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Improved single transistor flyback driver

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Marko

Wed Jan 23 2008, 12:35PM

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

If I had to build a blocking oscillator like that, first thing I'd do is to use a small saturable current transformer for feedback instead of saturating the power transformer's core. That should help efficiency especially with slower transistors.

Dr. Dark Current

Wed Jan 23 2008, 04:30PM

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

Marko wrote ...

If I had to build a blocking oscillator like that, first thing I'd do is to use a small saturable current transformer for feedback instead of saturating the power transformer's core. That should help efficiency especially with slower transistors.

Marko, if I wanted to go one complexity step higher I would use a 555 driving a MOSFET

Really this is just a "extension" of the classic STFD and it can push approx. 3 times as much power through the flyback without the transistor exploding

DuartmaN, do you mean driving the coil from the output of the flyback? If so, this would work but you need to rectify the flyback (if you already don't have a rectified one).

flannelhead

Wed Jan 23 2008, 05:36PM

Registered Member #952 Joined: Mon Aug 13 2007, 11:07AM
Location: Finland
Posts: 388

Can this circuit produce dangerous currents if a small FBT and 12V is used? I really wanna build this circuit but first I want to know how dangerous it is.

Dr. Dark Current

Wed Jan 23 2008, 06:07PM

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

spark wrote ...

Can this circuit produce dangerous currents if a small FBT and 12V is used? I really wanna build this circuit but first I want to know how dangerous it is.

No circuit is dangerous if you observe safety precautions

(like draw arcs to earthed wire and stay away from the HV output) I think not too dangerous currents are involved, probably below 20mA on the output.

I still need to fix the schematic, this 0.68 cap is too big, probably around 0.33 to 0.47 would be better. And to add that the flyback must use an air gap.

Littlew

Mon Apr 21 2008, 06:11PM

Registered Member #1448 Joined: Sat Apr 19 2008, 01:16PM
Location: Russia/Moscow
Posts: 21

Wow
I'll try to made this circuit at holidays

Tonic

Mon Apr 21 2008, 06:43PM

Registered Member #528 Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166

If we're back on topic, let me ask one thing - how diode and capacitor improve this circuit? I guess that capacitor bypasses spikes to supply rails, but diode? At first glance I thought that it bypasses current from primary's magnetic field to negative and helps with heating transistors, but diode is placed in way that blocks this current, so I have no clue what it does.

Dr. Dark Current

Mon Apr 21 2008, 07:05PM

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

Tonic wrote ...

If we're back on topic, let me ask one thing - how diode and capacitor improve this circuit? I guess that capacitor bypasses spikes to supply rails, but diode? At first glance I thought that it bypasses current from primary's magnetic field to negative and helps with heating transistors, but diode is placed in way that blocks this current, so I have no clue what it does.

The diode is there to extend the transistor's life - bipolar transistors don't like negative spikes on collector (I believe this is the reason why it sometimes fails in the original circuit).

The cap and the primary winding form a parallel resonant circuit and the driver starts to operate in "pseudo class E" mode, not exact Class E switching, but it improves performance greatly and decreases transistor heating (mainly because it has more time to turn off - Toff switching losses are minimised).

Firefox

Wed Apr 23 2008, 12:41AM

Registered Member #1389 Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346

I recently tried this circuit after my first few HV projects (two Tesla coils and three CW multipliers-threads to come) when a friend was getting rid of an old CRT monitor. My flyback is the crappy encased kind, but overlooking that it is rated for 26kV output with a 2A 250V input. However, being new to the hobby I didn't have any buckets of the seemingly abundant 2N3055, so I went searching through my broken computer PSU and other boards looking for a suitable replacement, and came across what I think is a real find in the Sanyo 2SC4106. While it is only rated for 7A collector current and 3A base current, the C-B rating is 500V and the C-E rating is 400V. E-B is 7V. The performance with 12V at up to 6A (computer PSU) in is reasonable with arcs starting at 1-3mm and drawing to 10-15mm, I would really like opinions on how far I can push the input with this transistor. The resistors are a chain and a bank of various 3-7 watt resistors (Req are 465 ohms and 22 ohms and are made up of 6 resistors each) so there is no worry of burning them out before anything else. Answers?

teravolt

Wed Apr 23 2008, 01:48AM

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

the circuit looks like a class E tesla circuit and the 8 primary turns and .68uf cap detemins the oscillating frequency. if the biasing were changed why not use a mosfet insted if it cuold be made to oscillate

Antonio

Wed Apr 23 2008, 03:05AM

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

This circuit does not require core saturation to work. The current in the transistor is interrupted when it leaves saturation. The oscillation frequency does not depend significantly on the capacitor, that just widens the pulse that appears at the collector when the transistor cuts the current. It can be smaller. I recommend adding another diode in series with a zener diode between emitter and base of the transistor, to avoid excessive inverse voltages there.

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