# MOSFET gate , offset PWM questions

#### Engelsheidt

Joined Jun 26, 2017
3
Hello!

A little about me , i haven't formally studied electronics, i do not own or have access to an oscilloscope.

I have been researching about MOSFETs to use an IRLZ44N as a switch in a basic boost converter, on the IRLZ44N datasheet Vgs(th) = 1v to 2v, as i undertand this Vgs(th) is the V needed for the MOSFET to START conducting, in order to fully turn on the MOSFET higher V is needed , for the IRLZ44N 5V at the gate is enough (i think) . On the LTSpice file MOSFET_offset.asc i am using Q2 as an emitter follower to reduce the 12v from 5v to 6v, then Q1 is configured as a level shifter and there's a voltage offset in the node between R6 and R3 , the voltage source pin3 represents an arduino UNO pin 3 with a PWM frequency of around 31kHz and 50% duty cycle, my thinking is : since Vgs(th) = 1v to 2v , if a square wave instead of reaching 0V has a minimum voltage of lets say 0.8v and is used to drive an IRLZ44N , will this allow the gate to have something similar to a preload? Will this result in a faster gate charging (since it has capacitance)? Will the MOSFET get hotter if the gate V is not reaching 0v?

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#### KeithWalker

Joined Jul 10, 2017
1,393
You show in your diagram that the load that the MOSFET is switching is just a LED in series with a 2K resistor. With only about 5.5Ma flowing, the MOSFET will never warm up whatever the gate voltage is. If you are using a load the draws a much heavier current, the temperature of the MOSFET will depend on the power it is dissipating during the "Off" period and on the duty cycle.
Regards,
Keith

#### cspwcspw

Joined Nov 8, 2016
78

#### crutschow

Joined Mar 14, 2008
25,992
for the IRLZ44N 5V at the gate is enough (i think)
Yes, that should be quite sufficient as that is a logic-level type MOSFET.
since Vgs(th) = 1v to 2v , if a square wave instead of reaching 0V has a minimum voltage of lets say 0.8v and is used to drive an IRLZ44N , will this allow the gate to have something similar to a preload? Will this result in a faster gate charging (since it has capacitance)? Will the MOSFET get hotter if the gate V is not reaching 0v?
Yes, an 0.8V preload should cause no heating of the MOSFET and provide some precharging of the gate capacitance, but it will likely make only a small difference in the switching speed of the MOSFET.