PRINCIPLES OF OPERATION
REV.-A
(5) Current
I
L
in the primary winding increases, but because the potential at point 6 of coil 6-2 is fixed, Q1
base current I
B
cannot surpass a specified level. As a result of this, the value of current Ic (= I
L
) flow-
ing from coil 4-3 to the Q1 collector cannot surpass a maximum of hfe
(1~).
Therefore, the 4-3 current
value stops changing, and the coil voltage drops. At the same time, a reverse voltage is applied to coil
6-2, I
B
drops, and current flows through D3 opposite to direction B. At this time, the potential at point
A is higher than that at point E, and C10 absorbs the current flowing in the A-E direction. Q1 is quickly
shut off by the resulting sharp drop.
(6) The above process causes the energy previously induced in the secondary side (in step (4), above) to
be released from coils 7-8, 8-11, and 9-10 in the direction opposite to D, and current flows in the easy-
flow direction of the diodes. Therefore, the secondary side outputs a voltage.
(7)
The release of energy declines linearly over time. When energy release is completed, all T1 coil voltages
momentarily reach zero. R3, however, again induces switching current lB in the direction of B, and Q1
conducts. Because the potential at point E then surpasses that at point A, the energy in C10 is released,
so that I
B
is maintained.
(8)
The sequence returns to the stage described in (3) above. This repetition enables the circuit to maintain
oscillation. R7 is involved, however, only at the start time.
The above sequence is generally known as a self-excitation type ringing choke converter (RCC) configuration.
Note that, at the instant when Q1 goes off (in step (5), above), the potential at point F jumps violently upward,
but because of the action of D2, the energy is fed into R5 and consumed.
Below are shown the waveforms for each part of the circuit.
As Figure 2-21 makes clear, the output voltage is controlled by the time period that Q1 is off. In other words,
the circuit is controlled by controlling the period during which Q1 is off.
2-16
LQ-510
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