Hobby builder

[Edwin Fitzpatrick]

I built a PWM motor speed control ciruit and it uses 10 IRFZ48 hexfets.in parallel. My question is--- How many amps should the gate driver be.? I would like to use a IC for this driver,Like a Micro chip TC4420 C-mos driver or larger in this type of circuit
TNX,Fitz

 

[BobK]

Depends on your switching frequency. You need enough amps to make the switching time a low percentage of the PWM period because the MOSFETS will dissapate a lot of power which switching.

The total gate charge of the MOSFET tells you how much charge you source / sink to turn them on / off. The charge will be current * time. So do the math.

Bob

 

[Edwin Fitzpatrick]

PWM drivers

Hi Bob K,
I will run 8 Khz. I don't know how to do the math. You can buy mosfet driver IC's in 4,8 and 10 amp sizes.I am using 10 IRFZ48 hexfets in parallel.They have 33 ohm resistors in series with each gate.I use a LM339 op amp as a square wave generator to feed the mosfet driver IC,like a Microchip TC4422. Thes chips are cheap,Maybe bigger is better.
TNX,Fitz

 

[(*steve*)]

With 33 ohm gate resistors, you're probably not going to be able to drive 4A to the combined bases.

It does depend on your supply voltage though. If it's 13.2V or higher, you'll be able to hit 4A.

 

[Edwin Fitzpatrick]

Hexfet driver

Hi Steve,
The supply is 12 volts. I can change the 33 ohm resistors,the design is still on paper. Some circuits have another resistor in series with a reverse fast diode accross the resistor,I need to look at the total resistance in series with the gates. How low can I go with the series resistor and prevent ringing ?
TNX,Fitz

 

[BobK]

You can calculate how long the MOSFET will be in the linear region by dividing the Qg (total gate charge) by the gate current. For your device Qg is 42nC. So at a gate current of 1A it will take 42nsec to charge the gate fully. This is an upper bound on how long the device will be in the linear region.

Next, you can make a high estimate of the power dissapated during transition by taking the full current at half the voltage (i.e. your device is dropping half the voltage). So, say your Id is 10A. The power disspated during switching would be 10A * 6V or 60W.

That sounds like a lot, but with a switching time of 42nsec out of a 125usec PWM period, it averages to 60 * 42 / 125000 or 0.02W.

So lowering the current to even 0.1A would still need at most .2W during switching. Of course you have to add this to the dissapation when on, which is calculated from the drain current and the Rdson.

Edited to add: The 42 nsec calcluated above is less than the 67 nsec rise time, so you would use that number in the caculation instead.



Bob

 

[Edwin Fitzpatrick]

PDM driver

Hi Bob K,
What you say, that I can use the Micro chip IC TC4422 that is rated at 9 amps and with 12 volts supply,I will put a Small value resistor in series with the driver supply voltage,10 ohms or less, and bypass with a large elect. capacitor This will supply voltage during the high current surge of the gates. The drain voltage on the IRFZ48 is 36 volts.What I am building is a golf cart controller. Do you think I need a Zenor diode on each gate or could I use one at the input of the drive chain?
Also what about the resistor with the fast diode, in reverse accross it ?
TNX,Fitz

 

[BobK]

AFAIK that driver chip is designed to connect directly to the gates, nothing else should be needed.

Bob

 

[(*steve*)]

Just a low value resistor (and what you have seems fine) to ensure that the gates are not held at the same voltage as the device switches should be fine.

 

[Edwin Fitzpatrick]

PWM driver

Hi Steve,
I don't understand theory. I have 10 IRFZ48 Hexfets in Paralell. How many amps does the drive need ? I would like to use transistors for the driver circuit.I was thinking about a 2N3904 or a 2N2222 class A amplifier driving a totem pole output with a Tip 29C NPN and a tip 30 C PNP As the PA. The specs gives the tip's as 3 amps peak pulse output.This may not be enough. What do you think ? The gates have 22 ohm resistors.
TNX, for your help,Fitz

 

[BobK]

Before, you were using a MOSFET driver, now you want to build one out of discrete components. That sounds like a step backwards to me.

Also, I seem to recall having calculated for you that 0.1A per gate would be fine. You only need really high current if you are switching at hundreds of kilohertz or more.

Bob

 

[Edwin Fitzpatrick]

PWM circuit

Hi Bob,
The TC4420 cmos driver is driven with a LM339 Chip and has an open collector.What size resistor do I need (pull up) ? And I think it can be connected direct to the TC4420 input.The power supply is 12 volts.
Tnx,Fitz