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| Why are 900V semiconductors not common? When 1200 are. |
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| Siwastaja:
Depending on country, 230VAC line is expected to go up to 253VAC periodically, this condition is perfectly legal and acceptable. UK has 240VAC nominal and while tolerances are tighter with 254VAC max, 260VAC is not reportedly unusual. This is already around 367V peak. With 400V parts, there is basically no margin (just 9%). You can't filter this out using passive filters, because you have to dimension the filter not to activate at 370V, otherwise the filter would overheat and blow up during high (but barely within, or slightly exceeding spec) mains. A lot of blown units will ensue. Good luck shopping any passive filter component which does not conduct at 370V, but shunts significant currents still below 400V. This would need to be an active crowbar or series pass regulator circuit with voltage reference and comparator driving semiconductors, just to filter. It's less costly to just use said properly dimensioned semiconductors in the actual circuit. 400V rated parts are just simply unsuitable for 230VAC mains, but fine for 115VAC countries. What wraper shows is a clear design mistake, no way around that. It's only matter of place and time when the 400V rating is exceeded, and then all bets are of if the part survives or not, and how it reacts; it doesn't necessarily release magic smoke but can misbehave like described. Operation beyond maximum ratings is never specified in datasheets so that doesn't help. Regarding original question, 900V semiconductors are fine for 400V rectified three-phase when suitably filtered. The market for such parts is huge, so I guess the answer has to lie within manufacturing processes; possibly 900V rated part is not always much different to manufacture, or they get 1200V rating "for free" even if aiming for 900V. Another reason might be, 400V 3-phase systems are used in rough industrial environments where part cost is secondary consideration, so higher margins are in use. Whereas it's normal to use 550V part for consumer 230VAC device (i.e., 550/(230*1.1*1.41) ~ 54% margin), 900V part for 400VAC application would incur a tad less margin (900/(400*1.1*1.41) ~ 45%). This is undesirable, even if easily filterable, so 1200V part is easier to design with robustly. |
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