component heat

[Fritz Oppliger]

I run a heater wire, 3 ohms at 13VDC, using IRFZ44 FET. On some units the
FET gets hot, on others it does not. Given resistance and voltage and duty
cycle are the same, why the difference?

 

[John Larkin]

I run a heater wire, 3 ohms at 13VDC, using IRFZ44 FET. On some units the
FET gets hot, on others it does not. Given resistance and voltage and duty
cycle are the same, why the difference?

What's the gate drive: voltage, frequency?

John

 

[John Popelish]

I run a heater wire, 3 ohms at 13VDC, using IRFZ44 FET. On some units the
FET gets hot, on others it does not. Given resistance and voltage and duty
cycle are the same, why the difference?

That fet is certainly big enough to handle the 4 amp load
without much heat if it is fully on.
Either it is not being turned completely on
with enough gate voltage, it is transitioning between on and off
too slowly, or it is on the verge of oscillating.

 

[Fritz Oppliger]

Thanks for the responses!

The IRFZ44 gate is driven by a ULN2003A against a 10K pullup to the same
13VDC.
ULN2003A is driven by 8051 port against 4k7 pullup to 5VDC.
Frequency... 200Hz max.

 

[John Popelish]

Thanks for the responses!

The IRFZ44 gate is driven by a ULN2003A against a 10K pullup to the same
13VDC.
ULN2003A is driven by 8051 port against 4k7 pullup to 5VDC.
Frequency... 200Hz max.

Then there is a fourth possibility, in addition to the three I
listed. The darlington transistors do not pull all the way to zero
when they are on, so if some mosfets have a lower turn on threshold
than others, they may leak a considerable current in the off state
(when the darlington pulls down) and dissipate heat. You can test
this by forcing the darlington on, continuously (disconnect the
processor line) and see if the transistor still gets warm.

 

[peterken]

That fet is certainly big enough to handle the 4 amp load
without much heat if it is fully on.
Either it is not being turned completely on
with enough gate voltage, it is transitioning between on and off
too slowly, or it is on the verge of oscillating.

OR some of them might oscillate due to layout, this being continuously in
the analog region

 

[John Larkin]

Thanks for the responses!

The IRFZ44 gate is driven by a ULN2003A against a 10K pullup to the same
13VDC.
ULN2003A is driven by 8051 port against 4k7 pullup to 5VDC.
Frequency... 200Hz max.

Take a look at the gate risetime with a scope. Fets like that have a
lot of capacitance, so the fet may be turning on very slowly when
pulled up by a wimpy 10k resistor. Try a lower gate pullup resistor or
an active pullup.

I bet the gate waveform looks like...


----------- 13v
/
/
/
/
/
________/
/
/
0------/


and the plateau is a killer for dissipation.


But that wouldn't very well explain fet-to-fet differences. So see if
the gate drives look different some other way. Check the heatsinking,
too.

John

 

[John Larkin]

OR some of them might oscillate due to layout, this being continuously in
the analog region

Could well be; that might explain the differences. A small (47 ohms,
maybe) resistor in series with the gate lead, real close, usually
fixes that.

John