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Registered Member #42796
Joined: Mon Jan 13 2014, 06:34PM
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Posts: 195
I'm building a small but powerful SSTC and because my silicon graveyard is full i want to build the bridge as sturdy as possible. So i was thinking on using ultrafast diodes to protect the transistors, one in series with each transistor and another in antiparallel with both the transistor and first diode.
1st question: Is it worthed doing this for IGBT's because i only seen it done for mosfets?
2nd question: Does it mater where i place the series diode? In series with the collector/drain or in series with emitter/source? The arrangement i've seen so far is the fallowing:
but the problem with this arrangement is that it complicates the PCB layout to much Can this arrangement be used instead?
with this arrangement the PCB layout will be easier to work with all because of the pinouts of the parts
3rd question: is heat-sinking required for D3 and D4 (all dides are MUR1560) (D1 and D2 will be on the same heatsink with the transistors) because i used them without any heatsink in antiparallel with both mosfet's and IGBT's in several SSTC's with no isue
It is used so frequently with mosfets because mosfet construction forces the creation of a slow "body" diode. You want to blockade that body diode out of function, and prevent it from ever being reverse biased.
Not all IGBTs have a co-pack diode in them, and often this diode is as good as any schottky you will be able to find, so the "diode blockade" as I call it isn't required for use or beneficial with IGBTs that have an adequate co-pack diode. If your IGBTs have crappy co-pack diodes you'll need the full blockade (series and antiparallel), and if they have no co-pack diode then you only need the antiparallel (freewheeling).
You can technically use the series schottky between emitter/source and -bus/pri but then you have a diode drop that will be seen by Vgs, complicating and hindering gate drive. It is better to place the series diode on the drain/collector side.
It is my experience that at least in Tesla Coil work, the freewheeling (antiparallel) diodes do not need to be heatsinked as they won't be conducting significant current, but I think there are some applications for bridges where they will indeed require heatsinking.
FWIW I've built tons of bridges as you show it in the first diagram (series schottky on drain side) and they are indeed bulletproof when used with adequate and clean gate drive. I've even driven highly capacitive low-impedance loads (did not make anything upstream or downstream happy!) but the bridge didn't skip a beat (figuratively). It complained, of course, but kept on ticking.
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Bridge questions
