Mosfet IRF740 Overheats

Thread Starter

PemfD11

Joined Dec 30, 2020
15
Hello everyone,
I’m new to the forum and to the electronics world.

I recently built a PEMF device and everything worked as it should until I plugged in the coil .

the 740 transistor overheated and the device stopped working .
I followed the schematic perfectly, but I’m unable to figure out what went wrong .

many help is greatly appreciated!
Ed
 

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Reloadron

Joined Jan 15, 2015
5,934
The schematic is pretty rough. My guess would be your gate source is not turning on the MOSFET all the way so the MOSFET gets hot and dies. Depending on the relay coil current requirement you may want to use what is called a Logic Level MOSFET. Look at the data sheet for the MOSFET you have. Now what gate voltage are you driving it with? Note the Vgs shown on the data sheet. Also note the Drain-Source On-State Resistance. Consider a MOSFET like the FQP30N06L or any other logic level MOSFET. You also want about a 10K resistor from Gate to Ground to make sure when the MOSFET turns off it really does turn off. Again, a really poor schematic. Also what is the PWM frequency since you are driving a relay?

Ron
 

Thread Starter

PemfD11

Joined Dec 30, 2020
15
The schematic is pretty rough. My guess would be your gate source is not turning on the MOSFET all the way so the MOSFET gets hot and dies. Depending on the relay coil current requirement you may want to use what is called a Logic Level MOSFET. Look at the data sheet for the MOSFET you have. Now what gate voltage are you driving it with? Note the Vgs shown on the data sheet. Also note the Drain-Source On-State Resistance. Consider a MOSFET like the FQP30N06L or any other logic level MOSFET. You also want about a 10K resistor from Gate to Ground to make sure when the MOSFET turns off it really does turn off. Again, a really poor schematic. Also what is the PWM frequency since you are driving a relay?

Ron
Hello Ron and thanks for your assistance!

the frequency generator module specs :
Application:Computer
Condition:New
Type:Logic ICs
Operating Temperature:-40 ~ 85 Celsius
Model Number:XY-KPWM
Dissipation Power:none
is_customized:Yes
Supply Voltage:DC 3.3V-30V
Package:Other
Output Current:About 5-30mA
Size:79*43*37mm
Feature 1:LCD high definition display
Feature 2:Support UART
Feature 3:frequency adjustment,duty cycle adjustment
My power supply is a Laptop 19v 4Amp.
The coil is 16 gauge 100 turns .

thanks again
Ed
 

Audioguru again

Joined Oct 21, 2019
3,172
The signal generator is missing the most important spec: Its output voltage.
Your IRF740 Mosfet needs a Vgs of 10V to turn on properly.
Why are you using a Mosfet rated at 400V?
Use an IRL540 Mosfet if your signal generator output is only 5V.
 

Thread Starter

PemfD11

Joined Dec 30, 2020
15
Thank you for your help!
I’m a novice when it comes to electronics, learning as I go along , just copied what was in the schematic.
I will try the IRL540 it’s great fun working on these circuits.

I appreciate everyone’s wisdom
 

Thread Starter

PemfD11

Joined Dec 30, 2020
15
These are the only available specs on this particular signal generator module
Parameter:
Working voltage: DC 3.3V-30V
Frequency range: normal mode 1Hz-150kHz; accurate mode 1Hz-15kHz
Frequency accuracy: 2% in each range
Signal load capacity: output current about 5-30mA
Output amplitude: PWM amplitude equals to input voltage value
Working temperature: -20℃~+70℃
Duty cycle range: 0-100%
https://www.amazon.com/Generator-DR...quency+generator+module&qid=1609382903&sr=8-3
 

Audioguru again

Joined Oct 21, 2019
3,172
This is the PEMF device I’m working on , works great until I connect the coil.
1) Does the signal generator dim or shut down when you connect the coil?
2) What is the resistance of the heavy coil plus the skinny orange wires connected to it? If it is less than 19V/4A= 4.75 ohms then the power supply max current is too low and the high voltage Mosfet is not designed for high current.
3) Does the Mosfet have a large finned heatsink attached?

The max gate voltage of an IRL540 Mosfet is only 10V.
 

Alec_t

Joined Sep 17, 2013
12,060
Depending on the pulse rate and duty cycle, a generator current of only 5mA may not be ebough to charge the FET gate capacitance sufficiently quickly to prevent overheating of the FET.
 

Thread Starter

PemfD11

Joined Dec 30, 2020
15
1) Does the signal generator dim or shut down when you connect the coil?
2) What is the resistance of the heavy coil plus the skinny orange wires connected to it? If it is less than 19V/4A= 4.75 ohms then the power supply max current is too low and the high voltage Mosfet is not designed for high current.
3) Does the Mosfet have a large finned heatsink attached?

The max gate voltage of an IRL540 Mosfet is only 10V.
yes the LED lights dim when I connect the 16awg coil , that’s when the Mosfet overheating takes place .
The originator of the schematic just told me that the 16awg is making the Mosfet overheat, that I should use 32awg magnet copper wire .

thanks for everyone’s help !
Ed
 

sagor

Joined Mar 10, 2019
439
Even with the IRL740, powering the device with "19V" may have ripple on it, higher voltage at low loads, or some spikes which cause the PWM to spike over 20V, destroying either of the MOSFETs, especially the logic level IRL540 !
What we don't know is the PWM controller's output voltage. That is the key. Is it controlled within the PWM controller, is it settable? I suspect it is the same as the input voltage. A 19V laptop PSU may be closer to 20V without a large load, causing the MOSFETs to be destroyed by a too large a Gate voltage.
 

Thread Starter

PemfD11

Joined Dec 30, 2020
15
Yes it’s the same as the laptop power supply voltage , so should I replace the Mosfet 740 ?
If so what would be a proper replacement?

thanks for your help!
Ed
 

Audioguru again

Joined Oct 21, 2019
3,172
Obviously, the 4A power supply drops its voltage when overloaded with the the coil and draws a current much higher than 4A. Then the high voltage but low current Mosfet curls up and dies.
If you make the coil with thinner wire then the coil will get hot instead of the Mosfet.
 

Audioguru again

Joined Oct 21, 2019
3,172
If the coil draws 15A then the IRF740 on-resistance of 0.55 ohms heats with 124W. You do not need the 400V rating on the IRF740.
An IRFZ44 Mosfet is rated at 60V and has an on-resistance of only 0.028 ohms and with 15A it will heat with only 6.3W.
 

Thread Starter

PemfD11

Joined Dec 30, 2020
15
So replace the 740 with the IRFZ44?
And will the 16awg work with the z44 or should I use a thinner wire gauge ?

thanks so much !
Ed
 
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