TPS92410EVM-002: Top LED string is not working.

Punsara Mahawela

Part Number: TPS92410EVM-002

I have designed a custom TPS92410EVM-002. In there top LED (160V) string is not working.
When the board is powered up it goes to the OVP after a few seconds.

I removed the over voltage protection by connecting the DOV pin of TPS92410P to the GND for the testing purposes.
Changed the R25(RCS) to 60Ohm which limits the avg current to 20mA.(originally it had a 30Ohm resistor and problem is there in both cases)

I checked the bottom and middle LED strings and they are working properly.
I removed the Q3 BJT and gave 14V(across VIN and VS) to the top floating switch from a separate power supply to make sure U1 is always powered up.
I have attached the waveforms captured around the top switch.

waveform across GND and VS1.

waveform across VDS of Q1

waveform across R6.

VGS = 200mV in Q1.

1) What could be the problem of my design?

2) Please explain how the Q1 mosfet should be working in a normal setup?

over 1 year ago

0 Steven Li over 1 year ago

TI__Mastermind 22830 points

Hi Punsara,

I am currently out of office. I will reply to you tomorrow. Thanks for your understanding.

Best Regards,

Steven

0 Punsara Mahawela over 1 year ago in reply to Steven Li

Prodigy 40 points

Sure, Thanks Steven.

0 Steven Li over 1 year ago in reply to Punsara Mahawela

TI__Mastermind 22830 points

Hi Punsara,

Could you provide your input voltage and LED voltage & current specs (The I-V curve of the LED is preferred.)?

Also, could you provide your schematic and tell me what do components like Q1 and Q3 correspond to? (Shall I refer to the EVM schematics?)

Best Regards,

Steven

0 Punsara Mahawela over 1 year ago in reply to Steven Li

Prodigy 40 points

HI Steven,

input voltage = 230VAC

LED forward voltage = 6V

Since there 20V LEDs were not available I made the strings using 6V LEDs. Datasheets are not available for those LEDs. But I prepared a graph after testing those LEDs.

Number of LEDs each string contains are as following,

bottom = 7

middle = 14

top = 26 / 27 (tested for both, but didn't work)

Yes, for the schematics please refer to the EVM.

example of working middle and bottom strings.

0 Steven Li over 1 year ago in reply to Punsara Mahawela

TI__Mastermind 22830 points

Hi Punsara,

I am currently out of office due to Chinese Qing Ming Festival. I will reply to you this Sunday. Thanks for your understanding.

Best Regards,

Steven

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TPS92410EVM-002: Top LED string is not working.