Completed Project Choosing a MOSFET driver

Thread Starter

anishkgt

Joined Mar 21, 2017
529
Hello all,
i am planing on to use six MOSFETs in parallel and wish to drive it via a MOSFET driver but not how to select one. What are the specs to be considered when choosing one ? From what i learned would this from MICROCHIP do the job ?
Annotation 2020-05-16 202639.jpg
 

Solar Mike

Joined Apr 26, 2018
24
Driving 6 mosfets in parallel, can be somewhat fraught with potential problems, you haven't mentioned the switching frequency.
That driver chip only supplies 1.2 amps, when coupled into 6 gate circuits each with approx 8nf means switching isnt going to be fast. Maybe ok if you are switching at 100 hz, but if its into the 10's of khz region you will need more drive power to each gate input, either use a uprated driver that can supply 6 amps or more; or use 1 MIC4416 per mosfet, the last option allows hugely shorter wire trace lengths around each gate\source node, with less problems with ringing etc.
Remove C3, it will make things worse replace it with a 4 to 10k resistor, also pull the input pin 4 to ground with a 10k resistor to ensure the output stays low while your driver is starting up.
Logic level mosfets as per your circuit are rated for 10v Gs so D1-6 are not required.
Cheers
Mike
 
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ronsimpson

Joined Oct 7, 2019
1,024
Six 300A MOSFETs in parallel!
The Gate capacitance is 8nF/transistor. 100 ohms & 8nF makes a slow turn on/off. Data sheet example = 5 ohms.
Long gate wires can make this a 100mhz oscillator.
Considering how much money you have in MOSFETs ….. I would put a 8 pin driver on each transistor. (very close)

I am looking at getting a PCB with Aluminum in the center. It will help pull the heat out. I have never used one before.
 

Thread Starter

anishkgt

Joined Mar 21, 2017
529
Appreciate all the replies.

Thought i had the MOSFETs figured out but i was wrong. Looking further, i will go with 3 of these. The MOSFETs will be controlling the switch on time of 6 Maxwell Ultracapacitors BCAP0350 at a second or 2 apart. Switch-on time would be between 50 to 200ms.

Can one explain what needs to be looked at when choosing an appropriate driver. Besides the supply voltage and TTL signal, Is it the Total Gate Charge or the Input Capacitance ? and what determines the source and Sink current of the driver ?
 

Solar Mike

Joined Apr 26, 2018
24
Total gate charge is easiest to use here, in the devices you have selected its 340 nC @10v per device, so if you want to turn each one on in say 100nS then its gate driver will have to supply 3.4 amps over that period; approx 10 amps total for all three.

As your switching frequency is low, circuit layout etc isn't so critical, you could use a single gate driver, look for one that can supply the required source current peak. The sink current rating of these drivers is generally higher that the source so they will be able to turn off your device in same or better time.
 

ElectricSpidey

Joined Dec 2, 2017
1,335
Can one explain what needs to be looked at when choosing an appropriate driver.
Input side…
Input capacitance.
High or low side drive
Schmitt trigger if needed
Input gate threshold
Input gate drive voltage
Input voltage range

Output side…
Capacitance drive rating
Output current
Output voltage range
Output drive configuration (usually push pull)
Output drive symmetry
Output voltage charge pump if needed
Inverting, non-inverting

Other design considerations…
PCB layout
Additional components
Wattage
etc

I would consider putting some load sharing into your circuit.
 
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Thread Starter

anishkgt

Joined Mar 21, 2017
529
Total gate charge is easiest to use here, in the devices you have selected its 340 nC @10v per device, so if you want to turn each one on in say 100nS then its gate driver will have to supply 3.4 amps over that period; approx 10 amps total for all three.

As your switching frequency is low, circuit layout etc isn't so critical, you could use a single gate driver, look for one that can supply the required source current peak. The sink current rating of these drivers is generally higher that the source so they will be able to turn off your device in same or better time.
Based on this application note from MICROCHIP, on page 4 in the Design Example. It talks about choosing a driver based on the peak driver current requirement of the MOSFET, would the peak current be calculated as follows based on the MOSFET i have chosen,
Gate−to−Source Charge Qgs = 51nc
Turn-On Time Delay td(ON) = 45.6ns
based on the example in the application note
Annotation 2020-05-18 125359.jpg
here;
I = 51nC/45.6ns
= 1.1A
Would this be correct ? Not saying your assumption is wrong, just confused.
 
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