TDA8385 Datasheet, PDF (6/21 Pages) NXP Semiconductors – Control circuit for a Self-Oscillating Power Supply SOPS

Philips Semiconductors
Control circuit for a Self-Oscillating
Power Supply (SOPS)
Preliminary specification
TDA8385
For ‘T_on’ yields:
T_on = ^V-----c---^×--p---C--_×----^×-n----_h^n---p-_×---^×-V----R-_I----1---2--
(4)
For the primary current I_c yields:
I_c = -V-L--^I × T_on
(5)
Substitution (4) into (5):
I_c = ^C-L-- × ^1p-- × _n-n---^ph- × R12 × V_c
(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).
DELAY 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.
Vmv
(2)
(1)
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.
(3)
V _clamp
(4)
(5)
Vfb
V _Ton(min)
MCD405
DEMAGNETIZATION 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.
(1), (2), (3) = V_TCG.
(4), (5) = V_diff.
Fig.6 Reference voltage (V_mv) as a function of feedback voltage (V_fb).
March 1994
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