NEC LCD1550X-BK, Service Manual

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1.2 Detailed description of each circuit
1.2.1 Pseudo-resonance circuit method
With pseudo-resonance, when the discharge of energy (secondary coil current = 0) ends on
the secondary side of the transformer, the transformer ends the transformer connection.
Pseudo-resonance takes place between the primary side inductance and voltage resonance
capacitor (or parasitic capacity) between the drain and source. The drain voltage is reduced to
the sine wave. When the drain voltage reaches the bottom (valley) of this vibration waveform
and the next cycle turns ON, soft switching takes place. With the effect of the voltage reso-
nance capacitor, the dv/dt of the V
DS
during turn OFF becomes gradual. This series of opera-
tions is called pseudo-resonance. The turn ON timing is applied using the signal that has
passed through the delay circuit using the flyback voltage waveform of the auxiliary coil.
<Circuit and waveform>
C1
D2
2
D3
1
C3
C2
V
OCP
P
VD
T= LpC4(delay time)
D
D4
R3
C4
C5
R4
R5
VDS
4
5
V
DS
V
D
I
D
V
P
V
P
V
IN(DC)
fR=1/(2 LpC4)(resonance frequency)
V
DS
lower limit
V
OCP
3.4
OCP/FB terminal threshold voltage (2) 1.5
OCP/FB terminal threshold voltage (1) 0.76
V
P
V
IN(DC)