capacitors connected to rectifiers

[samcheetah]

ive seen a couple of power supplies in which capacitors are connected in parallel to the diodes of a full wave rectifier. what is the purpose of these capacitors.

ive heard someone saying that these capacitors should be soldered directly onto the leads of the diodes of the rectifier. why is that so?

and what are these capacitors called???? (is it snubber capacitor)

 

[Nigel Goodwin]

ive seen a couple of power supplies in which capacitors are connected in parallel to the diodes of a full wave rectifier. what is the purpose of these capacitors.

ive heard someone saying that these capacitors should be soldered directly onto the leads of the diodes of the rectifier. why is that so?

and what are these capacitors called???? (is it snubber capacitor)

I've no idea what they are called (or even if they have a name at all?), but they are there to help prevent mains spikes damaging the rectifiers. I've certainly seen plenty of short circuit mains rectifers, so I don't know if they help or not - but presumably they don't do any harm.

I never fit them in anything I build, and I've never had a rectifier fail, but a small number of projects, used occasionally, hardly compares with millions of commercial units powered up 24 hours a day!.

 

[zevon8]

I agree with Nigel, and would add that they are installed more for reasons of meeting RFI and safety standards than for alot else. Sort of a belts and braces approach to engineering.

 

[Styx]

ive seen a couple of power supplies in which capacitors are connected in parallel to the diodes of a full wave rectifier. what is the purpose of these capacitors.

ive heard someone saying that these capacitors should be soldered directly onto the leads of the diodes of the rectifier. why is that so?

and what are these capacitors called???? (is it snubber capacitor)

odd?

If it was a thyristor bridge those caps (with a resistor and a diode) would be a snubber since the Thyristor is a controlled switched and a snubber might be needed.

however, it sounds like these are EMC caps, prob small value and ceramic. They need to be close to the diode leads to reduce stray inductance between point of interest and the capacitor

 

[Roff]

Actually, they are to prevent spikes caused by the rectifiers' reverse recovery characteristics. Here's something I found:

From: Winfield Hill
Newsgroups: sci.electronics,sci.electronics.design
Subject: ceramic caps across the diodes on a bridge rectifier
Date: 21 Sep 2002 08:01:51 -0700
Organization: Rowland Institute
Message-ID:
References: <3d8b4028.29581636@news.btopenworld.com>
NNTP-Posting-Host: p-613.newsdawg.com
X-Newsreader: Direct Read News 2.97

Martin Griffith wrote...
>
> http://martingriffith.users.btopenworld.com/PREAMP.HTM

From his web pages Martin wrote, "I built this Elektor preamp
in 1980, and I'm still using it. ... The only difference I
would make, would be to put 4 100nF ceramic caps across the
diodes on the bridge rectifier."

That's wise - it's for the purpose of dealing with the spike
resulting from diode snapoff when its reverse-recovery time
is finished. During the reverse-recovery time the diode
continues to conduct, even though the current is reversed.
When all the charge is finally swept out of the diode and it
does stop conducting, this happens suddenly, creating a spike.

The spike comes the transformer's leakage inductance, recall
the formula, V = L dI/dt. The spike's risetime and magnitude
are determined by parallel capacitance. Risetime, dV/dt = I/C,
and magnitude, V = I sqrt L/C as determined by matching the
inductive and capacitive energies, E = 0.5 L I^2 = 0.5 C V^2.

The current I in these equations is the reverse current flowing
in the diode just before turning off, which can easily approach
the average dc current being delivered to the load (remember
that peak diode forward current greatly exceeds the average).

We see that increasing C reduces dV/dt and V. Without an added
capacitor as Martin suggests the only capacitance helping us is
a low diode back-bias capacitance and the transformer's small
winding capacitance. Without any additional capacitance, it's
possible for a 50V spike lasting 5 to 20us to occur once every
ac half-cycle, not a good idea inside quiet audio equipment!

Martin suggests 0.1uF on each diode. The same good result is
achieved with two capacitors, one across each secondary winding.

There's another important issue, the resulting L-C tank circuit
has a high Q and will ring for many cycles. Because the 100nF
capacitor results in low frequencies, the ringing lasts an even
longer time. To deal with this, I like to add a series damping
resistor, damping the ringing in one cycle if properly selected.


Thanks,
- Win

Also, do a Google search on 'rectifier "reverse recovery"'.