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Half-bridge flyback driver not working...

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LAN_403

Steve Ward

Mon Feb 20 2006, 11:47PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Have you scoped the 555 when the flyback is running? Ive had problems in the past with the 555 going crazy and spitting out just a mess, which could possibly kill the driver. My flyback was also "sputtering" at this point. I had forgot decoupling caps on the 555 if i remember correctly.

teravolt

Tue Feb 21 2006, 05:30AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

when I make circuits that run off the line directly I add some current limitting and a variac to bring the voltage up slowly. Current limiting can be done with capasitors or inductors on the AC side and resistors on the DC side. L and C don't disapate heat. depending on your circuit you can use flip-flops and one shots to change the ON time of the mosfets.

Dr. Shark

Tue Feb 21 2006, 08:40AM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

I have scoped the input to the gates and the waveform looks OK, usually I just get problems with 555s if I run them at higher frequencys.

I have got some current limiting in place, I am using a small isolation transformer. The only variac I have got here is a tiny 50V 1A one, I am not sure if that could take 230V mains without saturating? One way to find out I guess...

Sulaiman

Tue Feb 21 2006, 02:01PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

Just a quick bit of math in case it's too few primary turns;
To operate the ferrite at a comfortable dissipation level
+/- 0.2 Tesla peak for a 1cm x 1cm ferrite core, 0.2T x .01m x .01m = 20 uV.sec
A 10 kHz squarewave of +/- 150V requires 150 x 1/(2 x 10000) = 7500 uV.sec
so Npri = 7500/20 = 375 turns !

If freq. is trebled to 30 kHz.....125 turns.
with a 2cm x 2cm core ... 31 turns
running hot...20 turns
etc. etc.

Dr. Shark

Tue Feb 21 2006, 02:29PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Thanks, thats valuable information for me. Is there any deeper reason for the field-area product being equal to the volt/frequency? At least I would have expected some proportionality constant there, but I am not good with magnetics.

Well, looks like I need to go up both in turns count and in frequency for proper operation.

However, I am still mystified by the dying IGBTs because my PSU simply cannot provide enough power to cause thermal damage. Well, maybe I somehow managed to discharge my 470uF filter capacitor through the IGBTs...

Sulaiman

Tue Feb 21 2006, 03:42PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

I was a bit lazy with my units;
Flux (Webber) = Flux Density (Tesla) x Area (sq. m)
Webber, by the wonder of the MKS system = (Volt.seconds)
(per turn)
so no 'Constant(s)' are required.

EDIT
If this isn't enough to get Americans to convert to Metric I give in!

Hazmatt_(The Underdog)

Thu Feb 23 2006, 08:23PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

sulaiman what book would you recommend for this area of study?

When I took EE 460 (magnetics) my instructor was more interested in divergence theorem then doing anythink productive, as a result, I don't know ^^^ which you just pointed out, and I really need to know it.

Matt

Sulaiman

Fri Feb 24 2006, 09:55AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

I've no Idea what book to reccomend;
I do all my learning from the net.

Electromagnetism is poorly/rarely taught because most teachers don't really understand it,
the best information comes from ferrite manufacturers
since it's in their interrest for us to use their product well.
Magnetics inc.

is a start.

Kolas

Sat Feb 25 2006, 08:46AM

Registered Member #102 Joined: Thu Feb 09 2006, 08:15PM
Location: Raleigh, NC
Posts: 169

hi
i kinda started the movement on the half bridge as your using my circuit :D
do you ahve a scope?
Kolas
(edit:) oh i see you have scoped it
lets go over again what is happening here
fets are dying, and the out put is crap
what votlage does the 555 signal reach with out zeners verus with the zeners?
ive found those larger switches like you to swing them to their full raiting of 20v to -20v
improper saturation can cause this very problem

also the is the GDT putting out realitivly square wave?
Kolas

Dr. Shark

Thu Mar 02 2006, 09:01AM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Hi Kolas, thanks for the info, I just noticed your post now. I guess I really need to do somw work on getting those IGBTs saturated properly. They dont even get warm so I did not think this was the problem, but driving components out of spec is never a good idea.
Since cutting the primary turns on my GDTs from 20 to 10 always seems to cause problems, I'll have a try at increasing the secondary turns to 40. Lets see if this works...

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