Diode question

I need help with diode. Actually a question: if you put diodes in series for example 2 diodes of 300 V will they act as one diode of 600 V?

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nf1199 years ago
The voltage rating on a diode is it's breakdown voltage (which could be said as the reverse biased voltage drop) - the voltage required for the diode to switch from an inductor to a conductor in a reverse biased configuration - that is the voltage require for current to flow in the reverse direction. So, in short, yes, if the 300 V you are talking about is the reverse biased breakdown voltage of those diodes - which is very likely since Power Zener Diodes have a 300 V breakdown voltage. What are you using the diode for? I ask because the voltage rating for diodes rarely matters in most applications.
gmoon nf1199 years ago
Partly true; it more-or-less doubles the reverse blocking voltage, but doesn't double the forward current capacity. Two diodes in parallel will do that.

Unless the diodes are well-matched, the characteristics in each case won't really be doubled. In cases where the diodes are really pushed to their ratings, then voltage sharing resistors need to be added.

The same approach is used when two capacitors are connected in series to increase the current capacity. Unless "sharing resistors" are added, one cap will always carry a higher burden, leading to premature failure.

The series diodes forward voltage drop would be cumulative (as you thought, guyfrom7up.)
I would like to know how these "voltage sharing resistors" are supposed to work, since going by your diagram if the voltage supply is AC then the resistors are providing a current path when the diodes are off, you would not get rectification.
Reference 1
Reference 2

I've never had cause to explore this with diodes, so I can't elaborate on their effects; but their use with caps is pretty common...
gmoon gmoon9 years ago
Nah, same thing with both references, they show a DC source. With AC the current would just go thru the resistors and not be blocked.
First, who said anything about AC? Or rectification? Not the OP. Not me...

AKAIK, either diagram only makes sense if it's showing a reverse bias. The resistance values needed to 'balance' two nearly identical diodes would be very small, and likely not effect rectification (place a 5M resistor across a rectifying diode, and see if if prevents operation.)

Neither reference qualifies their use to only DC, and I believe that interpreting the diagrams only (when no distinction is drawn in the text between DC and AC) is possibly a mistake...
When looking at the diagram, with reverse bias, ideally then the diodes are not really there, they're open. I'm left looking at two resistors, which will allow current flow in the reverse direction. In the Reference 2 example, that's 9.4M-ohms total, 4000V, only 0.43mA. Except, real world, the diodes are allowing limited current flow, 50mA is specified, so I guess talking about reverse current flow is moot.

You're using a voltage divider circuit to "share" between the two diodes. While that may increase effective reverse voltage ratings, some reality is involved. Between differences in the diodes, even the tolerances of the resistors, you will not be doubling the rating. I sure wouldn't stick two 300V diodes together like that and expect them to handle 598V. Maybe 350V, if really pushed 400V, but not up near 600V.

(BTW, why is it with the 2nd reference there is a "W" showing up where an ohm symbol should be?)

Oh, some form of current direction alteration is implied. Those diodes are going in a real circuit somewhere. If all that was desired was blocking of current in one direction always, with a current source always going in that direction, nothing works better than an open circuit.
I agree, I'm sure there must be some lost with the resistors, and reaching the theoretical limit is impossible (it always is) ... I see series diodes in amplifier power source rectifiers all the time (without the balancing resistors, since they aren't running at the limit.) They must be there for a reason. Whatever the increased voltage tolerance, it's significant, or they would have used a single, up-rated diode instead.
They must be there for a reason.
Diodes are cheap enough in bulk. If only one is used and it's bad (current goes straight thru) other components could get killed during a full-power test, two in series is insurance. Also reduces customer complaints, they may never know one went bad in use. Even a marginal ratings increase could mean they can use the same diode as elsewhere in the board, or a standard size in-plant, reducing inventory. Many reasons possible.
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