Bose SoundDock 10 Service Manual page 72

SoundDock 10 Theory of Operation
(Q400). R406, R409, and R410 are 3 discrete resistors in series rather than a single
resistor so that we do not exceed the maximum voltage rating of the resistors.
R400, R401, R402, and C401 AC couple the line voltage into the D400 rectifying diode
which charges C408. This circuit works with U400's internal voltage clamp to create a
~9V supply rail. Note that this is negative with respect to pin 1 of the triac. U400 pulses
pin 5 to turn on the triac. It uses pulses rather than a constant voltage to minimize power
dissipation. C403, R408 work together to generate clock pulses for U400.
DC Set Point Resistors (R412, R412, R506, and R507)
DC set point resistors are needed when the bulk capacitors are charged in series which is
the case in the 240V variant or the dual voltage variant with a 240Vac input. These
resistors work to set the voltage on each bulk capacitor. Without them the DC voltage
would be set solely by the leakage current on each bulk capacitor which could result in
asymmetric charging which could cause an overvoltage condition in one of the capacitors.
Switching MOSFETS (Q500 and Q501)
Q500 and Q501 work together to send a square wave into the primary side of T502. The
duty cycle of each is controlled by the PWM controller and is proportional to the input
voltage and output voltage. It is critical that Q500 and Q501 are never on at the same
time.
These MOSFETS have a surface mount heat spreader attached to them to keep their
temperature down. These devices actually run hotter at lower output power levels
because they are run more into hard switching (switching while the voltage across them is
non-zero).
Transformer (T502)
The transformer is a key component in the asymmetrical half bridge. It takes the square
wave generated by Q500 and Q501 through the C515 DC blocking cap and sends it out
of the secondary windings as well as another primary side winding used to generate the
"self supply". This transformer has been carefully designed to ensure there is no cross
conduction and to minimize Q500 and Q501 switching losses. The transformer has
asymmetric secondary windings in order to minimize voltage ripple on the secondary/
keep the output inductor and capacitor size to a minimum. This comes at the expense of
generating a slight DC magnetizing current which has to be kept well away from the
saturation level of the core. Asymmetric half bridges work best when there is a fixed input
and output voltage. Our system has this to some degree but deviates when the AC line
voltage changes.
Output Diodes (D503)
The output diodes work to half wave rectify the square wave pulses sent out by the
transformer. They are rated at 400V because they need to block nearly 200V plus
derating and there are little to no 300V rated diodes. These diodes require heat sinking to
perform over full range of output power. Care should be taken to heatsink the TO-220
14