HVLED007 datasheet(16/33 Pages) STMICROELECTRONICS

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Application information

HVLED007

16/33

DS12866 Rev 1

As to the dynamic properties of a HVLED007-based Hi-PF QR flyback converter, the ICS

circuit is essentially transparent: it only affects the DC gain of the feedback loop. This is

intuitive if one recalls that one of the basic assumptions was that the time constant

associated to the ICS circuit were << 1/(2 fline) and that in Hi-PF converters the crossover

frequency of the loop is typically much lower than fline.

Assuming 100% efficiency and that the Hi-PF QR flyback converter powers one or more

downstream switching regulators that drive the LED strings, the control-to-output transfer

function is:

Equation 8

4.4

Shaping capacitor (Ct) selection (pin CT)

As previously mentioned, one fundamental assumption for proper operation of the novel

control method implemented in the HVLED007 is that the time constant Rt Ct associated to

the ICS meets the constraints:

Equation 9

The left-hand side inequality ensures that the voltage across the shaping capacitor Ct is

essentially a DC level in any given switching cycle; the right-hand inequality ensures that

this DC level is able to change quickly enough to closely follow the ideal reference given by

(Equation 6) and depicted in Figure 7.

Secondary rms current

Secondary DC current

Secondary AC current

Low-frequency secondary current

Low-frequency output voltage ripple

Eq. #

Parameter

Expression

Isrms = Iout 2

 K

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

Isdc = Iout

Isac = Iout 1

K

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

Io 

 = 2 I

out sin

2  = I

out (1 - cos 2

V

o pk

–

= Iout

 f

line

 C

out

--------------------------------------- =

I out

 f

line Cout

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

G s

 =

Iout

V c

-------- 1

sCout

-------------= 1

----

N pri

N sec

------------- Kv





KM KP

V out

------------ 1

sCout

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



 << R

t Ct << 1/ 2 f line



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