Bose SoundDock 10 Service Manual page 75

SoundDock 10 Theory of Operation
Gate Driver (U504)
Simply takes the PWM inputs and translates them into an output voltage that each of the
MOSFET's can use. It also makes sure that both MOSFET's are never turned on at the
same time. It does this by inserting a dead time between the turn off of one MOSFET to
the turn on of the other. This is true even if the input signals call for both MOSFET's to be
turned on at the same time.
The gate driver has a built in high voltage section that allows the circuitry to float at the
source voltage of Q500. The gate driver uses a charge pump to generate a voltage
higher than that of the source of Q500 to be able to turn the MOSFET on at the
appropriate time.
Self Supply Circuitry
The self supply circuitry has several functions. First, the dc set point resistor divider is
used to charge C500 on Vin. When Vin gets above approximately 20V, the PWM
controller turns on it's 8.5V regulator at Vcc and starts to operate. This Vcc signal is then
fed into the self supply circuitry via R551 which in turn turns on Q507. Second, once
Q507 is on, Q505 and Q506 along with associated resistors (and capacitor) act as a Vbe
multiplier to generate a rough 15V. This 15V then is fed to the gate driver which allows
the supply to start to work. As the primary is switched, the other winding on the primary
provides power to this self supply circuitry and is half wave rectified through D506.
Thirdly, this self supply waveform is rectified and fed to U503 which generates ~21V on
the Vin line. The power supply is at steady state at this point and the PWM controller will
be supplied through self supply and U503 rather than through the dc set point resistor
dividers.
5V LD Regulator (U401)
U401 is a low drop 5V regulator. It is used to power the NOR gate even when the system
is in low power mode and the +24V rail is at 6.9V.
NOR gate (U200)
Pins 11, 12, and 13 of the NOR gate are used to drive a signal from one side of the pulse
transformer to the other. The inputs NOR together both BIGMAX_OFF and
OL_PS_SYNCH_CLK. Under normal conditions, OL_PS_SYNCH_CLK toggles at
384kHz which synchronizes the PWM controller. When the +24V output is PWM'd down
through BIGMAX_OFF, OL_PS_SYNCH_CLK is turned off (low) and BIGMAX_OFF
sends a pulse through the pulse transformer. The reason for this is explained below in
the peak detect circuitry section.
Isolation Pulse Transformer (T501)
The isolation pulse transformer takes the AC couples pulses from the NOR gate on the
secondary side and outputs pulses on the secondary side through D502 which also
serves to reset the windings. This pulse is AC coupled to the sync pin of the PWM
controller through a 100pF capacitor (C529). The PWM controller operates at
approximately 150kHz (output switching frequency = input sync/2) in freerun mode which
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