Need Help Troubleshooting My First SMPS Design (220VAC to 12V 60W)

[synacle]

Hi everyone,

I’m working on my first switch-mode power supply (SMPS) design. It’s supposed to convert 220V AC input to 12V DC at 60W output.

I’m not an electronics engineer and I don’t have formal training in this field – I’m just a hobbyist trying to learn.

The transformer was custom-wound by a manufacturer in China. Here are the specs:

Core: EE33

Primary Inductance: 82 µH

Primary: 15 turns, 0.65 mm wire

Secondary: 3 turns, 1.5 mm wire

Auxiliary: 5 turns, 0.2 mm wire

When I powered up the PCB for the first time, the transistor and Rsense resistors burned out instantly. I suspect my snubber/clamp circuit calculations might be wrong.

I’ve attached a hand-drawn diagram of my transformer (see image).

Could you please help me troubleshoot this design? I’d appreciate explanations in simple terms since I’m still learning.

Thank you very much!

[Magnethicc]

Upload PCB too, it could be wrong connections.

I see no soft-start circuit - could be too high duty on the flyback x'mer saturating it and blowing up the FET and sense resistors.

see page 22 in https://www.ti.com/lit/ds/symlink/uc3842.pdf and follow their guidelines.

[temperance]

The primary inductance is probably too low.

Given the primary inductance, the di/dt is around 3.7 A / µs with an input voltage of 300 V. With 82 µH primary inductance and an on time of 1 µs, you are trying to transfer 115 W to the output when switching at 100 KHz.

The primary inductance must be around 450 µH for a DCM mode converter.

With 450 µH primary inductance, the transferred power is 77 W when operating at 100 KHz with an on time of 3.5 µs for the primary switch.