Switching MOSFETs for a buck converter

[not_in_asia]

I'm confused about how to make the MOSFET Q1 act as an ideal switch in this buck circuit:

(schematic 1)


I need to have the voltage at N2 equal to the voltage at N1 when the transistor is turned on.
I've tried using a frequency generator in a setup similar to that in schematic 2. At N2 I ended up getting a a pulse more like V2 (voltage of the frequency generator) - Veb. I tried some BJT's and MOSFETS for Q1, different resistor values, different setups like schematic 1, different pulse frequencies, and never really got anywhere.

If I get a pulse from a frequency generator or microntroller- for experimental purposes- then how do I set up the pulse at the base/ gate to make the transistor work as an ideal switch?

(schematic 2)

 

[MrAl]

Hi,

The pulse amplitude must be higher than the desired output because that is a voltage follower circuit.
The pulse amplitude with the MOSFET may have to be as much as 10v higher than the output to get it to switch to a low resistance, although it depends on the specific MOSFET used.

 

[not_in_asia]

Hi,

The pulse amplitude must be higher than the desired output because that is a voltage follower circuit.
The pulse amplitude with the MOSFET may have to be as much as 10v higher than the output to get it to switch to a low resistance, although it depends on the specific MOSFET used.

Thanks. I tried a simulation with a BJT and it worked like you said:



However, the same setup with a MOSFET didn't work (it didn't work when I upped V2 to 20 V either).



For switching converters MOSFETs are recommended instead of BJTs. Why doesn't the MOSFET circuit operate the same as the BJT circuit?

 

[not_in_asia]

Got it. Thanks! The pulse is running at 5V, vin is 12 v

VR ≈ Vin*duty cycle

 

[buckysparks]

you need to select a model for the mosfet.

 

[crutschow]

Typically a PMOSFET is used for a buck circuit with the source terminal connected to the positive input voltage and the gate voltage driven between the input voltage and ground.

 

[MikeMl]

If you want to use an NFET, the gate must be driven ~10V above its source. Since its source is biased to the converter's output voltage, then the gate must be driven to Vout+10V. That is why it is better to use a PFET, because it turns on fully when the gate is driven ~10V more negative than its source, which is usually connected to the converter's input voltage.

 

[not_in_asia]

If you want to use an NFET, the gate must be driven ~10V above its source. Since its source is biased to the converter's output voltage, then the gate must be driven to Vout+10V. That is why it is better to use a PFET, because it turns on fully when the gate is driven ~10V more negative than its source, which is usually connected to the converter's input voltage.

Sweet, thanks