ALTAIR05T-800 Datasheet, PDF (16/28 Pages) STMicroelectronics – Off-line all-primary-sensing switching regulator

Application information
ALTAIR05T-800
The output voltage of the auxiliary winding is used by the demagnetization block to generate
the control signal for the mosfet switch Q1. A resistor R in series with it absorbs a current
V_C/R, where V_C is the voltage developed across the capacitor C_REF.
The flip-flop’s output is high as long as the transformer delivers current on secondary side.
This is shown in Figure 15.
The capacitor C_REF has to be chosen so that its voltage V_C can be considered as a
constant. Since it is charged and discharge by currents in the range of some ten µA (I_CREF
is typically 20 µA) at the switching frequency rate, a capacitance value in the range 4.7-10
nF is suited for switching frequencies in the ten kHz.
The average output current can be expressed as:
I_OUT
=
-^N-----^P---^R----^I-
N_SEC
⋅
----------^V----^C----^R----^E---^F-----------
(2 ⋅ R_SENSE)
(2)
Where N_PRI is the primary's turns number.
This formula shows that the average output current does not depend anymore on the input
or the output voltage, neither on transformer inductance values. The external parameters
defining the output current are the transformer ratio n and the sense resistor R_SENSE.
Figure 14. Current control principle
.
R zcd ^ZCD/FB
Rfb
A ux
DE MA G
LOGI C
Ir ef
R
S
Q1
Q
R
To PWM Logic
CC
F rom Rsense
IREF
Cref
16/28
Doc ID 17957 Rev 1