Philips EM5A P/M - Page 121

Circuit Descriptions and Abbreviation List
EN 121EM5A P/M 9.
The +33V is the tuning voltage for the Tuner.
The +5VT is derived from the +8V with stabiliser 7912 (see
diagram A8), and is used to supply the tuner only.
SSB Supply
There are several voltages going to the SSB: +8V6, +5V2 and
+3V3.
The +5V2 and +8V6 (always present) come directly from the
Standby power supply.
The +3V3 is derived from the +5V with stabiliser 7910 (diagram
A8).
9.3.4 Main Supply (Diagram A1)
The main power supply is able to deliver a continuous power
between 100 W and 160 W.
Some important notes on beforehand:
V
BAT
is not isolated from the main supply ('hot').
V
BAT
is alignment free.
Principle
The Main Power Supply, generates the 141 V (V
BAT
) and the +/
- 16 V for the audio part. It is based on the so-called
“downconverter” principle or the “buck-boost-convertor”
principle, dependant on the set type.
Figure 9-8 Down-converter principle
1. After closing switch 'S', the linear in time increasing current
I
T
, will charge capacitor C.
2. Opening switch 'S' will generate a counter-e.m.f. in coil L,
trying to maintain current I
T
. This is possible via diode D
(this diode is also called “freewheel diode”). Therefore,
after opening 'S', the magnetic energy stored in coil L will
be transferred to electrostatic energy in capacitor C. The
V
IN
will only supply current during the time that 'S' is closed
while a constant current is flowing through R
L
.
3. V
BAT
is directly proportional with V
IN
and the time that 'S' is
closed and reverse proportional with period time 'T'.
Therefore, by changing the duty cycle, it will be possible to
control V
BAT
.
Figure 9-9 “Buck boost” principle
1. After closing switch 'S', a linear in time increasing current
I
T
will flow through inductor L.
2. Opening switch 'S' will generate a counter-e.m.f. in coil L,
trying to maintain the current. This is possible via diode D
(this diode is also called “freewheel diode”.) Therefore,
after opening 'S', the magnetic energy stored in coil L will
be transferred to electrostatic energy in capacitor C.
3. So, by changing the duty cycle, it will be possible to control
V
BAT
.
4. ?
Implementation
At start-up of the main supply, C2515 (diagram A1) can be
assumed as being a short-circuit. U
AB
will be 15 V (R3513,
D6510) and U
GS
of the FET will be +5.4 V (via D6515). The FET
will be driven into saturation (same as closing switch 'S'). The
drain-current will increase linear in time. With other words:
resistors R3513 and R3518 will start the oscillator.
The voltage across the co-coupled coil (4, 5) will keep the FET
into conductivity.
The TS7502 is a low-voltage semiconductor, which drives the
MOSFET TS7504. To bridge the different voltage levels, an
opto-coupler (item 7507) is used. Via this opto-coupler, the DC-
current through R3504 is influenced. The changed current
through R3504 changes the V
BE
of TS7502, which will
influence the drive of MOSFET TS7504.
The sudden current interruption in the primary coil will induce a
counter-e.m.f. that wants to maintain the current via the
“freewheel” diode D6534. This current is linear decreasing in
time and, as it is also flowing through R3514//R3515, TS7502
will be blocked after a certain period. The gate of the FET will
be again made positive, is driven into conductivity and the cycle
starts again.
For safety reasons, transistor TS7530 is added as a back-up
solution for TS7502. If B-E of TS7502 is shorted, TS7530 takes
over its function.
Stabilisation of V
BAT
The output voltage V
BAT
is determined by: V
BAT
= V
IN
* (T
ON
/
(T
ON
+ T
OFF
)) = V
IN
* duty-cycle.
To stabilise the output voltage, a feedback loop is
implemented, which will reduce T
ON
when V
BAT
increases and
vice versa.
Via a voltage divider, existing of (1 %) resistors R3507, R3510,
and R3527//3549, a voltage of 2.5 V (when V
BAT
= 141 V) is fed
to the input of precision shunt regulator 7506. This regulator will
conduct, and a current will flow through the diode part of the
opto-coupler 7507. The base of TS7502 will now be set at a
certain positive voltage. As this transistor switches the FET
TS7504 “on” and “off”, this circuit can determine the duty-cycle.
E.g. when the load increases, V
BAT
will decrease.
Consequently, the input voltage of regulator 7506 will
decrease, resulting in a lower current. Via opto-coupler 7505
and transistor TS7502, T
ON
of the FET is changed (will
increase). The output voltage V
BAT
will rise.
96532156_022.eps
060100
R
L
I
T
I
T
V
BAT
Vin
S
C
L
R
L
I
D
I
D
V
BAT
Vin
S
C
L
S
closed
S
open
δT
V
IN
.
δT
T
+
+
D
D
V
BAT =
T
CL 26532058_022.eps
210602
R
L
V
BAT
Vin
S
+
-
C
GND_SUP
L
GND_STB
+
D