Current sensing and primary side transistor driver transformer

[japlytic]

In most PC power supplies around the early to mid 90s, they had separate current sensing and primary side transistor driver transformers. Most new PC power supplies use one transformer for current sensing and primary side transistor driving. I don't know how that would be possible (without the current sensing circuit being affected by the isolated primary side transistor driving pulses from the PWM controller)

An example of such a unit is on http://www.technick.net/public/code/...=cir_ps_atx200

[PeteS in CA]

Re: Current sensing and primary side transistor driver transformer

Well, T2 in that circuit doesn't sense the primary current for current limit purposes. However, as you spotted, the primary current does flow through one of the windings. This is a half-bridge inverter, and the two transistors turn on in alternation. When the top transistor is turned on by the TL494, the windings of T2 are phased so that the flow of primary current induces an aiding (regenerative) current in the base winding for the top transistor. Then when the bottom transistor is turned on by the TL494, the flow of primary current induces an aiding current in the base winding for the bottom transistor. The ratios of the primary current winding to the two base windings are designed so that the base current is a set proportion of the emitter current. This drive scheme is called "proportional drive", and the designed proportion is called "forced beta". This lessens stress on the TL494 O/P drive circuits and ensures adequate base drive current under low-line conditions.