Understanding a Single Transistor Flyback Transformer Driver?

[doggy]

TL;DR

Circuit problem in Flyback Transformer Driver

I've been watching a few analyses of CRT flyback transformer drivers and can't understand why the transistor would shut off. To my naive perspective, it appears that the transistor should never shut off due to the positive DC voltage applied constantly to the base. I'm aware that this isn't the case since transformers need changing currents to induce currents in their other coils. I'm aware that the coils have inductance which dampens current buildup and charges the inductor core, but I can't wrap my hear around the transistor region.

(Reference: https://www.kynix.com/Blog/The-Complete-Explanation-of-a-Flyback-Transformer.html)

Circuit Diagram:

 

[Baluncore]

Notice that the transistor is only just biased on.
You do not indicate the polarity of the primary and feedback windings.
At some frequency, the feedback loop through the transformer will have a gain greater than one. The turn on transient, or any noise, will be amplified until the circuit oscillates continuously.

 

[DaveE]

Try searching google for "self oscillating flyback power supplies". You'll get a bunch of more realistic versions (also more complicated). I thought https://www.electroschematics.com/diy-rcc-smps-circuits/ had a good description.

I think the idea is that the base initially has positive feedback to turn on, but eventually the base drive current is limited so at some point the collector current grows too large to keep the transistor saturated. The collector voltage then rises and causes a decrease in base drive, so now there's positive feedback to turn it off. You will get a big voltage imposed on the collector when the magnetizing current starts to flow in the secondary winding (the flyback part). This will act to turn the transistor off via the base winding. When the transformer energy has dissipated into the load the secondary diode turns off and the cycle can repeat starting from the small resistor bias. In practice these circuits are more complicated than they look though.

I thought the link you provided was a good introduction to more modern flyback SMPS design. This sort of self-oscillating version has been long since replaced with IC based control circuits. Frankly, these self oscillating designs are a really poor way of driving switching devices efficiently.

 

[alan123hk]

The DC bias provided by R1 & R2 starts transistor Q1 conducting as soon as power is applied. Current begins flowing through the transistor and the Collector winding L1, which induces a voltage in the Feedback winding L2. This induced voltage on L2 translates a positive voltage to the base of the transistor, forcing the transistor to increase current flow. As the collector current flow increases, the voltage coupled through the transformer saturates the Base.

As the collector turns full on, the transistor output drops to zero. The current has been increasing through the transformer. When the collector current reaches maximum and/or the coil becomes saturated, magnetic flux between the windings, L1 to L2, begins to stop changing. The base voltage and current drop sharply, and than shutting off the Collector current.