Drive an fet with a 555 and a different power source/not the same one that powers fet

thingmaker3

Joined May 16, 2005
5,084
If he leaves the 100 ohm resistor in series with the drain, the Rds(on) is irrelevant. He could reduce it to 10 ohms and still get away with a much smaller FET.
I apologise for my misunderstanding. When you said "smaller FET" and "less power," I thought you meant a FET incapable of handling as much current.

Can i use any any fet driver? i have a tps2811
The TPS2811 should work fine.
 

nomurphy

Joined Aug 8, 2005
567
But it did "fly" at those speeds.
How hot did it get? How many did you smoke? If it was flying, the pilot is dead.

See the attached to add a smaller FET with less input capacitance as a pre-driver to the IR540 FET.

The reason to use a higher voltage, such as 12V, is to quickly overcome the gate capacitance, turn the FET on/off as quickly as possible so as not to waste energy in a FET that is partially on.
 

Attachments

Ron H

Joined Apr 14, 2005
7,014
How hot did it get? How many did you smoke? If it was flying, the pilot is dead.

See the attached to add a smaller FET with less input capacitance as a pre-driver to the IR540 FET.

The reason to use a higher voltage, such as 12V, is to quickly overcome the gate capacitance, turn the FET on/off as quickly as possible so as not to waste energy in a FET that is partially on.
If a 555 with +/- 200mA drive capability is insufficient, do you really think 1k to +12V will do the job?
 

nomurphy

Joined Aug 8, 2005
567
The problem is the 555 trying to drive the IR540 gate capacitance at ~700KHz. The 555 will obviously burn out.

Go back a number of pages in this thread where I actually provided Arthur with the calculations. At 17, he should have been able to grasp the algebra and plug in his own numbers to help understand and perfect his design.

But at this point, I think Arthur is more of an Edison than a Tesla. :)
 

Ron H

Joined Apr 14, 2005
7,014
The problem is the 555 trying to drive the IR540 gate capacitance at ~700KHz. The 555 will obviously burn out.

Go back a number of pages in this thread where I actually provided Arthur with the calculations. At 17, he should have been able to grasp the algebra and plug in his own numbers to help understand and perfect his design.

But at this point, I think Arthur is more of an Edison than a Tesla. :)
I agree with your assessment of Arthur, but my point was, with a 1k pullup, the IRF540 will never turn on, because its gate capacitance is too high.
 

nomurphy

Joined Aug 8, 2005
567
If you're referring to the RC time constant relative to the Fsw, most likely your right.

I was doing my Edison-best to provide him a basic circuit, without much thought on my part.
 

Ron H

Joined Apr 14, 2005
7,014
If you're referring to the RC time constant relative to the Fsw, most likely your right.

I was doing my Edison-best to provide him a basic circuit, without much thought on my part.
Yes, you could lower Fsw and the FET would turn on, albeit slowly, but with lower Fsw you could drive the IRF540 with the 555 directly and get much faster switching speed.
 
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