ALTAIR04-900 Datasheet, PDF (16/29 Pages) STMicroelectronics – Off-line all-primary-sensing switching regulator

Application information
ALTAIR04-900
5.5
Constant current operation
Figure 14 presents the principle used for controlling the average output current of the
flyback converter.
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 Cref.
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 Cref 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
⋅
---^G-----^I---^â‹---^V----^C----^R---^E---^F----
(2 ⋅ R_SENSE)
(2)
where N_PRI is the primary turn's 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.
Current loop gain G_I and current reference voltage V_CREF are internally defined.
Figure 14. Current control principle
.
R zcd ^ZCD/FB
Rfb
Aux
D EMAG
LOGI C
S
Q
R
Ir ef
Gi
To PWM Logic
CC
R
F rom R sense
Q1
IREF
C
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Doc ID 18211 Rev 2