Application information

HVLED805

18/29

Doc ID 18077 Rev 1

5.5

Constant current operation

Figure 15

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

The flip-flop’s output is high as long as the transformer delivers current on secondary side.

This is shown in Figure 16.

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:

Equation 2

side and T is the switching period.

Equation 3

Equation 4

Equation 5

⎟

⎠

⎞

⎜

⎝

⎛

⋅

=

T

T

2

I

I

ONSEC

S

OUT

P

S

I

n

I

⋅

=

()

0

T

R

V

I

T

T

I

ONSEC

C

CLED

ONSEC

CLED

=

⋅

⎟

⎠

⎞

⎜

⎝

⎛

−

+

−

⋅

ONSEC

CLED

C

T

T

V

V

⋅

=