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| MOSFET N - channel; half AC driving |
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| tester43:
Hi, I am driving N-Channel mosfet normally from micro-controller using optoisolator. All ok. MOSFET is conducting half AC voltage from transformer. Then I realised that I need to cut off all potential from Load- that leads me to options: buy P-channel mosfet or build high side N-Channel driver. I wake up today with this question: What would happen with MOSFET if I would put Gate voltage for a very short time (i.e. 1ms) at the level of 12V. Does it continue working with low resistance while Source voltage is rising (like Triacs do)... or does it overheat and stop due to low gate voltage when compared to Source? ps. It seems that due to use of AC I can't use traditionnal Bootstrap capacitor and diode. Capacitor would get charged to more or less 30-40V, then AC would fall to zero leading to immediate destruction (max Vgs = 20V). ps2. LOAD is resistance + thermocuple is series. To read TC voltage i need to shut off all other voltage from this area. See the drawing for basic schematic |
| tester43:
the longer I think about it - the longer it does not make sense. I would need to have a separate voltage source higher by 10-12 volts than 24V peak following the AC all the time to avoid overvoltage on gate vs source. I even tested that using lm7812 with gnd tied some voltage gives me expected outcome - lm7812 output is gnd voltage + 12; somebody has a better idea? |
| David Hess:
That is going to be a real mess. You have a couple of options: 1. Use a TRIAC and optocoupler for driving TRIACs. They will work fine at 24VAC and this is the same thing as a TRIAC based solid state relay. 2. Use back to back MOSFETs as a power switch. They can be driven by a transformer or photo voltaic optocoupler. This is the same thing as a power MOSFET based solid state relay which can work with AC or DC. 3. Same as 1 and 2 above but use an SCR or single MOSFET inside of a bridge rectifier as shown in the example below. 4. Is this suppose to be a battery powered application like a thermostat? In that case, a latching relay or something like what is shown below is appropriate to minimize power drain. |
| Audioguru:
The way it is now, the body diode in the Mosfet will conduct on the half-cycle you want it to be turned off. |
| tester43:
but there is no other half of ac - it's rectified(?) |
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