TDA8385 Datasheet, PDF (5/21 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
Sawtooth generator (Block II)
CURRENT SIMULATION
(SEE FIGS 5 AND 16)
The current of the power supply
switching transistor is detected on the
secondary side by an indirect method
of current sensing.
Information of the collector current (I_c)
is obtained by integrating the voltage
of an auxiliary winding of the
transformer during transistor on-time
(T_on). An external capacitor C on pin 5
is charged during T_on by the current
source I_sim. The current I_sim is the
reflection of the current which flows
into pin 12. This current is obtained by
connecting an external resistor R12 to
the auxiliary transformer winding.
During transistor on-time this current
is related to the input voltage V_I.
During transistor off time (T_off) the
capacitor C is discharged by switch
sw1. This switch is ac^ht^aiⁿv^de^bood^ku^{, f}r^ui^lln^pg^ageth^wie^dth
total T_off time. In this way a sawtooth
voltage V_c is formed across C. This
sawtooth is a measure for the
collector current of the switching
transistor T1.
For the voltage V_c yields:
V_c = -I-^s---ⁱ-^m-----C-^×----T----^o---ⁿ-
(1)
I_sim = p × _n-n---^hp- × _R---V--1---^I-2-
(2)
Where: p = reflection factor;
p = -I-I^s--₁-ⁱ-_2m-- = 0.2
(2) → (1) gives:
V_c = _C-p-- × ^nn----hp- × _R---V--1---^I-2- × T_on
(3)
latch
initialization
5.2 7.5
operation
VP (V)
20
MCD403
Fig.4 Latch initialization as a function of supply voltage V_P.
L
np
nh
VI
Ic
T1
Vc
R12
12
I ₁₂
5
C
I _sim
sw1
Vc
Ton
Toff
t
MCD404
Fig.5 Determination of the peak current I_c.
March 1994
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