TTI EX752M, Service Manual

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Circuit Description
Main Pcb
Mains Input, Filtering and Rectification
The AC input is via a pcb mounted IEC plug, PJ3.
X-capacitors C1, C9 and C47, together with the Y-capacitors C6, C7 and common mode
inductors L1 and L2 comprise an input filter which ensures that the supply meets both conducted
emission and conducted immunity EMC requirements.
VDR1 clips mains spikes for component protection, while R1 discharges the X-capacitors after
removal of the mains supply.
Pcb mounted fuse FS1 limits damage on boost-stage or forward converter failure. The front panel
mains switch is connected via PJ4.
The bridge rectifier BR1 has C11 and C12 connected across it to bypass noise signals and to
snub reverse recovery transients in the bridge rectifier.
C89 further decouples the input filter and rectifier from the boost converter, while R113
discharges C89 after removal of the mains supply.
RT1 is an NTC which limits the initial inrush current into the boost stage output capacitor C88.
Primary Control
The primary control is by means of IC8.
This contains all the processing circuitry to shape the current taken by the power supply from the
mains supply to be sinusoidal and in phase with the voltage.
Initial start-up is via R79 and R80 which charge C32. When the voltage on C32 exceeds the
under voltage lockout level of 16V, IC8 starts up and supplies drive pulses to the boost MOSFET
Q5 . The gate drive pulses to Q5 are given a slight negative offset by C94, D52, D53 and R112 to
give extra noise immunity.
C117 provides a degree of soft start.
Once the boost converter is running, the supply to IC8 is derived from an auxiliary winding on the
boost inductor L3, rectified and filtered by D7, C2 and C15 and also from a power back winding
on the main converter transformer, rectified and filtered by D6, C87 and C93.
R131, R132, R133, R71, C4, and C34 form a two stage RC filter to feed a voltage proportional to
the mains voltage to pin 7 which, after a 1/V
2
transformation, is fed to the multiplier . For a
universal mains input, the voltage at this pin will vary between 1.5V and 5.5V.
R135 and R136 supply a current derived from the full wave rectified mains voltage to pin 4 to be
used by the internal multiplier.
Also fed to the multiplier is the output of the voltage error amplifier of pin 13. The input to the error
amplifier at pin 14 is from potential divider R74, R75 and R93. This potential divider monitors the
voltage on the boost stage output capacitor C88 and this is compared to an accurate internal
reference voltage. Thus a steady voltage of about 380V is maintained across C88.
Another potential divider R81, R82, R98 monitors the voltage across C88 and feeds into pin 3 of
IC8 which is the over voltage shutdown.
Pin 8 is the output of the multiplier and is the programming current for the internal current error
amplifier. This signal is taken to the mains supply side of the current sense resistor R83 via R8.
The other input of the current error amplifier is taken to the other side of R83 via R72. The current
error amplifier thus compares the signal across the current sense resistor R83 with the
programming current and adjusts the PWM drive to the boost MOSFET Q5 accordingly. R73 and
C86 with R72 form the compensation network around the current error amplifier.
When the mains supply falls below about 85Vrms, the circuit comprising of long tail pair Q6 and
Q10 and Q1 act on the soft start input of IC8 to curtail operation.
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