MOSFET gate , offset PWM questions

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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
 

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.
 
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