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Fault with LM5109 - High Side pulse issue

Aslesh Kumar

Jul 30, 2021
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Hi,
I am working on LM5109 gate driver for a buck converter operation. The gate driver circuit is providing appropriate pulses when connected on no load operation. But, the moment load is connected, high side pulse is missing, but low side is good enough. My bootstrap capacitor voltage is around 2 volts, while it should be 12 volts. My bias capacitor is also 10 times higher than bootstrap. I tried by reducing input resistance, gate driver resistance, it didn't help much. It'd be helpful if anyone can suggest the possible solution.
 

bertus

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Nov 8, 2019
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Hello,

Can you post the actual schematic?
Keep the size of the image around 300 kB.

Bertus
 

Aslesh Kumar

Jul 30, 2021
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Here is the driver schematic. As I said, the problem is with the bootstrap capacitor C10 in the picture
Thank you.
 

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  • LM5109.JPG
    LM5109.JPG
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bertus

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Hello,

Remove the 4.7 Ohms resistor from the bootstrap line.
It will likely slow down the charging of the bootstrap capacitor.

Bertus
 

Aslesh Kumar

Jul 30, 2021
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Hi Bertus,
I replaced it with a jumper resistor 0 ohms, Not helping. I The load point voltage is 30 volts, When I trigger the high side MOSFET, the voltage at HS pin is 30 V, causing negative transient voltage at bootstrap capacitor ( Vcc = 12V, Vhb = 30V). Is there any solution to this? I am experiencing this for the first time.

Aslesh Kumar
 

bertus

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Hello,

How are the mosfets connected?
You have not drawn them in your circuit.

Bertus
 

Aslesh Kumar

Jul 30, 2021
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I have added the converter circuit. Its a simple buck converter. the U1, U8 and U12 are parallel capacitors. 100 V/60 V conversion
 

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  • buck.JPG
    buck.JPG
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bertus

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Hello,

As you only posted two partial schematics, I have "connected" them as I would see it.
aslesh LM5109 full comment.JPG
There also can be a problem due to the inductive load, that causes spikes.

Bertus
 

bertus

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Hello,

What is the exact version number of the LM5109?
The LM5109 has a maximum voltage of 100 Volts.
The LM5109B has a maximum voltage of 90 Volts.

Bertus
 

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  • lm5109.pdf
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  • lm5109b-q1.pdf
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Aslesh Kumar

Jul 30, 2021
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Hello,
It's LM5109B which can operate on 90 Volts.. It is not due to the inductive spikes. I can operate the system for V input up to 20 volts. I could vary the output voltage from 1 to 20 volts based on duty cycle.. but higher than 20 volts... I can see the driver is burning up.
 
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