UCC28C45: Design not working as per webench design suggestion.

pujit gandhi

Part Number: UCC28C45

Hey,

We are designing an isolated 12V (aux battery) to 12V (1.5A output) power supply using TI Webench design tool for our electrical sub system. However power supply is not behaving as per the design suggested by webench.

1. 12V isolated output not coming when we give 12V to key switch pin.

2. After shorting R72 and R77 , Removing R86 we do get isolated output of 12V. However MOSFET overheats at load current around 1A.

3. Diode D20 blow while testing. Supply works after shorting D20 track.

Kindly review and tell the points to improve.

over 2 years ago

0 JCHK over 2 years ago

TI__Guru 50190 points

Hi Pujit,

When I run a Webench design assuming 12VIN, 12VOUT at 1.5A, I get a different schematic recommendation. Please see the attached. WBDesign59.pdf

Notice how the transformer is wound differently. Your schematic appears to be tying the primary side and secondary side grounds together. Is this intentional?

0 pujit gandhi over 2 years ago in reply to JCHK

Prodigy 30 points

Input voltage may vary from 10V to 13V , will it have any effect on the schematic shared above?

0 JCHK over 2 years ago in reply to pujit gandhi

TI__Guru 50190 points

Hi Pujit,

Yes. You will need to follow the schematic and transformer windings shown in the webench design I sent.

0 pujit gandhi over 2 years ago in reply to JCHK

Prodigy 30 points

The start threshold voltage for UCC28C45 is 7.8V. I guess current circuit won't give the threshold voltage. I tested it as well. When I am changing the resistor divider it starts working, else it's just turn on n off

0 JCHK over 2 years ago in reply to pujit gandhi

TI__Guru 50190 points

Hi Pujit,

DId you get it working? I am not sure which resistor divider you are referring to,

0 pujit gandhi over 2 years ago in reply to JCHK

Prodigy 30 points

R72 and R77. When these are connected in circuit. Output across C51 is 0V. However if we short R72 and R77. We do get mosfet switching and 12V at output.

0 JCHK over 2 years ago in reply to pujit gandhi

TI__Guru 50190 points

HI Pujit,

Glad to hear you got it working.

0 pujit gandhi over 2 years ago in reply to JCHK

Prodigy 30 points

Hi,

I tried capturing Vds waveforms for mosfet in reference power supply (Left) and our power supply (Right) @400mA load , I can see something is not right when it comes to our circuit.

0 JCHK over 2 years ago in reply to pujit gandhi

TI__Guru 50190 points

Hi Punjit,

It looks like something is causing the on-time of your gate drive to be cut short. Have you looked at what is happening at the CS and FB pins?

0 pujit gandhi over 2 years ago in reply to JCHK

Prodigy 30 points

Yes !!! Just found transistor in compensation network was not assembled right. Some ringing is still there. Need to tune snubber now and shunt resistance for

proper line and load regulation.

0 JCHK over 2 years ago in reply to pujit gandhi

TI__Guru 50190 points

It looks like you found it! By the way, we have released the UCC28C55 which is pin-to-pin with the UCC28C45. The key differences are:

1. VDD raised to 30V - This can make D23 in your schematic unnecesary.

2. Maximum startup current is now 75uA (down from 100uA)

3. Maximum operating current is now 2mA ( down from 3mA) - This will marginally help improve efficiency.

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UCC28C45: Design not working as per webench design suggestion.