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參數(shù)資料
| 型號： | TDA8385 |
| 廠商： | NXP SEMICONDUCTORS |
| 元件分類： | 模擬信號調理 |
| 英文描述： | Control circuit for a Self-Oscillating Power Supply SOPS |
| 中文描述： | SPECIALTY ANALOG CIRCUIT, PDIP16 |
| 封裝： | PLASTIC, DIP-16 |
| 文件頁數(shù)： | 6/21頁 |
| 文件大?。?/td> | 199K |
| 代理商： | TDA8385 |
March 1994
7
Philips Semiconductors
Preliminary specification
Control circuit for a Self-Oscillating
Power Supply (SOPS)
TDA8385
For ‘T
on
’ yields:
(4)
For the primary current I
c
yields:
(5)
Substitution (4) into (5):
(6)
Equation (6) shows that by limiting the
voltage V
c
the collector peak current
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:
Resistor R12 on pin 12
Capacitor C on pin 5
Capacitor on pin 7
Transfer ratio n
h
/n
p
Inductance L
Before comparing the sawtooth
voltage V
c
with the control voltage V
r
in the pulse width modulator, a
voltage of 100 mV is added to V
c
. In
this way it will be possible for V
r
to
become smaller than V
sim
, which is
important for a stabilized no-load
operation (see Fig.6 area 3).
D
EMAGNETIZATION INPUT
(
PIN
15)
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.
T
on
V
--------p
C
×
n
×
h
R12
V
I
×
×
=
I
c
V
I
L
----
T
on
×
=
I
c
C
L
1
p
×
n
p
n
h
-----
×
R12
×
V
c
×
=
D
ELAY SETTING
(
PIN
6)
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
delay
) on pin 6.
The switch-on moment of the
switching transistor can be
determined by capacitor C
delay
.
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.
Control part (Block III)
The differential amplifier,
sub-section 3, compares the
feedback voltage (V
fb
) with the
reference voltage V
ref
. The output of
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.
Fig.6 Reference voltage (V
mv
) as a function of feedback voltage (V
fb
).
(1), (2), (3) = V
TCG
.
(4), (5) = V
diff
.
(5)
(4)
(3)
(2)
(1)
MCD405
fb
V
Vmv
VTon(min)
Vclamp
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