STMicroelectronics Double output SMPS for power line application using ALTAIR04-900 primary controller EVLALTAIR900-M1 EVLALTAIR900-M1 Data Sheet

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Doc ID 18211 Rev 2

 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 

.

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:

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. 

         

(2)

I

OUT

N

PRI

N

SEC

--------------

G

I

V

CREF

⋅

2 R

SENSE

⋅

(

)

---------------------------------

⋅

=

 

.

IREF

R zcd

R f b

Aux

Q1

Ir ef

R

Gi

D EMAG

LOGI C

To PWM Logic

F rom  R sense

ZCD/FB

+

-

CC

C

S

R

Q