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Why two diodes per branch on a full wave rectifier?

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why two diodes per branch on a full wave rectifier?

I have seen several designs that use a center tap transformer and two identical diodes in series on each hot leg wired as a full wave rectifier.
What is the advantage of this over just one diode per hot leg?

I have attached a diagram of a ciruit that uses this for reference
Marc
 

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In that attached circuit diagram the reason is probably to meet the requirements on reverse voltage, 2 diodes in series can withstand twice as high voltage as one diode. Note! it is adviceable to connect high value resistors in parallell with each diode in order to equalise the voltage obver each diode.

Regards Hans
 
With the 275-0-275 transformer, the diode will see a peak inverse voltage or PIV of 275*1.4*2 = 770V. The rating for the diode is 1000V, I think. So the extra diode is supposedly unnessary, but perhaps the designer just wanted a bigger safety margin, as other have suggested. Note that with a bridge and a 275V secondary, the PIV is 275*1.4 = 385.

Sheldon
 
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In an instance like this one might put two diodes in series with a snubber resistor across each, the purpose would be generally to provide some additional margin for line transients. (load dumps, distant lightning strikes, inductive surges) I highly recommend this, diodes are cheap.. :D

Kevin
 
So, let me see if I understand this, with an example. I have a centre tapped secondary with 415VRMS per side. Using 2*sqr(2)*VRMS that's 1174V which is apparently the peak to peak swing from the entire secondary, and the total peak inverse voltage that a diode here would see.

I am using a 5V4, but want to extract a negative rail which I plan to do with a pair of silicon diodes on the same secondary. I have a pair of RGP02-20s which have a Vrrm of 2000V. I am concerned with safety margins as a failure of my negative supply might mean output device destruction in this application.

Can I trust these diodes?
 
Years ago when diodes were replacing the common rectifier tube, it was found a minimum of 2x the PIV was far better for reliability. Most issues did show up in higher voltage ham transmitters, transceivers and RF amplifiers at 750 volts & higher.

A MOV on the input can help those nasty line spikes and/or bypass the diodes with a .01uF ceramic cap of same voltage rating.
 
Sheldon said:
With the 275-0-275 transformer, the diode will see a peak inverse voltage or PIV of 275*1.4*2 = 770V. The rating for the diode is 1000V, I think. So the extra diode is supposedly unnessary, but perhaps the designer just wanted a bigger safety margin, as other have suggested. Note that with a bridge and a 275V secondary, the PIV is 275*1.4 = 385.

Sheldon

my take here is that since the secondary has a total of 550v, in effect the highest absolute voltage value the diode will see in reverse conduction is 550 x 3 or 1650volts so two 1kv diodes in series makes a lot of sense imho.
 
As Kevinkr states on a previous post. If you do use two diodes in series to increase the PRV/PIV value for margin, a high value resistance divider should be used. He refers to them as "snubber's". The idea is: the reverse leakage may not (is probably not) the same for each diode. Putting a dominant leakage resistance (snubber) across each diode will insure that the peak reverse voltage appearing across each diode will be much closer to being properly divided. If the diodes you're using are 2kV PIV, I'd use just the one, but that's just me.

P.S. by resistance values, 2.2 MegOhm should be very dominant when compared to reverse leakage resistances
 
I have beenlooking and looking for a book... but can't find it.

When 2 diodes in series are reversed biased, what establishes the voltage across each diode is its capacitance and its reverse leakage. In the case of power rectifiers, this capacitance is neither controlled, tested, or specified. The capacitance is small but it is the major thing that governs the voltage division across 2 diodes in series (this depends on dv/dt involved). The diode with least capacitance will have the proportionately larger share of the voltage. Keep in mind that under static conditions, leakage would be the prime factor.

Swamping these reverse capacitances/leakges with external caps in parallel or high value resistors is not a bad idea. You can't assume the reverse voltage will develop across each diode equally... and you need this to buy voltage rating.



Am I makin' sense here?

:)
 
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