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Old 18th March 2008, 05:56 PM   (permalink)
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k7elp60's resistor approach looks good, simple but effective.
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Old 18th March 2008, 10:53 PM   (permalink)
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k7elp60, I understand the resistor approach on the gate to source to overcome the capacitance which will allow the mosfet
to turn off. When you added three p channel fets that is where i am confused, especially when i would need individual mosfet drivers to boost the gate drive voltage. I think i will get a few and play around with them.
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Old 18th March 2008, 11:41 PM   (permalink)
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Quote:
Originally Posted by momo1
k7elp60, I understand the resistor approach on the gate to source to overcome the capacitance which will allow the mosfet
to turn off. When you added three p channel fets that is where i am confused, especially when i would need individual mosfet drivers to boost the gate drive voltage. I think i will get a few and play around with them.
The 180 ohm resistor from the gate to the voltage source helps compensate for the gate to source capacitance. The 1M resistor from the gate to source is there for two purposes. 1 to prevent false triggering by noise and to help with the turn off the mosfet when the gate voltage is removed.
I chose P channel mosfets so I didn't have to use individual drivers because the gate has to be about 10V higher than the source voltage. By using P channel mosfets and connecting the source to the supply and the drain to the load it overcomes that problem, even thou it is a postive voltage on the gate is at least 10V more negative than the source.
If you use N channel mosfets for the top 3 switches you have two approaches. 1. use individual drivers or 2. use a voltage pump to boost the gate voltage to about 46V.
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Old 20th March 2008, 07:33 PM   (permalink)
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How fast do you want this to switch?

Ten of those MOSFETs will have a gate capacitance of 24nF, assuming the driver puts out just 30ľA and the MOSFET turns fully on, I calculate the turn on time to be 8ms. This is no good if you're building an SMPS or a PWM motor speed controller but if it's just a simple relay type application, then it's more than fast enough.
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