IR2125 problem

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bkylin

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I try to drive NMOS IRF540N with IR2125. With VCC=12V. and 5V square wave input .but unfortunately the output of the HO(pin7) is only 3.25V.and the voltage of CS (pin6) and Vs(pin5) are 3.25V too. at the begining Verr=0 but after several minutes. Verr=Vcc. does anyone know what happen?
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The IR2125 circuit is not supposed to have a Cmos 555 connected to it like you have.
I cannot see the value of C3 on your fuzzy JPG file type schematic and if it is too small or if the square-wave Vin is too low in frequency then the IR2125 charge-pump will have a low output voltage like you have.

I think you need a logic input swing that matches the Vgs of your Mosfet which is 10V. Or you can use a "logic level" Mosfet with your Vin swing of only 5V.
 
Why is there 10K between the source of the NFET and ground???

The source must go to ground with no more than a few milliΩ. in that path?
 
MikeMl said:
Why is there 10K between the source of the NFET and ground???

The source must go to ground with no more than a few milliΩ. in that path?
Click to expand...
It is the load. The Mosfet is a source-follower and cannot drive a load as low as milli-ohms.
 
audioguru said:
R3 and R5 are very low value resistors (in parallel to make one very low value resistor) used for current sensing.
Click to expand...

They are supposed to be low value. That is why I was asking the OP.
 
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yes,R3 and r5 are low value resistor ,IR2125 has overcurrent protection functionality.

today, I changed R4 to 1K. it can work now. but when the input signal increase to several KHZ.the output is terrible.some guy suggest to decrease R4. is it ok?

may be i need post a photo next monday.
 
R4 is the load. The 1k resistor causes a load current of only 6mA to 30mA.
But the circuit is designed for a load current of up to about 30A.
 
You should pay more attention on the supplying for the IR driver. Probably the supply current on the Vcc driver is too much or too low. It makes the IR being break down. I suggest you to use especial for the drivers DC/DC converter.
 
You need to move Vs to the source of the FET. You might google 2125 application note. It needs a minimum load of 500 ohms for the boost to work. Are you trying to make a charge pump out of the 555 or ????. The 2126 will not run with dc input so you must always have an input square wave or it will shut off.
 
the charge pump is recommeded by IR2125 application note.

how to generate a current up tp 30 change the load to 1ohm?
 
I don't think your 555 circuit works. It is an interesting idea, but I don't think it works. The 15 volt zener from source to ground will get a little hot. The boost needs to "float" on top of the source voltage. Why do you need the 555. What are you trying to drive with the circuit.
 
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