(L-3) Dc/Dc Converter For H Deflection Circuit - NEC MultiSync 4FGe Service Manual

1-3. DC/DC converter for H deflection circuit
This circuit supplies DC voltages to H deflection circuit and controls H size and side-pin cushion correction by this
output voltage.
This DC/DC converter circuit is composed of a step down chopper circuit type, FET drive circuit and control circuit.
This chopper circuit is composed of Q510, L504, L505, D508, C521, and C522.
This circuit supplied DC voltage of 35-120V to the H deflection circuit. The output voltage level is in proportion to H
size and horizontal frequency.
The relationship between output DC voltage of the chopper circuit and horizontal frequency is shown in Fig l-3-4.
The FET drive circuit is composed of the buffer circuit of 0518, Q519, R570 and level shift circuit of R522, R523,
C520, D506, D507, and ZD503. The DC level of the output pulse from the control circuit pin @ of IC5A4 is shifted
and is supplied to FET Q510 by this circuit. (Fig 1-3-2)
The control circuit is a negative feedback circuit and supplies PWM (Pulse Width Modulation) pulse through FET
drive circuit to FET Q510.
The output pulse of T502 is clamped by C526, D510 and is rectified by D511 and C527.
The value of this rectified voltage is same as the crest value of T502 output pulse.
This rectified voltage is divided by R527, R528, D523, VR503 (H SIZE VR) and is supplied through buffer amp to
the error amp in IC5A5. H size control voltage and sidepin cushion signal is supplies to the negative input of error
amp.
The output voltage of the error amp is modulated.
The output voltage of the error amp is supplied to PWM circuit of IC5A4.
The output of error amp is converted to PWM pulse by IC5A4 and the PWM pulse of IC5A4 output is supplied
through FET drive circuit to FET Q510.
44