STMicroelectronics Double output SMPS for power line application using ALTAIR04-900 primary controller EVLALTAIR900-M1 EVLALTAIR900-M1 Fiche De Données

Doc ID 18211 Rev 2

15/29

The IC is specifically designed to work in primary regulation and the output voltage is 
sensed through a voltage partition of the auxiliary winding, just before the auxiliary rectifier 
diode.

 shows the internal schematic of the constant voltage mode and the external 

connections.

Due to the parasitic wires resistance, the auxiliary voltage is representative of the output just 
when the secondary current becomes zero. For this purpose, the signal on ZCD/FB pin is 
sampled-and-held at the end of transformer’s demagnetization to get an accurate image of 
the output voltage and it is compared with the error amplifier internal reference.

The COMP pin is used for the frequency compensation: usually, an RC network, which 
stabilizes the overall voltage control loop, is connected between this pin and ground.

The output voltage can be defined according the formula:

         

Where N

SEC

 and N

AUX

 are the secondary and auxiliary turn’s number respectively.

The R

ZCD

 value can be defined depending on the application parameters (see “

”).

 

2. 5V

R zcd

F rom  Rsense

Aux

+

-

EA

R

To PWM Logic

S/ H

Rf b

D EMAG

LOGI C

+

-

C V

C

COMP

ZCD/FB

         

(1)

R

FB

V

REF

N

AUX

N

SEC

-------------- V

OUT

V

REF

–

⋅

------------------------------------------------------ R

ZCD

⋅

=