Philips MCM595/21 Service Manual page 50

Class-D Circuit Description (
B P
ASED ON
Functional Description:
Refers to the left channel in schematic diagrams.
The first stage of the modulator is an error integrator which
compares the input to the (24 dB amplified) output signal of
the power stage. The difference is leading to a current,
which loads the integrator 7122-A. The second stage
(7122-B) adds the 500kHz rectangular oscillator signal,
creates high gain and low distortion and is again
integrating. The output signal leads to a triangle wave form
(see oscillogram E). The DC value of this triangle signal is
floating, dependent on the amplitude of the analog input
signal. The next stage is a comparator, which compares the
integrated voltage with the internal switching levels - thus
creating a voltage controlled duty cycle. 7122 C and D
improve the shape of the pulses. For details see
oscillograms A-F.
At pin 8 of IC7122 there is a square wave with the same
frequency and duty cycle as the desired output.
The next task is to feed this information to the output FETs.
Both FETs are n-channel types, so they are modulated by
feeding the gate in respect to the source connection.
Inverters 74LV14 are used as drivers. The driver for the low-
side FET (7121) is supplied by the negative supply -VL2
and a voltage +VL (generated by 7115 and 6113), which is
7.5V higher than -VL2. The digital signal is level-shifted by
7128 to the low side driver stage. 3142, 6111 and 2126
together form a delay circuit for rising edges by approx.
100ns for the low side FET. 3154, 6109 and 2137 delay the
rising edge by 50ns for the high side FET. This to
compensate the switch-off delay of the FETs and ensures
that both FETs are not conducting at the same time.
The high-side FET (7109) is controlled by the inverted
signal taken from pin 2 of 7118, which is level-shifted by
transistor 7119. The driver for the high-side FET is supplied
by a floating voltage between the amplifier output -V1L and
+V1L, created by the charge pump 6110, 2114 regulated by
7114 and 6114 to a 7.5V higher level. The pump is
additionally supplied (via 3151) by +45V to ensure supply at
start-up (no signal). The last stage in the gate driver
consists of three gates in parallel for increased output
current for the capacitive load of the FET. For additional
increase of the switching speed push/pull transistors
7132/7111 are added.
Protection Circuits:
The amplifier is protected against low load impedance
(including short circuit). Current is sensed by shunts 3101,
3130 in both supplies. Overcurrent at the positive supply is
then sensed by 7104, the negative supply overcurrent
triggers 7117, which then also triggers 7104. The collector
current in turn triggers the monoflop 7122-5 and -6, giving a
"High" pulse at pin 10. This shuts off level-shifter 7128 and
blocks transistor 7129 and 7131, which draws current into
the emitter resistor 3134 of level-shifter 7119. It is now also
shut off. So, both FETs are shut off within approx. 0,2 sec.
The monoflop can be reset by:
- switching mains off and on again
- pressing Standby button and afterwards any source button
- plugging headphone in for a short moment
When a headphone is used the amplifier is shut off. This is
done by pulling pin 13 of 7122 via signal line "AMP_OFF"
and transistor 7130 to high level. The line "AMP_OFF" is
controlled by the port expander 7406 which detects the
headphone via signal line "HP_DET".
The loudspeakers are protected against DC voltages
resulting e.g. from defective FETs. Voltages higher than ±2V
are detected by 7110 respectively 7112. The transistors
conduct and pull the "DC_PROT" line down, blocking
transistor 7243 which in turn disables speaker relay 1201.
11-2
2003 M 75-150W C D)
OWER ODULE LASS
Adjustments:
The gain of the class-D amplifier is 24dB, adjusted by the
feedback resistors 3155, 3136, 3149 and the input resistors
3139, 3340. The input reference voltage for 7122-A is
approx. half the supply, therefore 3144, 3148 are used for
offset compensation. This compensation has to be fine-
tuned with trimpotentiometers 3306 and 3307 to obtain
<1mV DC output.
Service Hints:
The analog part of the Combi Board can be repaired
without opening the metal shielding box. In case of a 'Class
D' problem it is advised to disassemble the board first,
desolder the metal bottom cover of the shielding box and
assemble the board again. This takes a few minutes only.
To de-solder the metal bottom cover proceed as follows:
1)Remove top cover of shielding box to reduce heat flow
2)Do not use de-solder wick
3)Simply hold the board upright down as shown in picture 1.
Heat up solder joints and move tip of soldering iron slowly
along the edge of the metal frame.
Solder will flow along the soldering tip and rest on the
metal plate. A small amount will drop off.
A small gap will become visible as indication that the
solder connection is released.
When all solder joints are released the cover can be
removed by help of a screwdriver. Begin at the corner
indicated by an arrow → see picture 2.
solder will rest
on the metal plate
pict. 1
desolder 1
put a screwdriver underneath
the plate and lift it
pict. 2
To re-mount the metal bottom cover proceed as follows:
1)The solder connections are not distributed evenly.
Therefore the cover has to be mounted in that way that
the arrow indicator on the cover is positioned in line with
the arrow printed on the printed board → see picture 3.
Service Hints
2)Press the metal cover smoothly down until the gap
between cover and printed board becomes a minimum.
This is important for proper shielding.
3)Heat up the residual solder on the metal cover. The solder
will flow back to the solder areas. If necessary apply
additional new solder.
4)Take care that all solder joints are re-soldered again.
Attention: Poor soldering of the metal shielding box results
in disturbance of the tuner.
press metal cover down
until gap is a minimum
gap
pict. 3
In most cases the FETs 7109 and/or 7121 for the left
channel, respectively 7218 and/or 7231 for the right channel
will be defective. This can easily be checked with an
ordinary Ohm-meter.
L C :
EFT HANNEL
In case 7109 is defect replace following parts:
7109, 7111, 7132, 7105, 7119, 7104, 3101, 3103 and 2106
In case 7121 is defect replace following parts:
7121, 7113, 7133, 7118, 7117, 3129, 3130 and 2118
R C :
IGHT HANNEL
In case 7218 is defect replace following parts:
7218, 7221, 7244, 7209, 7228, 7208, 3205, 3209 and 2206
In case 7231 is defect replace following parts:
7231, 7210, 7245, 7235, 7227, 3241, 3243 and 2220
Attention: Do not forget to adjust the DC-offset after
replacing the FET!
If none of the FETs is defective the fault is most probably
located in the modulator. To check the operation - follow the
given signals.
General description of Oscilloscope setup:
Ground Ch1 & Ch2
desolder 2
Y-Deflection
CH1 = 20V/DIV
Y-Deflection
CH2 = 20V/DIV
Timebase = 1µs
Triggerlevel =-33,6V
Triggered on Ch1
11-2
The following signals are measured on condition:
AUX in = 500mV/1kHz, Volume = -28dB
Load = 2 x 6Ω
A
Measuring point can be found on circuit diagram
All other measuring points are shown on circiut diagram
5
respectively .
A
Measuring point : Output pre-amplifier
align arrows
Normal analog signal measured (1kHz- Timebase 1µs). If
this signal can´t be measured - the fault is outside the
shielding box.
B
Measuring point : Input Modulator
The 1kHz signal is not visible anymore. Reducing the
timebase to 1µs shows the oscillogram below.
3
.
4