TTI QPX1200 Service Manual page 12

Boost Stage
Three boost inductors L5, L6, L7 are each driven by a pair of MOSFETs Q3+Q6, Q2+Q5, Q1+Q4
respectively to raise the incoming, full wave rectified, sinusoidal mains voltage to a value some 30V
higher than the peak of the maximum rms input voltage. This is achieved by storing energy in the
boost inductors when the MOSFETs are conducting and transferring it to C19, C20 via boost diodes
D5, D6, D33 when the MOSFETs are turned off.
When the MOSFETs turn on, the boost diodes D5, D6, D33 are still conducting current. To prevent
large switch on losses, delay inductors L9, L10, L11 are placed in series with the MOSFETs. This
ensures that the current in the boost diodes is reduced at a gradual rate and the reverse recovery
time is minimised at the same time as the drain voltage of the MOSFETs is allowed to fall rapidly.
The energy stored in L9, L10, L11 is transferred to C18, C61, C62 via D19, D7, D8 and dissipated in
R23, R6, R9.
Snubber networks C22, R15, R19, C23, R16, R21, C24, R18, R22 reduce switch-off losses in the
MOSFETs.
The main boost heatsink is protected against excessive temperature by PTC2, IC12 and associated
components; once triggered it is necessary to turn the mains power off and on again to restart.
Power Board
Forward Converter
The 400V HT supply from the PFC board is connected to the Power board via FAS1 and FAS2 and
decoupled by C2.
C1 and C3 with their respective bleed resistors R149 and R150 form the voltage divider to feed the
converter transformer T1 of the half bridge forward converter.
The half bridge configured power MOSFETs Q28 and Q29 are connected to the converter
transformer T1 via the current transformer CT1 and ZVS inductor L2 and operate at a fixed duty
cycle of about 42% at a switching frequency of 73kHz. See the gate drive waveform in
Photograph1.
C4 and C12 work with L2 to facilitate zero voltage switching for Q28 and Q29.
R11, C16 and FB3 damp high frequency ringing across T1 primary. See photograph 02.
The gate drive pulses for Q28 and Q29 are generated by oscillator IC2-B/IC2-C feeding into IC3-A
to produce complimentary square waves at IC3-A pins 1 and 2.
The leading edges of these anti-phase square waves are slowed by R39/C37 and R42/C38 and
buffered by IC2-A,E,D,F to produce anti-phase pulse trains of the correct duty cycle to drive the gate
drive transformer T3 via Q13, Q15, Q17, Q18 and C35. The supply to the gate transformer drivers is
decoupled by C17, C28, C29, C30. The gates of Q28 and Q29 receive their anti-phase drive from
the secondaries of T3 via R52 and R73 and respective anti parallel diodes D1 and D2.
The output of the current transformer CT1 is full wave rectified by D16, D17, D22, D23 and loaded
with burden resistor R26. The voltage developed across R26 is utilised by two different circuits.
The first is to turn on Q11 via zener diode D27 when the current in the primary power switches
exceeds a certain value determined by R157 and R158. Q11 collector charges C26 negatively via
R21. When the voltage across C26 exceeds a threshold set by zener diode D10, latch Q9, Q14
turns on and pulls signal SDWN down to the P0V rail via D20. This in turn causes the power factor
correction circuit to shut down – requiring the removal of the mains supply in order to restart. This
would only happen under repeated short-circuiting of the output. Q9, Q14 latch can also be
activated by the signal OPTO1 via R102 – as would be the case when the voltage on the secondary
exceeds a value set by zener diode D5. The primary heatsink is protected against excessive
temperature by PTC TH3 which turns on Q16 and this in turn activates latch Q9, Q14.
The voltage developed across R26 is also utilised to regulate the fan speed. As the secondary
loading increases, the voltage across R26 is peak detected by D50 and C31 and sets a demand
11