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I'm trying to simulate an LC filter for a PWM output and I'm having problems with MOSFET used for controlling the voltage. I'm using LTSpice for simulation. When I filter the PWM output directly, it works OK, nicely smoothing out voltage. The schematic looks like that in that situation:

schematic

simulate this circuit – Schematic created using CircuitLab

When I want to use N channel MOSFET, however, it doesn't work at all, ie. I only see noise on R1 resistor. The schematic in this case looks like that:

schematic

simulate this circuit

What's wrong with the second schematic? And if it's not a proper way of handling PWM with MOSFET, how should it work? I saw similar circuits for filtering PWM output in an H-Bridge and it looks like the same situation - in H-Bridge PWM is supplied to lower side MOSFETs.

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  • \$\begingroup\$ what voltage on V2? \$\endgroup\$
    – user16324
    Apr 18, 2015 at 18:23

2 Answers 2

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In your first circuit V1 acts like the output of a synchronous buck power modulator, or a half bridge. No matter the state of V1, current in L1 has a good path.

In your second circuit, when M1 is turned off, current in L1 has no place to go, and an impossible situation occurs. That's what Andy is talking about, using a flyback diode to give L1 current a path to flow when M1 is off.

If you want to continue to use an N-Channel FET and switch it referenced to ground, you could change the circuit to something like this:

enter image description here

This is a buck with Vout referenced to V1 (12V) so, Vout (voltage across C1 referenced to ground) will be V1 - D V1, where D is the duty cycle.

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  • \$\begingroup\$ Thanks! Interestingly, I already had a very similar circuit working, only with L1 between D1 and C1 on the top, but I thought that there's a difference between an LC filter and a buck converter, which now I see was wrong. \$\endgroup\$ Apr 19, 2015 at 9:14
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You need a flyback diode to catch the back emf when the mosfet open circuits. The flyback diode also allows inductor energy to flow into the load. It's fundamental to this circuit to use a diode. Try googling buck convertor circuits.

H bridges using mosfets are configured so the the mosfet's internal parasitic diode can be used for flyback purposes. Same is true with half bridge synchronous buck converters but not true of the single mosfet converter.

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