Driving an N channel FET? - Electronic Circuits Projects Diagrams Free

bigfarmerdave · 17th August 2009, 05:50 PM

If I want to drive a generic N channel FET, say an Infineon IPB70N10SL-16. This is a 100V, 70 amp, 16mOhm RDSonn part. If I use it as a high side switch to a 12 volt source, I know that I need my gate voltage to be higher than the 12 volt rail, but what I don't know is how to calculate/find how much current I'll need to drive the gate. Any help is appreciated.

Hero999 · 17th August 2009, 05:55 PM

Is speed important?

What's this for?

You could bootstrap it but the duty cycle can never be 100%.

bigfarmerdave · 17th August 2009, 06:26 PM

Speed is not important.
I'll be using it to control a 12 volt solenoid.
How do you calculate what the max duty cycle would be?

blueroomelectronics · 17th August 2009, 06:32 PM

Duty cycle of a solenoid?

bigfarmerdave · 17th August 2009, 06:44 PM

Max duty cycle is probably going to need to be about 85%

blueroomelectronics · 17th August 2009, 07:04 PM

Quote:

Originally Posted by bigfarmerdave

Max duty cycle is probably going to need to be about 85%

How did you arrive at that number?

bigfarmerdave · 17th August 2009, 07:21 PM

Right now it is a rough estimate. We anticipate that with an 85% duty cycle we should be able to open the solenoid up more than what we think will be needed to get the max oil flow that we'll ever need.

blueroomelectronics · 17th August 2009, 07:24 PM

Solenoids are more of an all on or all off affair. Do you have a model number?

bigfarmerdave · 17th August 2009, 07:45 PM

Unfortunately, I don't actually have any model number in front of me right now. In response to a solenoid being "all on or all off", I want to regulate oil flow and turning the solenoid all on or all off would be like flipping a switch, all or nothing. I want to regulate oil flow more precisely than that.

blueroomelectronics · 17th August 2009, 07:49 PM

Then a solenoid isn't going to do it, PWM on a relay or solenoid is handy for reducing power required to operate it as the holding current is lower than the pull in current.

smanches · 17th August 2009, 09:47 PM

The amount of current needed is the Qg / switching time. Your mosfet has a 160nC gate charge. If you wanted a 1us turn on, then your gate current needs to be...

160nC / 1000ns = 0.16A.

Obviously, if you want it to turn on faster, you'll need to push more current.

bigfarmerdave · 17th August 2009, 10:26 PM

Smanches, That's what I was looking for. Thanks a lot!

PatM · 17th August 2009, 10:48 PM

Quote:

Originally Posted by bigfarmerdave

Unfortunately, I don't actually have any model number in front of me right now. In response to a solenoid being "all on or all off", I want to regulate oil flow and turning the solenoid all on or all off would be like flipping a switch, all or nothing. I want to regulate oil flow more precisely than that.

How about several solenoids in parallel lines to incrementally increase the flow?
ie:
1/4, 1/2, 3/4, or full, or isn't that accurate enough?

PatM

smanches · 17th August 2009, 10:55 PM

Just PWM it like I think you're trying to do. I've done this exact same thing to implement a soft turn-off for hydrolic systems so the rams don't slam when it's shut off.

You need to find the inductance of the coil though.

blueroomelectronics · 17th August 2009, 11:01 PM

But you still won't be able to control the flow rate. It'll be either on or off.

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17th August 2009, 05:50 PM	#1
bigfarmerdave	Driving an N channel FET? If I want to drive a generic N channel FET, say an Infineon IPB70N10SL-16. This is a 100V, 70 amp, 16mOhm RDSonn part. If I use it as a high side switch to a 12 volt source, I know that I need my gate voltage to be higher than the 12 volt rail, but what I don't know is how to calculate/find how much current I'll need to drive the gate. Any help is appreciated.

17th August 2009, 05:55 PM	#2
Hero999	Is speed important? What's this for? You could bootstrap it but the duty cycle can never be 100%. __________________ I do not answer private messages asking for help because no one else can: benefit from advice I may give or correct me if I'm wrong. Please ask on the open forum if you have a question and I'll be happy to help, if I know the answer.

17th August 2009, 06:26 PM	#3
bigfarmerdave	Speed is not important. I'll be using it to control a 12 volt solenoid. How do you calculate what the max duty cycle would be?

17th August 2009, 06:32 PM	#4
blueroomelectronics	Duty cycle of a solenoid? __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com/

17th August 2009, 06:44 PM	#5
bigfarmerdave	Max duty cycle is probably going to need to be about 85%

17th August 2009, 07:21 PM	#7
bigfarmerdave	Right now it is a rough estimate. We anticipate that with an 85% duty cycle we should be able to open the solenoid up more than what we think will be needed to get the max oil flow that we'll ever need.

17th August 2009, 07:24 PM	#8
blueroomelectronics	Solenoids are more of an all on or all off affair. Do you have a model number? __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com/

17th August 2009, 07:45 PM	#9
bigfarmerdave	Unfortunately, I don't actually have any model number in front of me right now. In response to a solenoid being "all on or all off", I want to regulate oil flow and turning the solenoid all on or all off would be like flipping a switch, all or nothing. I want to regulate oil flow more precisely than that.

17th August 2009, 07:49 PM	#10
blueroomelectronics	Then a solenoid isn't going to do it, PWM on a relay or solenoid is handy for reducing power required to operate it as the holding current is lower than the pull in current. __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com/

17th August 2009, 09:47 PM	#11
smanches	The amount of current needed is the Qg / switching time. Your mosfet has a 160nC gate charge. If you wanted a 1us turn on, then your gate current needs to be... 160nC / 1000ns = 0.16A. Obviously, if you want it to turn on faster, you'll need to push more current. Last edited by smanches; 17th August 2009 at 09:47 PM.

17th August 2009, 10:26 PM	#12
bigfarmerdave	Smanches, That's what I was looking for. Thanks a lot!

17th August 2009, 10:55 PM	#14
smanches	Just PWM it like I think you're trying to do. I've done this exact same thing to implement a soft turn-off for hydrolic systems so the rams don't slam when it's shut off. You need to find the inductance of the coil though. Last edited by smanches; 17th August 2009 at 10:56 PM.

17th August 2009, 11:01 PM	#15
blueroomelectronics	But you still won't be able to control the flow rate. It'll be either on or off. __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com/