AN2450 datasheet(7/35 Pages) STMICROELECTRONICS

7 / 35 page

DocID12784 Rev 6

7/35

AN2450

FHA circuit model

Equation 14

Thus, in the end, we have transformed the non linear circuit of Figure 1 into the linear circuit

of Figure 2, where the ac resonant tank is excited by an effective sinusoidal input source

and drives an effective resistive load. This transformation allows the use of complex ac-

analysis methods to study the circuit and, furthermore, to pass from ac to dc parameters

(voltages and currents), since the relationships between them are well-defined and fixed

(see Equation 3, Equation 5, Equation 6, Equation 10 and Equation 12 above).

Figure 2. FHA resonant circuit two port model

The ac resonant tank in the two-port model of Figure 2 can be defined by its forward transfer

function H(s) and input impedance Zin(s):

Equation 15

Equation 16

For the discussion that follows it is convenient to define the effective resistive load reflected

to the primary side of the transformer Rac:

Equation 17

Roac

voFHA t

irect t

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

VoFHA

Irect

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



------

out

Pout

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



------Rout

controlled

switch

network

dc output

H (j )

n :1

ac resonant tank

Vdc

Iout

Rout

Ro.ac

rectifier &

low-pass

filter

Vi.FHA

Irect

Irt

Vout

dc input

Vo.FHA

Ii.dc

Zin (j )



VoFHA s



ViFHA s



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

---

oac

sLm



Zin s



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

Zin s



ViFHA s



Irt s



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

sCr

--------- sLr n

oac

sLm



Rac

oac

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