TO220--thin leads--high current--How?
in [Design]
|
From: MooseFET on 3 May 2010 10:24 On May 2, 12:07 pm, n...(a)puntnl.niks (Nico Coesel) wrote: > "Tim Williams" <tmoran...(a)charter.net> wrote: > >I once accidentially burned out a capacitor lead at 50A. It was 0.6mm > >diameter, active section about 2-3mm long. > > >I was testing a high current power supply, which was operating happily at > >50A, with unusually low output voltage. Ten seconds later, *phut*, poof. > >;-) My fault for clipping a large alligator clip to a fairly small > >capacitor. > > >The only thing you need to know about MOSFETs is use them at half rated > >current. As an added bonus, the Rds(on) is relatively small, so you need > >even less heatsinking. > > The counterside is that the gate capacitance is much larger so the > turn on/turn off time is larger which results in extra heat losses. I > like to choose the smallest MOSFET that fits the worst case scenario. There is some poorly documented impedance between the gate leg and the actual controlling gate element. The reverse transfer capacitance is not a constant with voltage or time. It conspires to stop the gate electrode from moving just at exactly the time when the conditions are the worst from a power point of view. The body diode is lossy and slowish. If the body diode is being used even if it is just to pass a small spike from ringing, the losses are increased. > > -- > Failure does not prove something is impossible, failure simply > indicates you are not using the right tools... > nico(a)nctdevpuntnl (punt=.) > --------------------------------------------------------------
From: John Larkin on 3 May 2010 13:09 On Mon, 3 May 2010 07:24:04 -0700 (PDT), MooseFET <kensmith(a)rahul.net> wrote: >On May 2, 12:07�pm, n...(a)puntnl.niks (Nico Coesel) wrote: >> "Tim Williams" <tmoran...(a)charter.net> wrote: >> >I once accidentially burned out a capacitor lead at 50A. �It was 0.6mm >> >diameter, active section about 2-3mm long. >> >> >I was testing a high current power supply, which was operating happily at >> >50A, with unusually low output voltage. �Ten seconds later, *phut*, poof. >> >;-) � My fault for clipping a large alligator clip to a fairly small >> >capacitor. >> >> >The only thing you need to know about MOSFETs is use them at half rated >> >current. �As an added bonus, the Rds(on) is relatively small, so you need >> >even less heatsinking. >> >> The counterside is that the gate capacitance is much larger so the >> turn on/turn off time is larger which results in extra heat losses. I >> like to choose the smallest MOSFET that fits the worst case scenario. > >There is some poorly documented impedance between the gate leg >and the actual controlling gate element. > >The reverse transfer capacitance is not a constant with voltage or >time. It conspires to stop the gate electrode from moving just at >exactly the time when the conditions are the worst from a power >point of view. > >The body diode is lossy and slowish. If the body diode is being used >even if it is just to pass a small spike from ringing, the losses >are increased. > > Body diodes can also snap on reverse recovery, which can blow out gates. John
From: Tim Williams on 3 May 2010 13:24 "MooseFET" <kensmith(a)rahul.net> wrote in message news:a255d645-99e3-4207-8aad-dbe3d49c2988(a)h20g2000prn.googlegroups.com... > There is some poorly documented impedance between the gate leg > and the actual controlling gate element. In other words, gate spreading resistance. And maybe some inductance, and other squirrelies, depending on how detailed you want to get. More important also is the source lead inductance (which is sometimes specified at 5nH or so). I get the impression that spreading resistance has dropped over the years. Back in the day, you didn't see FETs spec'd for very impressive rise/fall times. But they also used fairly large Rg's, so is it just that they didn't try? > The reverse transfer capacitance is not a constant with voltage or > time. It conspires to stop the gate electrode from moving just at > exactly the time when the conditions are the worst from a power > point of view. Cgd is fun. http://myweb.msoe.edu/williamstm/Images/PFC_Gate1.png DCM PFC circuit, somewhere in the middle of a half cycle (well, 4.3ms in, about the middle of a 8.33ms half cycle), 10 ohm Rg, transistor FDH44N50. When the inductor current drops to zero, voltage sinks down from +boost (410V) to Vin (maybe 160V at this point). So the gate voltage drops by about 0.2V. Then the PFC chip turns on, current rises, then off it goes. (Ringing is turn-on trash from the catch diode, which needs a snubber.) Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: John Larkin on 3 May 2010 15:28 On Mon, 3 May 2010 12:24:57 -0500, "Tim Williams" <tmoranwms(a)charter.net> wrote: >"MooseFET" <kensmith(a)rahul.net> wrote in message >news:a255d645-99e3-4207-8aad-dbe3d49c2988(a)h20g2000prn.googlegroups.com... >> There is some poorly documented impedance between the gate leg >> and the actual controlling gate element. > >In other words, gate spreading resistance. And maybe some inductance, and >other squirrelies, depending on how detailed you want to get. More >important also is the source lead inductance (which is sometimes specified >at 5nH or so). > >I get the impression that spreading resistance has dropped over the years. >Back in the day, you didn't see FETs spec'd for very impressive rise/fall >times. But they also used fairly large Rg's, so is it just that they didn't >try? > >> The reverse transfer capacitance is not a constant with voltage or >> time. It conspires to stop the gate electrode from moving just at >> exactly the time when the conditions are the worst from a power >> point of view. > >Cgd is fun. > >http://myweb.msoe.edu/williamstm/Images/PFC_Gate1.png > >DCM PFC circuit, somewhere in the middle of a half cycle (well, 4.3ms in, >about the middle of a 8.33ms half cycle), 10 ohm Rg, transistor FDH44N50. > >When the inductor current drops to zero, voltage sinks down from +boost >(410V) to Vin (maybe 160V at this point). So the gate voltage drops by >about 0.2V. Then the PFC chip turns on, current rises, then off it goes. >(Ringing is turn-on trash from the catch diode, which needs a snubber.) > >Tim I generally assume that mosfets are infinitely fast; you just have to drive the gates hard enough. You can get fets to switch in a fraction of the datasheet values if you whack them hard enough. Here's a 100 volt pulse into 50 ohms, transformer coupled. http://www.highlandtechnology.com/DSS/T760DS.html The output stage uses a couple of BSS123s, SOT23 fets which literally cost 2 cents each. We're actually getting under 1 ns rise and fall at 100 volts. How can anybody sell a mosfet for 2 cents? John
From: John Fields on 3 May 2010 16:33
On Mon, 03 May 2010 12:28:19 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >Here's a 100 volt pulse into 50 ohms, transformer coupled. > >http://www.highlandtechnology.com/DSS/T760DS.html --- 8nm prop delay? JF |