Gate resistor
in [Design]
|
Prev: UART Multiplexing
Next: User forum design
From: Tim Williams on 11 Apr 2010 06:02 Neet. I increased the gate resistors on a half bridge MOSFET inverter, and switching loss dropped significantly. Tim
From: Bill Sloman on 11 Apr 2010 07:12 On Apr 11, 12:02 pm, Tim Williams <tmoran...(a)gmail.com> wrote: > Neat. I increased the gate resistors on a half bridge MOSFET > inverter, and switching loss dropped significantly. Interesting. The gate resistors we designed in as a mattter of course on MOSFETs we intended to prevent the devices from oscillating. Presumably your original resistors were big enough to suppress sustained oscillation, but not big enough to completely suppress ringing during switch-on and switch-off. Do you hae a scope that is good enough to see ringing at a feww hundred MHz? And acess to a point on the circuit where you could see it? -- Bill Sloman, Nijmegen
From: Tim Williams on 11 Apr 2010 07:29 200MHz. But the reason is much more ordinary than that. ;-) Incidentially, I don't think I've ever seen a MOSFET oscillating. I've always included the gate resistor. I should be daring some time and skip it. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms "Bill Sloman" <bill.sloman(a)ieee.org> wrote in message news:33700d87-c407-4772-b27d-40faba093817(a)i37g2000yqn.googlegroups.com... On Apr 11, 12:02 pm, Tim Williams <tmoran...(a)gmail.com> wrote: > Neat. I increased the gate resistors on a half bridge MOSFET > inverter, and switching loss dropped significantly. Interesting. The gate resistors we designed in as a mattter of course on MOSFETs we intended to prevent the devices from oscillating. Presumably your original resistors were big enough to suppress sustained oscillation, but not big enough to completely suppress ringing during switch-on and switch-off. Do you hae a scope that is good enough to see ringing at a feww hundred MHz? And acess to a point on the circuit where you could see it? -- Bill Sloman, Nijmegen
From: John Larkin on 11 Apr 2010 12:15 On Sun, 11 Apr 2010 06:29:26 -0500, "Tim Williams" <tmoranwms(a)charter.net> wrote: >200MHz. But the reason is much more ordinary than that. ;-) > Something to do with shoot-through? What were the resistor values? But you are top posting again. Shows sorry lack of discipline. John
From: MooseFET on 11 Apr 2010 12:22
On Apr 11, 3:02 am, Tim Williams <tmoran...(a)gmail.com> wrote: > Neet. I increased the gate resistors on a half bridge MOSFET > inverter, and switching loss dropped significantly. Another way that switching losses can go up is if the MOSFET doesn't actually oscillate but that the fast edge of the switching is at a frequency way above what the inductors are good for. In that case, you are going to see a really big current spike at the turn on. I have found that this is the case often enough to check for it. The glitch capacitively coupled to the gate can go into the driver chip and make it do funny things. I have seen a thing that looked a little like half a cycle of a sine wave on the gate just after the MOSFET has actually switched. The gate pin pulls the output of the driver below ground briefly and then ringing pulls it up. If the driver can't hold the pin near ground just after the backwards current you get this bump. |