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Old 09-29-2009, 01:27 PM   #2320 (permalink)
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Michael's Electric Beetle - '71 Volkswagen Superbeetle 500000
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The freewheel mosfets are much more efficient at lower currents. Around 500 amps or so, there's a break even point, and then after that, the diodes are more efficient.

But the new mosfets' legs are rated for 160 amps. The inside is rated for 230 amps. The diodes I have been using are rated for 60 amps. So, even though the mosfets are less efficient at really high currents, they can just brute force take more current at higher powers. And for normal around town 100-200amp driving, the freewheel mosfets are quite a bit more efficient.

Let's say you are cruising around town at low power. look at 10 amps per freewheel mosfet:
The voltage drop is about...
1.8*0.0075*10 = 0.135v

And the heat loss:
10*0.135 = 1.35 watts

(the 1.8 comes from the graph on the datasheet assuming 100degC operational temperature)

The freewheel diode at 10 amps:

1v (or so).

Heat loss:
1*10 = 10 watts.

Now, let's say a short burst of 100 amps per freewheel mosfet:
Voltage Drop is about...
1.8*0.0075*100 = 1.35v
Heat loss:
100*1.35 = 135 watts! hahaha (VERY BRIEFLY!!!!)

Now, 100 amps per freewheel diode!
IMPOSSIBLE! SINCE IT WILL BLOW UP! This is WAY over the max rating.
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