Problem with LM3150
in [Design]
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From: Wimpie on 16 Jan 2010 06:37 On 15 ene, 22:59, Raveninghorde <raveninghorde(a)invalid> wrote: > On Fri, 15 Jan 2010 11:40:26 -0800, John Larkin > > > > <jjlar...(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. 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.
From: Jamie on 16 Jan 2010 10:16 Wimpie wrote: > On 15 ene, 22:59, Raveninghorde <raveninghorde(a)invalid> wrote: > >>On Fri, 15 Jan 2010 11:40:26 -0800, John Larkin >> >> >> >><jjlar...(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. > > > 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.
From: Wimpie on 16 Jan 2010 12:02 On 16 ene, 16:16, Jamie <jamie_ka1lpa_not_valid_after_ka1l...(a)charter.net> wrote: > Wimpie wrote: > > On 15 ene, 22:59, Raveninghorde <raveninghorde(a)invalid> wrote: > > >>On Fri, 15 Jan 2010 11:40:26 -0800, John Larkin > > >><jjlar...(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. > > > 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. Hello Jamie, I forgot to mention that I ment the diode inside the low-side switch (the active rectifier). Just before the high-side switch turns on, all load current flows through the body diode of the lower side switch. Depending on how hard the high-side switch is driven, huge current may flow through the body diode of the low-side switch (in reverse direction). Many modern MOSFETS have negative temperature coefficient in there Id versus Vgs characteristic up to high current (so I think they are not as robust as the older ones). Best regards, Wim PA3DJS www.tetech.nl
From: Raveninghorde on 27 Jan 2010 10:23 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.
From: Wimpie on 27 Jan 2010 13:30
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. |