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Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
There is a huge difference, HV diodes do not like much current at all. Just think about that all plastic housed diodes are roughly the same size, so their wattage ratings will also be about the same throughout the whole voltage/current scale.
A randomly picked 9kV diode from my stash is rated 350mA continues use and can withstand 50A surges of 8.3mS length, but recurrent its only 10A.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Diodes approximate to a constant forward voltage drop, so their "wattage rating" *is* expressed as a current rating.
Due to limitations of semiconductor physics, single junctions don't stand much more than 1000v reverse, so the HV microwave oven diode tends to be built from a series stack of perhaps 10 junctions, with a corresponding increase in forward voltage drop, and so lower forward current for any given power dissipation.
Voltage rating has nothing to do with power disspiation.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Due to limitations of semiconductor physics, single junctions don't stand much more than 1000v reverse, so the HV microwave oven diode tends to be built from a series stack of perhaps 10 junctions...
Same principle applies to state-of-the-art high voltage DC power transmission. Converter stations use valves with ratings up to at least 500 kV and 3600 amperes. Individual semiconductor junctions are at least 5 inch diameter but can block not much more than 9 kV, so there are dozens in series. Fiber optics is popular for controlling the gate drive electronics, and [edit] today there are even standard fiber interfaces for direct optical triggering. Great overview of gate drive issues at
Utility-scale power semiconductor fabrication is more demanding than microelectronics in many ways. For example, putting a shallow but polished bevel around the wafer edge, to limit electric field strength where junction meets surface. Special potting material bonded to that surface. And back in the ovens, these devices require process parameters to be uniform across the whole wafer, much more strictly than digital processors or memory or even image sensor chips.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I'd warn that this last post was off-topic, but I have no idea what the original topic was, or what the original poster was asking.
Voltages and currents of diodes have the following meanings:
The voltage rating is the voltage that the diode can stand across it in the reverse direction. Ie, when the diode is supposed to be "off".
The current rating is the current that it can stand in the forward direction, when it's on.
Multiplying the two numbers to make a wattage is more or less meaningless. You use them to choose a diode for your application as follows. Let's say that you're designing a 6-pulse 3-phase rectifier to power a subway third rail. The subway train has 500hp worth of motors, requiring up to about 600 amps at 600 volts DC.
So straight away we know that we need some diodes with a rating of at least 600A, and 600V. In practice we would use 1200V to give a safety margin. And, each diode only conducts one-third of the time, so the theoretical current per diode is 200A average and 346A RMS. (600 divided by the square root of 3) In practice we'd probably choose 600A, again for a safety margin.
So we go to radio shack and get six 600A, 1200V diode pucks, three fuses the size of shotgun cartridges, some giant MOVs, a bunch of big heatsinks, a torque wrench, and a 500kVA substation to power it. Except in practice, Radio Shack are usually out of substations.
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high voltage and high current diodes
