UCC28C43-Q1: Efficiency and heat issue

Michael Santamaria

Part Number: UCC28C43-Q1

Dear all,

I made a Flyback converter with the UCC28C43-Q1. The design was made thanks to the Webench tool using the following input :

VinMin = 16.0V
VinMax = 36.0V
Vout = 24.0V
Iout = 5.0A
Iout_nom = 2A
Tamb = 70°C

I try to use all the recommended components, excepted for the custom parts (Transformer, Rsense) where I select standard components. I select the Wurth 750311592 for the transformer which provide the closest parameters to those computed by Webench. However, the output power is limited to 45W. The complete schematic is the following :

The board seems to works correctly, but the power components (Mosfet, snubber,Transformer and Rsense) becomes very hot (over 100°C with 2A current drawn in a 25°C room). The efficiency is around 80% which is smaller than expected (Webench computation is around 90%).

I've measured some signals on the primary side Vin = 28V, Vout = 24V @ 2A :

Yellow = gate of the mosfet (TP13)

Blue = source (TP18)

Here are the drain voltage and the gate voltage :

Yellow = gate

Blue = drain

Thank you very much for your help.

Best regards,

Michael

over 5 years ago

0 Sonal Singh over 5 years ago

TI__Expert 6080 points

Hello Michael,

May you please share the link to your Webench model for this particular design for better understanding.

The reason for the Snubber, transformer and MOSFET getting hot can range from: that they either one of them is not rated for the correct conversion power, to the transformer is hitting its saturation current limit. Have you followed the guideline for size of the heat sink on the MOSFET as mentioned in the Layout example of the datasheet?

Regards,

Sonal

0 Michael Santamaria over 5 years ago in reply to Sonal Singh

Prodigy 120 points

Hello Sonal,

Thank you for your very quick answer.

Here is the Webench link of the design : https://webench.ti.com/power-designer/switching-regulator/customize/12.

For the Snubber there are 2 2010 resistor of 1W each in parallel, the Rsense is a 2512 package able to dissipate 2W. The Mosfet is the same reference as Webench suggestion. For the transformer, I've notice one misunderstanding : I tie pin 1-2 and 2-4 on the primary, and 12-11-10 and 7-8-9 on the secondary. The datasheet gives an inductance Lp= 8µH but I don't know if I should tie the winding or not because the datasheet is not clear to me. Thus I've unconnected pin 2-4 on the primary, and pin 11-10-7-8 on the secondary in order to only keep one winding on each side. This doesn't solve my problem and the board keep warming.

Concerning the saturation current of the transformer, it's given at 18A. On the oscilloscope screenshot above, you can observe the voltage across the Rsense which give the image of the current flowing through the primary side of the transformer. The voltage reach around 400mV peak across a 27mR resistor, which gives 400mV/27mR = 14.8A.

Best regards,

Michael

0 Michael Santamaria over 5 years ago in reply to Michael Santamaria

Prodigy 120 points

Hello Sonal,

Did you look at the Webench design ? I'm looking for a transformer who matches the requirements but I can't find it on the market. Do you know if a custom transformer was already made to works with UCC28C4x ?

Thank you for your help.

Best regards,

Michael

0 Sonal Singh over 5 years ago in reply to Michael Santamaria

TI__Expert 6080 points

Hello Michael,

Looking at your Wurth 750311592 transformer; it looks like the secondary is rated for 24V and 1.9A, which is why you are not able to deliver more than ~45W and the transformer is getting overheated at higher load values.

Please make sure that your transformer is rated for the right current levels for both input and output.

I would suggest contacting Wurth Electronics if your custom transformer design.

https://www.we-online.com/web/en/passive_components_custom_magnetics/products_pbcm/new_design_request/new_design_request_1.php?_ga=2.29829814.8026066.1570697503-1490965270.1570697503

Regards,

Sonal

0 Michael Santamaria over 5 years ago in reply to Sonal Singh

Prodigy 120 points

Hello Sonal,

Thank you for your answer.

