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 #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
I've been trying to get the output stage of the TL494 under control for so long. I made an output stage a while ago which preformed well, and I thought I had finally conquered the TL494. But when I put it in a smps today the primary drive transistors heated excessively. I tried swapping the 15k base resistors with 180 ohm ones, but the heating just got worse. Within 5 seconds the transistors where near 100 degrees. I haven't needed basic electronics help for a while, but now I need it bad, the TL494 headache has got to end. Has anyone successfully used a TL494 + PWM + non-centertap GDT? Ever? Because I've only seen it done once before, and involved a complicated system which I never understood. To top things off I threw in the towel and bought one SG-something or other, the type with just two outputs that can be connected directly to the GDT. But one of the outputs had died somehow, so all I got was a single sided pulse! Could somebody please help me!
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
To me it seems that your PNP transistors never turn off... Try to put your 15k resistors in bases of both your pnp and npn transistors, not only for the pnp ones. Reduce the value of the pull-up resistors to at least 1k or less (though i'm not exactly sure on this part) Also I recommend to put diodes across C-E of all transistor to dampen the oscillations/spikes that may occur. Or better, buy a SG3525/KA3525 which has "totem pole" output stages :)
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
The basic design looks ok but;
Is the duty very near 50%? Even if it is very close, unless it's EXACTLY 50% you will get 'flux walking' each cycle will add a little dc current to the primary. The cure is a capacitor in series with the primary. the value of capacitance should be very large compared to the value that would resonate with the GDT inductance at the operating frequency.
You could instead add resistance (say 5 to 10 ohms) in series with the GDT primary which would serve the same purpose.
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
The npn gets base drive from the 15k pull-up. The values are large, and should be about 180-220 ohms for the given transistors. Next time I go shopping I'm definitely getting SG3525s.
The driver is designed so the primary is shorted to ground during the off-time. (at least it is supposed to be.) Would the "flux still walk" with the primary shorted, or does it get reset? The design did work well from what I remember, tomorrow I'll do some breadboarding and try your suggestions. Hopefully it's just a faulty connection somewhere.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
You need to decrease the pull-up resistors on the TL494 outputs, and turn all your transistors round so they are operating as emitter followers instead of common emitter amplifiers.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
jmartis is correct, the pnp transistors will allways be ON (via either the TL494 or the npn base-emitter). Steve's suggestion is the most common/simple way to go, I still think a capacitor in series with the primary will be required. I'd also put diodes from 0V to both sides of the transformer, and another two from the transformer connections to +V to protect the transistors from inductance spikes from the primary.
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
Yes! Steve's suggestion worked perfectly! The waveform looks better too. I'll put in some diodes and a DC blocking cap and it will be ready. I can't thank you guys enough!
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.
TL494 output stage headaches
Could somebody please help me!
