Philips PM 3266 - 3.2.10. Power supply

138
3.2.10. Power
supply
Input
circuit
The
power
supply
input
circuit is
matched
to the 1
15 V or
230
V range
with
selector-switch
SI 801
which is
located
at the
power supply
unit
at
the rear
side.
The
mains
voltage
is rectified
with
the
diode
bridge
VI
801
and Cl
802,
Cl
803,
which
form a
voltage
doubler
in the 1
15 V
position
of SI
801,
and
a
standard
bridge
rectifier
circuit
in the
230
V
position
of
SI
801.
The
voltage
across
the series
circuit
of Cl 802
and
Cl 803
amounts
250
to 400
V for
both
mains
voltage
ranges.
Switching
circuit
The
unregulated
d.c.
voltage
is
applied
in the
form
of
pulses
to a resonance
circuit
consisting
of
the
primary
coil of the
convertor
transformer
T1
801
,
combined
with
C1807
and C1808,
via
switching
transistor
V1806.
The
sine-wave
voltage
(approx.
800V
p.p.)
across
the
primary
coil
of
T1801
is kept
constant
by
regulating
the
duty
cycle of
the
base
current
of VI
806.
The
primary
coil
of LI
806 which
is in
series
with
the
switching
transistor,
limits
the
current
through
this
transistor.
The energy
stored
in LI
806
is
fed-back
to the mains
rectifier
circuit, during
the
cut-off
time
of VI
806 via
diode VI
811.
VI 808 and
VI
809 keep
the
dissipation
during
the
switching
moments
out
of
transistor
VI
806; instead
of
these
losses
are
dissipated
in
R 181
4 and R1816.
VI
807
improves
the
base
drive for
VI
806.
Regulator
circuit
The
regulator
circuit
itself
consists
of
integrated
circuit
D1801
(type
TDA
1060), the
output
of
which
(p.15)
supplies
a
square-wave
current
with
variable
duty-cycle
to
the
base
of
VI 81 2.
The
duty-cycle
of
this
signal
is
variable.
The
collector
signal
of VI
81
2 is
applied
to
the
switching
transistor
via
transformer
LI
803.
The
regulator
circuit
is
controlled
by:
—
Feed-back
voltage
(p.3)
This is
the
regulator
control
voltage
and
is
taken
from
the
rectifier
circuit
at
the
feed-back
winding
of
T1801.
This
control
voltage
depends
on
the
setting
of R1826
(V out).
—
Feed-forward
(p.16)
This
voltage
is
derived
from
the
mains
voltage
and
provides
direct
mains
voltage
variation
compensation.
—
Over-voltage
protection
(p.13)
This voltage
is
also
derived
from
the
mains
voltage
and
inhibits
the
regulator
output
at too
high
mains
voltages
(the
trip-level
on
p.13 is
600
mV).
—
Current
limit
(p.1
1)
The
voltage
drop
across
the
current-sense
resistor
R181
1 controls
the regulator
circuit
in
case of
overload.
—
Frequency
(p.7)
The
resistance
between
p.7
and
gnd
estimates
the
convertor
frequency.
R1827
(Freq.)
has
been
adjusted
to obtain
a
frequency of
20 kHz.
The resonance
frequency
of Cl
807,
1808
and
the
primary
coil of
T1801
is wide
enough
to tolerate
this.
Under normal
working
conditions
the
power
supply
voltages
for
the
regulator
circuit
are
delivered
by the
rectifier
connected
to the
feed-back
winding
of
T1801.
VI
804
is then
conducting
so
that VI
803
does
not deliver
current.
Switching-on
When switching-on
the
instrument,
no
supply
voltages
are
available
in the
regulator
circuit,
from
T1801.
At
this
moment
VI 804
is
not
conducting,
so that
VI 803 is
fully conducting,
and
the regulator
circuit
gets
current
via R1804
and R1806.
As
soon
as the
converter
circuit
is
working
VI
804
becomes
conducting
and
VI 803
is
not conducting
anymore.