If your math is good, try this ap note:
http://www.hagtech.com/pdf/snubber.pdf
see section titled "snubber Capacitor".
http://www.hagtech.com/pdf/snubber.pdf
see section titled "snubber Capacitor".
It basically says that using a resistor in series with a capacitor connected in parallell with each diode is better then just using a single capacitor as the resitor helps by damping out the ringing after less then one cycle.
I have tested this in an amplifier power supply and can confirm that the theory seems to be correct, in my experience the resistor is not that critical but it is important to have it there, for a power supply of 220VAC 2A I used 100 ohm and 10nF and for a power supply of 350VAC and 100mA I used 10nF and 470 ohm, all components selected for min ringing.
Regards Hans
If you are going to use snubber caps on the rectifier for the mains 50/60 Hz VAC I would suggest you to use simple and plain 100 nF lossy ceramic capacitors with high dielectrica in parallel with each rectifier diode.
The PDF doc at
http://www.hagtech.com/pdf/snubber.pdf does not address the real issue with diodes, talking about only the intrinsic capacitance in the diode is actually a lack of "knowledge"
But the math is ok and applicable.
To be accurate you must measure the di/dt for the diodes Dtrr if you really want to calculate on the proper snubber, but I don't think this is a matter for you and your 50/60Hz rectifier, it's something else when designing SMPS's.
BTW, don’t use too large cap, the capacitors own resonance frequency decrease and thereby the snubbering capability for high frequency energies, eg. a small one can actually be more efficient.
Good luck!
The PDF doc at
http://www.hagtech.com/pdf/snubber.pdf does not address the real issue with diodes, talking about only the intrinsic capacitance in the diode is actually a lack of "knowledge"
But the math is ok and applicable.
To be accurate you must measure the di/dt for the diodes Dtrr if you really want to calculate on the proper snubber, but I don't think this is a matter for you and your 50/60Hz rectifier, it's something else when designing SMPS's.
BTW, don’t use too large cap, the capacitors own resonance frequency decrease and thereby the snubbering capability for high frequency energies, eg. a small one can actually be more efficient.
Good luck!
Re: Cap value
Thanks for bringing it to my attention James, and you may be right , however wouldnt it be more helpful if your response was more like PaulB's whose attitute was more of providing a solution than merely pointing out the issue?
Also I know enough not to make blanket statements like "it's very unlikey that you have caps which are 0.474 uF" when we all know tolerances in caps are inadequate enough for a cap marked 474 to actually be anywhere from 0.376uF to 0.564uF based on a 20% tolerance level... Sorry to be irked here but I had to make the point.
Regardless, the input is well taken and I would not know what to do without the help of a lot of good guys out there!
nemestra said:
Thanks for bringing it to my attention James, and you may be right , however wouldnt it be more helpful if your response was more like PaulB's whose attitute was more of providing a solution than merely pointing out the issue?
Also I know enough not to make blanket statements like "it's very unlikey that you have caps which are 0.474 uF" when we all know tolerances in caps are inadequate enough for a cap marked 474 to actually be anywhere from 0.376uF to 0.564uF based on a 20% tolerance level... Sorry to be irked here but I had to make the point.
Regardless, the input is well taken and I would not know what to do without the help of a lot of good guys out there!
Ultima Thule said:If you are going to use snubber caps on the rectifier for the mains 50/60 Hz VAC I would suggest you to use simple and plain 100 nF lossy ceramic capacitors with high dielectrica in parallel with each rectifier diode.
The PDF doc at
http://www.hagtech.com/pdf/snubber.pdf does not address the real issue with diodes, talking about only the intrinsic capacitance in the diode is actually a lack of "knowledge"
But the math is ok and applicable.
To be accurate you must measure the di/dt for the diodes Dtrr if you really want to calculate on the proper snubber, but I don't think this is a matter for you and your 50/60Hz rectifier, it's something else when designing SMPS's.
BTW, don’t use too large cap, the capacitors own resonance frequency decrease and thereby the snubbering capability for high frequency energies, eg. a small one can actually be more efficient.
Good luck!
U-T,
Isn't the issue the switch-off of the rectifier current at the end of the charging cycle? I always thought that because of wiring L the current shutt-off is not instanteneous, so you get oscillations that are decaying because of the losses. So, what I wonder, if you use fast recovery diodes, do you get less oscillations? I mean, you still have the current to shut-off and the wiring L. Am I overlooking something?
Jan Didden
Re: Re: Cap value
Sorry K-amps, didn't mean to annoy. I just wanted to highlight that someone who is reading cap snubbers papers should know how to read cap values. I just thought it might lead to a discussion on how caps are marked, which might help some novice users.
