UC2845A: VCC drop when load current change instantly

Cindy Chang

Part Number: UC2845A

Hi,

We faced a a problem when we just start up UC2845A as flyback circuit for fan.

When the system start up, the fan will make load current from 0 to 3A within 200ms.

The secondary side voltage will be from 24V up to 25V and this will cause UC2845A stop switching so the auxiliary side voltage (VCC) will drop dramatically from 16V.

The schematic is shown as below.

Is there any suggestion of modifying the circuit to solve this case?

Thank you.

Best Regards,

Cindy

over 3 years ago

0 John Griffin1 over 3 years ago

TI__Mastermind 34465 points

Hi Cindy,

Are C126 and C127 not fitted ?

Regards

John

0 Cindy Chang over 3 years ago in reply to John Griffin1

TI__Intellectual 2475 points

Hi john,

I didn't get your point.

We build our circuit just as what schematic shows.

Do you mean there might be some problems about C126 and C127?

Thank you.

Best Regards,

Cindy

0 John Griffin1 over 3 years ago in reply to Cindy Chang

TI__Mastermind 34465 points

Cindy,

The schematic shows that these parts are ghosted in.
Without a bill of material for the pcb assembly, the implication is that the parts are not fitted.

Regards

John

0 Cindy Chang over 3 years ago in reply to John Griffin1

TI__Intellectual 2475 points

Hi John,

C126, C127 are 22UF/25V and on PCB board.

Others NC are really not on PCB.

Thank you.

Best Regards,

Cindy

0 John Griffin1 over 3 years ago in reply to Cindy Chang

TI__Mastermind 34465 points

Hi Cindy,

The power supply is cycling on/off because there is either a fault somewhere or because the voltage on VCC is dropping below the UVLO value.

Check VCC with a scope. It may be the auxiliary winding is no providing enough voltage.
Check the voltage on ISEN pin. It may be triggering the over current shutdown.
The power supply is set for a peak primary current of 1.8 A .
Is this the correct value for the application ?

Regards

John

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UC2845A: VCC drop when load current change instantly