ltspicemosfetswitching
I'm trying to design a MOSFET based switch which allows a sinewave through, however I noticed that when I was simulating it in LTSpice, whenever I try to turn off the FET the negative half of the sinewave still passes through.
Why is this and how do I prevent this from happening?
If I use the procedure I wrote at the beginning the temperature rise becomes
ΔT = P × θ = 30 × 62 = 1861°C
But this is insane. ... Do I need a heat-sink here?
You are neglecting to consider the heat capacity of the chip. This acts like a capacitor in the thermal equivalent circuit, connected between Tj and (some arbitrarily defined) ground, which prevents the chip from heating up instantaneously. Unfortunately you are unlikely to find good data on exactly what the value of this capacitor should be.
You can refer to the safe operating area curve for your MOSFET:
Based on this it's probably safest to keep your switching time below 10 ms, instead of close to 1 s as you have currently.
Start with this: -
And then invert the output of the OR gate.
Both gates should be schmitt trigger types. The RC time constant and the schmitt trigger high and low thresholds produce the deadband timing.
Best Answer
Note that the source and body of your MOSFET are internally connected together. When you apply a negative voltage to the drain you will forward-bias the internal PN junction between the MOSFET body (p-type) and the drain (n-type).
You might want to just use an analog switch that is designed for this purpose rather than build your own.