Mosfets getting damaged and i don't know why

Thread Starter

idrbur

Joined Jan 11, 2018
63
Hi,
I made a really simple PWM generator but the mosfet placed in the circuit keep getting damaged even though they are not heating up.
The load in the schematic is a 20ohm heating element but i also tried to replace it with much led and resistor but the mosfet still get damaged after 1 or 2 runs.
The mosfet is cj3400. Any suggestions are appreciated.
Thank you.

There's also a 10k pull down resistor between gate and source with is not present in the schematic
Screenshot_2023-03-10-12-24-32-379_com.everycircuit.free.jpgScreenshot_2023-03-10-12-24-24-084_com.everycircuit.free.jpg
 

Pyrex

Joined Feb 16, 2022
74
Hi,
cj3400 is SMD style and 0.35W only. Load takes 0.6A ,so regulated power is 12*0.6=7W. Transistor operates at margin...
And, in addition, the multivibrator's fronts are not so perfectly shaped, so mosfet goes in active mode periodically, dissipating significant heat
 

Audioguru again

Joined Oct 21, 2019
5,815
Where did you measure 10.8mA waveforms?
Your schematic is missing 5V and 12V supply bypass capacitors very close to the pcb.
I hope you built it on a compact pcb and not in a mess on a breadboard.

The Mosfets are made and sold by unknown Chinese companies. Have you tried using name-brand Western Mosfets?
 

wayneh

Joined Sep 9, 2010
17,262
@Pyrex has identified your main problem.
Your drawing shows the 5V and 12V sources sharing a common ground. Does your build actually accomplish that? I only ask because it's commonly overlooked.
 

crutschow

Joined Mar 14, 2008
32,026
I think the problem may be that the MOSFET gate voltage from the transistor is not going to 5V when it is off, because of the current going through the 100nF timing capacitor from the collector resistor.
Have you looked at that voltage?
You need to add a buffer transistor at the oscillator output
 

Thread Starter

idrbur

Joined Jan 11, 2018
63
Hi,
cj3400 is SMD style and 0.35W only. Load takes 0.6A ,so regulated power is 12*0.6=7W. Transistor operates at margin...
And, in addition, the multivibrator's fronts are not so perfectly shaped, so mosfet goes in active mode periodically, dissipating significant heat
I monitored the mosfets for heating but they always remain only a few degrees above room temp
 

Thread Starter

idrbur

Joined Jan 11, 2018
63
I think the problem may be that the MOSFET gate voltage from the transistor is not going to 5V when it is off, because of the current going through the 100nF timing capacitor from the collector resistor.
Have you looked at that voltage?
You need to add a buffer transistor at the oscillator output
They do share common ground on pcb
 

Thread Starter

idrbur

Joined Jan 11, 2018
63
Where did you measure 10.8mA waveforms?
Your schematic is missing 5V and 12V supply bypass capacitors very close to the pcb.
I hope you built it on a compact pcb and not in a mess on a breadboard.

The Mosfets are made and sold by unknown Chinese companies. Have you tried using name-brand Western Mosfets?
It's made on a pcb.
I am using same mosfet in a different circuit and it's working quite well.
 

Pyrex

Joined Feb 16, 2022
74
I monitored the mosfets for heating but they always remain only a few degrees above room temp
Mosfet periodically goes into active mode. Peak power is 1/2 of load power, i.e. 7/2=3.5W. The crystal inside case is very small and heats up quite quickly, in miliseconds. When transistor is saturated, or closed, it cools down. External temperature is just an average and do not indicate extremal rises during active mode.
Anyway, it's not a good idea to drive 7W load with 0.35W transistor, imho
 

sarahMCML

Joined May 11, 2019
242
It's made on a pcb.
I am using same mosfet in a different circuit and it's working quite well.
Show us a picture of your board, please. Have you tried a small (say 20 Ohm) resistor directly between the gate of the MOSFET and the collector of the transistor, to stop any potential parasitic oscillations?
 

Pyrex

Joined Feb 16, 2022
74
Bypassing capacitors across 5V source and 12V source installed? Ceramic 0.1 uF and electrolytic 100 uF at least for each source
 

Thread Starter

idrbur

Joined Jan 11, 2018
63
Mosfet periodically goes into active mode. Peak power is 1/2 of load power, i.e. 7/2=3.5W. The crystal inside case is very small and heats up quite quickly, in miliseconds. When transistor is saturated, or closed, it cools down. External temperature is just an average and do not indicate extremal rises during active mode.
Anyway, it's not a good idea to drive 7W load with 0.35W transistor, imho
I get what you are saying but as I said in other reply I have used the same mosfet with a PWM circuit made with 555 ic. which is running on 24v (12w ) load and it's been working without problem for more than a month.
Though I will add the capacitor you mentioned so, thanks for the suggestion
 

Thread Starter

idrbur

Joined Jan 11, 2018
63
I
Show us a picture of your board, please. Have you tried a small (say 20 Ohm) resistor directly between the gate of the MOSFET and the collector of the transistor, to stop any potential parasitic oscillations?
I will post the picture on Monday.
Though I etched the pcb myself so it's not that clean.
 
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