UCC256402: Vcc voltage issue

Jitendra Suthar

Part Number: UCC256402

I am using Ucc256402 for half bridge LLC converter, where Vhv = 150V.

I referred datasheet where it is mentioned that Vcc supply voltage range is Min: 13V, Nom: 15V, Max: 26V in recommended operating condition table.

But IC didn't work when I gave 24V at Vcc pin, then I came across another table which mentioned VCCStartSelf: min :25V, TYP: 26V, Max: 28V !!!!!!

So my question is if IC doesn't work below 26V ( which I observed so far) then what is the meaning of recommended operating condition ?

Regards,

Jitendra suthar

over 3 years ago

0 Mike O' over 3 years ago

TI__Guru 66135 points

We have received your inquiry and are looking into it.

Regards,

0 Mike O' over 3 years ago in reply to Mike O'

TI__Guru 66135 points

Hello,

I reviewed the data sheet and the absolute maximum rating for the VCC pin is 30V, please refer to section 7.

The recommended operating range for VDD is 13 to 26 V. To keep power dissipation down I would recommend biasing the VCC pin with 15V.

The reason why the VCC startup threshold is so high it because of trickle charge starting and having to hold up VCC until the converter has time to be up and running and the VCC voltage can be supplied by the bias winding. Reviewed section 10.1 for sizing the VCC capacitor this will help to understand why the startup threshold is so high. Please note it is still below the 30 V absolute maximum rating.

Regards,

0 Jitendra Suthar over 3 years ago in reply to Mike O'

Intellectual 295 points

Thanks Mike for reyply.

But When I decrease (after start up) Vcc voltage from 26V to 24V IC stops generating switching waveforms.

I always need to apply 26V to Vcc pin in order to make it work.

Thanks.

0 Mike O' over 3 years ago in reply to Jitendra Suthar

TI__Guru 66135 points

Hello,

Could you share you schematic for this design?

It sounds like you are not powering the UCC256402 correctly.

Regards,

0 Jitendra Suthar over 3 years ago in reply to Mike O'

Intellectual 295 points

0 Jitendra Suthar over 3 years ago in reply to Jitendra Suthar

Intellectual 295 points

Vcc supply voltage is applied through Variable power supply and it is set to 26V at power ON condition and when switching waveform starts and then if I decrease voltage from 26V to 24V it stops generating waveforms.

Regards,

Jitendra

0 Mike O' over 3 years ago in reply to Jitendra Suthar

TI__Guru 66135 points

Hello Jitendra,

Is this actually in a LLC or converter or is this just an open loop test circuit?

LO and HO should be driving pulses at roughly 50% duty cycle and should work with a VDD voltage well below 24V.

Could you send your complete schematic and I will review it for you?

Did you use any of our design tools?

0 Jitendra Suthar over 3 years ago in reply to Mike O'

Intellectual 295 points

LLC_Resonant_Cir.pdf

0 Jitendra Suthar over 3 years ago in reply to Jitendra Suthar

Intellectual 295 points

Hi Mike,

LO and HO are generating pulses at 50% duty cycle with 12V amplitude.

Yes, We referred your excel design tool.

0 Mike O' over 3 years ago in reply to Jitendra Suthar

TI__Guru 66135 points

Hello,

I reviewed your schematic and I don't see anything that could be causing the unit not to startup.

Things that will prevent startup.

Input under voltage sampled at the BWH pin, VBKLStart = 3V, VBKLSto= 2.2V
Output over voltage sampled at the BW pin, VBWOPos = 4V, VBWONeg = - 4V
Over current sampled at the ISNS pin, VISNS_OCP1 = 4V

Evaluating the ISNS, BW, BLK, SS, Vout and VCC with an oscilloscope during startup will help determine why the converter is not starting up.

Regards,

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UCC256402: Vcc voltage issue