Philips Chassis EM1.1A AA Service Manual Download Page 102

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Circuit Descriptions, Abbreviation List, and IC Data Sheets

Figure 9-3 Flyback principle

After closing switch 'S,' the current ID will increase linearly in
time. The magnetic energy in the primary coil is directly
proportional with the self-inductance of the coil and current ID
(thus with the time the switch is closed).
The voltage polarity at the secondary winding is negative (due
to different winding direction), meaning that diode D will block.
Capacitor C will discharge via R

, and U

OUT

will decrease.

Opening switch 'S' will generate a counter-e.m.f. in the primary
winding, trying to maintain current I

. Through this the polarity

of the secondary voltage will inverse. The magnetic energy
stored in the coil will now be transformed to the secondary side.
Diode D will now conduct, capacitor C will be charged, and
U

OUT

will increase.

Implementation

Figure 9-4 Standby supply circuitry

To apply this on the EM1.1A(diagram A2): replace switch 'S' by
FET TS7102, coil L by L5100, diode D by D6111, and C by
C2104.

•

Time interval t0-t1: After switching 'on' the TV-set, the
gate of MOSFET TS7102 will be high (max. 15 V due to
zener diode D6105). This will drive the FET into saturation
(UDS = 0 V). The DC-voltage UMAINS will be transposed
across the primary winding of L5100 (3, 5), resulting in a
linear increasing current through this coil. The voltage
across the co-coupled coil (1, 2) is also positive and will
keep the FET in conductivity via C2101, R3103/3105/3102
and R3117 for some time. The self-induction of the coil and
the magnitude of the supply voltage (+375 V) determine the
slope of the primary current. The maximum current is
determined by the time the FET stays in conductance (t0-
t1). This time is directly determined by the voltage across
R3108//R3118 (0.7

Ω

). This voltage is a measure of the

current, and if it exceeds 1.4 V, TS7101 will be driven into
conductivity and consequently connects the gate of

TS7102 to earth. The FET will block. The current is: 1.4 V
/ 0.7

Ω

= 2 A. The voltage across the secondary winding (8,

9) will be negative, diodes D6111 and D6107 will block.

•

Time interval t1-t2: The sudden current interruption in the
primary coil will induce a counter-e.m.f. that tries to
maintain the current. The voltage on the drain of the FET
will increase. The secondary voltage (8, 10) will become
positive and will charge C2104 via D6111. All energy that
was stored in L5100 during t0-t1 will be transferred into the
load. Due to the transformer principle, a voltage will now
be induced in the primary winding (3, 5) and the co-coupled
winding (1, 2). This voltage will be N* U

SEC

(N= winding

ratio). The voltage across the co-coupled coil will be
negative, keeping the FET blocked.

•

Time t2: At t2, the current through the secondary coil will
be reduced to zero, as C2104 is no longer charged.
Consequently, the voltages will decay and will change
polarity. The gate of the FET will be again made positive, is
driven into conductivity, and the cycle starts again.

96532156_020.eps

210100