UCC256402: UCC256402

Jianping Fan

Part Number: UCC256402
Other Parts Discussed in Thread: UCC256404, UCC27524, UCC25640EVM-020

I have a start up problem with UCC256402 in a DC/DC converter design. When I apply input voltage, VCC rises to about 15V and stay flat, but RVCC stays at zero, BLK volatge is >3V. At beginning UCC256404 was chosen as the primary controller, it is changed to UCC256402 trying to avoid problem with AC zero crossing detection and X-cap discharging function, but the circuit still cannot start up. Apart from the AC zero crossing detection/X cap discharge function (which UCC256402 does not have) and BLK voltage for UVLO, I cannot find other clues that could affect the start up.

over 4 years ago

0 Hong Huang over 4 years ago

TI__Mastermind 47655 points

Can you get an EVM to help your debugging? Your circuit prevents VCC reaches needed to start up so you need to find out what circuit you made wrong to prevent that and resolve that.

0 Jianping Fan over 4 years ago in reply to Hong Huang

Prodigy 120 points

Hi, Hong:

Many thanks for your prompt response. Sure, I will look into buying an EVB and also appreciate if you could advise on the following:

1. Would the EVB be able to start up with DC input voltage, instead of AC input on J3?

2. From the EVB user guide, the LLC controller is UCC256404. If I change to UCC256402, would it be able to start up with DC input on J3?

3. Will RVCC only be enabled when VCC reaches 26V at first start up?

4. In my design there is no external load (except a 22uF bypassing capacitor) pulling down VCC, but VCC rises only to 15V and then stay at 15V afterwards. So I feel VCC is clamped by internal circuitry of the controller. Which part (or function) of the internal circuitry clamps VCC?

Best regards.

Jianping

0 Jianping Fan over 4 years ago in reply to Jianping Fan

Prodigy 120 points

Jianping Fan said:

Hi, Hong:

Many thanks for your prompt response. Sure, I will look into buying an EVB and also appreciate if you could advise on the following:

1. Would the EVB be able to start up with DC input voltage, instead of AC input on J3?

2. From the EVB user guide, the LLC controller is UCC256404. If I change to UCC256402, would it be able to start up with DC input on J3?

3. Will RVCC only be enabled when VCC reaches 26V at first start up?

4. In my design there is no external load (except a 22uF bypassing capacitor) pulling down VCC, but VCC rises only to 15V and then stay at 15V afterwards. So I feel VCC is clamped by internal circuitry of the controller. Which part (or function) of the internal circuitry clamps VCC?

Best regards.

Jianping

0 Hong Huang over 4 years ago in reply to Jianping Fan

TI__Mastermind 47655 points

The difference between UCC256402 and UCC256404 are listed in the datasheet page 3. It is ok you use DC input on J3.

UCC256402 should be ok with HV connect to DC.

If VCC (UCC256402 and UCC256404) cannot reach 26V, the IC cannot start.

Between VCC and RVCC there is an LDO. Do you have how much load on RVCC?

I do not think it is latched by internal circuit at very initial start - so you may probe very initial VCC to see it can reaches 26V.

I think you can use the EVM to find out all correct waveforms also compare your circuit to the EVM to see if anything different and possible something needed to change.

0 Hong Huang over 4 years ago in reply to Hong Huang

TI__Mastermind 47655 points

I noticed you said VCC has a 22uF - is it the only component on VCC? Do you have a bias circuit to VCC? You can refer to the EVM VCC bias circuit and compare to yours.

0 Jianping Fan over 4 years ago in reply to Hong Huang

Prodigy 120 points

Hong:
thanks for your advice. I captured the VCC waveform over the whole start up process. After DC input voltage is applied, VCC rises to 15V monotonically and stays flat all the time. RVCC is constantly 0V, I believe it is most likely because VCC has never reached 26V.

VCC has a bias supply from a NPN transistor with E connected to VCC, C connected to the cathode of the rectifier from bias winding and a 10uF bypass capacitor, and B set to 15V with a 15V Zener diode.

RVCC is conected with a 4.7uF bypass capacitor, supplying to VDD of two UCC27524 driver chip and a resistor divider for LL/SS pin.

The voltage level at BLK pin is about 3.5V. I have downloaded the EVB user guide and studying in details.

Please let me know if you see other other clues and need more information of the design.

Have a good night.

Jianping

0 Hong Huang over 4 years ago in reply to Jianping Fan

TI__Mastermind 47655 points

Can you provide your schematics for me to review? I need to review your schematics to see if there are something needed to modify. UCC256402 should be ok to work with DC input.

0 Jianping Fan over 4 years ago in reply to Hong Huang

Prodigy 120 points

Hi, Hong:

I can send our schematics to you. But your customer support advice me on early mail that E2E is a public forum. Be mindful of the proprietary information that it might be sharing publicly. Attached please find your customer support email as below for your reference. In the meanwhile, Could your please send me your TI email address to me and then I can send our schematics to you.

Jianping

hank you for contacting TI. We appreciate your business.

Your case CS0136313 has been updated. In order to provide your response, we encourage you to click this LINK or send an email by replying to this email notification. Thank you.

Short description: UCC256402 Converter design
Update:

Hello Jianping,

Thank you for contacting TI Customer Support and for choosing our products as part of your solution. This is to acknowledge the receipt of your support request.

I've read the transcript of your conversation with my colleague earlier and I understand that you are having start-up issues with your UCC256402 design.

