Barco SLM R12 Plus, Service Manual, Page: 119 / 281

15. Lamp Power Supply R7633705
The rectifier bloc on the other board R7632113 is terminated with a small capacitor C400/C401 just to remove the HF components
(130khz) generated by the DC / DC converter.
The 50 or 60Hz of the mains voltage is not removed by these capacitors and the rectified sinus waveform is presented to the input
of the DC/DC converter. The amplitude is the peak value of the sinus (eg. +325V for a 230Vrms).
This input voltage is boosted up by the DC/DC converter to +380V independent of the mains input voltage.
The boost converter power stage
This power stage consists of
• switcher Q400-Q401
• Coil L400
• rectifying diode D404
• smoothening capacitors C410-C414.
The switching frequency is approximately 130kHz and the duty cycle is regulated by the controller I420.
Controller I420
I420 (L4981A) is an average current mode controller designed for power factor correction. The oscillator is tuned at 130kHz ( R432
/ C429).
The power supply voltage is ++15VM to pin 19. This supply voltage is monitored with the P-UVLO input pin 15. When this pin is
below 13.5V the drive to the Mosfets is stopped. If the PFC does not work, the LPS cannot work neither. Thus, this is a protection
for a too low 15V on the LPS circuit as well.
In standby position the pin is low via D421, because the PFC_ON_M line is at a low level.
The capacitor C433 is the soft start capacitor. The voltage across this capacitor determines the status of the controller.
If too low, the drive is interrupted. This will happen when the mains voltage drops below 160V.
The VAC is applied on pin 6 of the comparator I370 with the divider R350/R351/R352/R354/R353. Pin 7 of I370 can discharge C433
as soon the mains voltage drops below 160V. From 170V onwards the controller starts up again, with a soft start, by charging up
C433.
The current in the switcher Q400-Q401 is measured with the sense resistors R400/R401/R402. When a maximum is got, the mosfets
are turned off. This maximum amplitude changes in step with the measured voltage. The controller knows the mains voltage via the
IAC pin 4 through R424/R425/R426. Hereby is guaranteed that we have a sinus shaped current in phase with the sinus voltage.
Besides correction of the power factor, we need to make sure that this maximum current is sufficient to feed the output load. A
voltage feedback loop R7447/R448/R449/R450/R451P420/R452 supplies the VFEED pin 14 with this information.
C432/R438 is a compensation for possible oscillations in this feedback circuit.
The voltage divider R442 - R446 feeds the overvoltage pin (OVP, pin 3). The threshold is hereby set at 420V maximum.
The controller starts up again without a soft start from the moment the voltage drops below 420V.
The filter R420/R421/C420/C421/R422/R423/C422/C423 feeds an RMS (Root Mean Square) information to the VRMS pin 7. With
this information, the PFC circuit will show a saturation or limitation of the delivered power independent of the RMS value.
Auxiliary circuit for zero loss (zero volt) switching
This auxiliary circuit eliminates the switching losses of Q400-Q401 during switching on and switching off. The same circuit equally
minimizes the switching power in the diodes and the auxiliary switcher itself Q403.
Hereafter we”ll discuss in detail the switching on and off phenomenons.
Timing Diagram
Controller
PWM Out
(M460)
Q403
DRIVE_AUX
Q400 - Q401
DRIVE_IN
0
t3
0 t2
t1
t3
Image 15-13
Timing diagram
R5976820 SLM R12+ PERFORMER 08/03/2005 117