Honeywell bendis king KLN 94 Maintenance Manual page 23

BENDIX/KING
4.3.1.3.8 Power Interrupt Switch
The Power Interrupt Switch provides the control logic and power switches to turn on the Capacitor
Backup circuit during power interrupt. U6003-A senses the output voltage from the Input Hash
Filter and compares it to a reference voltage that is generated by U6002 pin 14 (5V_LVPS_REF).
R6060 provides about 0.7V of hysteresis. Since U6003-A is powered by the Bootstrap Power
Supply, the output is low at unit power-up. Under this condition, the output remains low until V_IN
drops to about +10.1VDC. At this voltage, the output of U6003-A goes high and cannot return low
until the input voltage returns to about +10.8VDC. When the output of U6003-A goes high,
CR6009 and CR6026 stop conducting. This causes Q6021 to turn on, sinking current from the
Capacitor Backup circuit through R6033 and R6040. The resulting negative gate-source voltage
on Q6012 turns it on, connecting the Capacitor Backup circuit to the input power steering diode
CR6016. CR6016 couples the Backup Power Supply circuit output to the Low-Voltage (LV) Fly-
back Transformer (XFMR_HI) and to the front panel On/Off switch (PWR_SW_HI). This also
shuts down the Capacitor Charger circuit through CR6009.
4.3.1.3.9 Capacitor Charger
When FDBK_13V is present, this circuit charges the Capacitor Backup circuit through R6014,
R6015, Q6018-B, and CR6007. The Capacitor Backup circuit is sufficiently charged at +10.5VDC
to sustain a one-second interrupt. The Capacitor Charger circuit will take approximately 3 minutes
to recharge the Capacitor Backup circuit to +10.5VDC after a power interruption and approximate-
ly 6 minutes to charge up to this voltage on initial power-up.
4.3.1.3.10 Button Backlighting Supply Circuitry
The Button Backlighting Supply generates the required unit DC power for the front panel nomen-
clature and buttons. This power supply is a simple switcher using a dedicated current-mode con-
trol pulse width modulator (PWM) integrated circuit (IC) (U6001).
Power supply voltage to the Button Backlighting control circuitry is provided through CR6005 from
the +13V output when the LVPS is running. Power is provided through Q6001 from the
FILT_AC_PWR filter whenever the unit is shut down. Voltage FDBK_13V is applied to U6001
through CR6005 only if the ENABLEn line from the On/Off Control circuit is low.
The PWM IC generates an internal reference voltage that is accessible at pin 14. The PWM IC
operates at 128 kHz as set by R6010 and C6010. Output pin 10 drives Q6008 (an N-channel
switching FET) through R6008 and R6022. During the time that Q6008 is turned off, current is
sourced to C6007 which is fed back to U6001 to maintain a constant voltage (approximately
12.5VDC). R6023 and R6025 divide the fed back sense voltage for U6001 to maintain a 2.5-volt
reference at pin 3 of U6001.
Charging current for L6001 is delivered through Q6002 during those portions of the PWM cycle
when Q6006 is turned on. Resistor dividers R6002 and R6006 work with CR6004 to make sure
Q6002 is properly turned on and that Q6002's gate-source voltage limit is not exceeded. When
Q6008 is turned off, the energy stored in L6001 as magnetic flux causes current to flow in CR6010
to C6007. Q6006 shuts off the incoming current during those portions of the PWM cycle when
L6001 is delivering its stored energy. During this time, CR6002 conducts and provides a ground
for the negative-going end of L6001.
The button backlighting requires two different current levels, one for normal operation, and one for
the situation where the unit's aircraft power is on but the unit is turned off. Q6009-B is the pass
transistor providing these current levels. Resistors R6038 and R6039 apply gate-source voltage
to Q6009-B to turn it on.
During normal operation, current flows from CR6010 through Q6009-A and current limit resistors
R6028 and R6030, as well as current limit resistor R6029. This current then flows through Q6009-
B and on to the front panel via J6005-15.
Page 4-24 15599M00.JA
KLN 94
Rev 0, Sept/2000