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Problem with Flyback Driver

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Mike

Wed Mar 08 2006, 04:56AM

Registered Member #58 Joined: Thu Feb 09 2006, 05:40AM
Location: Tri-Cities, Washington, US
Posts: 317

Hi, I am trying tobuild this High Voltage PSU.

I'm only building the flyback part, and my input for the 555 is 12v, input on other side of primary is 37v. Instead of the B208D transistor used there (datasheet:

)
However I am usinga slightly different transistor BU252DAF (datasheet:

)
However it is not working correctly, the output of 555 is 10v@15khz with the pot tuned one way, but the output of the transistor is only 2khz.
Flyback is not working at all.
Also please don't reccomend another circuit for flyback driver unless its extremely simmilar, I have this one almost done and I have a friend who made one and I've used it and it works very well.
Thanks,
Mike

...

Wed Mar 08 2006, 05:46AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

I am guessing that the 555 does not have enough power to turn on the transistor.
You could solve this by using a mosfet (easy drop in solution), or by putting in a driver between pin 3 of the 555 and the transistor. This could be a simple pair npn/pnp transistor or n/p Chanel mosfets, or a specially designed mosfet driver (since you are not using the transistor in the linear range you can use mosfet drivers).

One thing to mess with the circuitry between the transistor and the 555. You could try just using a resistor in series with the transistor, but if I am reading the datasheet right you need about at least 100ma (at 5v that is a 50ohm resistor), probably more to drive it into saturation...

Good Luck!

Mike

Wed Mar 08 2006, 06:25AM

Registered Member #58 Joined: Thu Feb 09 2006, 05:40AM
Location: Tri-Cities, Washington, US
Posts: 317

Ahhg, I am sorry I did not say the correct frequency of the 555. Max frequency is 5khz@10V
Which is like nothing. also im using Cap values for c1 and c2 as 104

Mike

Well I finally got it so the 555 is oscillating and im getting, up to 28khz from the 555 but it still is sucking and the flyback gives only the tiniest arc, so i guess it probably is that the transistor isnt getting enough voltage, I'll have to get something else to power the 555.
Mike

Sulaiman

Wed Mar 08 2006, 09:41AM

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

Do not increase the 555 power supply voltage.
555's work best between 5V and 15V, more than 18V could be fatal.

Look at fig.11 in the datasheet for BU2520AF;
to minimise transistor overheating enough current must flow into the Base of the transistor
to ensure low voltage from Collector to Emitter, hence lower heating power.
Suppose you want 4 A through the primary, assume the transistor will be hot;
I would ensure AT LEAST 0.6 Amps base current.
(if you want 8 Amps then about 3 Amps would be required for the Base)

A 555 is only rated for 0.2 Amps maximum from the output pin,
so it cannot fully turn on the transistor.

Option 1 (Makes life simple)
Use a suitable MOSFET or IGBT transistor. They require very small drive currents.

Option 2. (make good use of the BU2520)
Put a driver/buffer between the 555 and the BU2520.
It could be a full buffer like a TC4422
or a transistor (mosfet or bipolar)

I used a BU2522 like this for a while,
a great benefit is the very high voltage rating (1500V)
which allows extremely high secondary voltages for a given flyback or ignition coil.

Dr. Shark

Wed Mar 08 2006, 10:12AM

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

Without actually reading through the thread, may I suggest simply not building a HV supply based on a bipolar transistor? This has never worked properly for anybody, and never will, since we have MOSFETs these days, which perform MUCH better.

Marko

Wed Mar 08 2006, 11:00AM

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

Connecting base directly to pin 3 of NE555 is very bad idea in any mean.
One more or two better transistors in darlington are must, this way you will burn 555 and possibly transistor that doesnt turn on fully.

Posted: Wed Mar 08 2006, 11:12AM
Without actually reading through the thread, may I suggest simply not building a HV supply based on a bipolar transistor? This has never worked properly for anybody, and never will, since we have MOSFETs these days, which perform MUCH better.

If enough current is fed to their base bipolars can outpreform msfets, especially at lower frequencies.
With constant voltage drop they are much similar to IGBT's but (maybe) harder to drive.

As they dont have gate capacitance that must be charged and discharged they don't need huge peak currents like mosfets, simple darlington pair can be used to drive them efficiently.

I built some of such small drivers that worked nicely with 2N3055's, so i dont see real problem here.

Steve Conner

Wed Mar 08 2006, 12:50PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Bipolars work fine in flyback drivers, if you use the right part (a BU208 or similar transistor that was actually designed for flyback driving, is much better than a 2N3055) and make sure it gets plenty of base current (they can need up to 1-2A)

btw- don't forget the snubber cap between the transistor C and E

Marko

Wed Mar 08 2006, 12:56PM

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

Ironically I found even 2N3055 driving flyback in some old TV's.
Also found one interesting transistor, 2SD951, 1500 V (!) 3A, needs hell of base current to go.

Sulaiman

Wed Mar 08 2006, 12:59PM

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

joe doh, if you read the post before yours (mine)
you would realise that I have had success with bipolars,
in fact bipolars are BETTER than mosfets for flybacks in some respects,
why do you not see MOSFETS in TV amd Monitor flybacks? (IGBTs may take over)
because high-voltage MOSFETs have a high ON resistance for a given cost/amount of silicon.
If folks can get some performance out of a poorly driven 2N3055 with 60V rating
imagine what you can do with a 1500V rated device.

The need for current-drive makes bipolars a little harder to drive
than voltage-drive mosfets/igbts.
But the benefits can be worth it.
(e.g. anyone can salvage a suitable part from an old TV or Monitor)

Because I had some IRFP460A mosfets originally intended for SSTC use
(donated via Adam Horden)
(I found them too 'fragile') I now tend to use a MOSFET for a flyback.

The BU25xx range of transistors are cheap and comon
(any of the hv HOT transistors will do)
and are good candidates for Royer/zvs on UK 240Vac too.
(feedback winding(s) required)

Mike

Wed Mar 08 2006, 03:02PM

Registered Member #58 Joined: Thu Feb 09 2006, 05:40AM
Location: Tri-Cities, Washington, US
Posts: 317

Well my power supply for the 555/bipolar is 12v@3A AC rectified.
Mike

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