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MOSFET power supply issues debug request

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jabba99

Electrical
Jul 5, 2024
11
Hello guys,

As I said in my previous request I'm fairly new to electronics. I was tasked to figure out the issue with this supply. Note it is part of a bigger schem.
supplypower_p4i82x.png


Here Q1 gets a PWM signal from an IC and the output voltage from the supply is fed to a TPS62120DCNR DCDC converter to get 3.3V
I haven't before worked with MOSFET as a power supply and I'm struggling to find out why R1 burns out and Q2 overheats and sometimes it expoldes as well.

PS: it's a 2wire circuit

Any tips or articles that help with this are very welcome

Thanks in advance
 
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@IRstuff I don't know the designer.
R1 is Thick Film 2512 470 Ohm 5% 2W -±100ppm/C Pad SMD T/R

rect_bridge2R1_piz4gm.png
14VC_ppeeze.png

These are about the only 2 wave forms i managed to get.
I tried the see the PWM but i only get 80mV steady
 
You have 470K of resistance (R3) in series with the gate of a power switching transistor. That's not switching, it's swamped in the RC time constant of the resistor and the capacitance associated with the MOSFET gate input. The transistor Q2 is neither on nor off most of the time and is going to get hot and burn up.

Also is the input a sinusoid? If so there should be a filter capacitor after the diode bridge.

Why is there a 470 ohm resistor (R1) in series with the power supply? Except for really low currents that is going to limit your current output a lot.

Jerry Twomey, Author of:
"Applied Embedded Electronics Design Essentials for Robust Systems"
 
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