AN2450 datasheet(17/35 Pages) STMICROELECTRONICS

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DocID12784 Rev 6

17/35

AN2450

ZVS constraints

Due to the phase lag of the input current with respect to the input voltage, at the end of the

first half cycle the inductor current Irt is still flowing into the circuit and, therefore it can

deplete CZVS so that its voltage swings from V to zero (it will be vice versa during the

second half cycle).

In order to guarantee ZVS, the tank current at the end of the first half cycle (considering the

dead time negligible as compared to the switching period, so that the current change is

negligible as well) must exceed the minimum value necessary to deplete CZVS within the

dead time interval TD, which means:

Equation 40

This current equals, of course, the peak value of the reactive current flowing through the

resonant tank (it is 90° out-of-phase); the one that determines the reactive power level into

the circuit:

Equation 41

Moreover, as the rms component of the tank current associated to the active power is:

Equation 42

we can derive also the rms value of the resonant tank current and the phase lag

 between

input voltage and current (that is the input impedance phase angle at that operating point):

Equation 43

Equation 44

Thus we can write the following analytic expression:

Equation 45

which is the sufficient condition for ZVS of the half-bridge Power MOSFETs, to be applied to

the whole operating range. The solution of Equation 45 for the quality factor Qzvs that

ensures ZVS behavior at full load and minimum input voltage is not convenient.

Izvs

irt

Tsw

---------





Czvs



-------

2COSS Cstray



Vdc

---------

Izvs

2Irt



sin

Iact

Irt



cos

ViFHA

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

Irt





Irt





sin

cos

ViFHA

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





Izvs

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



ViFHAIrt

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





cos





tan

Im Zn fn  Q







Re Zn fn  Q







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

Czvs

T

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

Vdc

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



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