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Bu508d breaking down.

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Tiemes

Thu Mar 04 2010, 04:07PM

Registered Member #2718 Joined: Thu Mar 04 2010, 03:36PM
Location: Netherlands
Posts: 3

Hi,

For a big school project I wanted to make an ionocraft. But for an inocraft you need around 20Kv and because school does not have a high voltage power supply I had to create my own. By using a flyback transformer. I have used this diagram to build my power supply.

I am only using the bottom part of the diagram, The top part is replaced by 2 school power supply's (12V and 0-30V) . So far so good, I have build the diagram and it works. With a input of 5V I get around 5000V output. But the problem is that the Bu508d is breaking down the whole time. I have replaced it for 4 times now but after some test it breaks down. I have looked around the internet but nobody seems to have the same problem with their high voltage power supply.
Some time I also have the problem that the output voltage isn't stable but hovers around a certain altitude. Most of the time it stables out if I change the frequency but not all the time.
Does somebody know why my BU508D is breaking down all the time.

Here are some pictures of my High voltage lunch box

Thank you very much and sorry for my pour English.
mod edit: all your pix are belong to OVERSIZED

Dr. Dark Current

Thu Mar 04 2010, 04:50PM

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

What frequency is the circuit running at? It might be too low, around 20kHz is best.
What is the number of turns on the primary winding and the core gap? The inductance might be too low, or the core could be saturating.

Edit: The 10 ohm resistor is loading down the output of the 555 too much.

Edit2: You need a FAST DIODE anti-parallel to the base! Replace it with e.g. an UF4007.

MinorityCarrier

Thu Mar 04 2010, 05:14PM

Registered Member #2123 Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312

The 2N3055 is the wrong transistor for your 60V supply, its BvCE is 60 volts. You should use a 100V device at least.

Your schematic, if I am reading it correctly, shows the 12v supply is fed by the 0-60V supply. You are exceeding Vmax input (35V) for the 78L12, plus when the 0-60V output is dialed down to ~12V your 12V regulator will stop regulating.

uzzors2k

Thu Mar 04 2010, 05:48PM

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

I would use a mosfet instead of a BJT, or at least put a buffer between the 555 and BU508D. During my newbie days when I built 555 flyback drivers I found BJTs would always die if simply connected to the 555 with a resistor and no protection diodes. After trying some mosfets the problems stopped.

Proud Mary

Thu Mar 04 2010, 06:14PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

BU508 likes a couple of amps base current - no way you can squeeze that out of 555, so the BJT won't be switching properly.

cjk2

Thu Mar 04 2010, 09:25PM

Registered Member #51 Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263

The BU508 will need more base drive as said before. Try a Darlington arrangement or use a larger driver. I would also put a clamp across the C-E of the transistor so it isn't overvolted. String some TVS devices together to get maybe a 1200v rating.

hboy007

Fri Mar 05 2010, 02:15PM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374

Tiemes,

I have successfully built a flyback driver circuit, unfortunately I am not allowed to give you the exact schematic, but here are a few hints:

use the BU2508DFI or BU2508AFI with an external freewheeling diode / TVS diode rated 1.2 .. 1.5 kV. The "F"/"FI" types have insulated packages so you don't have to worry about the collector tab insulation, the D-Types have internal freewheeling diodes. Remember, these are no fully rated TVS diodes! The BU2508 is a revised version of the BU508. C6 is too small, R6 is too large, D2 is too slow.

although the BU2508 has improved switching characteristics, both versions have really low current gain factors, usually around 10, dropping to 3-5 as the collector current rises. You want to drive the base with a low voltage, low internal resistance current source. The NE /LM555 can source and sink around 200mA, the CMOS LMC555 can handle no more than 10/50mA.

You can find current transformers with ratios of 10:1 to 20:1 (I found one that is 16:1) in television set cirucits - they are there for some good reason! In addition, they generate a negative kick to turn off the transistor which I think is very convenient. These transformers are nearly impossible to buy off the shelf, so you are perhaps better off with a step-down converter and a high current half bridge controlled by the NE555.

By the way: you can really get 40W output without the need to apply heat sinking to the BU2508 once you get the base current waveform right

ps. start with adding a little ceramic capacitor (220-1000pF / 2kV) in parallel to the collector-emitter tabs to clamp the rate of rise / peak voltage the transistor sees - this will protect the transistor from overvoltage and improve your chances of switching it off properly. Otherwise the transformer may resonate with its parasitic capacities and pulse the base through the miller capacity

Tiemes

Fri Mar 05 2010, 02:54PM

Registered Member #2718 Joined: Thu Mar 04 2010, 03:36PM
Location: Netherlands
Posts: 3

Thank you for all you reply's
@ Dr. Kilovolt I already changed the potentiometer, by a 10k version to get more frequencies.
@ MinorityCarrier you are taking about the top part of the diagram, but i have changed that by separate power supply's.
@Uzzors I think I am going for your solution by changing the BU508d BJT by a MOSFET I have found that the IRFPC50 is the mosfet "version" of the BU508D.
@hboy007 The problem is that the BU2508DFI or the BU2508AFI aren't available in Holland and because my knowledge isn't that big I am first going to try the easiest solutions. If they fail I am going to try your approach.
Conclusion.
I am going to change the BU508D by a IRFPC50 MOSFET(they are expensive though, 10 euro), and I am going to chance the diode by a UF4007.

uzzors2k

Fri Mar 05 2010, 03:56PM

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

If you try a mosfet you can/should remove C6 and D2. Change R5 to 68 ohms to protect the 555 in case the mosfet shorts from gate to source.

Antonio

Fri Mar 05 2010, 09:41PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

I made some power supplies to power "lifters". They can be seen here:

The first version is not very different from yours. I used a better transistor, taken from the same monitor board where I found the flyback transformer, and mounted it with adequate protection (D3, C3). The other version shown has a more efficient driver for the transistor.

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