drive mosfet

siavashs · Oct 30, 2019

hi
I want to turn on and off a MOSFET at 100khz, but MOSFET turns on and off at a slow speed.
Where's the problem?
Thankful.

shortbus= · Oct 30, 2019

siavashs said:
Where's the problem?

Your circuit?

ronsimpson · Oct 30, 2019

siavashs said:
MOSFET turns on and off at a slow speed

How do you measure the speed?
Please define slow speed. What are you measuring? What do you want?
What are you using to measure the speed? Scope? Probes?

----edited----
The rise time is slow on the MOSFET, you only have 1.5k pulling up. The MOSFET capacitance + scope probe capacitance being charged by 1500 ohms, that seems slow. I did not do the math. The data sheet measured rise time at 2 amps.
I think the voltage fall time should be fast.
Can you send a picture of the parts in use. Bread board? PCB?

crutschow · Oct 30, 2019

What does the signal at the MOSFET drain and gate look like?

siavashs · Nov 1, 2019

crutschow said:
What does the signal at the MOSFET drain and gate look like?

i sent the signal shape as a photo.thanks

siavashs · Nov 1, 2019

ronsimpson said:
How do you measure the speed?
Please define slow speed. What are you measuring? What do you want?
What are you using to measure the speed? Scope? Probes?

----edited----
The rise time is slow on the MOSFET, you only have 1.5k pulling up. The MOSFET capacitance + scope probe capacitance being charged by 1500 ohms, that seems slow. I did not do the math. The data sheet measured rise time at 2 amps.
I think the voltage fall time should be fast.
Can you send a picture of the parts in use. Bread board? PCB?

i measure by scope.
the fall time is slow too.
that is,the 1.5k ohm resistor in drain causing slow rise time?
i sent drain source signal

siavashs · Nov 1, 2019

shortbus= said:
Your circuit?

ronsimpson · Nov 1, 2019

siavashs said:
the fall time is slow too.

Interesting. If the RC of 1.5k and C of the mosfet + scope probe + bread board +etc was the cause the rise time should be bad but the fall time should be fast.
It is also interesting that the rise time has two things happening. (jumps up fast for 30 volts then slowly) Also the fall has a fast jump of 15V then slowly.

Please test you scope probes and inputs. Are you using the same probe on both pictures? Same input to scope. Something is wrong/different.

I can get the same picture (drain picture) by not connecting the probe but just laying the probe tip against the drain, no connection. Using the smallest air gap for insolation.

siavashs · Nov 1, 2019

ronsimpson said:
Interesting. If the RC of 1.5k and C of the mosfet + scope probe + bread board +etc was the cause the rise time should be bad but the fall time should be fast.
It is also interesting that the rise time has two things happening. (jumps up fast for 30 volts then slowly) Also the fall has a fast jump of 15V then slowly.

Please test you scope probes and inputs. Are you using the same probe on both pictures? Same input to scope. Something is wrong/different.

I can get the same picture (drain picture) by not connecting the probe but just laying the probe tip against the drain, no connection. Using the smallest air gap for insolation.

yes,i use a probe in both picture.the direct probe connects to the circuit.
do you think how much of this is related to effect millers?

ronsimpson · Nov 1, 2019

Miller effect should be much faster.
The MOSFET looks to be running in linear, not switched on/off. Maybe the Gate was damaged by static. Do you have another one?

shortbus= · Nov 1, 2019

No I meant your schematic IS the problem. Why a pullup 1K ohm resistor from 15V to the drain of the bottom driver mosfet? Why the 12ohm resistor in the same drain? Both will slow down your turnoff time.

ronsimpson · Nov 1, 2019

shortbus= said:
No I meant your schematic IS the problem. Why a pullup 1K ohm resistor from 15V to the drain of the bottom driver mosfet?

Disagree. While the circuit is strange it works. See the picture. The gate voltage has rise and fall edges that are 1000 time faster than the power MOSFET's Drain.
I think the gate signal should be faster but it appears the signal slices through the Gate turn on voltage level in 50 to 100nS. The power MOSFET is turning of off in 50uS. We have nS compared to uS differance.
I think the MOSFET is dead.

siavashs · Nov 2, 2019

shortbus= said:
No I meant your schematic IS the problem. Why a pullup 1K ohm resistor from 15V to the drain of the bottom driver mosfet? Why the 12ohm resistor in the same drain? Both will slow down your turnoff time.

At zero volts, both driver mosfets are momentarily bright.To stop the burning of MOSFET, I put up 12 ohm resistance . 1k resistor to correct MOSFET performance .
why rise time is very slow?

siavashs · Nov 2, 2019

ronsimpson said:
Disagree. While the circuit is strange it works. See the picture. The gate voltage has rise and fall edges that are 1000 time faster than the power MOSFET's Drain.
I think the gate signal should be faster but it appears the signal slices through the Gate turn on voltage level in 50 to 100nS. The power MOSFET is turning of off in 50uS. We have nS compared to uS differance.
I think the MOSFET is dead.

View attachment 121523
[/QUOT
Does it mean that Mosset is burnt?

i used a different mosfet,but it didnt matter.
also,if i used 20 volt instead 300 volt rise time and fall time will very fast.

ronsimpson · Nov 2, 2019

Please send a picture of your parts. I think your parts are not like the schematic.
It is clear you have 14V on the Gate and 150v (about) on the Drain. It is almost like the Source is not connected to ground.

siavashs said:
if i used 20 volt instead 300 volt rise time and fall time will very fast.

I can see that is likely. I don't know why.

rjenkinsgb · Nov 2, 2019

ronsimpson said:
It is clear you have 14V on the Gate and 150v (about) on the Drain. It is almost like the Source is not connected to ground.

Is the ground of the 300V supply also connected directly to the ground of the low voltage logic/driver supply?

siavashs · Nov 3, 2019

rjenkinsgb said:
Is the ground of the 300V supply also connected directly to the ground of the low voltage logic/driver supply?

yes,is connected.

siavashs · Nov 3, 2019

ronsimpson said:
Please send a picture of your parts. I think your parts are not like the schematic.
It is clear you have 14V on the Gate and 150v (about) on the Drain. It is almost like the Source is not connected to ground.

I can see that is likely. I don't know why.

The circuit is closed on the board.The photo does not specify connections.
but source is connected to ground.
can you introduce a drive circuit for 100khrz switching?

ronsimpson · Nov 3, 2019

Your gate drive is not the problem.
Look at the +300V with the scope. Look at the return of the 300V supply. Look for something that is wrong.

siavashs · Nov 5, 2019

ronsimpson said:
Your gate drive is not the problem.
Look at the +300V with the scope. Look at the return of the 300V supply. Look for something that is wrong.

what do you mean by return supply?

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