Yes I know the transformer is limited for this load, but even at 1A load it becomes very hot. I want to make sure the design will fit for my application before making custom transformer. I've found this one : https://katalog.we-online.de/pbs/datasheet/750312872.pdf which is able to provide 4A @28V (112W power). However the inductance is 14µH instead of the 8µH recommended by webench. Is this an issue for the design ?

Best regards,

Michael

0 Sonal Singh over 5 years ago in reply to Michael Santamaria

TI__Expert 6080 points

Hello Michael,

The webench tool is suggesting 810uH of primary inductance for converting the load to 5A. May you please double check this for me because 8uH is a very small value to convert 125W of power.

Regards,

Sonal

0 Michael Santamaria over 5 years ago in reply to Sonal Singh

Prodigy 120 points

Hello Sonal,

I've checked my Webench design and the recommanded value is 7.44µH for Lp (cfr complete webench report in attachement)

WBDesign12.pdf

Best regards,

Michael

0 Sonal Singh over 5 years ago in reply to Michael Santamaria

TI__Expert 6080 points

Hello Michael,

I think there is a discrepancy in the webench report here.

Please check the webench link you attached earlier that have a different transformer value.

Here is the Webench link of the design : https://webench.ti.com/power-designer/switching-regulator/customize/12.

May you please help me understand which one is the correct file.

Regards,

Sonal

0 Michael Santamaria over 5 years ago in reply to Sonal Singh

Prodigy 120 points

Hello Sonal,

The report comme from the webench design on the link. I think the transformer values are round on the webench design but not in the report : we have Lp = 7.44µH vs 7.436µH etc for the other values.

Best regards,

Michael

0 Sonal Singh over 5 years ago in reply to Michael Santamaria

TI__Expert 6080 points

Hello Michael,

May I please ask you to use a transformer with a primary inductance of 810uH as suggested in the webench model.

Regards,

Sonal

0 Michael Santamaria over 5 years ago in reply to Sonal Singh

Prodigy 120 points

Hello Sonal,

How did you get this value ? When I look at my design the parameters are :

Best regards,

Michael

0 Sonal Singh over 5 years ago in reply to Michael Santamaria

TI__Expert 6080 points

Hi Michael,

Below is the link that you provided to me in your previous post:

https://webench.ti.com/power-designer/switching-regulator/customize/12

It is weird if you see different recommendations than below:please check the BoM I attached generated from the webench link.

e2eBOM.xlsx

Regards,

Sonal

0 Michael Santamaria over 5 years ago in reply to Sonal Singh

Prodigy 120 points

Hi Sonal,

Yes it's really weird because I don't have the same value. However when I decrease the output current, the Lp value increase. Could you verify the design parameters are correct :

VinMin = 16.0V
VinMax = 36.0V
Vout = 24.0V
Iout = 5.0A
Iout_nom = 2A
Tamb = 70°C

Maybe can you send me the complete webench report you get ?

Thank you for your help.

Best regards,

Michael

0 Sonal Singh over 5 years ago in reply to Michael Santamaria

TI__Expert 6080 points

Hello Michael,

Please refer to the webench link below:

https://webench.ti.com/power-designer/switching-regulator/customize/79?AppType=DC&Flavor=none&O1I=5&O1V=24&Topology=FLYBACK&VinMax=36&VinMin=16&base_pn=UCC28C43&isolated=Y&lang_chosen=en_US&op_TA=30&optfactor=3&origin=pf_panel&refpage=http:%2F%2Fwww.ti.com%2Fproduct%2FUCC28C43&source=DC

I would also suggest double-checking these values with another calculation tool: below is the link to the Power stage designer tool and the txt file with your spec loaded already.

http://www.ti.com/tool/POWERSTAGE-DESIGNER

Fullscreen Flyback_example.txt Download

FLYBACK
16
36
24
5
100
0.7
66
40
3
0.4
7

I hope this helps.

Regards,

Sonal

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UCC28C43-Q1: Efficiency and heat issue