AN3093 datasheet(24/29 Pages) STMICROELECTRONICS

24 / 29 page

Modes of operation

AN3093

24/29

Doc ID16539 Rev 1

A.2

Constant current operation

Figure 22 presents the principle used for controlling the average output current of the

flyback converter.

The output voltage of an auxiliary winding is used by a demagnetization block to generate

the control signal for the MOSFET switch Q1. A resistor (R) in series with it absorbs the

current VC/R, where VC is the voltage developed by capacitor CREF.

The flip-flop's output is high, as long as the transformer delivers current on the secondary

side (Figure 23).

If the value of the capacitor (C) is sufficiently high to consider the voltage VC as constant,

and the system works in DCM (this is the case because the IC is in QR mode), it is possible

to obtain the equation:

Equation 5

Where n is the primary-to-secondary turn ratio and VCREF = IREF x R.

Equation 5 shows that the average output current no longer depends on input or output

voltage, nor on the transformer inductance values. The parameters defining the output

current are the transformer ratio n and the sense resistor RSENSE, while the current

reference voltage VCREF (equal to IREF x R) is internally defined.

Figure 22.

Current control principle

OUT

---

CREF

SENSE

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AN3093 Datasheet(PDF) 24 Page - STMicroelectronics