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Mains Flyback trouble

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Marko

Thu Mar 02 2006, 06:21PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

If core is saturated then problems will occur, otherwise flyback at certain frequency tends to draw only a little current. Single transistor can be later upgraded to bridge too.
If transistor blows number of turns must be increased or voltage adjusted to some level.

I did such driving methods and bipolar transistors even showed to heat less than mosfets, and I used primary ballast inductor.
That was low voltage SMPS but flyback is not far from there (high voltage winding).

uzzors2k

Thu Mar 02 2006, 08:44PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Hey I'm in luck! The mains side of the PSU has a voltage divider in it! I should have realized this before... It gives 150 volts, at what I assume is low current.
I have wound the new primary, but since it was intended for 300v use, its 80 to 90 turns. The resistance is 2 ohms and I've also got an inductor in place, so I guess the power problem is solved.

Now I have to get that driver fixed up...

EDIT: Turns out the 555 and my MOSFET blew.

I guess I really need to build for something better.

Kolas

Thu Mar 02 2006, 09:26PM

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

the 555 cant drive that mosfet
if the 555 dies, it's output becomes "on" (usually)
that turns the transistor on, and boom.

Marko

Thu Mar 02 2006, 09:46PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

We said nicely that NE555 cant drive mosfet directly, you need DRIVER of any kind, two transistors pushpull as start and as shown in schematics.

You must take lots of precautions before powering unisolated circuit to full voltage, NE555 must be oscillaing at right frequency and duty cycle, and must never stop for any reason (thats why GDT's were invented).

uzzors2k

Sat Mar 04 2006, 12:25PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Sorry, my last reply was bit unclear. What I meant is that the 555 and FET had died from the first test, along with the load resistor. I wouldn't try the same driver twice without fixing it!
I managed to fit a push-pull setup on the tiny board though, despite a lack of space.... Anyway, I tested the system again, but at 150v I just get one thick spark, and then the driver seems to stop oscillating. I put a voltage regulator on the driver, but it still stops running. No parts are damaged, and it works again after I cut the power.

Could this be due to the MOSFET being overpowered in some way, or needing a stronger driver? I'm using a C5129

not the best choice, but what I have at hand....

1141475155 95 FT2664 Uzzors Driver Upgrade

Marko

Sat Mar 04 2006, 01:57PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

How many turns are you using for primary?
If there are not try to fit 470 ohm resistors to base of each transistor in push-pull, maybe NE555 overloads and hangs (it happened to me few times).
Or use gate driver IC if you have some.

uzzors2k

Sat Mar 04 2006, 08:04PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

The primary is 90 turns. I put a 1k resistor on the push-pull transistor bases, and I also put a MOV in parallel with the primary, but no dice. It still gives a *beep* along with a spark, and then dies.

Any other ideas? Should I try some diodes on the MOSFET, or change it all together?

Marko

Sat Mar 04 2006, 08:25PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

How did you get 150V?
Scope NE555 if it stops oscillating, and check power supply for flaws...

Dr. Dark Current

Sat Mar 04 2006, 10:04PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

uzzors wrote ...

Hey I'm in luck! The mains side of the PSU has a voltage divider in it! I should have realized this before... It gives 150 volts, at what I assume is low current.
I have wound the new primary, but since it was intended for 300v use, its 80 to 90 turns. The resistance is 2 ohms and I've also got an inductor in place, so I guess the power problem is solved.

Now I have to get that driver fixed up...

EDIT: Turns out the 555 and my MOSFET blew.

I guess I really need to build for something better.

Sorry, now I don't have time to read all posts, but it looks like you are trying to drive a flyback with single MOSFET with 150V in!

This will generate HUGE voltage spikes across the fet, at this voltage well over 1kV, maybe even 2kV. Unless you have special high voltage mosfet and a snubber across the D-S, it will die from overvoltage instantly.

Marko

Sat Mar 04 2006, 10:17PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Diode across mosfet + zener/TVS + cap is usually full protection, sometimes clamp diode with resistor can be added across primary.
All flybacks in TVs work with single transistor very nicely, with good protection circuit will live forever.

As I can figure out you are trying to use 150V from capacitive divider of SMPS, that is for half bridge and single transistor just drains one cap and losse spower.
You get one arc and after that you must disconnect power so other capacitor can bleed out, then they charge again for another spark.

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