Adding an RC snubber in parallel with a diode

[KerimF]

Hi,

Is there a reference for choosing the values of R and C which could be added to a 4-diode bridge (an RC for each diode) when working at high voltages?

I didn't add yet the 4 RC snubbers on a diode bridge in any circuit I built, because the reverse breakdown voltage of the diodes are usually much higher than the voltage applied on them.

Now, I look using a diode bridge on mains voltage (380 Vrms) so I wonder if it is better adding the 4 RC snubbers to it.

Thank you

Kerim

 

[chuckey]

To protect the diodes in bridge it is traditional just to use a 1000 PF capacitor across each diode, so any incoming spike bypasses the diode and is dissipated in the reservoir capacitor.
Frank

 

[FvM]

As an effective overvoltage protection, I would refer to MOVs instead of snubbers or capacitors. A small capacitor (e.g. 1 nF) is mainly used to reduce conducted RF from the rectifier and can be often found in radio and TV receivers.

 

[KerimF]

Thank you for your kind replies.

In power electronics (50 Hz), do you think the 1n capacitors could also be enough in dividing equally the reversed voltage on each diode pair (4-diode bridge)?

Naturally, simulating the circuit of interest can give a good idea on their effect (capacitors) on the diodes and the circuit as a whole. There is a chance they also create sort of resonance for example. In this case, damping series resistors may reduce the oscillations if not eliminate them.

I thought that perhaps others had the same problem and provided some hints on it. I usually don't like re-inventing the wheel if there is one around

Thank you anyway.

Kerim

 

[andre_luis]

Are you sure it is really necessary ?
In a 4-diode bridge, allways at least 2 diodes are conducting, and a reverse voltage will be never present at no one device.


+++

 

[KerimF]

Hi Andre,

I couldn't get your idea very well, so let us analyse a simple example:

D1 AC1 to DC+
D2 AC2 to DC+

D3 DC- to AC1
D4 DC- to AC2

If V(AC1) > V(AC2)
And for simplicity we set V(AC2)=0V (as ground) and Vd=0V (ideal diode forward voltage)

D1 is forward biased
DC+ = V(AC1)

D4 is forward biased
DC- = 0V

In this case:
The reverse voltage on D2 is V(AC1)
The same applies on D3.

Now I realized that my mistake was in thinking that the reversed voltages were V(AC1)/2 on each diode.
As we see here, the diode should stand the full AC peak voltage.
So thank you for pushing me rethink about it

Kerim

 

[FvM]

Andre is right, no diode reverse voltage can be higher than the DC output voltage. If you have any bus capacitor, it will absorb surges in a first order.

 

[dselec]

Another reason is that high volt diodes have large capacitance that needs to be discharged.

 

[KerimF]

Hi FvM,

It seems there is a misunderstanding somewhere, perhaps the language

Let us assume I have an AC voltage with a peak of 220V * 1.414 = 311V and I apply it on a 4-diode bridge to power a DC load.
I have to choose the diodes with Vbrk > 311V.

True or false?

Kerim


Added:
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Hi dselec,

Sorry... Another reason is for what?

Do you mean the added capacitor in parallel can discharge the diode high capacitance?

Thank you.

 

[FvM]

I don't see a misunderstanding, perhaps you overlooked that I assumed an output capacitor. In this case Vpeak is equal Vdc. Even if you calculate some ripple, the output capacitor will absorb surge voltages more effective than a (necessarily small) snubber capacitor.

P.S.: Vbr must be > Vpeak in any case. There would be a misunderstanding, if you see Vdc as averaged rectified value. But I used the term referring to the voltage across the output capacitor.

 

[dselec]

Sorry meant a parallel resistor

 

[vimalkhanna]

Ref to your snubber design.This element is to dissipate the diode's rev capacitance charges when the diode is switching OFF
and the reverse diode is tending to switch ON.Pl. see data sheets of the bridge rectifiers for capacitance and reverse recovery.
The snubber should be 3x or 5x this value ,,not 20times this ratings .It will load the additional charges and slow down the operations.
As such ,for a 385VAC feed ,DC at the storage cap. is 580VDC ,say 600VDC ..
The reverse peak shall be 1100VDC as the PIV of the diodes .The same needs be for the capacitor used as snubber .In addition, you must ensure that the cap used is foil wound polyester or polycarbonate which is self recovery type rather than ceramic which has low ripple handling and liable to puncture .
In addition, kindly see the charge handling for the rectifier diodes in the main bridge .If the current ratings are 10Amps /25Amps etc. you will need to put 10 ohms in series with the snubber capacitor as the pair shall cater to heavy charging/discharging without breakdown.