UC2843: Failure to turn on and drive

Siji Oluwadara

Part Number: UC2843

HI,

I am having a couple of problems with the UC2843. I am trying to step 12V down to 8V with output current of about 4A.

1. It is trying to turn on but failing to do so. At the RT/CT pin I have the RT as 8.25K and CT as 1nF.

At VFB and Comp I have a 510K resistor and a 100pF cap and for the resistor divider at VFB I have a 22k resistor as R2 (going to VCC) and 4.7K resistor as R1 (going to ground)

2. It is not providing enough current. I have a 1 ohm resistor as my sense resistor and I am using an NMOS to drive.

Thanks

over 7 years ago

0 John Griffin1 over 7 years ago

TI__Mastermind 34465 points

Hello Siji,
Can you please post a schematic with component values ?

Thanks
John

0 Siji Oluwadara over 7 years ago in reply to John Griffin1

Prodigy 60 points

The transformer is this: https://www.digikey.com/product-detail/en/pulse-electronics-power/PA3856.005NLT/553-2139-1-ND/3906266

0 John Griffin1 over 7 years ago in reply to Siji Oluwadara

TI__Mastermind 34465 points

Hello Siji,
Apparently this transformer can be configured as a flyback or forward converter.
You incorrectly configured it as a forward. And there is no proper current sense waveform on R43.
Please configure as a flyback by switching the pinouts on pin 4 and pin 5 and also pins 9,10 and 6,7.
Then check the current sense signal on R43.
You probable need to replace Q1.
Regards
John

0 Siji Oluwadara over 7 years ago in reply to John Griffin1

Prodigy 60 points

Hi John,

Thanks for pointing that out. Just to clarify should I make pin 4 ground and pin 5 my source and make pins 9&10 isolated ground and pins 6&7 my isolated source? or should I swap the pins such that 9&10 and 6&7 becomes the auxiliary side and pins 4&5 become the secondary side?

Out of interest, do you mind explaining why the pins of the transformer can be switched to make it a flyback ?

Thanks

0 John Griffin1 over 7 years ago in reply to Siji Oluwadara

TI__Mastermind 34465 points

Hi Siji,

You should go with your first interpretation.

This will make your circuit a flyback.
The operation of the flyback is described in this TI training link:

Regards

John

0 Siji Oluwadara over 7 years ago in reply to John Griffin1

Prodigy 60 points

Hi John,

I tried what you suggested and it is still not working. I see that it is trying to turn on. I also wanted to note that I replaced R41 with a 0 ohm resistor and removed C51.

I also tried replacing the transformer altogether with a flyback transformer from the series and it still does work. Although in this case, it is not trying to turn on. VCC is about 1.2V.

Lastly in both cases I replaced R43 with .5 ohms.

Any other suggestions of what to do?

0 Siji Oluwadara over 7 years ago in reply to Siji Oluwadara

Prodigy 60 points

Are there any other suggestions of what to do?

0 Siji Oluwadara over 7 years ago in reply to John Griffin1

Prodigy 60 points

I am still having trouble with this. Does anyone have any suggestions of what to do?

0 Joe Leisten over 7 years ago in reply to Siji Oluwadara

TI__Expert 3095 points

Hi Siji,
Please post waveforms of the VCC pin voltage.
The MOSFET Drain voltage and the ISENSE pin voltage.
The OUTPUT pin voltage.
Having all of these waveforms on the same scope picture should help us to figure out what is going wrong.
Thanks
Joe Leisten

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UC2843: Failure to turn on and drive