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9. Circuit descriptions and List of abbreviations
9.1
Switched-mode power supply PS (PS Part)
9.1.1
Technical data:
Mains voltage
Maximum output
Operating frequency
Efficiency
Six different direct voltages are supplied on the power supply
outputs.
9.1.2
Functional principle:
This power supply functions in a similar way to a blocking
oscillator. In the supply voltage part [1300 to 2318], the mains
voltage is rectified and buffered in the capacitor [2318]. From
this direct voltage [2318] energy is transferred into the
transformer [5301, pins 1-3] during the conductive phase of
the switching transistor [7302] and is stored there as
magnetic energy. This energy is passed to the secondary
outputs on the power supply in the in the blocking phase of
the switching transistor [7302]. With the switch-on time of the
switching transistor [7302], the energy transferred in every
cycle is regulated in such a way that the output voltages
remain constant regardless of changes in the load or input
voltages. The power transistor is activated using the
integrated switch [7303] ( see figure 9-1 ).
3318
MC44608
8
VI
C demag
start - up
management
Vcc
management
start - up
phase
200 µA
1
0
&
stand - by
200 µA
1
0
2
leading edge
blanking
NC
7
Circuit descriptions and List of abbreviations
: 195-264 Vrms
: 15W / 40W
(continuous /
maximum output)
: 40 kHz
: approx. 75 % at
maximum output
1
DEMAG
current mirror
65mV/45mV
current mirror
&
stand - by
management
PWM
comp
9.1.3
Supply voltage part
The supply voltage part extends from the mains socket
[1300] to the capacitor [2318]. Using the diodes [6310, 6311,
6312 and 6313] the a.c. supply voltage is rectified and
buffered using the capacitor [2318]. The line reactor [5305]
and capacitor [2316] create a filter to keep interference
arising in the power supply away from the mains.
Components [1302], [3326] and [3323] protect the power
supply against short-term overvoltages in the mains, e.g.
caused by indirect effects from lightning.
9.1.4
Start-up with Mains-on:
Following connection to the mains, the capacitor [2310] is
loaded via the start-up resistor [3318] and a current source
between pin 8 and pin 6 on the IC [7303]. Once the voltage
on [2310] and therefore the supply voltage Vcc on the IC
[7303] has reached approx. 13V, the IC starts up and issues
pulses to its output on pin 5. These pulses are used to control
the gate on the power transistor [7302] ( see figure 9-2 ). The
frequency has a fixed setting in the IC (approx. 40 kHz). The
current input on the IC is approx. 5 mA in normal mode.
If Vcc drops to below approx. 10V (e.g. with power limitation)
or if Vcc exceeds around 15V (interruption of the control
loop), the output on the IC [7303, pin 5] is blocked. All output
voltages on the power supply, and therefore also Vcc,
decrease. Once Vcc has dropped to below approx. 6.5V, a
new start-up cycle begins. (See also "Overload, Power
Limitation, Burst Mode" section)
current and voltage
references
200 µA
t
thermal shutdown
&
&
PWM
latch
latch OFF phase
stand by
regulation
block
Figure 9-1
VR120
9.
VCC
6
6307
7300
5
buffer
DRIVER
4
GND
0
1
&
3
CTRL
GB 97
5301
7302
+
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