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Since X2 = Y2 = 0 the output voltage is:
(R261+R262+R260
VS + IS
PS=
x
10
(R261+R262)
VS x IS
PS=
+DIM
2.4
The feedback circuit starts regulating from 2.5V onwards (internal reference level of the controller I500). The PS output pin 5 of the
analog multiplier must therefore reach 4V (taken into account the divider R263 / P260 / R264).
Dimming
•
DIM = 0 V (max. power)
If DIM=0V and the PS output should be 5V, then VS x IS = 10V
Taken into account the ratio's between Vlamp / VS and Ilamp / IS we can calculate that:
•
DIM = 2
If we apply the above formula, we see that VS x IS should be 8V now. The output power for the lamp will consequently be
reduced to 1000W .
The DIM voltage is generated by the microprocessor I214 and added to the offset of the analog multiplier.
Current limiting
When the regulating circuit stabilises the lamp power, the current will increase heavily when the lamp voltage drops. However, there
are limits for the circuit and for the lamp itself. This limitation is realised with the voltage clamper built around I250. Let us illustrate
this with an example.
Assume we like to limit at 80A. This corresponds with a lamp voltage of 25V when the power is stabilised at 2000W.
In this case the controller will install a VL_MIN = 0.1 x min. voltage, or, 2.5V.
D250 remains reverse biased as long the VL_MIN voltage is higher than this limit. But, when VS would like to drop below the pin 5
level, the diode is forward biased and clamps pin 1 of the analog multiplier at 2.5V.
This means that the output current, irrelevant the lamp voltage being lower than 25V , will be limited at 2000W/25V = 80A.
In general, the VL_MIN is calculated by the processor in function of the lamp power as follows:
A second clamper implements a 2V at the comparator input pin 5. This is a second protection of the maximum current on a higher
level.
In the unlikely event that the microprocessor would fail to generate the VL_MIN voltage.
At a 2000W installed power, the 2V level means a current limitation at 100A.
Controller I500
I500 is a controller IC from Unitrode generating the four drive outputs for a phase shift modulated full bridge (4 power Mosfet switch-
ers).
The oscillator frequency is tuned at 200 kHz (C800-C809), the transformer works at 100kHz and thus the converter again at 200kHz
(full phase rectifier).
The controller can be stopped and turned on via the Soft Start (SS) pin 19. The dead time between the A/B and the C/D outputs is
installed with the resistors R508 and R509 and the voltage at pin 11 (ADS) .
With an increase of the primary current, the dead time is shortened since the output capacitance of the FETs are much faster charged
up and discharged when the current goes up.
The analog multiplier output is adjusted with P260. The adjusted voltage on P260 is sensed by the isolation amplifier I270. A
differential output volatge is created on the other side of the I270 optical isolation barier. This differential voltage is converted to a
single-ended signal PENSE_M by I312 and feeds pin 1 inpuT of the controller I500.
The internal reference in the controller IC is 2.5V. This means that the feedback circuit will adjust the power until the voltage at pin
1 is 2.5V. To reach this 2.5V, the lamp voltage and lamp current are constantly adapted. By adding an offset to the Z-input of the
multiplier we can "mislead" the controller and this is used for dimming purposes.
This regulation circuit is relatively slow, a fast (dynamic) current limitation is necessary. A current transformer 1 : 100 feeds a current
proportional voltage to pin 12 at low impedance. This pin has a two-level detector.
From the 2V level onwards, the drive of the FETs is momentarily stopped. If however a 2.5V is applied, the controller completely
stops and re-starts with a soft start.
Drive of the full bridge mosfets
The respective outputs are directly supplying, via a 10 Ohm, the High (HIN) and Low (LIN) inputs of the FET drivers IR2110.
To speed up the discharge of the gate-source capacitors a discharge-transistor (Q552, Q553, Q562, Q563) is added in the gate
drive.
R5976820 SLM R12+ PERFORMER 08/03/2005
+DIM
15. Lamp Power Supply R7633705
8
V
x I
=
lamp
lamp
0.1 x 0.04
P
lamp
VL_MIN =
10 x I
max
=200W
(in A)
119
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