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From: Raveninghorde on 27 Jan 2010 16:16
On Wed, 27 Jan 2010 10:30:35 -0800 (PST), Wimpie <wimabctel(a)tetech.nl>
wrote:

>On 27 ene, 16:23, Raveninghorde <raveninghorde(a)invalid> wrote:
>> On Fri, 15 Jan 2010 13:43:02 +0000, Raveninghorde
>>
>> <raveninghorde(a)invalid> wrote:
>>
>> >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.
>
>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.
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