard recovery diodes, I think Motorola used to do it. It's not too difficult to measure and Tim, T3sl4co1l posted a design for a simple trr tester. The topic also reminded me of one of Mr Carlson's videos. Interesting part starts at 12min. --- End quote --- Unfortunately, reverse recovery time is not the problem; it is the snap-off time or hard versus soft recovery which is. Fast recovery rectifiers have less stored charge because of lower minority carrier lifetime and this seems to correlate with better results although the fastest parts do tend to recover very hard. |
| coppercone2:
I have been thinking about snubbers and stuff, but is the RFI rectification through the diode bridge serious? In that case maybe even if you snub it you still want some HF bypass in parallel with the snubber. I don't know what the pass frequency should be or how to treat the filter impedance properly. I think I might experiment with doing HF injections through a power amplifier into a diode bridge to see what happens to measure the output of a crude filter with a SA but I need to make some decent linear HV sine wave generator thats well behaved because I don't want to plug a expensive RF amp (100-500MHz @ 50W max) into something connected to the outlet. I need to get some APEX chips to make a low noise low power line simulator). Then when I get some data maybe the LISN can be used for something finally. |
| T3sl4co1l:
Rectifier acts as a PIN diode, gating noise in/out of the PSU. Therefore even if the PSU is clean and stable (making sharp tones at Fsw and harmonics), all of those tones have a 120Hz buzz modulated onto them. Tim |
| David Hess:
I first ran across this problem in one of my simple fixed voltage bench power supplies when using it for audio design work. The impulse noise from the snap-off of the standard recovery rectifiers proved impossible to filter out and was actually audible in circuits not even using the power supply. After filtering, I tried ceramic capacitors directly across the diodes and that worked fine. Later I found that fast recovery rectifiers solved the issue also. After that, finding small capacitors in parallel or ferrite beads in series with power line rectifiers in test equipment made a lot more sense. |
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