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mosfet failure while doing an inverter of 150watt

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charanbandi

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I am working on inverter 150w using pwm Ic3524 to switch IRFZ44 mosfets.I am not getting the output.

I have tested mosfet individually by connecting it to a battery i.e across drain and source using a 200ohm resistor without giving any gate signal i.e leaving the gate terminal and i am getting about 1amp current flowing across the source and battery negative terminal i.e Id is 1amp.
But according to the datasheet of mosfet zero gate voltage drain current is 10µA max.
Why i am getting like this??If that is the failure of that mosfet i have tested 8 mosfets and all are showing the same current??
Is there something wrong with my testing??BAttery is 12volts 32Ah
Thank you ..
 

Are you leaving the gate not connected to anything? If so then it is capable of picking up 60 cycle hum right out of the air.

Just like a static-detecting antenna.

This means the gate could alternate between a negative and positive volt level.

So it's possible you are getting 60 Hz pulses of 2A through the mosfet. Averaging out to 1 amp.

To get zero current, you must actively pull the gate low via a pull-down resistor.
 

i dont know how a static-detecting antenna will work..!
SRy i cant clearly understand ur concept??But i have found that we shouldnt left gate terminal unconnected coz due to the sray charges in gate capacitance it can take any voltage that m8 be the reason....
Thanks for the reply....:)
 

You probably left the gate floating. As mentioned by Brad, you need to pull the gate low.

When designing circuits with MOSFETs, you should ALWAYS use a pull-down resistor from gate-to-source. Value could be between 1k to 4.7k. You'll find plenty of threads of MOSFET failure, where the solution was the pull-down resistor.
 
now the problem with ic3524 11th pin is giving 7.5v as gate to source voltage but 14th pin giving only 1.8v as Vgs.mosfet is triggering at 3.76v..what should i do?if i change the value of pulling resistors what will happen??
 

This link shows a typical schematic for using the LC3524. It is driving 2 mosfets which sounds as though it is the way you are using the IC. It shows the values of resistors attached to the gates.

https://www.afiata.com/1500w-ups-circuit/

14th pin giving only 1.8v as Vgs
Click to expand...

This sounds way too low. Even if that were an averaged reading of alternating hi and low pulses, it's too low.

Does it show this without any load on it? If so then it could be broken.

You may need a new 3524 IC, unless you can devise a stage of voltage gain to add at pin 14.
 

BradtheRad said:
This link shows a typical schematic for using the LC3524. It is driving 2 mosfets which sounds as though it is the way you are using the IC. It shows the values of resistors attached to the gates.

https://www.afiata.com/1500w-ups-circuit/



This sounds way too low. Even if that were an averaged reading of alternating hi and low pulses, it's too low.

Does it show this without any load on it? If so then it could be broken.

You may need a new 3524 IC, unless you can devise a stage of voltage gain to add at pin 14.
Click to expand...

yes!I am using the same circuit but i have given feedback through drain to drain shorted ,removing that extra transformer used in the feedback

yes!I have tested with-out any load on it.Just giving supply to SG3524 and measuring the voltage at gate source terminal...
 

thanks!The problem got fixed now.I have changed the ic,now i m getting 8.2volts at 14th pin and 5.45v at 11th pin with feedback unconnected i.e pin1 left unconnected..

If i have given the 12volts battery supply to center tapped transformer,I am getting Id as 0.85amps in both mosfets and i have used 25ohm rheostat in series with drain but I couldnt get any output..I have used 10watt bulb connected across the output.what else i need to take care of???
I havent used any filter across the output for now.
 

I believe the output needs the capacitor, because a discharged capacitor appears as very low ohms. That will draw current through the transformer.

Apparently the load by itself will not do this. A 10W bulb will show as several hundred ohms.

Because if no current is demanded in the secondary, no (or little) current will pass through the primary.

Perhaps the control IC detects this? Or it should if you have installed the sense circuitry?

I do not have a clear picture what arrangement you are using. With a 25 ohms rheostat in the power path, wouldn't that drastically inhibit current flow?
 
I will consider those factors..:)
I am using 25 ohm rheostat just to make the current flow in both mosfets equal coz gate voltages for both are differing,even then i am putting very less resistance across them.
 

Are you using a p-mos at top?
Or N-mos?

An N-mos gate is referenced to the more negative terminal (source).

For it to work right, the source terminal has to be at a definite lesser volt level. This is hard to be sure of when a load intervenes.

Sometimes a doubler or tripler is used to boost the gate V.
 

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