Formula for minimum drive current for mosfet
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From: Jon Slaughter on 10 Apr 2008 19:33 Whats the formula? I = 1/2*F*Q*V? Trying to figure out if I can drive http://www.fairchildsemi.com/pf/FD/FDD8424H.html with a uC directly? (I think it can supply up to 20mA or so) V = 12V if I use pullup and 5V if not. Thanks, Jon
From: John Larkin on 10 Apr 2008 18:37 On Thu, 10 Apr 2008 17:33:44 -0600, "Jon Slaughter" <Jon_Slaughter(a)Hotmail.com> wrote: >Whats the formula? > >I = 1/2*F*Q*V? > >Trying to figure out if I can drive > >http://www.fairchildsemi.com/pf/FD/FDD8424H.html > >with a uC directly? (I think it can supply up to 20mA or so) > >V = 12V if I use pullup and 5V if not. > >Thanks, >Jon > How fast (rise/fall) do you need to switch? John
From: bill.sloman on 10 Apr 2008 18:45 On 11 apr, 01:33, "Jon Slaughter" <Jon_Slaugh...(a)Hotmail.com> wrote: > Whats the formula? > > I = 1/2*F*Q*V? > > Trying to figure out if I can drive > > http://www.fairchildsemi.com/pf/FD/FDD8424H.html > > with a uC directly? (I think it can supply up to 20mA or so) > > V = 12V if I use pullup and 5V if not. Check out the data sheet. Figure 7 shows the typical gate charge required to get the gate voltage up to a level where the part is turned on - something like 10nC. The worst case total gate charge listed earlier in the data sheet is 24uC. 20mA s going to take 1.2usec to deliver that 24uC of charge - this is slow switching by MOSFET standards, and you won't want to switch that slowly very often, because if you do there is a real risk that the switch will overheat. -- Bill Sloman, Nijmegen
From: Fred Bloggs on 10 Apr 2008 19:44 bill.sloman(a)ieee.org wrote: > On 11 apr, 01:33, "Jon Slaughter" <Jon_Slaugh...(a)Hotmail.com> wrote: > >>Whats the formula? >> >>I = 1/2*F*Q*V? >> >>Trying to figure out if I can drive >> >>http://www.fairchildsemi.com/pf/FD/FDD8424H.html >> >>with a uC directly? (I think it can supply up to 20mA or so) >> >>V = 12V if I use pullup and 5V if not. > > > Check out the data sheet. Figure 7 shows the typical gate charge > required to get the gate voltage up to a level where the part is > turned on - something like 10nC. The worst case total gate charge > listed earlier in the data sheet is 24uC. > > 20mA s going to take 1.2usec to deliver that 24uC of charge - this is > slow switching by MOSFET standards, and you won't want to switch that > slowly very often, because if you do there is a real risk that the > switch will overheat. > > -- > Bill Sloman, Nijmegen That thing looks like a cross conduction hazard and half at that switching speed, both FETs come on at less than 2V from their source rails, wonder if he's tying the gates together, definitely would want to switch as fast as possible then...
From: Jon Slaughter on 10 Apr 2008 23:56
<bill.sloman(a)ieee.org> wrote in message news:20e9d847-ea21-4aa1-bb24-4a2848d7a57a(a)q24g2000prf.googlegroups.com... > On 11 apr, 01:33, "Jon Slaughter" <Jon_Slaugh...(a)Hotmail.com> wrote: >> Whats the formula? >> >> I = 1/2*F*Q*V? >> >> Trying to figure out if I can drive >> >> http://www.fairchildsemi.com/pf/FD/FDD8424H.html >> >> with a uC directly? (I think it can supply up to 20mA or so) >> >> V = 12V if I use pullup and 5V if not. > > Check out the data sheet. Figure 7 shows the typical gate charge > required to get the gate voltage up to a level where the part is > turned on - something like 10nC. The worst case total gate charge > listed earlier in the data sheet is 24uC. > > 20mA s going to take 1.2usec to deliver that 24uC of charge - this is > slow switching by MOSFET standards, and you won't want to switch that > slowly very often, because if you do there is a real risk that the > switch will overheat. > Why is slower going to going to cause it to heat up? Its less current so less heat (same amount of charge). In fact it probably would be better because its spread out over time. (like, say, charging a battery at 1A for 1 year compared to 365A in one day. Same amount of charge but totally diffrent results) My switching is at most 100khz(its for motor control so anything about 20khz should be ok but I'm going for about 50khz). I figure I need about 5 to 10 times this but really it shouldn't be that important(don't need it to be exact). Really though, Can you explain to me why a slower switching speed will cause it to heat up more? It contradict's everything I know about transister switches and switching speed. |