LM5116: LM5116 no load situation

Chen Tse-Kai

Part Number: LM5116
Other Parts Discussed in Thread: CSD19534Q5A

Dear Sir,

When I use the Lm5116 IC, I found that if there is no load, the voltage will drop directly from the 24V I preset to about 3.6V, but it is normal when there is a load.

During the observation, it was found that the inductor has a reverse current that causes the high side diode to turn on and low side turn off. At this time, the bootstrap cap cannot be charged, and the output

slowly becomes 3.6V. But I don’t know what caused this situation.

Vin=30-60V

Vout=24/2A

Fsw=200k Hz

LM5116 Circuit-Dylan.pdf

When I tried to change the low side resistor from 33 ohm to 0 ohm, the output 24V does not fall down.

Best Regards,

Dylan

over 4 years ago

0 Timothy Hegarty over 4 years ago

TI__Guru 73752 points

Hi Dylan,

The low-side gate resistor should be 0 Ohms. Also, you can reduce the boot cap to 100nF to make it easier to refresh its voltage at light loads.

Regards,

Tim

0 Chen Tse-Kai over 4 years ago in reply to Timothy Hegarty

Prodigy 10 points

Is this situation caused by the inability of CHB to discharge completely?

0 Chen Tse-Kai over 4 years ago in reply to Chen Tse-Kai

Prodigy 10 points

I have tried to remove the resistance of the low side that the low side only has a diode, and the same situation will happen.

0 Timothy Hegarty over 4 years ago in reply to Chen Tse-Kai

TI__Guru 73752 points

The lower value should make it easier to keep the boot cap charged.

0 Chen Tse-Kai over 4 years ago in reply to Timothy Hegarty

Prodigy 10 points

Hi Timothy,

Thanks for your reply.

1. I have replaced 100nF but still have the same situation.
2. I also used a differential probe to measure VHB-VSW, the initial voltage was stable, until the output voltage started to drop, the VHB-VSW voltage also decreased...

3. I have also tried to remove the low side Rg resistance, and the same situation will occur.

Warm Regards,

Dylan

+1 Timothy Hegarty over 4 years ago in reply to Chen Tse-Kai

TI__Guru 73752 points

Hi Chen,

Please send the MOSFET part numbers. In particular, check the gate voltage Miller plateau as it should be < 4V (this is indicated by the flat level in the the gate charge vs. gate votlage curve). Note that during startup, during the changeover from VOUT to VCCX, the gate drive will operate at the changeover threshold of 4.5V, which may be insufficient for the MOSFETs.

Also, please send a completed LM5116 quickstart calculator so we can review compensation etc.

Regards,

Tim

0 Chen Tse-Kai over 4 years ago in reply to Timothy Hegarty

Prodigy 10 points

Hi Timothy,

BSC160N15S5

Infineon-BSC160N15NS5-DataSheet-v02_03-EN.pdf

Dylan

+1 Timothy Hegarty over 4 years ago in reply to Chen Tse-Kai

TI__Guru 73752 points

Hi Dylan,

This MOSFET has a 6V Miller plateau (see diagram 14 in the datsaheet). This is quite high, mainly because this is a 150V FET (typically there should be no need to choose a FET more than the rated voltage of the controller). Can you try a 100V FET (for example, CSD19534Q5A) with a MIller plateau less than 4V. Also, note the VCC voltage pulls down to the 4.5V changeover threshold during the transition from internal VCC to the 12V supply as it starts up.

Regards,

Tim

+1 Chen Tse-Kai over 4 years ago in reply to Timothy Hegarty

Prodigy 10 points

Hi Timothy,

Thank you for your tracking and suggestions these days, I will try.

Best Regards,

Dylan

0 Timothy Hegarty over 4 years ago in reply to Chen Tse-Kai

TI__Guru 73752 points

Thanks, Dylan.

Tim

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LM5116: LM5116 no load situation