# PC SMPS Startup by self oscillation

Hello every one,

I was puzzled with the PC SMPS circuit that is found in most
taiwanis PSU, i can't figure out how does it start, so i tried some
experiments to know how it works but with no success till now,i am
seeking help in understanding this self oscillation startup.

The first thing i did was making a schematic of my old AT PSU; i put
it online at www.mathworld.8m.net/AT_PS_Primary.pdf
I collected info from the web and from this group which can be
summurized into:
1) R107 & R109 give start up current to drive BJT.
2) Proportional base drive move one of the BJT into stauration.
3) Oscillation is not by resonans between C107 and T101 or T102.
4) Oscillation is due to transformer T102 but how; no one tell.

The experiments i tried was:

1)I removed R107 & R109 and as was expceted the PSU didn't start.

2)I disconnected power from TL494 in secondary then i found that:

2.1)The PSU work ok with output about 4.8V when unloaded but when
loaded this voltage drop to 2.5V @ 1A.

2.2)The frequency and duty cycle while self oscillating was about
1.4khz when unloaded and 1.6khz when loaded but the ON time is nearly

2.3)When I shorted C107 the PSU work ok with the same frequency,C107
have about 40V constant DC when not shorted.

Any help will be very helpfull, thanks very much.

2. ### leggGuest

With a TL494 controller in the circuit, you have to have overlooked
two items:

A control winding on the proportional base drive transformer.

A supply winding for the TL494 on the main transformer.

There may also be a tickle winding capacitively coupling the main
transformer to the lower base drive winding, though this is not
strictly neccessary..

The circuit properly self-oscillates with 50/50 duty at the natural
frequency set by the saturable volt-seconds of the base winding.

When these voltseconds are exceeded, the drive voltage collapses and
'off' bias is provided by the electrolytic capacitors C109C110. The
resetting base drive transformer turns on the opposite switch -
reinforced by the current feedback winding on the same transformer.

In the self-oscillating mode, sufficient power is developed by a main
power transformer winding housekeeping supply, in order to wake up the
control circuit. The original TL494 had no undervoltage lock-out,
which could be either a help or a hindrance in this circuit, depending
on the control configuration.

The tickle winding mentioned earlier is required only in situations
where the main transformer saturates first, or is severely overloaded.
Reducing volts on this winding can be used to turn the switch off
early.

The TL494 controls duty cycle to less than the saturation limit on
each cycle and sets a resulting increased operating frequency. It does
so by effectively shorting the (push-pull centertapped) control
winding, that is not present in your drawing, to turn both switches
off. The control winding is large turn and low current, so a short is
a low-power control function - this winding can even supply power to
the controller.

The TL494 turns the opposite switch on by opening the winding with the
correct phase, starting the regenerative proportional base drive
transformer on to it's next opposing phase.

This particular circuit was popular in Lambda, Boschert and Todd
bipolar designs originating in the 70's. They all had patents on
specific functions of the circuit - some even covering the range of
cross-sectional areas of practical drive transformer cores sizes.

It's a two-transformer converter developed by Jensen, in turn a
variation of the early Royer converter.

RL