LM51561-Q1: problem with Webench schematic, max current and efficiency

Jurgen Stelbrink

Part Number: LM51561-Q1
Other Parts Discussed in Thread: LM5156

I have created the schematics with Webench and the following settings:

Vin: 12V

Vout: 48V (at 1.5A)

I have noticed that Ccomp2 capacitor is 100nF. This is expremely high. In the demo board schematics you use 22pF. I have 2 problems:

1. I only get a max current of 0.7A. At higher currents the voltages drops significantly (37V at 1A)

2. I have very high losses (approx. 30%). The heat is generated in the power magnetics and the MOSFET.

I have tried two magnetics: SER1390-103ML and SER1390-473ML. I am using the BSC252N10NSFGATMA1 MOSFET.

Can it be that the MOSFET is enabled too long - wrong timing? Is the gate charge of the MOSFET critical?

Thanks, Jurgen

over 4 years ago

0 Yinsong Cai1 over 4 years ago

TI__Expert 7450 points

Hi Jurgen,

Thank you for using LM5156.

Do you use the same schematic as the Webench design you show here for your hardware? I can help review the schematic.

Thanks,

Yinsong

0 Jurgen Stelbrink over 4 years ago in reply to Yinsong Cai1

Prodigy 80 points

Hi Yinsong,

I have follwowed the Webench schematics closely. I just added the Vcc diode to 12V to be able to externally power the device. My diodes D5 and D19 are a little weak. I am addressing that now.

I have tried to remove the gate resistor R49, but no change. I also changed the current sense resistor R5 from 15mOhm to 10mOhm. Also no change.

Best regards, Jurgen

0 Yinsong Cai1 over 4 years ago in reply to Jurgen Stelbrink

TI__Expert 7450 points

Hi Jurgen,

Thank you for the information. I'll give a quick review to see if there is anything not good and let you know.

Thanks,

Yinsong

0 Yinsong Cai1 over 4 years ago in reply to Yinsong Cai1

TI__Expert 7450 points

Hi Jurgen,

The schematic looks fine. Do you use internal or external VCC?

Thanks,

Yinsong

0 Jurgen Stelbrink over 4 years ago in reply to Yinsong Cai1

Prodigy 80 points

Hi Yinsong,

we have tried both, with internal and with external Vcc. No change. The latest tests were performed with internal Vcc.

Observations:

1. best case we get an efficiency of 85%. This is a lot less than the spec of 95%.

2. at Vin = 24V we can source up to 1.5A at 85% efficiency.

3. at Vin = 12V we can only source 0.7A. At 1.5A the output voltage drops down to 22V. And the efficiency to less than 50%.

Please help. We need to resolve this quickly.

Kind regards, JurgenTest_Report_LM5156.xlsx

0 Yinsong Cai1 over 4 years ago in reply to Jurgen Stelbrink

TI__Expert 7450 points

Hi Jurgen,

1. Just to clarify, there is no efficiency spec for LM5156 since it's just a controller. The efficiency depends on the design and external FETs and diode used.

2. It sounds like the current limit issue. To debug the issue, please show the FB, COMP, CS, and SW waveforms at whatever load when the Vout drops below the target.

Thanks,

Yinsong

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LM51561-Q1: problem with Webench schematic, max current and efficiency