Primary Lps - Barco SLM R12 Plus Service Manual

15. Lamp Power Supply R7633705
Switching on of Q400-Q401.
A short while before Q400-Q401 are driven into conduction, Q403 stops conducting (see timing diagram). Just before
Q403 stops conduction, there flows a current in L401/Q403/C404. This current is modulated on the input current
flowing through L400/L401/D404 towards the output.
The above current charges up the D-S capacitor of the MOSFETS and this means that Q403 can switch off with a
minimum of losses.
Even when the mosfet Q403 fully stops conducting the coil L401 tries to maintain the current . This current now flows
in the following circuit : D404/C408,C409/GNDM /D-S cap. of MOSFETS and the parallel capacitors C402, C403.
The above current discharges C402/C403 from 380V to 0V. As soon these capacitors are discharged the inverse
diodes in the Mosfets Q401/Q402 start conducting and this is the moment to drive the mosfets in conduction (see
timing diagram t1).
The input current, flowing in L400 via D404 to the output, gradually is taken over by the drain current of Q401-Q402.
The di/dt decrease of the diode current is slow due to the L401, hence its dissipation is low. There flows however
an inverse recovery diode in D404. This current finds its way through C404 D-S cap. of Q403. The drain-source
capacity is discharged from 380V to 0V and the inverse diode of Q403 starts conduction. This is the moment to
drive the mosfet on (t2).
When Q403 is conducting, the current in L401/C404/Q403 reverses and becomes a forward current in Q403 (from
drain to source). This is the current mentioned in the start....
Switching on of Q400-Q401.
The switching off time of the main switchers happens when the PWM output of the controllers goes low. Q403 remains
in conduction to maintain the current in the mentioned circuit (see "switching on").
Elimination of the switching losses happens by adding the capacitors C402/C403 in parallel with the drains of the
mosfets.
At switching off, the drain current must now charge up first these capacitors. The drain voltage increases slowly
and, hence, the drain current decreases equally slowly.
Above only is possible on condition these capacitors are discharged by the above-mentioned diode reverse recovery
current (see switching on).
Short voltage peaks are removed by D403/C405. The physical position of these components is such that they are very
close to the drains in order to limit the peaks to 500V. R413 provides a discharge path for the capacitor to the output.
Drive of the power Mosfets
The timing diagram of the drive of the main switchers Q400-Q401 and the auxiliary switcher Q403 can be as seen (see timing
diagram).
• Floating drive Q403 (OUT A of I470).
The off time of Q403 must be constant and start coincident with the rising edge of the PWM output. This time (0-t2) is pro-
grammed with monostable I460 to input A of driver I470.
Since we require a floating drive between -30V and +380V, a pulse transformer is used.
Drive of the main switchers.
•
The delay between the rising edge of the PWM output and the switching on time of the main Mosfets is programmed with the
monostable I460 to input B of driver I470.
I470 is a non-inverting mosfet power driver generating drive pulses having 15V amplitude.
For the main (powerful) switchers a buffer is necessary to drive power FETs with a high input capacitance at a high frequency.
Temperature protection
An NTC resistor, mounted on the heatsink of the converter, is part of a resistive divider (R326 TEMP_HTSNK_PFC and R336
TEMP_HTSNK_LPS). The output of this divider is checked by the microprocessor which calculates the heatsink temperature from
the non-linear input (NTC is a non-linear device).
As soon a programmable threshold is reached , both, the PFC and the LPS are switched inactive.

15.6.4 Primary LPS

Feedback (see sheet 2/5).
The feedback informations IS and VS enter via J603 from module R7632113 and need to be multiplied in the analog multiplier I231
(74HC4051) on sheet 2/5.
IS is proportional with the lamp current and related as 0.04V / A
VS is proportional with the lamp voltage and relates as 0.1V / V
The output voltage W (PS) (pin 5) is function of the feedback voltage returned to Z or pin 4 of I251.
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lamp.
R5976820 SLM R12+ PERFORMER 08/03/2005