calculation of MOSFET gate current at high frequency question

Thread Starter

psoke0

Joined Mar 31, 2017
162
guys i have a mosfet that i think i have to drive it with higher voltage then 3v - 4 volt and i have to use 3v to 4v power source because its needs to be small and it is a lithium polymer battery. the curcuit is a boost converter i have to generate about 200 volt without load. i searched alot for finding a mosfet that have very low Gth because frequency will be generated with microcontroller but no luck in my country i guess. so i thought maybe there is a way of generating 10v or so just for driving the mosfet and after some research i found out that MOSFETs have capacitance to be charged that at high frequency. and i need to know how much current do i going to need at frequency i want. so how do i calculate it. i found this thread https://forum.allaboutcircuits.com/...urrent-the-gate-of-a-mosfet-will-draw.165357/
someone says that Ig = Qg/t = Qg*F i mean https://pdf.direnc.net/upload/pju7na65-datasheet.pdf this mosfet i have have 16,8 nC so am i suppose to make calculation like this ? if i have 10 kHz so 16,8/10000 ? and this will give me the current ? about 1.6 mA ?
 

ronsimpson

Joined Oct 7, 2019
1,157
Here is a link to a part that will increase your 4V so you can drive the Gate better. There are similar ICs from other sources.
switched cap
Do you have RS232 on board. Some low voltage RE232 drivers have this same circuit inside to make +/-10V from 3.3V.
 

Thread Starter

psoke0

Joined Mar 31, 2017
162
Here is a link to a part that will increase your 4V so you can drive the Gate better. There are similar ICs from other sources.
switched cap
Do you have RS232 on board. Some low voltage RE232 drivers have this same circuit inside to make +/-10V from 3.3V.
yes but 125 mA may not be enough because i dont know how much current will my mosfet draw at given frequency
 

Ian0

Joined Aug 7, 2020
1,579
It's an easy calculation. Look in the datasheet for the maximum GATE CHARGE. Multiply by the frequency.
Current = Charge/time, so Current = Charge * frequency.
 

Thread Starter

psoke0

Joined Mar 31, 2017
162
so if its in coulombs times frequency in hertz. 16,8 nC is 0,0000000168 coulombs times 10000 hertz = 0,000168 Amps is that right ? so at 10 kHz i need 168 uA of current ?
 

Ian0

Joined Aug 7, 2020
1,579
Yes. That's the average current. Say 1.68A to charge the gate in 10ns, 10000 times a second. Seems about right.
The process isn't perfectly efficient because of resistive losses in whatever semiconductor is charging the gate capacitance, and the driver will require some current to run.
 

Thread Starter

psoke0

Joined Mar 31, 2017
162
Yes. That's the average current. Say 1.68A to charge the gate in 10ns, 10000 times a second. Seems about right.
The process isn't perfectly efficient because of resistive losses in whatever semiconductor is charging the gate capacitance, and the driver will require some current to run.
so my pulse needs to able to give 1,68 A of current of 10ns of short time
Edit: i think its have to be 100ns not 10ns
 
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