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Old 27th March 2006, 05:27 PM   #41
Eva is offline Eva  Spain
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They discharge C35 (100nF) to 0 volts by configuring RA0 as an output and pulling it down for the right amount of time. Then, they configure RA0 as an input and leave the capacitor to charge through R48 (220k but acting mostly as a current source).

IC2B and IC2D act as comparators and allow the PIC to know how much time C35 takes to charge from 0V up to the voltage corresponding to each temperature level. That's all.
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Old 27th March 2006, 05:35 PM   #42
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Thanks for the description, Eva....you are the best!

K a n w a r
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Old 27th March 2006, 05:44 PM   #43
subwo1 is offline subwo1  United States
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Your explanations are excellent, Eva.
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Old 27th March 2006, 11:51 PM   #44
subwo1 is offline subwo1  United States
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Default MOSFET driver buffer

I have revised the description of the MOSFET driver buffer by moving the resistor from the gate of the N-channel MOSFET to its source.

Click the image to open in full size.
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Old 28th March 2006, 03:56 AM   #45
mzzj is offline mzzj  Finland
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Default Re: MOSFET driver buffer

Quote:
Originally posted by subwo1


Steve,
I think probably yes. In a standard type design (not ZVS) for efficiency, it can be good to have extra drive, especially to turn off the MOSFETs quickly because that is when they dissipate maximum power. Also, you can be more confident that you won't blow your IR211x MOSFET driver if you should decide to experiment with the circuit. If you would like me to clarify anything about adding the IRF7343 buffers, feel free to ask since I am not sure if any information is available from other Internet sources.

Low impedance low-state drive is also good for preventing capacitive coupling shoot-trough at turn-on of opposite switch. Gate-drain capacitance kicks gate voltage over threshold and very short duration shoot-trough occurs between switches. These can be hard to see on scope, occuring easily on 100Mhz region.

BTW. Magnetic EMI-probe can be very handy when tracking this kind of like problems. Linear website has somewhere in appnotes exellent EMI-probe constrution shown, but I cant find it right now as Linears website is sloooow once again.

High-voltage mosfets(esp. older ones like IRF450) are pain in **** to drive nicely, one reason why I recommend forward instead of half-bridge... Or go for bipolars as Eva does
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Old 28th March 2006, 06:28 AM   #46
Eva is offline Eva  Spain
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Indeed my first SMPS project had 5 IRFP460 of the good old ones in it, those with 4200pf input capacitance and 350pF reverse transfer capacitance for a bare 0.27ohm Rds-on It also had a BYV29-400 buck diode with late 1980's manufacturing code, over which I was hard switching

That project got me very disappointed about high voltage MOSFETs. I have never used them again. Also, bipolars produce nice low conduction losses when properly driven (0.1 ohm "rds-on" at 10A in a 400V TO-220 device) and 50ns fall times may be achieved for that current level, but that's not easy to reach either.

I think that the easiest to drive devices currently available are by far IGBTs. They feature low input capacitances below 1nF and reverse transfer capacitances as low as 10pF. That's what I strongly recommend for everyone into DIY SMPS.

BTW: I'm considering paralelling a fast IGBT of the ones featuring fall times around 80ns at the expense of an apparent "Rds-on" in the 0.25 ohm range with a slow one of those featuring less than 0.08ohm apparent Rds-on at the expense of fall times around 800ns (both in TO-220 case). The trick will be to turn on both at the same time but turn off the slow IGBT 2us before the fast one, effectively forcing the fast device to do the switching and the slow one to do the conduction. What do you think about that, mzzj? The "fast" device may be also something like SPP20N50S5.
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Old 28th March 2006, 07:12 AM   #47
mzzj is offline mzzj  Finland
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Quote:
Originally posted by Eva
Indeed my first SMPS project had 5 IRFP460 of the good old ones in it, those with 4200pf input capacitance and 350pF reverse transfer capacitance for a bare 0.27ohm Rds-on It also had a BYV29-400 buck diode with late 1980's manufacturing code, over which I was hard switching

That project got me very disappointed about high voltage MOSFETs. I have never used them again. Also, bipolars produce nice low conduction losses when properly driven (0.1 ohm "rds-on" at 10A in a 400V TO-220 device) and 50ns fall times may be achieved for that current level, but that's not easy to reach either.

