Harris Performance 7000 - Manual (Page 49)

6800/7000 Series - Frames and Power Supplies Installation and Operation Manual 23
Buck Converter Chapter 4: CF-6801 Cooling Frame
Buck Converter
The buck converter specifications are as follows:
The buck converter is protected against voltages in excess of 15 V by the
Zener diode D13 and by resistor R43. This structure clamps the voltage
across U3 in the case of an over-voltage. This protection is added as a
redundant fail safe measure in the event that the Over-Voltage Protection
(OVP) circuitry provided in the 6804PS fails to activate. U3 will not
regulate properly until the over-voltage condition ceases.
Over-Current (OC) and Under-Current (UC) Sensing
A 0.05 W sensing resistor across a window comparator gives a voltage
reading that indicates the current through the fans:
• Vsense = Rsense x Ifans
Vsense is then multiplied by the OP AMP gain factor of about 174 (set by
resistors R16 and R18), to become:
• Vcompare = 174 x Vsense
This voltage is compared to the set limits which are approximately 5.1 V
and 7.5 V. It is an indication of an OC condition (there a short-circuit on
the distribution board) if Vcompare goes above the 7.5 V limit and an
indication of a UC condition (one or more of the fans is not working) if
Vcompare is below 5.1 V. Either of these conditions (OC or UC) will
cause the red error LED to illuminate and the relay will close (to make
contact for the GPI output). The expected average value of current draw
for the 10 fans is about 0.73 A. The sensing circuitry allows for about
20% tolerance on either side before triggering.
V in (max.) 15 V
V out 9.5 V (3%)
Efficiency 96.4%
Frequency 100 KHz
Imax 1.1 A