Why can't this mosfet (IRF3205) receive a good pulse at its GATE-SOURCE terminal?

Thread Starter


Joined Sep 28, 2021
Dear all, as you can see in the high frequency square wave (maybe 100kHz) inverter that I saw online, the 4 mosfet they used is IRF 3205 and its datasheet here. I have tried to attach a high frequency square wave between the Gate and Source of the Mosfet, however all I got was a sawtooth wave which I saw. Furthermore, when I attach a resistor between the Gate and Source of Mosfet together with mosfet and I remove the Mosfet, I saw a very very good square wave (100kHz) on the resistor. I believe the problem is with the mosfet. Are there any ways to solve this issue that is to produce a good square wave (100KHZ) on the Gate-Source of this IRF 3205 mosfet that i bought which is used in high frequency inverter. I am pretty surprised it didnt work because these mosfet is used in high frequency inverter. :)
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Beau Schwabe

Joined Nov 7, 2019
A schematic you are using would help? The Gate to Source voltage needs to be around 10V to 15V not to exceed 20V. The Threshold Gate to Source Voltage is around 4V which is where the MOSFET just begins to conduct. My suspicion is that you are not adequately driving the Gate.


Joined Feb 20, 2016
I think the driver IC needs a buffer between it and the FET as there is a fair gate capacitance to charge up and down.
What is the IC used?
I am having trouble reading the type number.
EDIT: I did a screen grab and expanded the picture to see it is an SG3525.
What is the resistor value on pins 11 and 14?
Can you show the waveform there?
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Joined Feb 20, 2016
I had a look on Ebay and there are some adverts that show a circuit but it is for a different unit.
This is a pretty simple board, so, as an exercise, trace out the circuit and post your effort.
The Ebay pictures make it easy to do.
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Joined Jan 23, 2018
I see FOUR power devices, did you remove all four of them? If the transformer is the source of the feedback you will not have a good square wave.
AND, is the system working? Or has it failed to deliver the stated output? and why are you expecting a nice square wave?
In addition, that assembly does not look to be able to handle 500 watts at all. 500 watts out means at least 500 watts in and at 12 volts 500 watts is over 40 amps. (12V x 40A=480W) And that is presuming 100% efficiency.