Honeywell bendis king KLN 94 Maintenance Manual page 21

BENDIX/KING
4.3.1.2.4 ON/OFF Control
The On/Off Control circuit monitors the signal from the front panel On/Off switch (PWR_SW_LO),
the avionics power bus (AIRCRAFT_POWER_MONITOR), and the microprocessor-controlled
shutdown line (POWER_SHUTDOWNn). If these inputs allow the unit to turn on, the On/Off Con-
trol circuit outputs an active-low ENABLEn signal. When the ENABLEn signal is low, the LV On/
Off Switch circuit allows the LVPS to start up. Q6028 and Q6030 bring the ENABLEn line to the
low state when both are on. Q6028 is turned on when the front panel On/Off switch is pushed in
to the ON position, connecting PWR_SW_LO to PWR_SW_HI. While in normal operation, Q6030
is held on by the presence of +5V through R6096 and CR6038.
However, during initial startup, Q6030 can only be turned on by the presence of at least +6VDC
on the AIRCRAFT_POWER_MONITOR line, or by C6058 being charged through R6080 and
CR6037 while Q6028 is off (front panel On/Off switch pulled out to the off position). Under this
condition, Q6030 is held on by C6058 long enough for the LVPS to start once Q6028 turns on
(front panel On/Off switch pushed in to the ON position). The circuit operates this way to allow the
unit to restart (if the KLN 94 is connected to an external emergency battery supply) by cycling the
On/Off switch after the avionics power bus drops out and the unit has been shut down due to a
POWER_SHUTDOWNn being asserted.
A POWER_SHUTDOWNn can be asserted by the KLN 94 Host microprocessor when the KLN 94
is connected to an external emergency battery supply and the AIRCRAFT_POWER_MONITOR
line indicates a loss of avionics bus power. This is done to conserve battery supply life. A low
level on the POWER_SHUTDOWN line causes R6079 to conduct base current from Q6023, turn-
ing it on. This causes C6057 to rapidly charge through R6098, turning on Q6029. When Q6029
turns on, it pulls the gate of Q6030 low through R6095, turning off Q6030. As the LVPS shuts
down and +5V falls to 0V, C6057 slowly discharges through R6099 keeping Q6029 on long
enough for the unit to remain shut off.
4.3.1.3 LVPS Circuitry
4.3.1.3.1 LV Controller/Switcher
The Low-Voltage Power Supply (LVPS) generates the required unit DC power outputs. This pow-
er supply is a Flyback topology using a dedicated current-mode control pulse width modulator
(PWM) integrated circuit (IC) (U6002). The LV Controller/Switcher circuit is made up of this IC and
U6003-B which provides undervoltage shutdown. Power supply voltage to the control circuitry is
provided through CR6028 from the +13V output when the LVPS is running or through CR6029
from the Bootstrap Power Supply during startup.
This voltage is switched by the LV On/Off Switch (Q6018-A) and is applied to U6002 and U6003
only if the ENABLE line from the On/Off Control circuit is low. The PWM IC generates an internal
reference voltage which is accessible at pin 14 (5V_LVPS_REF). The PWM IC operates at 128
kHz as set by R6055 and C6033, and the output at pin 10 drives Q6016 (an N-channel switching
FET) through R6050. During the time that Q6016 is turned on, current is sourced from C6024,
C6025, and C6020 on the XFMR_HI line through the primary windings of the LV Flyback Trans-
former (T6001) and returned to ground through current sense resistors R6048 and R6049. These
resistors generate a current sense voltage that is filtered by R6051 and C6041 and provided to
U6002 for current-mode regulation.
When Q6016 is turned off, the energy stored in T6001 as magnetic flux causes current to flow in
the secondary windings. This is known as Flyback. Parasitic elements in the transformer also
cause a Flyback voltage spike to occur in the primary of T6001. A snubber network consisting of
C6021, R6047, and CR6019 capture this energy and return it to power input capacitors C6024,
C6025, and C6020. Ferrite inductor L6005 reduces unwanted RF emissions.
Page 4-22 15599M00.JA
KLN 94
Rev 0, Sept/2000