TDA8385 Philips Semiconductors, TDA8385 Datasheet - Page 6

• Manufacturer Part Number: TDA8385
• Description: Control circuit for a Self-Oscillating Power Supply SOPS
• Manufacturer: Philips Semiconductors

Philips Semiconductors
For ‘T
For the primary current I
Substitution (4) into (5):
Equation (6) shows that by limiting the
voltage V
can be limited. The peak current is
limited by means of the clamping
circuit in the transfer character
generator (TCG); see Fig.1
sub-section 4.
The clamping level can be externally
influenced by means of a resistor
on pin 7.
The collector peak current can be
influenced in several ways:
Before comparing the sawtooth
voltage V
in the pulse width modulator, a
voltage of 100 mV is added to V
this way it will be possible for V
become smaller than V
important for a stabilized no-load
operation (see Fig.6 area 3).
D
This input prevents the switching
transistor from conducting during
demagnetization of the transformer in
order to prevent the transformer from
going into saturation. The output of
comparator (11) is HIGH as soon as
the voltage of the transformer winding
exceeds 115 mV.
March 1994
T
I
I
c
c
EMAGNETIZATION INPUT
on
Resistor R12 on pin 12
Capacitor C on pin 5
Capacitor on pin 7
Transfer ratio n
Inductance L
Control circuit for a Self-Oscillating
Power Supply (SOPS)
=
=
=
V
---- -
C
--- -
L
L
on
I
V
---------------------------------------------- -
’ yields:
c
c
c
1
-- -
p
T
the collector peak current
with the control voltage V
p n
on
C n
n
----- -
n
p
h
h
p
h
R12 V
/n
V
p
R12
I
sim
c
(
PIN
yields:
, which is
c
15)
r
c
to
. In
(4)
(5)
(6)
r
D
The output of sub-section 11 is
extended by the delay circuit of
sub-section 12. The starting
(reference) point of the delay circuit is
the falling edge of the output of
demagnetizing comparator (11) The
delay can be determined externally by
capacitor (C
The switch-on moment of the
switching transistor can be
determined by capacitor C
A minimum delay time is required to
prevent transistor T1 from switching
during demagnetization of the
transformer because of oscillations
caused by the leakage inductance.
ELAY SETTING
(1), (2), (3) = V
(4), (5) = V
Fig.6 Reference voltage (V
diff
delay
V mv
.
TCG
(1)
(
PIN
) on pin 6.
.
7
6)
delay
.
(2)
mv
) as a function of feedback voltage (V
Control part (Block III)
The differential amplifier,
sub-section 3, compares the
feedback voltage (V
reference voltage V
the differential amplifier is available
on pin 11 to allow gain setting. The
differential amplifier is internally
compensated for 0 dB feedback
stability.
The feedback input (pin 9) is also
used as the input for the TCG
(see Fig.6) with which a current
foldback characteristic can be
obtained as shown in Fig.7.
(3)
V
fb
(4)
(5)
Preliminary specification
V clamp
V Ton(min)
ref
fb
TDA8385
MCD405
) with the
. The output of
fb
).