If your cap is marked "474", the first two digits indicate the first two numbers of the value and the third digit is the number of zeros - in this case 4. This gives a value of 470000, normally in pf, so the caps are likely to be 0.47uF (470nF). This is the text I should have written in the first place.
Your 470n caps are probably too big to be used successfully as snubbers. The use of resistors as suggested by Jim Hagerman is a good one.
Jan's point about the use of alternate diode types is well made. There have been a number of threads about the use of Schottky rectifiers and in particular about soft recovery types. These have not had any significant traffic for a while, but a search should locate them. If you have already purchased bridges, then this may not be useful.
Apologies,
James
K-amps said:
Thanks for bringing it to my attention James, and you may be right , however wouldnt it be more helpful if your response was more like PaulB's whose attitute was more of providing a solution than merely pointing out the issue?
Also I know enough not to make blanket statements like "it's very unlikey that you have caps which are 0.474 uF" when we all know tolerances in caps are inadequate enough for a cap marked 474 to actually be anywhere from 0.376uF to 0.564uF based on a 20% tolerance level... Sorry to be irked here but I had to make the point.
Regardless, the input is well taken and I would not know what to do without the help of a lot of good guys out there!
Sorry K-amps, didn't mean to annoy. I just wanted to highlight that someone who is reading cap snubbers papers should know how to read cap values. I just thought it might lead to a discussion on how caps are marked, which might help some novice users.
If your cap is marked "474", the first two digits indicate the first two numbers of the value and the third digit is the number of zeros - in this case 4. This gives a value of 470000, normally in pf, so the caps are likely to be 0.47uF (470nF). This is the text I should have written in the first place.
Your 470n caps are probably too big to be used successfully as snubbers. The use of resistors as suggested by Jim Hagerman is a good one.
Jan's point about the use of alternate diode types is well made. There have been a number of threads about the use of Schottky rectifiers and in particular about soft recovery types. These have not had any significant traffic for a while, but a search should locate them. If you have already purchased bridges, then this may not be useful.
Apologies,
James
Re: Re: Re: Cap value
James, I really am not sure what the effect would be of fast recovery. Do you think it would give less oscillations? I think the determining factor is the value of the current to shut off, and the wiring L. So, with faster diodes, does it just shift upwards the oscillation frequency, or will there be less of it? I tried to work it out in my head, but don't know. I would GUESS higher freq oscillations.
Jan Didden
nemestra said:
James, I really am not sure what the effect would be of fast recovery. Do you think it would give less oscillations? I think the determining factor is the value of the current to shut off, and the wiring L. So, with faster diodes, does it just shift upwards the oscillation frequency, or will there be less of it? I tried to work it out in my head, but don't know. I would GUESS higher freq oscillations.
Jan Didden
Re: Re: Re: Cap value
Here is a rather extensive thread from nearly a year ago that deal with this subject.nemestra said:
janneman said:
Jan,
sorry for not answering earlier!
Roddyama reffered to a thread that cover the diode issue well I think, have just red through only the first couple posts so i don't know how much they did cover.
The diodes reverse recovery is there anyway wethere there is an inductance or not in conductors.
The resonance frequency is probably quite low for a mains PSU's with big lytes, transformers, chokes etc so I don't think it's an issue in such a case and won't probably react, the problem is the trr of the diode.
What happens when you switch off the diode (because of the altering sine wave) is that you get a "current spike" in the opposite dirrection with a verry short durration, and this particular high frequency energy is the problem, it's harmful RF energy which can have frequency components up to several MHz and every single transistor (in your nice amplifier etc) is actually a potential AM radio receiver with it's emitter diode and intrinsinc capacitor and distorts the audio signal.
The eventual LF resonance oscillation is probably not the problem, good amps also have very good PSRR so they can handle probably quite well frequencies in the audible spectra.
Should mention that soft recovery is what you should look after for audio applications (also mentioned in the mega thread started by FD).
And a capacitor in parallel with the diode up to max 100 nF is ok, fast diodes might even be happier with a cap down to 10 nF because of it's lower resonance f.
I don't know how much is covered up in the mega diode thread, but since I know this issue quite well I think I would like to spare my time, some threads take days to read through!
Here is an onsemi app note about fast recovery, soft recovery and snubbers.
http://www.eetkorea.com/ARTICLES/2000DEC/2000DEC08_AMD_AN8.PDF
http://www.eetkorea.com/ARTICLES/2000DEC/2000DEC08_AMD_AN8.PDF
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