Problem with LM3150
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From: John Larkin on 15 Jan 2010 18:44 On Fri, 15 Jan 2010 21:59:55 +0000, Raveninghorde <raveninghorde(a)invalid> wrote: >On Fri, 15 Jan 2010 11:40:26 -0800, John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> 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. >> >>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. Could the fet be fried? Maybe add a small series gate resistor and see if there's any gate current? John
From: Raveninghorde on 15 Jan 2010 18:54 On Fri, 15 Jan 2010 18:57:44 -0500, Jamie <jamie_ka1lpa_not_valid_after_ka1lpa_(a)charter.net> wrote: >Raveninghorde 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. > 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.
From: dagmargoodboat on 15 Jan 2010 19:03 On Jan 15, 6:57 pm, Jamie <jamie_ka1lpa_not_valid_after_ka1l...(a)charter.net> wrote: > Raveninghorde 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. > > 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. Correct. The high-side drive is bootstrapped by (Vcc - a diode drop). Since Vcc is internally set to +6v, Vgs(hi side) will be ~5v, max. > Maybe a different choice of fet? Yes. > 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. Not allowed. V(BST to SW) must be under 7v, absolute max. -- Cheers, James Arthur
From: legg on 15 Jan 2010 22:18 On Fri, 15 Jan 2010 21:59:55 +0000, Raveninghorde <raveninghorde(a)invalid> wrote: >On Fri, 15 Jan 2010 11:40:26 -0800, John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > <snip> > >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
From: Robert Baer on 16 Jan 2010 03:44
Raveninghorde 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. ....a 9V gate drive would guarantee a decent Rds(on) with a cold (-55C) FET or a hot (+125C) FET. |