PMP40766: How to test the constant voltage and constant current of the PMP40766 post-stage LLC

Yinzhang Peng

Part Number: PMP40766
Other Parts Discussed in Thread: UCC256403

Hi，

May I ask how to test the voltage loop and current loop of the post-stage LLC of PM40766 respectively? When I am testing the voltage loop now, I have soldered the components of the main circuit, the voltage loop and the overvoltage protection part. The components of the current loop are not soldered. During the test, I replaced D10 with a single-phase diode to connect the optocoupler and U7A. To stabilize U7, I soldered a 0-ohm resistor at the position of C33, making U7B a voltage follower. During no-load and load tests, the output is 0. After testing, the MOS tube has no drive. The RVCC pin voltage of UCC256403 is 13V, the VCC pin is 14.5V, and the BLK voltage is 3V, which are normal values. The BW pin voltage is 0V, and the FB pin voltage is 6.7V. I think the voltages of these two pins are not normal. I don't know how to solve it, and I hope your company can help me. Is there a problem with my testing method? If all the components are soldered, will there be any output voltage during no-load operation later? When a pure load is connected later, does only the voltage loop work? Please answer these questions for me.

Thanks.

2 个月前

0 Lydia 2 个月前

TI__Mastermind 29705 points

您好，已经收到了您的案例，调查需要些时间，感谢您的耐心等待。

0 Yinzhang Peng 2 个月前

Prodigy 60 points

The LL-SS pin remains at 1.26v continuously.

0 Yinzhang Peng 2 个月前回复 Lydia

Prodigy 60 points

I look forward to your prompt reply. Thank you.

0 Vivian Gao 1 个月前回复 Yinzhang Peng

TI__Mastermind 37140 points

You need to populate the entire feedback circuit for both CV and CC loops. C33 is part of the CC loop, not CV loop. You should see ~71.4V during no load operation.

0 Yinzhang Peng 1 个月前回复 Vivian Gao

Prodigy 60 points

Yes, I have already soldered both the constant voltage (CV) and constant current (CC) loops. During no-load operation, the output voltage is 71.4V. When a 30-ohm pure resistive load is applied, the circuit operates normally with the red indicator light on, the fan running, and the output voltage remaining stable. However, when the load is changed to 20 ohms (approximately 254W), the fan stops, the output voltage drops to 0V, and even after switching back to a 30-ohm load or no load, the output voltage remains at 0V. Upon inspection, the voltage at the LL/SS pin of the UCC256403 is measured at 1.26V. After replacing the chip, the output voltage returns to normal and the fan starts again. What could be the cause of this issue? Additionally, is the UCC256403DDBR a latching-type chip? I would appreciate it if an expert could clarify these questions for me. Thank you.

0 Vivian Gao 1 个月前回复 Yinzhang Peng

TI__Mastermind 37140 points

Is the PFC operating correctly and providing ~390V to the LLC? If it is not, the LLC will not operate correctly.

UCC256403 is not a latched-fault controller.

0 Yinzhang Peng 1 个月前回复 Vivian Gao

Prodigy 60 points

The pre-stage PFC can provide a voltage of 390V

0 Vivian Gao 1 个月前回复 Yinzhang Peng

TI__Mastermind 37140 points

Thanks for the additional info. Could you look at the LLC switch node on an oscilloscope and check if the LLC is in burst mode with 30Ω load?

0 Yinzhang Peng 1 个月前回复 Vivian Gao

Prodigy 60 points

When the load is 20 ohms, the output voltage of the front stage is unstable, fluctuating between 360V and 390V

0 Vivian Gao 1 个月前回复 Yinzhang Peng

TI__Mastermind 37140 points

Could you try disabling the LLC for now and load the PFC directly? This will help isolate where the problem is on your board.

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PMP40766: How to test the constant voltage and constant current of the PMP40766 post-stage LLC