Connecting a capacitor in parallel with each diode in a FWBR?
Connecting a capacitor in parallel with each diode in a FWBR?
(OP)
What is the purpose of adding a capacitor in parallel with each of the 4 diodes in a full-wave bridge rectifier? Does it help with noise rejection, or what? Any help will be greatly appreciated.





RE: Connecting a capacitor in parallel with each diode in a FWBR?
Yes, I know, it could have been better phrased. But that is what they are for.
If you google "reverse recovery" you will find curves showing how the tiny reverse recovery current goes to zero very suddenly - in nanoseconds or less. This sharp current edge produces high frequency noise. It is shorted out by the capacitors.
Gunnar Englund
www.gke.org
RE: Connecting a capacitor in parallel with each diode in a FWBR?
RE: Connecting a capacitor in parallel with each diode in a FWBR?
The RC network's corner frequency (the -3 dB frequency) is 1/RC when expressed in radians/second or 1/(2*PI*RC) when talking Hz.
If there is no HF pollution concern (might be in R mode) then you do not have to care about capacitors. On the other hand, if RF pollution is a concern, make them one order of magnitude bigger than the diode zero volt capacitance. Use HF capacitors. Ceramic and such stuff. No R, I would say. SCR:s need R but not diodes.
Gunnar Englund
www.gke.org
RE: Connecting a capacitor in parallel with each diode in a FWBR?
RE: Connecting a capacitor in parallel with each diode in a FWBR?
Are you designing a high voltage power supply where there are 4 diodes in series in each leg? (If high power, there would be 4 legs/16 diodes if single phase, and 6 legs/24 diodes if three phase-USUALLY-)
Where there are more than one diode in series, capacitors and resistors are used to control the DC and AC voltages across each diode.
The resistance of the resistors depends on the DC leakage current of the diode.
The capacitance of the capacitors depend on the stray capacitance of the rectifier assembly to ground.
Another approach is to use transient voltage suppressors instead of resistors and capacitors.
RE: Connecting a capacitor in parallel with each diode in a FWBR?
If this is the case, avalanche diodes are more suitable. So, which one of these applications is it you are designing for?
Also; do not post same question in several fora. It only brings confusion. And it is against site policy.
Gunnar Englund
www.gke.org
RE: Connecting a capacitor in parallel with each diode in a FWBR?
My question is really for understanding the purpose of the capacitor and how to calculate the capacitance value to be used in the circuit. Yes, each diode will have a resistor and a capacitor in parallel with it.
According to Carl I should calculate the reactance for the cap. Any explanation of an example for a better understanding would be extremly helpful.
Sorry for the confusion with the posting . . . my mistake.
RE: Connecting a capacitor in parallel with each diode in a FWBR?
There will be stray capacitance from each resistor to ground. If the impedance of the stray capacitance is less than the resistance of the resistor, there will be unequal voltage across each resistor. The resistor connected to the 60 KV will have much higher voltage across it than the resistor connected to ground.
To correct the voltage unbalance, a capacitor with much larger capacitance than the stray capacitance is connected across each resistor.
To really understand what is being described, you should draw the circuit and do some calculations.
The same thing also happens with a rectifier string, except that the calculations are considerably more difficult.
Avalanche diodes as suggested by Gunnar are sometimes used (instead of resistors and capacitors)for low current high voltage rectifiers and the use of transient voltage suppressors (instead of resistors and capacitors) is probably a more modern approach.
You may discover that is is lower cost to purchase a complete rectifier assembly rather than design and build it yourself.
Good Luck, it sounds like a fun job.
Carl Pugh
RE: Connecting a capacitor in parallel with each diode in a FWBR?
Long and short of it is to swamp out diode-to-diode variations in the reverse characteristics.