AN2644 datasheet(18/64 Pages) STMICROELECTRONICS

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The LLC resonant half-bridge converter

AN2644

18/64

deadtime and ZVS are related is actually more complex and depends on converter's

operating conditions.

It is instructive to see this in Figure 13, which shows typical node HB waveforms occurring

when working in the inductive region but too close to the capacitive region, so that ZVS is

not achieved. They refer to the Q1

→OFF, Q2 → ON transition; those related to the opposite

transition are obviously turned upside down.

●

Case a) is very close to the boundary between inductive and capacitive regions. Tank

current reverses just after Q1 is switched off, a portion of node HB ringing appears as a

small "dip", then the tank current becomes negative enough to let the body diode of Q1

start conducting. When Q2 turns on there are capacitive losses and the recovery of the

Q1's body diode with all the related issues.

●

Case b) is slightly more in the inductive region but still IR crosses zero within the

deadtime. The node HB ringing becomes larger and the body diode of Q1 still conducts

for a short time and its recovery is invoked as Q2 turns on.

●

Case c) Is even more in the inductive region but still not sufficiently away from the

capacitive-inductive boundary. The ringing of the node HB is large enough to reach

zero but IR reverses within the deadtime and the voltage goes up again. At the end of

the deadtime the voltage does not reach Vin, hence the body diode of Q1 does not

conduct and Q2, when turned on, will experience only capacitive losses.

●

Case d) Is further in the inductive region and IR crosses zero nearly at the end of the

deadtime. Q2 is now almost soft-switched with no losses. This can be considered as

the boundary of the operating region where ZVS can be achieved with the given

duration of TD.

Note that the resonant tank's current during node HB ringing is lower than the one flowing

through Lp. This means that their difference is flowing into the transformer and,

consequently, that one of the secondary half-windings is conducting. Therefore, CHB is

resonating with Ls only.

This analysis shows that there is a "border belt" in the inductive region, close to the

boundary with the capacitive region (fR2 < f < fR1, R = Rcrit) and that as converter's operation

is moved away from the capacitive-inductive boundary and pushed more deeply in the

inductive region there is a progressive behavior change from hard-switching to soft-

switching. In the cases a and b the inductive energy in the resonant tank is too small to let

the node HB even swing "rail-to-rail"; moving away from the boundary, as shown in case c,

the energy is higher and allows a rail-to-rail swing, but it is not large enough to keep the

node HB "hooked" to the rail throughout the deadtime TD. If the converter is operated in this

border belt, Q1 and Q2 will be hard-switched at turn-on and, in cases such as case a and

case b, the body diode of the just turned off power MOSFET is injected and then recovered

as the other power MOSFET turns on.

Case b and, especially, case c highlight that it is possible to look at the deadtime TD also

from another standpoint: looking at those waveforms, one might conclude that the current IR

at the beginning of the deadtime is too low or, conversely, that the deadtime is too long. In

case c, for example, if the dead-time had been approximately half the value actually shown,

Q2 would have been soft-switched at turn-on. Of course, the more appropriate interpretation

depends on whether TD is fixed or not.

These cases are related to heavy load conditions.

Figure 14 shows a case typical of no-load conditions, where ZVS is not achieved because of

a too slow transition of the node HB so that it does not swing completely within the deadtime

TD. In this case the situation seems less stressful than operating in the capacitive region.

AN2644 Datasheet(PDF) 18 Page - STMicroelectronics

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