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push-pull transformer for 12v to 300v
By kell, Section Homebrewed Electricity Posted on Sun Sep 12th, 2004 at 03:50:04 PM MST
U-core, toroid or E-core?

Aiming for 100 watts, so the secondary will take 2 or 3 hundred turns of wire and don't want to tangle with ten yards or more of fine wire, looping it through a toroid so many times. Yuck! Did anybody ever try it with two u-cores, wind the primary on one of them and the secondary on the other, and put them together how should I secure them.
http://www.amidon-inductive.com/aai_ucores.htm

push-pull transformer for 12v to 300v | 8 comments (8 topical, 0 editorial)

Re: push-pull transformer for 12v to 300v (3.00 / 0) (#1)
by K3CZ on Sun Sep 12th, 2004 at 09:01:54 PM MST
(User Info)

Actually, puting up to several hundred turns or more on a toroid with a 1"or larger hole isn't that big a deal; I used to do it all the time in the '60's for aerospace power supplies. Just use a tongue depressor with notches in the end as a bobbin. Make sure your core material is proper - powdered iron (Amidon) usually cannot be pulled down to power frequencies; a tape wound core is what you are looking foor. Mated "C" cores also work well(they are basically tape wound, too.) For general sine wave work, good old E - I laminations salvaged from old audio or power transformers, with a single bobbin are FB. Turns ratios and numbers can be calculate from info from a number of sources, incl. the ARRL handbook, along with info on the core.

Re: push-pull transformer for 12v to 300v (3.00 / 0) (#2)
by nack on Sun Sep 12th, 2004 at 09:06:53 PM MST
(User Info)

The cores you linked to are used in pairs, you wind on bobbins and slip the cores in from both ends so they meet in the middle of the bobbins then strap it all together - the core cross-section shown on the right has a groove for a tie wire to run down the outside of the legs. Same is generally true for ferrite E cores, except you only have the one bobbin on the middle leg.

Re: push-pull transformer for 12v to 300v (3.00 / 0) (#3)
by kell on Sun Sep 12th, 2004 at 09:54:03 PM MST
(User Info)

This will run at switch mode freq, > 20 kHz. Going to use Amidon's #77 ferrite core material.
K3CZ I like the shuttle idea.
About the circuit --
Wish I could drive a push-pull with the good old 555, but not sure I can, because of the deadtime requirement.
Looking at TL494 and UC3525, but need to get up to speed on them. The 3525 datasheets have no smps wiring diagram examples, though I saw the post a page or so back with the example of a 3525 circuit. TL494 seems to have more info around.

Nack, I'll have to ask Amidon if they have plastic bobbins to fit u-cores.

[ Parent ]

Re: push-pull transformer for 12v to 300v (3.00 / 0) (#4)
by commanda (alwynne at unwired dot com dot au) on Mon Sep 13th, 2004 at 12:16:24 AM MST
(User Info)

I've built a small inverter using a 555 driving a 4017 counter. 4017-Q0 drives first transistor, 4017-Q2 drives second transistor. 4017-Q1 & 4017-Q3 are not used. 4017-Q4 tied to 4017 reset.

So, in sequence, its first transistor, space, second transistor, space, reset pulse back to first transistor.

The pwm design I'm working on is similar. The pwm from the 555 clocks the 4017, Q0 is anded with the pwm and drives the first transistor, Q1 is anded with the pwm and drives the second transistor. Q2 is tied back to the reset. You need a small RC network betwqeen the pwm and the inputs to the and gates, to negate the propagation delay of the 4017 (it's noticeable when running on 5 volts), otherwise you get a glitch on the output.

Amanda

[ Parent ]

Re: push-pull transformer for 12v to 300v (3.00 / 0) (#5)
by kell on Mon Sep 13th, 2004 at 12:41:58 PM MST
(User Info)

If I use a timer, counter and and gates I'll have a mess of wires on perf board or else have to make a pcb. I'd rather use a dedicated pwm controller chip and minimize the component count.

But I was also contemplating driving the n-channel mosfets for the push-pull directly from a 555. One mosfet would have a pull-up resistor on the gate and a npn transistor to shunt it to ground: on when 555 output low, off when 555 output high. The other mosfet would get direct drive from the timer: off when 555 output low, on when 555 output high. Wired this way the two mosfets would alternate. Dead time would be negligible (however many nanoseconds it takes the 555 to switch). But even if the mosfets overlap, shoot through might not be a problem because in the push-pull configuration both mosfets are on the low side. I think it's worth a try. I wouldn't be able to implement pwm, but I can figure out some other way to achieve regulation.

The thing I like about Amidon is they cater to the small guy (hams). They'll talk to you on the phone, and they only have a $10 minimum order.

[ Parent ]

Re: push-pull transformer for 12v to 300v (3.00 / 0) (#6)
by kell on Mon Sep 13th, 2004 at 09:42:40 PM MST
(User Info)

I tried it and the npn apparently shunts enough current to ground by way of its base current to kill the timing signal. I found out too that a pullup resistor isn't a good way to turn on a mosfet.

Here's a link with a description of an inverting totem pole that would make it possible to drive alternating mosfets from a single timer signal:
http://www.techonline.com/community/ed_resource/feature_article/5652
Maybe the shoot through problem they mention could be alleviated by putting a zener in series with the gate of each driver mosfet.

Re: push-pull transformer for 12v to 300v (3.00 / 0) (#7)
by commanda (alwynne at unwired dot com dot au) on Mon Sep 13th, 2004 at 11:22:24 PM MST
(User Info)

I tried it and the npn apparently shunts enough current to ground by way of its base current to kill the timing signal.

Sorry to ask what hopefully is a really silly question, but you do have a resistor between the 555's output and the base of your NPN?

Amanda

[ Parent ]

Re: push-pull transformer for 12v to 300v (3.00 / 0) (#8)
by kell on Tue Sep 14th, 2004 at 03:28:44 AM MST
(User Info)

Well, no. I'll need that, and I'll need something that will turn on the mosfet better than a pullup. This whole thing has to be redesigned. I have an idea and sometime I'll put it together and test it out.

Some of these timers only source 10 ma. Kind of makes it hard to run one inverting driver of discrete components and still have enough left over to drive the other mosfet. Mosfets drivers need enough oomph to suck the capacitance out of the gate, so even though it's only for an instant they still need to be able to sink or source about an amp. The idea that there's "no gate current" is misleading; one needs to drive a mosfet just as hard as a bjt.
If I can't get a circuit with solid performance out of this timer I'll do the sensible thing and go with a dedicated pwm chip.

push-pull transformer for 12v to 300v | 8 comments (8 topical, 0 editorial)

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