mosfetswitchingtotem-pole
I have a simple switching circuit around 500Hz with 120V supply using Push Pull mosfet with high resistive load showed below
but the output was not as expected, like this
the blue line is the voltage across R1
i have searched for totem pole circuit and the result exactly same like my circuit, but i do not know why my circuit didn't work, any ideas?
big thanks
If you are trying to use the FETs as a source follower current buffer for the Opamp, the sources of the FETs need to be connected to each other and the coil.
The IRF9510 is upside down as drawn in your schematic. That would allow current to flow through the coil (or N FET if it is turned on) and through the body diode of the P FET. This will happen no matter what the state of the gate of the P FET is.
After you correct the orientation of the IRF9510 the circuit should be able to apply about +/-11V across the coil. Gate to source threshold voltage of these FETs is about 4V, so that will limit the voltage range of the source to 4V less than the bias voltage. Also, since the two gates are tied together there will be a large area for crossover distortion between conduction of the IRF510 and the IRF9510, which may or may not be a problem for you.
Edit: Also just noticed that the ground reference at the batteries in the schematic must be wrong. Ground should be at the battery +/- common point to get +/-15V.
+5V with respect to GND is not high enough to turn on Q1, which is an N-channel MOSFET in the original schematic. The O.P. was thinking in the right direction, when he had looked-up the VGS. But with respect to what is the VGS taken? It's not taken with respect to GND. It's taken with respect to MOSFET's source is at +7.5V (or some such voltage required for the 7805). So, the if you drive the gate to +5V, then VGS < 0. It's negative. The N-channel MOSFET doesn't stay on, because it doesn't turn on in in the first place.
A high-side switch like this is usually done with a P-channel MOSFET and a second small transistor to control its gate. Like so:
(I didn't redraw all of the original circuit, but this should be enough to get the idea across.)
Best Answer
It does not work because:
The two first problems are easy to solve. The last one, however, is a bit more difficult to fix. You need some kind of level translation to provide the appropriate gate signal to the top P-channel FET (which needs to go from 120V to ~115V), from the 0-5V signal you have as input.
What I can suggest, if the input signal is a square wave continuously oscillating, is to use a simple capacitor to shift the 0-5V signal to 115-120V. It is the simplest way (but won't work with continuous input signals).
The requirements for this to work is to choose small mosfets with very low gate charge, because they will switch fast enough. Otherwise, the rise/fall time will be too slow and both FETs will end up conducting at the same time, which is bad. The part suggestions I made on the above LTspice simulation show it can work up to the MHz range. Probably higher with some adjustments and compromises on the power consumption.
Note that there are other possible topologies (e.g. N-channel both at the top and bottom), but they will be a bit more complicated to implement.