LM5116: Does LM5116 support External gate driver

Li hongsheng

Part Number: LM5116

My application needs high voltage input, like 50-300Vc and output 28Vdc, 4A. I selected LM5116 because of its synchronous rectifier and high efficiency. Since its max input is 100Vdc, I added external gate driver in my application. SW is connected to GND for that reason. I have attached the schematic. There is other battery(25V) connected to the output in my system. So I need LM5116 starting normally with pre-biased voltage and not consume the battery.

Questions are

1. Is Diode Emulation still supported if DEMB connected to ground through a 10K resistor like schematic, like in start up and light load?

2. Does adaptive deadtime feature still work since SW is grounded?

If those above two stop working, does my circuit work as expected except the efficiency? If so, how much efficiency I lost?

The third question, can I use GaN Mosfet to replace SIC Mosfet in circuit? If so how to calculate the switch loss of GaN Mosfet? The switch loss in my application is very high per page 27 of datasheet.

Thanks

Please ignore +18V_Start, it should be 15V.

over 1 year ago

0 EricLee over 1 year ago

TI__Guru 58851 points

Hello Li

1. Unfortunately, it is not. Diode emulation function works when the device is directly connected to the MOSFETs without the external gate driver.

1. Unfortunately, it is not. Adaptive dead-time function works when the device is directly connected to the MOSFETs without the external gate driver.

The third question is related with the external gate driver which you will use.

-EL

0 Li hongsheng over 1 year ago in reply to EricLee

Prodigy 30 points

Hey Eric,

Does my circuit work as expected except the efficiency? If so, how much efficiency I lost?

I believe it works since it is configured as full synchronous.

The questions are

1. Does it work as pre-biased output voltage?

2. How much efficiency lost?

3. Do you know the switching loss is different between standard Mosfet and GaN? Per page 27 of lm5116 datasheet. The switch loss is related to Vin, Io, frequency and tR and tF. I didn't see any better tR and tF in GaN compared to my Mosfet, C3M0065090J. or different equation is used for GaN mosfet switch loss?

0 EricLee over 1 year ago in reply to Li hongsheng

TI__Guru 58851 points

Hello Li

1. No, the diode emulation should work properly during the start-up to cover the pre-biased start-up condition, but the diode emulation function works properly when the device is directly connected to the MOSFETs without the external gate driver.

2. I'm not sure how much efficiency you can get by using the IR2113 gate driver.

3. I haven't seen a good article which explains GaN MOSFET switching loss. Please refer the efficiency calculator of GaN manufacturer. https://epc-co.com/epc/design-support/tools-and-calculators/gan-power-bench/gan-fet-selection-tool-for-buck-convertors

-EL

0 Li hongsheng over 1 year ago in reply to EricLee

Prodigy 30 points

Eric, Sorry I didn't get email which indicate you replied for unknow reason.

1. What happened if pre-biased output voltage there, won't startup?

0 EricLee over 1 year ago in reply to Li hongsheng

TI__Guru 58851 points

Hello Li

Then, the current can flow back from the pre-bias power supply to the converter using LM5116+external gate driver.

-EL

0 Li hongsheng over 1 year ago in reply to EricLee

Prodigy 30 points

Eric,

It only can flow back when bottom Mosfet turn on per LM5116 datasheet. It means efficiency lost or LM5116 won't start up when pre-biased (output have 25V for example)?

0 EricLee over 1 year ago in reply to Li hongsheng

TI__Guru 58851 points

Hello Li

That happens only during the start-up with pre-biased condition. After the start-up, the efficiency will be back to normal value.
LM5116 will be able to start-up if there is no damage caused by the back-flow current. In general, the output of the pre-bias supply drops due to the back-flow current and allows the converter (using LM5116) to start-up.

-EL

0 Li hongsheng over 1 year ago in reply to EricLee

Prodigy 30 points

Eric,

My case is complicated. Its output paralleled with Battery output. Battery has serial diodes while LM5116 output doesn't have serial diodes. Any suggestions?

0 EricLee over 1 year ago in reply to Li hongsheng

TI__Guru 58851 points

Hello Li

If your pre-biased supply is a battery, it means there is no current limiting. How about using ORing controller ? https://www.ti.com/power-management/power-switches/ideal-diodes-oring-controllers/overview.html . If you have a question about the ORing controller, please create a new post.

-EL

0 Li hongsheng over 1 year ago in reply to EricLee

Prodigy 30 points

Eric,
It is complicated in my application. Current limiting is not necessary on LM5116 when battery is there.

I try to make sure the buck circuit is working when battery is there and no damage to battery.

0 EricLee over 1 year ago in reply to Li hongsheng

TI__Guru 58851 points

Hello Li

Since the system is complex, it might be better to measure how much reverse-current flows during the start-up. I guess you can judge based on the amount of reverse current.

-EL

0 Li hongsheng over 1 year ago in reply to EricLee

Prodigy 30 points

Thanks Eric! My board is still in design phase. I will update when I get board.

I just worry about it doesn't work since the prebias voltage.

0 EricLee over 1 year ago in reply to Li hongsheng

TI__Guru 58851 points

Hello Li

As I mentioned, please consider ORing controller www.ti.com/.../overview.html if you worry about the reverse-current flow.

-EL

Power management

Power management forum

LM5116: Does LM5116 support External gate driver