UC2844: Vds up to 770v, please help to analyze.

Yue TANG

Part Number: UC2844

Hi,

My customer use UC2844 in their new design, 346VDC input,24V2ADC output. during the test, They found the Vds voltage of the MOS is up to 770V. They don't know the root cause and how to lower this voltage. below is the schematic, transformer spec and Vds waveform. please kindly give your suggestion. thanks.

2844.Schematic.pdf

Transfomer.pdf

over 5 years ago

0 user5988235 over 5 years ago

Prodigy 95 points

from the schematic, the transformer is not a flyback.

but from the wave form, it is a flyback

0 user5988235 over 5 years ago in reply to user5988235

Prodigy 95 points

it is normal

0 Yue TANG over 5 years ago in reply to user5988235

TI__Expert 8695 points

Hi,

Thanks for your reply. but My customer's question is how to reduce this Vds voltage? BTW, could you be more specific from which SPEC you can see that this transformer is not for flyback? thanks.

0 Peter Meaney over 5 years ago in reply to Yue TANG

TI__Mastermind 31580 points

Hi Yue,

The power supply is a flyback but the dot notation in the schematic is incorrect for a flyback.

The customer appears to have applied 346Vac (= 498Vpk) instead of 346Vdc to the input. The reflected voltage from the output through the transformer turns ratio should be 132V given by (80T (Np) /14.5T (Ns) * 24V (VOUT) but it looks like 285V in the waveform above so please check the turns ratio and output voltage of the design.

The only way to reduce the reflected voltage on the mosfet is to modify the transformer turns ratio.

Regards

Peter

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UC2844: Vds up to 770v, please help to analyze.