If you need assistance, please send an email to forum at 4hv dot org. To ensure your email is not marked as spam, please include the phrase "4hv help" in the subject line. You can also find assistance via IRC, at irc.shadowworld.net, room #hvcomm.
Support 4hv.org!
Donate:
4hv.org is hosted on a dedicated server. Unfortunately, this server costs and we rely on the help of site members to keep 4hv.org running. Please consider donating. We will place your name on the thanks list and you'll be helping to keep 4hv.org alive and free for everyone. Members whose names appear in red bold have donated recently. Green bold denotes those who have recently donated to keep the server carbon neutral.
Special Thanks To:
Aaron Holmes
Aaron Wheeler
Adam Horden
Alan Scrimgeour
Andre
Andrew Haynes
Anonymous000
asabase
Austin Weil
barney
Barry
Bert Hickman
Bill Kukowski
Blitzorn
Brandon Paradelas
Bruce Bowling
BubeeMike
Byong Park
Cesiumsponge
Chris F.
Chris Hooper
Corey Worthington
Derek Woodroffe
Dalus
Dan Strother
Daniel Davis
Daniel Uhrenholt
datasheetarchive
Dave Billington
Dave Marshall
David F.
Dennis Rogers
drelectrix
Dr. John Gudenas
Dr. Spark
E.TexasTesla
eastvoltresearch
Eirik Taylor
Erik Dyakov
Erlend^SE
Finn Hammer
Firebug24k
GalliumMan
Gary Peterson
George Slade
GhostNull
Gordon Mcknight
Graham Armitage
Grant
GreySoul
Henry H
IamSmooth
In memory of Leo Powning
Jacob Cash
James Howells
James Pawson
Jeff Greenfield
Jeff Thomas
Jesse Frost
Jim Mitchell
jlr134
Joe Mastroianni
John Forcina
John Oberg
John Willcutt
Jon Newcomb
klugesmith
Leslie Wright
Lutz Hoffman
Mads Barnkob
Martin King
Mats Karlsson
Matt Gibson
Matthew Guidry
mbd
Michael D'Angelo
Mikkel
mileswaldron
mister_rf
Neil Foster
Nick de Smith
Nick Soroka
nicklenorp
Nik
Norman Stanley
Patrick Coleman
Paul Brodie
Paul Jordan
Paul Montgomery
Ped
Peter Krogen
Peter Terren
PhilGood
Richard Feldman
Robert Bush
Royce Bailey
Scott Fusare
Scott Newman
smiffy
Stella
Steven Busic
Steve Conner
Steve Jones
Steve Ward
Sulaiman
Thomas Coyle
Thomas A. Wallace
Thomas W
Timo
Torch
Ulf Jonsson
vasil
Vaxian
vladi mazzilli
wastehl
Weston
William Kim
William N.
William Stehl
Wesley Venis
The aforementioned have contributed financially to the continuing triumph of 4hv.org. They are deserving of my most heartfelt thanks.
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
wrote ... its powered by an ATX psu so the ground terminal is acutally connected to the mains ground. the gate is 10v relative to ground and source is 6v relative to ground.
If you ever want to directly measure a voltage like this and you only have a scope with grounded probes you can put one scope on the gate, the other at the source, and ground both probes, then put the scope in A-B mode.
wrote ... UPDATE: Given the above pic... i think you need a diode between the mosfet and inductor, with the polarity arranged so that the inductor back EMF cant get to the mosfet (reversed biased horizontal diode) but then the Back EMF gets conducted through the vertical diode (forward biased) and to the load, i think this will solve your heat problems.
Nope, the "back emf" is required for the circuit to work, the diode from ground to the inductor's left side will conduct the inductor's current when the FET turns off, and the voltage at this node will go below that of circuit ground, which will further reverse bias the body diode of the FET.
If you're getting lots of heating there are two things: conduction loss and switching loss. If you're fully turning the FET on (usually requires about 10V Vgs) then you expect a power loss of Iload^2*Rds,on=4A^2*.04R=640mW (assuming low current ripple with a large enough inductor, with higher ripple current it would be a bit higher). So you could try getting a higher Vgs. With a NMOS device you have to use tricks like the battery trick Patrick suggested, or other high side driver techniques including gate drive transformers or bootstrap capacitor circuits, because you want that gate voltage when turned on to be optimally about 10V higher than the input at the drain. As Sulaimain pointed out a PMOS device you are pulling the gate to ground which is much easier, but PMOS devices cost slightly more and are a bit slower for the same current.
So if conduction loss is not causing your heating then it might be slow switching of the gate. Take a look at Vgs using the technique above and zoom in on the time scale to see how long it takes to charge and disharge the gate. While it charges up and down it goes into an intermediate "linear" state with high power dissipation, and if the gate drive is slow it will result in higher power dissipation in the FET.
Edit:
wrote ... at around 30% duty cycle the current through 12v supply and drain is 1.3A, voltage at source relative to ground is 7 volts
Oh, that's the problem then. If you have 12V in and you're seeing only 7V at teh source your FET is dropping 5V. At 4A that's 20W, times your duty cycle so with 30% that's 6W which seems kind of high for a circuit with these specs. It likely means your FET is not fully on so you need a higher gate voltage still, so either more batteries, the isolation transformer, bootstrap cap, or PMOS device.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Mattski wrote ...
wrote ... its powered by an ATX psu so the ground terminal is acutally connected to the mains ground. the gate is 10v relative to ground and source is 6v relative to ground.
