DocID12784 Rev 6

13/35

AN2450

Voltage gain and input impedance

35

Figure 5. Normalized input impedance magnitude

The ac analysis can also help to estimate converter's efficiency

changes with the load. Considering the generic model similar to the one in Figure 2 on

page 7, where the resonant tank includes also the dissipative elements (i.e. series resistors

for magnetic components winding losses and capacitor's ESR, and parallel resistors for

magnetic losses of inductors and transformer), we can compute the transfer function

Equation 29

expressed as:

Equation 30

Equation 31

The region on the left-hand side of the diagram in Figure 5, i.e. for a normalized frequency

hand side region, the input impedance is inductive, and the resonant tank current lags the

input voltage. In the region between the two resonance frequencies the impedance can be

either capacitive or inductive, depending on the value of the impedance phase angle.



-----------



2





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

=

=

.

.



cos

1



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

=

=

.

.

.

.



V

2

oFHA





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

V

2

iFHA



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

 H



2

=

=

=

.