STMicroelectronics HVLED805 Evaluation Board EVALHVLED805 EVALHVLED805 Data Sheet

18/29

Doc ID 18077 Rev 1

 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 C.

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

.

The capacitor C has to be chosen so that its voltage V

C

 can be considered as a constant. 

Since it is charged and discharged by currents in the range of some ten µA (I

CLED

 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 I

S

 is the secondary peak current, T

ONSEC

 is the conduction time of the secondary 

side and T is the switching period.

Taking into account the transformer ratio n between primary and secondary side, I

S

 can also 

be expressed is a function of the primary peak current I

P

:

As in steady state the average current I

C

:

Which can be solved for V

C

:

Where V

CLED

=R 

• I

LED

 and is internally defined.

As V

C

 is fed to the CC comparator, the primary peak current can be expressed as:

⎟

⎠

⎞

⎜

⎝

⎛

⋅

=

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

⋅

=