Small pull-down resistor for mosfet (fast switching, high capacitance)

Thread Starter

MrsssSu

Joined Sep 28, 2021
181
Hi readers,
When I used this IRF 3205 which has high capacitance between the Gate-Source of the mosfet, whenever I add a lower pull-down resistor, the switching becomes faster and faster (visible nice square wave). For my case, for IRF 3205 to fast switching at 20kHz (nice square wave), it would require a 50 Ohm pull-dwon resistor. Unfortunately, And assuming driving with 12V for Gate-Source, it would means that 12V^2/50=3 Watt on the resistor and that's huge? May I know whether this is how to use the mosfet correctly that is to add a very low pull-down resistor for fast switching or are there any other ways?

Cheers:)
 

crutschow

Joined Mar 14, 2008
29,481
whether this is how to use the mosfet correctly that is to add a very low pull-down resistor for fast switching or are there any other ways?
There are other, lower power ways.
You use a high-current push-pull driver (discrete or dedicated IC).
Which would you prefer?

What will be the source of the signal to the driver?

Note that MOSFET needs a 10V gate-source voltage to be fully turned on.
 

Thread Starter

MrsssSu

Joined Sep 28, 2021
181
There are other, lower power ways.
You use a high-current push-pull driver (discrete or dedicated IC).
Which would you prefer?

What will be the source of the signal to the driver?

Note that MOSFET needs a 10V gate-source voltage to be fully turned on.
The gate actually have enough drive since I am driving the Gate Source of mosfet using a 2n2222a transistor. The Voltage comes from 12V lithium ion battery with low impedence, so I am pretty sure I am driving the mosfet with enough current. However, I am not getting why only when i place a low value resistor across the gate source of mosfet where the resistor dissipates more power or get hot, the switching time becomes better and i got a much nicer square wave? Thank you, sir:)
 

crutschow

Joined Mar 14, 2008
29,481
The gate actually have enough drive since I am driving the Gate Source of mosfet using a 2n2222a transistor.
But only in one direction.
The transistor provides a low impedance to drive the gate high.
The resistor to ground is what pulls the gate low.
So if you want to avoid the resistor, you need a transistor to pull the gate low as well as high.

Post your complete circuit diagram.
 

k1ng 1337

Joined Sep 11, 2020
420
The gate actually have enough drive since I am driving the Gate Source of mosfet using a 2n2222a transistor. The Voltage comes from 12V lithium ion battery with low impedence, so I am pretty sure I am driving the mosfet with enough current. However, I am not getting why only when i place a low value resistor across the gate source of mosfet where the resistor dissipates more power or get hot, the switching time becomes better and i got a much nicer square wave? Thank you, sir:)
Other users will know more.. MOSFET has Gate to Source capacitance so any resistance instead of a direct short from Gate to GND while OFF will slow/alter the discharge by some amount the same way a resistor from input to Gate (if used) does so when sourcing and sinking current. So your using a short circuit for the charging phase and then a resistor for the discharging phase. As the resistor approaches zero, the signal will become less attenuated down to where some resistance is needed to prevent ringing which may be taken care of by the output impedance of the driver at the input to Gate.

Using a push pull output to drive Gate eliminates the pull down resistor because the input sinks the current as well as it sources it instead of this "battle" between V+ and resistor to GND. Since you are using a pull down resistor to dump this tiny amount of charge of the MOSFET at high frequency, a low value resistor will get hot if the power rating is mismatched.
 
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crutschow

Joined Mar 14, 2008
29,481
Below is the LTspice simulation of an example driver using the paralleled outputs of one CD4050 CMOS buffer IC (which has push-pull outputs).
The MOSFET has about the same gate capacitance as the one you are using.
The power used is mostly just that needed to charge and discharge the gate capacitance at 20kHz (<50mw).

1642204204945.png
 

k1ng 1337

Joined Sep 11, 2020
420
Below is the LTspice simulation of an example driver using the paralleled outputs of one CD4050 CMOS buffer IC (which has push-pull outputs).
The MOSFET has about the same gate capacitance as the one you are using.
The power used is mostly just that needed to charge and discharge the gate capacitance at 20kHz (<50mw).

View attachment 257696
Can you explain why you choose 6 buffers for this scenario and not 1-5 / 7+?
 

ronsimpson

Joined Oct 7, 2019
1,999
Can you explain why you choose 6 buffers for this scenario and not 1-5 / 7+?
Inside the 4050 there are MOSFETs that have a RDSon of ____I don't remember but 100s of ohms depending on the supply voltage. One buffer can only drive so much current, set by the resistance of the buffer. Using all the buffers increases the drive current by 6x.
 

crutschow

Joined Mar 14, 2008
29,481
Can you explain why you choose 6 buffers for this scenario and not 1-5 / 7+?
Simple.
There are 6 buffers in one IC package.
Using fewer buffers would likely leave some unused.
Using more buffers would require more than one package.
Six buffers in parallel appear to provide a sufficiently low output impedance to charge/discharge the fairly large gate capacitance of the example MOSFET with a fast rise/fall time signal.
 
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