EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: GelatinCapacitor on August 15, 2023, 04:58:05 am
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I was dissasembling a SONY CRT TV, model KV-21R22/5
Before so, I was dumbfounded by the power supply section in the schematic; how the heck do you drive a transistor-driver-transformer with nothing but a TL431?
(https://i.imgur.com/9BtntRK.png)
Of course, this looks to me like a half bridge forward, but where the half bridge is exactly isn't very apparent to me.
I'm also not sure how do Q603 and Q604 not quickly kill each other. Of course I assume that is prevented somehow at T604.
I remade the circuit in Falstad just for the sake of more clarity (also attached here the falstad file just in case).
(https://i.imgur.com/rN4aeLb.png)
I'm aware of transistor driving transformers thanks to ATX power supplies, but I'm yet to study the details.
However, this transformer (T604) doesn't look like a typical transformer:
(https://i.imgur.com/6C2WSWu.png)
It's tiny, looks like a common mode choke, and has a weird shaped core, with weirdly wounded coils.
Also, it has a separator between the core pieces, so it could also be a flyback somehow?
Does this arrangement have a name? Or could you link me to other examples of this, or explanations?
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Self oscillating push-pull?
Just instead of tapping off the drive pulses from main transformer (T603) it have its own pulse transformer. Bottom half of L3 is the "primary", L2 and top half of L3 is the complementary drive secondary. Regulation is achieved by "loading" the pulse transformer from L1, modifying the L3_bottom to L3_top and L3_bottom to L2 coupling.
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It reminds me of some ancient color TV circuitry with inductors of variable inductance, controlled by a DC current through a helper coil. Maybe the strange transformer shape wants to avoid the helper coil being a secondary of the transformer.
It saves an optocoupler.
Regards, Dieter
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Our very own Tim has a note on this exact topology on his website, with a good explanation of how it works. https://seventransistorlabs.com/Monitor/index.html