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"Ideal" diodes

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sbkenn

sbkenn

Mechanical
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Feb 22, 2010
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Opinions please:
Should the Groundless controllers be regarded as "Ideal" when they switch off the MosFET every few 65mS to recharge the capacitor ?
I bought some modules, intending to use them for alternator rectifiers, but with AC power going into them, they don't work much better than a regular diode.
 
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An incandescent light-bulb is pretty close if you need heat and light !
 
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How is that supposed to work then? AC line frequency is either 50 Hz or 60 HZ, depending on where you really are in the world. That's either 20 ms or 16 ms per cycle. If your component is mucking around at 65 ms, that would seem to be a serious conflict

TTFN (ta ta for now)
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IRstuff : exactly. Some controllers use 0v and supply V, and used a switched-capacitor circuit to derive the greater-than-Vin gate voltage. The ones that I have don't have a 0/negative connection, so have to switch off the FET in order to derive power from the 0.7V drop across the body diode. The supplier just specified the on resistance of the FET, with no mention of it being switched off for a significant proportion (about 15%) of the time.
 
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I suspect that the ones I have are intended, and only suitable for, DC blocking diodes. The Bosch/Infineon ones are for car alternators and chargers i.e. low frequency rectifiers, and I haven't found any way of procuring a small quantity. I think they are only going to manufacturers in that industry.
 
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10k RPM -> 160Hz x 6 pole-pairs -> 1kHz. Not HF in electronics terms.
 
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If there is no Gnd connection and you want to keep the ideal diode "ideal" for an extended time, then it is getting challenging. "Ideal" diodes are basically body diodes of Mosfets with the channel of the Mosfet beeing turned on in parallel. The device will stay on as long as the gate is charged. This may work for minutes, but not forever if there is no chance to replenish the charge lost by leakage.
 
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The other variety, ones which DO have a Gnd connection, work with a small forward voltage, the ones that I looked at has 30mV. What there response time is, I haven't determined. I presume there is a switched-capacitor converter to generate a decent Vgs.
 
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