Bally 5000 Plus, Manual

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11.
Appendices
Appendix 5: POWER SUPPLY
2.
The Power Converter
The power converter uses a pulse-width-modulation (PWM) technique. Transistor Q I
functions as a switch controlling the amount of energy sent to the secondary of transformer
Tl. When Q I is on, current increases linearly in primary winding " IF- I S" of transformer T l .
When Q I turns off, current flows i n the secondary windings via D I8/D I 9, D20/D2 1 ,
D22/D23, and D27. The capacitors o n the secondaries are charged up, and current flows to
the load. Q I switches on again when current in the secondary reaches zero and the cycle
repeats itself.
The tertiary winding "2S-2TP" and the diodes D I and D30 provide transformer
demagnetization when Q I is switched off by returning the transformer magnetic energy into
the input DC bus.
In transformers, there is a parameter called "leakage inductance" which results from the
magnetic flux in the primary winding not coupling to the secondary windings. In actual
operation of the transformer, the magnetic flux related to leakage inductance kicks the
magnetic energy back to the primary winding " IS- I F" in the form of high-rising voltage
spikes appearing on the dotted terminal " IS" when Q I is switched off. These voltage spikes
may exceed the col lector-emitter voltage rating of Q I and destroy it. Since the dotted
terminals of the windings are in the same polarity and the tertiary winding is tightly coupled
to the primary winding, the diodes DI and D30 conduct when the spike voltage at terminal
"2S" is above the input DC voltage. This clamping action of the tertiary winding and the
diodes limit the peak voltage at the collector of QI to twice the DC input and hence protect
Q I from being destroyed.
DI and D30 are I N600V diodes. Two of them are put in series to obtain a peak reverse
voltage of 1200V because the highest PRY for a single fast-recovery diode attainable from
the market is lOOOV.
When the power supply is first turned on, Q I gets a "kick start" from the voltage divider
consisting of R2, R3 and R5. The voltage across R5 provides a starting base current for Q l .
Further base current is provided by the voltage induced in winding 2TP-2F. The base
current is limited by R7 and C 12. Transistors Q3, Q4 and their associated circuitry comprise
a booster for ensuring the turn-on of Q l.
The power converter is regulated by a "control signal" fed back from windings W3 and W4
as well as the 5 V DC output voltage.
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