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[SOLVED] Lower frequency of self resonant flyback driver

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Junior Member level 2
Feb 28, 2011
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Campbell, CA
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I would like to slow this circuit down to about 1-2KHz. At present it's about 38KHz with the transformer I have.

Is there a simple way to do this with a single capacitor ?


cap across CB is amplified more than across CE and 0 deg phase shift at resonance, so smaller value is needed. Compute impedance of Zc and turns ratio to determine effective Q due to Z ratio on collector due to load R. Higher R results in higher Q.
The frequency of a blocking oscillator like that depends on the rate of current rise through the primary winding, until the core saturates.
Frequency depends on the applied voltage, the inductance, and magnetic characteristics of the core.

Nothing you can do to slow it down except use a much larger higher inductance transformer.
Its not really resonant in the normal sense, it relies on alternate saturation and then self turn off of the transformer.
Thank you both for the response. Out of curiosity I threw a large (2uF) cap from B to E and the result was a fairly sharp high amplitude square wave at about 200Hz. I was excited till the secondary opened up. Good thing I have a box of these transformers.

I was going for very low parts count but I think I will try a 555 controlling a transistor on the primary. I only need about 500V peak at 1KHz or lower. Thanks again.

You will want to prevent saturation and monitor current and core temp.
If you are loading secondary, then likely burning up the fine wire.

try a microwave oven transformer.

You will want to prevent saturation and monitor current and core temp.
Saturation is only o/k when its a part of a normal oscillation cycle, where it will instantly turn itself off and then repeat the cycle.
That is a perfectly safe mode of operation.
If you try to drive it directly with a 555 at a much lower frequency, core saturation is just about guaranteed. And its going to burn things up fairly readily.

Your best bet might be to run your 555 astable at whatever frequency you want, and try coupling into the primary with a suitable capacitor.
The spikes produced will not have much energy, and it will not be very efficient, but it may provide some worthwhile voltage on the secondary if the secondary load is very light.

Conceptually you could use a 555 one-shot to delay the turning back on of the transistor at the end of the cycle (when the transistor normally turns off due to the start of transformer saturation).
You could use the saturation spike at the end of the cycle to trigger the 555 one-shot, with the 555 output inhibiting the turn-on of the transistor for the desired time.
After the one-shot times out it would allow the transistor to turn back one and complete another single fly-back cycle.
This would allow the normal fly-back operation but for a low frequency rep rate of the pulses. The transistor ON pulse width is not controlled with this scheme, only the rep rate, thus avoiding excess transformer saturation.

Thanks for all the suggestions. I ended up using a primary oscillator (555 type) to operate a mosfet switch on the XFMR primary so I can pick the frequency. This made things a lot more straight forward and the application is working so far.

Thanks again all...

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