UCC2897A: UCC2897A asn not boot.

Rock Shao

Part Number: UCC2897A
Other Parts Discussed in Thread: TPS2373,

Hi Sir,

I design a TPS2373+UCC2897A as the following schematic, which is a 90W with 12V output and 7.5A but now has something wrong in UCC2897A, it's can't boot.

The following is the experiment I have done.

1. T1 11.5uH(Np:Nbias:Ns=1:0.666:0.5) , L3 33uH.

2. Disable U1 TPS2373, R17, T1, L2.

3. Change SR MOSFET to schottky diode and disable peripheral components ( D16, D17, D18, D19, Q5, Q6, Q7, Q10, R32, R33, C32, C40)

4. Gives 48V at C30

5. Give 12V at C52

After the above experiment, the output is 3~5V, but I designed it to be 12V. Where did I neglect or where should I start debugging? thanks.

Thank you & best regards

over 5 years ago

0 John Griffin1 over 5 years ago

TI__Mastermind 34465 points

Hello Rock,

I cannot read the photograph you embedded in the post.

Please attach a pdf of the schematic so that I can properly view an engineering document.

Regards

John

0 Rock Shao over 5 years ago in reply to John Griffin1

Expert 2995 points

Hi John,

Please refer to the following file, thanks.

UB012D_1V1_F_20200616-01.pdf

0 John Griffin1 over 5 years ago in reply to Rock Shao

TI__Mastermind 34465 points

Hi Rock,

It looks like the power supply is going into current limit.

With 48V input and a 90W load the average input current is about 1.9A
With a Np:Ns ratio of 1:0.5 the duty cycle is 0.5 for 48V input and 12V output.
This will result in a peak primary current of 1.9/0.5 = 3.8A

For simplicity I have not accounted for the ripple d to the output inductor.
The CS pin has a trip level of 0.4V min and in this application there is a DC bias of 0.05V on CS
This means the CS limit is 0.35V min.
So the power supply will go into current limit at 0.35/0.1 = 3.5A and not be able to provide the full output voltage

I recommend that you lower the value of R43 from 0.1R.
Use the Excel calculator for the UCC2897A to calculate the correct value.
The DC offset provided by R48 is not required.

Test at lower power levels and check the switching waveforms.

It is a lot easier to understand what is happening when you can see the scope waveforms.

Regards

John

0 Rock Shao over 5 years ago in reply to John Griffin1

Expert 2995 points

Hello John,

Could you provide the excel calculator for the UCC2897A, thank you very much.

Best regards,

Eock Shao

0 John Griffin1 over 5 years ago in reply to Rock Shao

TI__Mastermind 34465 points

Hello Eock,

You will find the calculator Here:

https://www.ti.com/product/UCC2897A?keyMatch=UCC2897A&tisearch=Search-EN-everything&usecase=GPN#design-development##design-tools-simulation

SLUC662A zip file is about halfway down the page

Regards

John

0 Rock Shao over 5 years ago in reply to John Griffin1

Expert 2995 points

Hello John,

Thank you for your reply.

I confirmed the primary side waveform was found the abnormal waveforms,

there should be no wave mode like burst mode for UCC2897A of no-load, and when the load is 30mA, the voltage of Vds will reach -188V.

How can I solve this problem? Thank you.

CH1: Main MOFET Vgs, CH2: Aux MOSFET Vgs, CH3: Aux MOSFET Vds.

output load at no load

output load at 30mA

Waiting for your answer, thank you very much

Best regards

Rock

0 John Griffin1 over 5 years ago in reply to Rock Shao

TI__Mastermind 34465 points

Hi Rock,

The second waveform shows the power supply going into some fault situation.
The power supply starts up with a soft start and shuts down.
Check the LINEOV , LINEUV pins.
Check CS.
Noise on the CS pin can cause the pwm to turn off and this will give you low output voltage
Any of these signals can cause the shutdown

Regards

John

0 John Griffin1 over 5 years ago in reply to John Griffin1

TI__Mastermind 34465 points

Hello

I am closing this post since no further communication was received.

Regards

John

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UCC2897A: UCC2897A asn not boot.