High Voltage DC converter for CDI

Hi

Here is a schematic of a DC-CDI.

The upper part is pulse-shaping module and the lower part is High Voltage converter, red box. The transformer is a tiny EE core with 22 turns of 0.3mm wire in primary and 140 turns+30 turns of 0.14mm in secondary.

The pin#1 of the comparator IC is issuing final shaped pulse, which is fed to the gate of the SCR. This shaped pulse is also sent to the lower part via 1N4007 to pause the oscillation while the HV capacitor is being discharged.

I simply need help to understand the working of the lower part of the circuit, especially the network connected to 30T secondary.

The transistor at the bottom and to the left of the transformer is the HV oscillator. When the DC output exceeds the voltage of the two zenner diodes (200V) the transistor to the right of it conducts and shorts out the feedback, so the transistor stops oscillating.
Frank

chuckey said:

The transistor at the bottom and to the left of the transformer is the HV oscillator. When the DC output exceeds the voltage of the two zenner diodes (200V) the transistor to the right of it conducts and shorts out the feedback, so the transistor stops oscillating.
Frank

Click to expand...

Thanks for your response.

I need to understand the how the oscillation starts,
what is the function of the feedback secondary and
what is the function of 8.2V zener?

Here is a similar circuit with a slight change at the feedback secondary, rest exactly same. I believe both circuits doing same. Just for the sake of learning, what is the difference between these two approaches.

one end of the zener is biassed to 1/2 X 12V = +6V, when the other end goes to -2V the zenner conducts and protects the base of the transistor from negative volts that would blow it up.
oscillators need two things, a source of "gain" and some feed back mechanism, to make sure that any noise is fed back to be amplified and the amplified version adds to the original noise. In the end some limiting happens so the signal stops at some level such as the collector voltage going from 0 to Vcc. As this is happening at some frequency the gain is higher then at all others, so the oscillations stabilize at this frequency. The transformer performs the function of making the phase of the voltage at the base equal to the phase of the voltage at the collector. If you swopped the connections to 1 and 3 on the transformer it would stop working as aan oscillator.

Frank