MOSFET does not seem to turn on in buck converter setup.

Thread Starter

Fewandfar

Joined Mar 27, 2023
2
As far as I can tell I've assembled this buck converter correctly in Multisim, but the MOSFET does not seem to turn on, and in other simulations has a large voltage drop across it, when ideally, Vsw should be ~= Vin during on the phase. Any help is greatly appreciated. In the graph blue is Vsw (point 1), red is the PWM input to the MOSFET and green is Vin.
1679903908240.png
1679903920087.png1679903973281.png
 

StefanZe

Joined Nov 6, 2019
191
Hi,
because the n-MOSFET is a high side switch in this configuration the voltage to switch the MOSFET needs to be higher than V1. If this is not possible for your circuit use a p-MOSFET
 

Irving

Joined Jan 30, 2016
4,051
As said already, but by way of further information, for an N-channel MOSFET the gate must be +ve to its source, in this case Vsw, to turn on, by 4 - 10v depending on the actual device spec. You can see in your simulation that Vsw rises to approx 6v in a spike as the gate voltage rises, so Q1 does turn on for a few uS but turns off again as Vsw rises.

A P-channel MOSFET, source to supply, gate driven from Vin to ground to turn it on, solves the problem for most lower power solutions, but P-channel tend to have higher losses than N-channel so the N-channel is preferred for higher power solutions. To alleviate the extra complexity a 'high-side' gate driver is used which provides the extra gate voltage via a bootstrap technique (look at the datasheets for IR2110 high/low gate driver, an old but common device) which also requires replacing D1 with a MOSFET (so called synchronous rectification) which again reduces overall losses, as a turned-on MOSFET is generally better than a diode for conduction losses at high currents.
 
Last edited:

Thread Starter

Fewandfar

Joined Mar 27, 2023
2
Upon increasing either the PWM input voltage, or replacing the N MOSFET with a P MOSFET, I still get the same peaks. I attached the negative terminal of the functional generator to the gate, and the source pin is connected to the DC power supply of 12V.
 

DickCappels

Joined Aug 21, 2008
10,246
The MOSFET will not work well in the configuration you show in post #1. The source voltage will always be several volts below the gate voltage so the MOSFET will not actually switch like you want..

If you take the good advice already given by others, change the MOSFET to a P-channel type with the source connected to V1+, switch the gate between V1+ and ground, the FET will be able to saturate and give you good efficiency.

Something like this:
1679916821523.png
When the gate voltage = source voltage, the MOSFET is OFF.
When the gate voltage > source voltage (in the negative direction with respect to sourceO the MOSFET is ON.

(By the way, R8 would probably be better if it were something smaller like 10k -not my circuit).
 

ronsimpson

Joined Oct 7, 2019
3,209
The problem is typical of people just learning MOSFETs.
Q1 does not care what the voltage is from Gate (4) to Ground (0). It only can see the voltage from Gate (4) to Source (1).
Place the signal source from Source to Gate as shown in red. Increase the voltage from 6V to 12Vpk and the offset to 0V.
1679924574425.png
 
Top