Problem with LM3150
in [Design]
|
Prev: PCB Cleaning
Next: White LED Spice Model
From: Raveninghorde on 15 Jan 2010 08:43 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.
From: Wimpie on 15 Jan 2010 11:09 On 15 ene, 14:43, 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. Hello, If you can decouple the ground from the circuit and the oscilloscope, measure the voltage across the high side MOSFET (with the oscilloscope ground connected to the drain of the MOSFET). This will help you to figure out whether conduction or switchting losses are dominant. Do you have sufficient supply voltage (as undervoltage dropout can be below 5V)? When conduction loss is not dominant, you may parallel M2 with a schottky rectifier as reverse recovery times of many MOSFETs are too long for operation in the 300 kHz range. With a slow freewheel diode, M1 is subjected to high peak power dissipation. Using the schottky rectifier avoids that the body diode inside M2 will conduct. Best regards, Wim PA3DJS www.tetech.nl Don't forget to remove abc in case of PM
From: John Larkin on 15 Jan 2010 14:40 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
From: Raveninghorde on 15 Jan 2010 16:59 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.
From: Jamie on 15 Jan 2010 18:57
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. |