UCC256403: Electrostatic Discharge Immunity Test During Operation

Hi Kurata San,

Which 1sec fault is getting triggered for this case?

Regards,

Sougata

UCC256403_Electrostatic Discharge Immunity Test During Operation　　　2025.08.05

Reply 1

Electrostatic discharge tests were conducted on prototype power supply knit products using this device.

When static electricity was applied to the front panel of the power supply unit, the IC's 1-second shutdown protection circuit malfunctioned, temporarily halting power output before restarting.

This malfunction undesirably triggered an alarm lamp and an alarm signal from the power supply system.

Since this system is designed for continuous operation, a malfunction caused by static electricity could not be tolerated.

As a countermeasure, we increased the capacitance of the filter capacitors on the BLK and Isns pins of the UCC256403 and shorted the BW pin to GND to disable overvoltage protection, but this did not improve the reset phenomenon observed during the electrostatic discharge test.

Furthermore, to prevent electrostatic noise from entering, we strengthened the filtering by inserting a low-resistance resistor (10 Ω) or ferrite bead near the Vcc pin's printed wiring pattern, and by inserting a ferrite bead near the GND pin's pattern.

However, the results showed almost no improvement, and we are unable to come up with an effective solution as we are unable to identify the route through which the noise that is causing the IC to malfunction is entered, so we are at a loss as to what to do.

Please let us know the cause and mechanism behind the malfunction caused by this electrostatic discharge test, as well as any fundamental ways to improve the situation.

Thank you in advance for your consideration.

Dear Ti Sougata,

This is Kurata.

Regarding your initial inquiry, you asked, "Which second triggered this?" We have only been able to confirm the issue when we performed an electrostatic discharge test, where the LLC output stopped for one second and then automatically recovered.

Based on the pin function descriptions in the device specifications, we believe the one-second pause and automatic recovery is due to the reset function for the one-second pause, triggered by one of the following protection functions: overcurrent protection for the Isns pin, overvoltage protection for the BW pin, UVLO operation for the BLK pin, or UVLO operation for the Vcc pin. In our current investigation of this issue, the BW pin is shorted to GND, so we believe the overvoltage protection from the BW pin is not affecting it.

Also, when monitoring the voltage drops on the BLK and Vcc pins with an oscilloscope, there was no voltage drop that would trigger UVLO during the one-second pause. Therefore, we believe that the noise entering the remaining Isns pin cannot be suppressed, causing the malfunction that causes the one-second outage, but we are not certain and do not understand the detailed design inside the IC, so we are currently trying to find ways to improve the circuit connections outside the IC by trial and error, and no solution has been found.

Therefore, based on your experience at the design and evaluation level, we would like you to advise us on what remedial measures are possible, whether the most likely cause is a malfunction of the Isns pin, or whether there are other possible causes of the malfunction.

Hi Kurata,

I had never experienced this type of failure previously.

To detect 1 sec fault, can you please check all the pins voltages at the point of 1sec fault to understand which particular fault is triggering?

Regards,

Sougata