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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I am hoping GeorgieBoy, who has been very helpful, can answer this.
I have a mosfet that is rated for 900v. This means that the voltage across the drain and source should not exceed 900v (I am not considering margin of safety factor here). I am using a 240vac supply which I double to 480vac, and then convert to DC. This give a maximum voltage of 670vdc.
When the half-bridge is running, on leg conducts, and then the next. What I see when I probe across the load is a square wave with the peak equal to the DC input voltage. If I am using 100vdc I see a square wave with +100/-100 peak-to-peak.
I would like to think that the half-bridge could survive full voltage at +670/-670, but I am not sure. Each leg is only seeing a maximu of 670vdc which is under the 900vdc. Is there an overshoot that I could be missing, or am I safe?
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
it depends on what load your diving. if its largly inductive, with high current, and high freq, then you should plan on seeing wicked waveforms. and if i were you i would use a variac to turn up the voltage, while you monitor it by o-scope, when or if you see bad signs (overshoot, cemf, and such) youll be able to see it, then start backing off. this way you can limit your transistor losses when they eat themselves, otherwise you may blow them by the dozen$ before you figure out whats wrong.
my above method, i use because you need to chracterise both the bridge you've built, and the load your driving, both can be unique, which leads to some factors being more important than others.
so without knowing what your using it for, i will cautiuosly say that 670vDC put into a 900 V MOSFET in to a mostly resistive load is probably OK, but you must be sure you have no excursions and or gate drive problems at your power/freq.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
A half-bridge develops a peak-to-peak voltage across the load equal to the DC supply voltage. A full-bridge develops a peak-to-peak load voltage equal to twice the DC supply voltage. In both cases this neglects voltage drops due to device on-resistance and stray inductance, but these are typically small.
In either case the devices need to be rated for the DC supply voltage being used plus some safety margin for voltage overshoots at switching instants. So for similar rated devices the full-bridge develops twice the peak-to-peak voltage across the load when compared to the half-bridge.
Typically loads that are at least partly inductive give the best behaved switching waveforms with MOSFETs because they encourage freewheeling of the load current and soft switching of load current between diodes and switches.
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Maximum mosfet voltage
