Circuit Descriptions - TTI QPX1200 Service Manual

PFC Board
Mains Input, Filtering and Rectification
The AC input is via PJ3, a pcb mounted IEC plug, and fuse FS1.
X-capacitors C1, C4, C7, C10 & C68, together with the Y-capacitors C5, C6, C11 & C12, common
mode inductors L1, L2 & L3 and differential inductors L4 & L8 comprise an input filter which ensures
that the supply meets both conducted emission and conducted immunity EMC requirements.
VD1 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 connectors W1, W2, W3 and W4.
The bridge rectifier comprised of D1, D2, D3 and D4 has C13 and C14 connected across it to
bypass noise signals and to snub reverse recovery transients in the bridge rectifier.
C15, C16 further decouple the input filter and rectifier from the boost converter.
R2 limits the initial inrush current into the boost stage output capacitors C19 and C20 and is by-
passed by relay PL1 once the boost converter starts.
Auxiliary Supplies
Primary Side
The primary auxiliary supplies are generated by IC5. This is a 7 pin TO220 style device which
incorporates the control circuitry and power switch to implement an off line flyback power supply.
It derives its power from the boosted HT rail.
R93 sets the power switch peak current to 0.4A. D20, R55 and C47 clamp the drain voltage of the
power switch at a safe level. The low voltage output of transformer T9 is peak rectified by D21 and
C49 . The output voltage is regulated to 24V by zener diode D22 and feedback to the control pin is
via opto coupler IC15. The 24V rail powers the fans.
IC6 generates an 18V rail for the PFC control IC4.
IC7 generates a 15V rail for the power pcb primary control circuit.
Secondary Side
A 5V supply for the front panel keyboard is generated by IC9. This is an 8 pin SMD style device
which incorporates the control circuitry and power switch to implement an off line flyback power
supply.
It derives its power from the AUX_HT line, which is generated by D36, D37 and C81, C37. This rail
is decoupled from the HT boost rail. When the mains supply is switched off, this AUX_HT rail falls
quickly and the front panel display vanishes very rapidly.
Start-up Control Circuit
D23 and R58 provide a 5.1V reference voltage to the negative inputs of all 4 op-amps of IC8.
When the mains is switched on, potential divider R59, R59B, R60, R60B and R61 feeds the
attenuated full wave rectified mains voltage to the positive input of IC8-A.
The bulk storage capacitors C19, C20 are being charged via inrush current limiting resistor R2.
When the mains voltage exceeds approximately 80Vrms, the output of IC8A begins to produce
pulses, which charge C21 via D24. When the mains voltage reaches approximately 87Vrms, the
voltage on C21 exceeds the reference voltage on IC8-B negative input and the output of IC8-B goes
high, charging C55,C56 via R67.

Circuit Descriptions

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