UCC28750: 0V output at 12V rail

Lucas VanAssen

Part Number: UCC28750

Tool/software:

I am attempting to use the UCC287502DBVR in order to generate 12V max 2A. When I plug in my PCB, I get 0V out at the output. Voltage (carefully) measured after the bridge rectifier is ~172VDC, and the VDD pin on the UCC287502DBVR drops from ~12.5VDC to ~9.5VDC before jumping back to ~12.5VDC. Below is the schematic for my PCB.

4 months ago

0 JCHK 4 months ago

TI__Guru 50040 points

HI Lucas,

If VDD is dropping down to ~9.5VDC, you might be triggering UVLO. Have you tried increasing the capacitance on C22 and C23? The EVM has 100uF here. Also I don't see the polarity dots on the transformer. Do the waveforms on the aux winding look correct?

0 Lucas VanAssen 4 months ago in reply to JCHK

Prodigy 20 points

JCHK,

I discovered the auxiliary windings were reversed and fixed that, and I began getting 1.8V out. I added 20uF more capacitance on C22 and C23 and I am still getting 1.8V out of the circuit. I am getting the same VDD oscillation as before as well.

0 JCHK 4 months ago in reply to Lucas VanAssen

TI__Guru 50040 points

HI Lucas,

Your Aux winding voltage should look something like this:

This is taken from the UCC28750EVM-071 Users guide.

If you see this and VDD is still dipping below 9.5V, What are you seeing at the node I labeled as Vaux2?

0 Lucas VanAssen 4 months ago in reply to JCHK

Prodigy 20 points

This is what I am getting on FLYB_VDD

This is what I am getting on Vaux2

0 JCHK 4 months ago in reply to Lucas VanAssen

TI__Guru 50040 points

Hi Lucas,

I looks like you might need to adjust the turns ratio on your aux winding. It looks like you are hitting the UVLO shutdown every switching cycle.

0 Lucas VanAssen 4 months ago in reply to JCHK

Prodigy 20 points

I just manually verified the transformer, and the primary:auxiliary ratio is 5.84:1, which matches the WEBENCH requirements for the transformer. Any thoughts on why the WEBENCH output could be wrong?

+1 JCHK 4 months ago in reply to Lucas VanAssen

TI__Guru 50040 points

Hi Lucas,

I will confirm the WEBENCH model and try to get it corrected. The Excel based calculator (attached) is better for selecting the turns ratio and component values.

By the way, your CS resistor R22 is about double what it should be. Why did you select such a high value?

UCC28750-CALC Lucas.xlsx

0 Lucas VanAssen 4 months ago in reply to JCHK

Prodigy 20 points

I appreciate the effort, the Excel spreadsheet is very useful.

R22 was chosen from the WEBENCH outputs. I see from the Excel spreadsheet that it is significantly higher than needed.

0 JCHK 4 months ago in reply to Lucas VanAssen

TI__Guru 50040 points

Can you share the parameters you entered to get your WEBENCH design? I will use that to get it fixed.

Thank you

0 Lucas VanAssen 4 months ago in reply to JCHK

Prodigy 20 points

Good Morning,

Input:

AC 85 V - 200 V at 60.0 Hz

Output:

Isolated 12 V at 1.5 A

Temp:

30 °C

0 JCHK 4 months ago in reply to Lucas VanAssen

TI__Guru 50040 points

Hi Lucas,

Thanks. I will use this to get the WEBENCH model fixed.

0 Lucas VanAssen 4 months ago in reply to JCHK

Prodigy 20 points

Good Morning,

I found a possible alternative transformer, but the primary inductance is about 1/3rd the value that the excel sheet recommends. How critical is the primary inductance to operation?

0 JCHK 4 months ago in reply to Lucas VanAssen

TI__Guru 50040 points

Hi Lucas,

The primary inductance impacts how much energy (1/2LI^2) is stored per switching cycle. So given the UCC287502 is a fixed frequency device, you will not be able to supply adequate power to the load.

It is usually best to work directly with a magnetics supplier to optimize the transformer for your application.

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UCC28750: 0V output at 12V rail