MOSFET power supply issues debug request

[jabba99]

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.

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

 

[IRstuff]

What does the designer say?

Have you used an oscilloscope to see the waveforms at different parts of the circuit?

What's the wattage rating of R1?

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[jabba99]

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

These are about the only 2 wave forms i managed to get.
I tried the see the PWM but i only get 80mV steady

 

[looseelectron]

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"

https://effectiveelectrons.com/book/

 

[IRstuff]

Why is there a 470 ohm resistor (R1) in series with the power supply?

Perhaps because OP's TPS62120 downstream load is only 75 mA

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[looseelectron]

downstream load is only 75 mA

Let's see: V = (75E-3)(470) = 35 volts....

Need I say more?

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

https://effectiveelectrons.com/book/