If you ever want to directly measure a voltage like this and you only have a scope with grounded probes you can put one scope on the gate, the other at the source, and ground both probes, then put the scope in A-B mode.
I didnt want to make this to complicated for him before i knew what he could really do.
Mattski wrote ...
wrote ... UPDATE: Given the above pic... i think you need a diode between the mosfet and inductor, with the polarity arranged so that the inductor back EMF cant get to the mosfet (reversed biased horizontal diode) but then the Back EMF gets conducted through the vertical diode (forward biased) and to the load, i think this will solve your heat problems.
Nope, the "back emf" is required for the circuit to work, the diode from ground to the inductor's left side will conduct the inductor's current when the FET turns off, and the voltage at this node will go below that of circuit ground, which will further reverse bias the body diode of the FET.
Now that im sober i realize the polarity is added at the time of switching...DUH. And yes i was wrong about the internal diode too... crap.
Mattski wrote ...
If you're getting lots of heating there are two things: conduction loss and switching loss. If you're fully turning the FET on (usually requires about 10V Vgs) then you expect a power loss of Iload^2*Rds,on=4A^2*.04R=640mW (assuming low current ripple with a large enough inductor, with higher ripple current it would be a bit higher). So you could try getting a higher Vgs. With a NMOS device you have to use tricks like the battery trick Patrick suggested, or other high side driver techniques including gate drive transformers or bootstrap capacitor circuits, because you want that gate voltage when turned on to be optimally about 10V higher than the input at the drain. As Sulaimain pointed out a PMOS device you are pulling the gate to ground which is much easier, but PMOS devices cost slightly more and are a bit slower for the same current.
So if conduction loss is not causing your heating then it might be slow switching of the gate. Take a look at Vgs using the technique above and zoom in on the time scale to see how long it takes to charge and disharge the gate. While it charges up and down it goes into an intermediate "linear" state with high power dissipation, and if the gate drive is slow it will result in higher power dissipation in the FET.
Ok lets revisit the gate conditions then, i thought having 6V across the gate with a spec of Vgs threshold of 4V would have minimized the linear region to a brief time. but if the drop of 12 - 7 = 5 V x I = W, is across the transistor then that seems to indicate a gate drive deficiency.
Mattski wrote ...
Edit:
wrote ... at around 30% duty cycle the current through 12v supply and drain is 1.3A, voltage at source relative to ground is 7 volts
Oh, that's the problem then. If you have 12V in and you're seeing only 7V at teh source your FET is dropping 5V. At 4A that's 20W, times your duty cycle so with 30% that's 6W which seems kind of high for a circuit with these specs. It likely means your FET is not fully on so you need a higher gate voltage still, so either more batteries, the isolation transformer, bootstrap cap, or PMOS device.
Im wondering if a bootstrap diode/cap would be possible for him to add using discrete components?
Registered Member #2063
Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352
the mosfet doesn't break a sweat if I do this. but as soon as I add filter cap parallel to the load, the mosfet heats up
also, if you take a look at the CPU power supply on motherboards, they look like buck converters, it has n-mosfets,inductors and filter caps but to diodes. but the mosfets have an internal drain-source diode.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
haxor5354 wrote ...
the mosfet doesn't break a sweat if I do this. but as soon as I add filter cap parallel to the load, the mosfet heats up
also, if you take a look at the CPU power supply on motherboards, they look like buck converters, it has n-mosfets,inductors and filter caps but to diodes. but the mosfets have an internal drain-source diode.
This is a great circuit! let me think about it for a minute.
OK, so why would it work fine as pictured above, yet not anyother way?
OK im reading some MOSFET switching PDF's right now, i think extra capacitence is being fed forward from the load to slow down the gate turn on/off. There is a special case of coupling im not sure though.
I think this is a good explantation, unless others feel differently. Im reading all four sections.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
haxor5354 wrote ...
wait hold on...... now it doesn't heat up with a cap parallel to load now im baked
Yeah i was really pressed for an explanation for why the cap makes such a difference... inductive or capacitive switching is more complicated then resistive switching, so the transistors phases change quite abit....
I dont see why we are having such difficulty with this circuit !?
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
For about $1 you can buy an IRF9Z34 which would easily do the job there are over 1000 different p-channel mosfets on the market choose one and use it instead of an n-channel !
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Sulaiman wrote ...
For about $1 you can buy an IRF9Z34 which would easily do the job there are over 1000 different p-channel mosfets on the market choose one and use it instead of an n-channel !
Yeah maybe, i think the gate switching is the problem not the internal diode. So maybe a P channel, or is it possible to reverse the circuit such that he can still use a N-channel device, if only temporary just to see what the problem/solution is?
Now that i think about it, just developing a capacitor bootsrtapper or GDT is the answer for verifying the solution\problem heating.
Registered Member #2063
Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352
Sulaiman wrote ...
For about $1 you can buy an IRF9Z34 which would easily do the job there are over 1000 different p-channel mosfets on the market choose one and use it instead of an n-channel !
ok yeah.... maybe substituting parts and using random parts isn't such a good idea
Registered Member #2063
Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352
so gate driving for FETs are like pre-amp for vacuum tubes? and whats different between mosfets and IGBTs? they both voltage controlled, maybe IGBTs can handle more voltage.
This site is powered by e107, which is released under the GNU GPL License. All work on this site, except where otherwise noted, is licensed under a Creative Commons Attribution-ShareAlike 2.5 License. By submitting any information to this site, you agree that anything submitted will be so licensed. Please read our Disclaimer and Policies page for information on your rights and responsibilities regarding this site.
help needed for a buck converter
now im baked