I think that the easiest to drive devices currently available are by far IGBTs. They feature low input capacitances below 1nF and reverse transfer capacitances as low as 10pF. That's what I strongly recommend for everyone into DIY SMPS.

BTW: I'm considering paralelling a fast IGBT of the ones featuring fall times around 80ns at the expense of an apparent "Rds-on" in the 0.25 ohm range with a slow one of those featuring less than 0.08ohm apparent Rds-on at the expense of fall times around 800ns (both in TO-220 case). The trick will be to turn on both at the same time but turn off the slow IGBT 2us before the fast one, effectively forcing the fast device to do the switching and the slow one to do the conduction. What do you think about that, mzzj? The "fast" device may be also something like SPP20N50S5.
Yeah, I have wasted my time with half-bridge IRFP450's once. Couldnt turn on that damn thing without triggering opposite mosfet also on. Modern mosfets are a lot better luckily. 600v 70mohm device with Crss=50pF sounds way better than IRFP450.
And APT60N60B specs sheet claim amazing Crss=5pF when VDS>75v

Parallei IGBT's? sounds good for effiency but I really dont like extra complexity. Maybe ok somewhere like pfc-boost switch but full-bridge..nooo way I would go for big $$$-mosfet if last percents in effiency count.
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Old 28th March 2006, 09:00 AM   #48
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Default Re: Re: MOSFET driver buffer

Quote:
Originally posted by mzzj

Low impedance low-state drive is also good for preventing capacitive coupling shoot-trough at turn-on of opposite switch. Gate-drain capacitance kicks gate voltage over threshold and very short duration shoot-trough occurs between switches. These can be hard to see on scope, occuring easily on 100Mhz region.
There would be lots of EMI indeed, with 100mhz power spikes.

Quote:
BTW. Magnetic EMI-probe can be very handy when tracking this kind of like problems. Linear website has somewhere in appnotes exellent EMI-probe constrution shown, but I cant find it right now as Linears website is sloooow once again.
It would be interesting to look into the magnetic probe. Alternately, a short piece of wire connected to a probe combined with viewing a TV set on a low VHF channel could to do well.

Quote:
High-voltage mosfets(esp. older ones like IRF450) are pain in **** to drive nicely, one reason why I recommend forward instead of half-bridge... Or go for bipolars as Eva does
I like ZVS because the switching times can be much slower and still get low or no switching losses. The intrinsic capacitances become much less relevant
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Old 28th March 2006, 09:59 AM   #49
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Hi Folks,

Here is a link to Quasi-Resonant half Bridge used in QSC amps for offline purposes...

http://www.qscaudio.com/support/libr...ms/plx3402.zip

regards,
K a n w a r
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Old 28th March 2006, 12:04 PM   #50
mzzj is offline mzzj  Finland
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Default Re: Re: Re: MOSFET driver buffer

Quote:
Originally posted by subwo1


It would be interesting to look into the magnetic probe. Alternately, a short piece of wire connected to a probe combined with viewing a TV set on a low VHF channel could to do well.

I like ZVS because the switching times can be much slower and still get low or no switching losses. The intrinsic capacitances become much less relevant
In case link outdates, try searching "an70" and look for "A Monolithic Switching Regulator with 100mV Output Noise"
http://www.linear.com/pc/downloadDoc...30,P1535,D4159
Starting somewhere page n. 55 is small EMI-probe. Small size makes it possible to track down even invidual high-dI/dt traces on board, crappy recovery behaviour etc.

Btw toroidal transformer construction shown on page 52...what a hell they were thinking?
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