AN2644 datasheet(53/64 Pages) STMICROELECTRONICS

53 / 64 page

AN2644

Resonant transitions of half-bridge midpoint

53/64

In the end, the approximation (Equation 36) provides a relationship that is sufficiently

accurate for design purposes under all operating conditions. Fortunately the conditions

where accuracy is worst (CCM) are not critical as far as ZVS is concerned because the

switched current IR(0) is greater than it is under all other conditions.

To achieve ZVS for both switches, a necessary condition is that TT≤TD. An additional

constraint comes from the condition that the tank current IR has to keep its sign unchanged

during the time interval (0, TD). Should the current become zero in that interval, the body

diode of Q1 would not be forward biased any more and the voltage VHB, no longer

constrained to Vin, would experience oscillations at an angular frequency equal to either

ωDD or ωCC. Q1 would then be turned on with a drain-to-source voltage in general greater

then zero.

In practical cases, when the converter is operated at or above resonance the tank current

crosses zero with a considerable delay after the end of the deadtime, especially in DCM

modes, where the phase lag

ϕ described by Equation 5 or 8 or 11 or 13 tends to π /2; hence,

this constraint can be disregarded. It becomes significant when working below resonance

and close to the boundary between the capacitive and the inductive mode (CCMB mode).

Under the assumption TT < TD, in the time interval (TT, TD) Equation 34 no longer apply and

IR is again a portion of sinusoid having frequency fR1, which can be described by an

equation of the type:

Equation 37

In order for the tank current to keep the same sign during the remainder of the deadtime, the

following condition must be fulfilled:

Equation 38

Figure 31.

Voltage of HB node vs time (see

Equation 32)

Figure 32.

Resonant current vs time (see

Equation 34)

R t

()

Rpksin 2πfR1t

–

()

πf

R1TD

ϕ 0

–

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