Problem with LM3150

[Raveninghorde]

I'm debugging a buck converter using an LM3150 simple switcher.

The switching FET is an N type, FDP060AN08A0 and gets hotter than
anticipated. The gate drive voltage is generated using a 470n X7R
boost capacitor as specified in the data sheet.

However the gate-source pulses are only about 4.5V peak which means
the FET is not being switched on hard enough.

4.5V gate drive appears wrong but as yet I haven't a clue why.

 

[Raveninghorde]

Post your schematic?

John

http://www.zen88234.zen.co.uk/design/Output-2.png

I am concerned that the gate/source voltage of Q2 is too low. However
it is governed by the LM3150.

 

[Raveninghorde]

After looking at that chip lay out, it appears that all you're going to
get is no more than 6 Volts to be driven to the FET on the high side.
Maybe a different choice of fet?

You could perform a bypass via a diode from a higher supply voltage to
the BST pin. Essentially, you'd be able to supply the driver and Cbst
cap with more voltage than what the rest of the chip has.

A basic SI diode with cathode to BST and anode from a higher source.
maybe the Vin pin could be used as the source.. It would then be a
simply by pass ..

The internal diode that resides there now, will block the voltage from
back tracking..

Just a thought.

Thanks for the responses everyone. Looks like I have enough to do on
Monday morning.

 

[Jamie]

I'm debugging a buck converter using an LM3150 simple switcher.

The switching FET is an N type, FDP060AN08A0 and gets hotter than
anticipated. The gate drive voltage is generated using a 470n X7R
boost capacitor as specified in the data sheet.

However the gate-source pulses are only about 4.5V peak which means
the FET is not being switched on hard enough.

4.5V gate drive appears wrong but as yet I haven't a clue why.

After looking at that chip lay out, it appears that all you're going to
get is no more than 6 Volts to be driven to the FET on the high side.
Maybe a different choice of fet?

You could perform a bypass via a diode from a higher supply voltage to
the BST pin. Essentially, you'd be able to supply the driver and Cbst
cap with more voltage than what the rest of the chip has.

A basic SI diode with cathode to BST and anode from a higher source.
maybe the Vin pin could be used as the source.. It would then be a
simply by pass ..

The internal diode that resides there now, will block the voltage from
back tracking..

Just a thought.

 

[legg]

On Fri, 15 Jan 2010 11:40:26 -0800, John Larkin

http://www.zen88234.zen.co.uk/design/Output-2.png

I am concerned that the gate/source voltage of Q2 is too low. However
it is governed by the LM3150.

You are correct. The bootstrap supply is a regulated 5V. You must use
mosfets designed for this kind of drive level. The FDP060AN08 is not.

RL

 

[Jamie]

Hello,

I am afraid, Legg and James are right; the gate drive voltage of the
LM3150 isn't compatible with your MOSFETs.

I overlooked the fixed 6 V regulator for generating the boost
voltage. This limits your MOSFET selection to "low gate drive"
devices only.

Regarding the body diode, it must be very fast (or bypassed with a
schottky rectifier). I have some bad experience with similar type of
step down converters (even with evaluation boards directly from the
manufacturer). They broke down under short circuit condition. Current
glitches (because of diode reverse recovery) were responsible.

Best regards,

Wim
PA3DJS
www.tetech.nl
without abc in the address, PM will reach me.

I don't think its a good idea to depend on the body diode for
protection of the device. I prefer to use a schottky diode in its
place.

 

[Raveninghorde]

I'm debugging a buck converter using an LM3150 simple switcher.

The switching FET is an N type, FDP060AN08A0 and gets hotter than
anticipated. The gate drive voltage is generated using a 470n X7R
boost capacitor as specified in the data sheet.

However the gate-source pulses are only about 4.5V peak which means
the FET is not being switched on hard enough.

4.5V gate drive appears wrong but as yet I haven't a clue why.

Following the suggestions here we've changed the FET to a logic level
one, SPP80N06S2L-07 which was the only one of the right
characterisitcs that farnell had in stock.

We have also added a schottky in parallel with the lower transistor.

The switching FET still gets too hot, dissipating around 3 Watts. The
gate drive rise and fall times are too slow. We have added a schottky
diode and transistor to the gate drive to speed up switch off which
helps.

So far it looks like the LM3150 doesn't have the drive capability to
drive FETs directly.

 

[Raveninghorde]

Hello,

Did you do hand calculations regarding the switching loss based on
your measurements (to make sure this is the problem)?

Your MOSFET needs about 6 times more gate charge then the one used by
National (see page 15 chapter 7 for details about gate charge
limitation versus frequency), so maybe you are right about the gate
drive capability of the LM3150 in your application.

I hope you will find the right answer why your circuit behaves
differently then expected.

Best regards,

Wim
PA3DJS
www.tetech.nl
in case of PM, please remove the obvious 3-letter combination.

Cheers. You made me look at the right part of the spec.

The problem is gate charge. The LM3150 needs a lower Qgd FET than the
one I'm using.

Just ordered something better from Digikey.