Marantz MAR770 - Manual (Page 26)

24
Circuit description of P816
Input circuit
The input circuit consists of a lightning protection circuit and an
EMI filter.
The lightning protection comprises R819.
The EMI filter is formed by C820, L820, C825 and C826.
It prevents inflow of noise into the mains.
Primary rectifier/smoothing
The AC input is rectified by rectifier bridge D802 and smoothed
into C821.
The voltage over C821 is approximately 300V.
It can vary from 100V to 390V.
Start up circuit and Vcc supply
This circuit is formed by R823, R834, C829, D828, R829,
R811, C833 and C811.
When the power plug is connected to the mains voltage, the
stabilised voltage over D829 (24V ) will charge C833 via
R829.
When the voltage reaches 14.5V across C811, the
control circuit of Q810 is turned on and the regulation starts.
During regulation, Vcc of IC7110 will be supplied by the
rectified voltage from winding 7-9 via L832, D832 and C833.
Control circuit
The control circuit exists of Q810, C802, C804, C807, C809,
C810, R802, R803, R804, R807, R808, R809 and R810.
C802 and R810 define the frequency of the oscillator.
Power switch circuit
This circuit comprises MOSFET Q825, Rsense R826, R827 and
R828, R825, C827, L825, R812 and R813. R825 is a pull-down
resistor to remove static charges from the gate of the MOSFET.
Regulation circuit
The regulation circuit comprises opto-coupler Q801
which
isolates the error signal from the control IC on the primary side
and a reference component D881. The TL431(D881) can be
represented by two components:
a very stable and accurate reference diode
a high gain amplifier
Figure 8-17 ‘TL 431’
TL431 will conduct from cathode to anode when the reference
is higher than the internal reference voltage of about 2.5V. If the
reference voltage is lower, the cathode current is almost zero.
The cathode current flows through the LED of the opto-coupler.
The collector current of the opto-coupler flows through R806,
producing an error voltage, connected to voltage feedback pin
14 of Q810.
Demagnetisation
The auxiliary winding (7-9) voltage is used to detect magnetic
saturation of the transformer core and connected via R801 to
pin 8 of Q810. During the demagnetisation phase, the output
will be disabled.
Overvoltage protection circuit
This circuit consist of D814, C814,
R814, R815 and R816.
When the regulation circuit is interrupted due to an error in the
control loop, the regulated output voltage will increase
(overvoltage). This overvoltage is sensed at the auxiliary
winding 7-9.
When an overvoltage longer than 2.0 (s is detected, the output
is disabled until VCC is removed and then re-applied. The
power supply will come in a hiccup mode as long as the error
in the control loop is present.
Secondary rectifier/smoothing circuit
There are 5 rectifier/smoothing circuits on the secondary side.
Each voltage depends on the number of windings of the
transformer.
The -8V supply is regulated by voltage regulator Q871.
On/off circuit
A
2.5V
R
K
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