Single transistor flyback driver. / High Voltage / Forums

Forums

4hv.org :: Forums :: High Voltage

« Previous topic | Next topic »

Single transistor flyback driver.

1 2 next

Move Thread

LAN_403

Platinum

Thu Mar 14 2013, 10:31PM

Registered Member #3926 Joined: Fri Jun 03 2011, 08:32PM
Location: UK.
Posts: 525

I know this is a very bad circuit to drive flyback in the first place, but I have a question, I'm using a HOT (BU4506AF) with 15primary and 7 feedback turns, 270Ohm and 33Ohm resistors, I've also found that higher turns on the primaries have better output, as less will just convert more energy into heat, the input voltage is 49vDC and the arcs are not very good, they start at 2cm and stretch to 3.5cm, but can anyone tell me why the output is so bad for such a high input voltage?

Also at 12-19vDC the driver can run for a longer period of time (10min), is the transistor inhibiting the power output of the flyback, as I have found that other transistors around the same power rating do have a better output. At 12vDC the transistor c5763 (I think) is better than my current transistor when now at 49vDC.

Also the arcs get slightly thicker when I add atleast a 1uF cap to the collector and emitter.

Ash Small

Fri Mar 15 2013, 09:20AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I think HOT's usually drive flybacks via what is effectively a GDT (sounds like your HOT is a BJT, so doesn't have a gate).

I think the 'GDT' is a step down transformer, so thus increases current.

This may be the reason, I'm guessing here.

Maybe someone else will comment?

Platinum

Fri Mar 15 2013, 03:46PM

Registered Member #3926 Joined: Fri Jun 03 2011, 08:32PM
Location: UK.
Posts: 525

I've increased the primary turns to 20Pri/10Feedback, and I've added a diode to collector and emitter and 1.7uF of capacitors also, the power has increased quite a bit. It's now beginning to start to flame like a hot arc, it also stretches further. And the transistor is cooler.

Is there anything else I can do to get this circuit even more efficient?

What topology to TV's use when the drive them?

Harry

Fri Mar 15 2013, 04:06PM

Registered Member #4081 Joined: Wed Aug 31 2011, 06:40PM
Location: UK
Posts: 139

Try adding more capacitance for more power. There will be an optimum value for capacitance which you can find experimentally.
TV's use a flyback topology to drive EHT (flyback) transformers.
Ash I've found the GDT's to be a smaller turns ratio on the output also. Normally potted in a small enclosure as well for some reason.

Ash Small

Fri Mar 15 2013, 04:12PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

This is the circuit from a Toshiba TV, if I remember correctly. The 'GDT' and HOT are in the bottom left hand side of the image below, the flyback is on the right hand side (the bit with lots of coils and diodes).

1363363978 3414 FT152015 Horizontal Drive Circuit

This image is from page 3 (I think) of the .pdf below, so you can download it, and zoom in, etc, to follow the full circuit.

]orega.pdf[/file]

EDIT: It's on page 4

Q404 is the HOT

T401 is the 'GDT'

Q402 drives the 'GDT'

HOT collector waveform:

1363365551 3414 FT1630 Collector Waveform

(At least, that's my interpretation of it)

Platinum

Fri Mar 15 2013, 04:13PM

Registered Member #3926 Joined: Fri Jun 03 2011, 08:32PM
Location: UK.
Posts: 525

For some reason smaller turns and the power decreases and the transistor get's very hot (saturation?). I've found the optimal rating of the capacitors any lower and power decreases and any higher and the power stays the same. Does the voltage divider have a part in how much power the circuit can put out? I just raised the input voltage to 60vDC, I'll be running out of power supplies soon.

Ash Small

Fri Mar 15 2013, 06:45PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Platinum wrote ...

For some reason smaller turns and the power decreases and the transistor get's very hot (saturation?).

The reason the HOT is getting hot (no pun intended

) is probably because there is insufficient current to fully turn it on.

It is deigned to be switched using the 'GDT' (actually base drive transformer, but readers of this thread will be more familiar with GDT's, it works on broadly the same principle)

Have you seen the edits to my previous post?

Platinum

Fri Mar 15 2013, 10:27PM

Registered Member #3926 Joined: Fri Jun 03 2011, 08:32PM
Location: UK.
Posts: 525

So there's not much that can be done? As in the circuit will not get any better? The base current of this transistor is 3A, surely my supply is supplying 3Amps to turn it on fully, or is this is how the circuit works?

Ash Small

Sat Mar 16 2013, 10:43AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Platinum wrote ...

So there's not much that can be done? As in the circuit will not get any better? The base current of this transistor is 3A, surely my supply is supplying 3Amps to turn it on fully, or is this is how the circuit works?

Post edited

The feedback coil turns the transistor 'off', the voltage divider turns it on.

You'd need to work out the current supplied by the voltage divider, and adjust the resistor values to give at least 3A at whatever voltage is required. I think these can require more current as the temperature rises, but I'm not sure about this point. Do you have a link to a datasheet fpr this particular HOT?

Platinum

Sun Mar 17 2013, 08:16PM

Registered Member #3926 Joined: Fri Jun 03 2011, 08:32PM
Location: UK.
Posts: 525

Yes here it is

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob