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I am using a string of six diodes between a 4 kV, current limited, power supply and a capacitor to charge it to 4 kV. I then shut the supply off, hoping to leave the capacitor at 4 kV making it available to power a flash lamp.

There is no other impedance in series, so the forward current during charging is power supply limited during charge (0.5 A). When the power supply is turned off, the diodes go into reverse recovery and current flows back into the power supply.

How much reverse current will there be? The diodes are fast recovery, 1000 V diodes (SF5408). I assume the duration of the reverse current will be within the RR spec (75 ns), but what is the peak current?

Another question I have is: Do diodes in a string all go into reverse recovery at the instant the string is reverse biased? That would result in the RR current all adding together and have a higher peak, with short duration (75 ns). Or do they get reverse biased in sequence, making the current peak lower, but longer duration (6 x 75 ns). Or somewhere in between?

SamGibson
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David Santos
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  • How fast does the power supply turn off? How fast the voltage drops on the capacitor, to estimate the reverse current? Have you measured voltage over each diode? As the diodes have manufacturing tolerances, there might be different voltages over them when leakage current flows. What type of capacitor it is and do you know how much it has leakage? – Justme Jun 24 '20 at 19:40
  • I am more concerned with the reverse recovery current, since the reverse leakage current should so small as to not be an issue. To answer your questions. The power supply shuts down instantaneously, but it has 12nF cap on it's output, that is discharged internal to the PS, so quick enough that the reverse bias occurs relatively quick. Not sure about the capacitor type and it's leakage, but I'm interested in understanding the reverse recovery current. – David Santos Jun 24 '20 at 19:46
  • Instantaneous does not mean much. Does it change from 4kV to dead short to 0V in 1 picosecond or just turn to high impedance that slowly fades away? At what rate does it fall off? The capacitor is good as it will slow the ramp down. – Justme Jun 24 '20 at 19:51
  • I believe it's a flyback type PS, so when shut down, the input side of the transformer stops switching and the output voltage is left on its output cap to be discharged internally. We don't make the supply, so I'm not sure how fast they do it. Third day on the job, so a lot of unknowns (to me) – David Santos Jun 24 '20 at 21:31

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