Philips MC-77/22 Service Manual page 56

Class-D Circuit Description (
Basically Class-D works by transforming the LF audio input
to a square wave signal with a fixed frequency and a
variable duty cycle. See simplified drawing below.
Modulated Digital Audio Signal
+VL
0
-VL
Duty cycle 9:1
(DC value nearly +VL) (DC value is zero)
Amplitude pos. max.
Amplitude zero
+VL
0
-VL
LF Analog Audio Signal
The amplitude of the square wave signal is equal to the
supply voltage of the amplifier. With the audio signal the
square wave signal is pulse-width modulated.
Compared to a conventional power amplifier the benefits of
the Class D amplifier are:
• higher effiency
• lower power dissipation
• smaller cooling fin
• smaller mains transformer
Disadvantage of this concept is:
• 500kHz square wave signal with high current requires a
shielding box to suppress radiation.
Blockdiagram Class D
analog audio signal
f= 20Hz ...20kHz
Analog Pre-amp.
Modulator
5V
0V
square wave
500kHz
Square wave
duty cycle 1:1
Oscillator
-32V
-38V
B P
ASED ON
Duty cycle 1:1 Duty cycle 1:9
(DC value nearly -VL)
Amplitude neg. max.
square wave
500kHz
variable duty cycle
+38V
-30V
square wave
500kHz
variable duty cycle
5V
0V
+5V
Level
OFF
shifter
ON
-32V
-38V
square wave
500kHz
-38V
variable duty cycle
11-2
2002 M - 70W C
OWER ODULE
Required Circuitries:
• 500kHz square wave oscillator.
The oscillator frequency is created by 7312-C; it is a
dual-frequency oscillator with ceramic resonators 5300
and 5302, which resonate at 500kHz and 425kHz
respectively. The resonators are switched by transistors
7309 and 7316, controlled by the "OZ_SW" line from the
port expander 7406.
The reason for 2 frequencies is to avoid tuner
disturbances in the AM-band.
The oscillator signal is shaped to square wave with
7312-B, afterwards buffered and fed to the amplifier
modulators (ROZ to the right channel, LOZ to the left).
One channel gets inverted clock to balance supply
loading.
t
• Modulator
The modulator forms the pulse width modulated signal.
For each channel a seperate modulation is required.
• Power FETs
The FETs require drivers which supply the gates. One for
the high-side-FET and one for the low-side-FET.
Because of the different supply voltages also an
additional level shifter is necessary per driver.
• Output filter
The output filter is necessary to block the 500kHz square
wave signal from the speaker.
It consists of a series-mode coil 5101 and a capacity of
approx. 500nF (2116, 2134), which forms a Chebycheff
filter with 40kHz cut-off frequency at 6Ω load. For EMC
reasons both, the speaker output and the return ground
are fed through a common mode coil 5102, the filter is
further improved by splitting the output capacity into 2116
before and 2134 after the common mode coil.
All above mentioned circuitries are located inside the metal
shielding box.
V1L+6V V1L+6V
Level
shifter
V1L
OFF ON
-32V
Driver
-38V
D)
LASS
Power-amp.
+AMP
square wave
500kHz
variable duty cycle
+38V
-38V
amplified analog audio signal
output filter
-AMP
f= 20Hz ...20kHz
Blockdiagram Class D 2002 04 05