We have checked your schematic design and as our evaluation, this issue would be best addressed by our product apps engineers handling our E2E support forum. They provide the best TI product practices and design methodologies which are beyond the device's documentations. Through the forum, you will be directly engaging with them and all the needed diagnosis on your UCC256402 desigin will be shared to you.

Here's the link to add your post.

https://e2e.ti.com/p/addpost

Please note that E2E is a public forum. Be mindful of the proprietary information that you might be sharing publicly. At best, please do post an initial description of the issue. And if in case our apps specialist would need the schematic for full evaluation, they will inform you of some other mediums where they can continue their support.

Once you have posted, please share with me the link so I could follow-up our product team for you to have the timely response.

Thank you for understanding our limitations here in the Support Center. We will be waiting for your updates.

Kind Regards,

Jejomar Ildefonso

Texas Instruments Customer Support

Original details: Customer full chat inquiry:

" I have a start up problem with UCC256402 in a DC/DC converter design.

When I apply input voltage, VCC rises to about 15V and stay flat, but RVCC stays at zero. BLK volatge is >3V.

I design the circuit based on the datasheet information. Most of my designs with TI controller were successful without referencing to eva module."

0 Hong Huang over 4 years ago in reply to Jianping Fan

TI__Mastermind 47655 points

hong_huang@ti.com

0 Hong Huang over 4 years ago in reply to Hong Huang

TI__Mastermind 47655 points

Can you check if your Q9 is in regulation which it looks Z1 is on 15V so Q9 emitter to VCC is 15V? If so, your converter is actually already start?

You can probe VCC with an oscilloscope before power on then power on, you should see VCC reaches its start threshold. Otherwise, you can try remove Q9 to see if VCC still stays 15V or not.

I suggest you get an EVM, change the IC to UCC256402, then tie TP14 to TP1, and apply DC voltage to J1-J2, to help your board debugging.

0 Hong Huang over 4 years ago in reply to Hong Huang

TI__Mastermind 47655 points

Sorry, tie TP14 to TP1 to work with DC input on J1-J2.

0 Jianping Fan over 4 years ago in reply to Hong Huang

Prodigy 120 points

Hi, Hong:

How I can get an EVM? There are three EVM on TI web. Which one will be use for me? Please advise.

Product matches

Part number	Status	Description	Product family
ESDEVM	ACTIVE	Generic ESD Evaluation Module	ESD protection & TVS surge diodes
EVMK2G	ACTIVE	66AK2Gx (K2G) Evaluation Module	C6000 DSP + Arm
EVMK2H	ACTIVE	66AK2H Evaluation Modules	C6000 DSP + Arm
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0 Hong Huang over 4 years ago in reply to Jianping Fan

TI__Mastermind 47655 points

The board part number is

UCC25640EVM-020

0 Jianping Fan over 4 years ago in reply to Hong Huang

Prodigy 120 points

Thanks, Hong.

Because RVCC in not established yet and the controller has not entered switching state yet, There is no supply from the transformer bias winding. I will double check and remove Q9 to see if anything changes, and will let you know. I will consider to get an EVB for my reference. How I can get EVB or EVM?

In meanwhile, Should I tie TP1 to TP14 or TP12?

Best regards.

Jianping fan

0 Jianping Fan over 4 years ago in reply to Hong Huang

Prodigy 120 points

Hi, Hong:

I am going to get an EVM for reference. Can you provide a EVM type for my test? Attached a EVM picture from TI web. I am not so sure if this is correct one.

Jianping

0 Jianping Fan over 4 years ago in reply to Hong Huang

Prodigy 120 points

Hi, Hong:

I disconnected the bias supply transistor Q9. The start up behavior is still the same - VCC rises to 15V and stay flat, and RVCC stays at zero volt all the time. It looks that VCC is clamped by internal circuitry of UCC256402. I noted two signals in the IC block diagram that could be related - VCCClampEn and RVCCEn. I appreciate if you can provide details of the functionality these two signals, especially the operating conditions for enabling and disabling them. It might be very helpful for me to further debug the problem.

Have a great weekend.

Jianping

0 Jianping Fan over 4 years ago in reply to Hong Huang

Prodigy 120 points

Hi, Hong:

Have a great weekend.

Jianping

0 Jianping Fan over 4 years ago in reply to Hong Huang

Prodigy 120 points

Hi, Hong:

Have a great weekend.

Jianping

0 Hong Huang over 4 years ago in reply to Jianping Fan

TI__Mastermind 47655 points

The IC VCC has to reach to its turn on threshold before enable the internal circuit.

You circuit has additional connections to the VCC you need to disconnect all of them to try. If you still cannot get VCC to reach its threshold, you can change to a new IC as the IC on your board may be damaged.

You need to have an EVM to help your debugging, as you circuit may have something else wrong. Again, The IC VCC has to reach to its turn on threshold before enable the internal circuit. It looks like your external circuit has something clamp the VCC or the IC on your board is already damaged. If you have an EVM, it is easy to test to find out if you put your IC on the EVM to find out if it is bad or you board has something not good.

0 Jianping Fan over 4 years ago in reply to Hong Huang

Prodigy 120 points

Hi, Hong:

How I can get a free shipping coupon code. Attached below from E2E support. I followed the below instruction to click this resolved my issue but there is not a coupon code. I am working on order the EVM now.

Jianping Fan

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