AN2644 datasheet(19/64 Pages) STMICROELECTRONICS

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AN2644

The LLC resonant half-bridge converter

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There is no body diode conduction and, consequently, no recovery. Q1 will be almost soft-

switched at turn-off, while Q2 will have capacitive losses at turn-on. It is true that the turn-on

voltage is lower than Vin, thus the associated energy of CHB is lower, but at no-load the

operating frequency is usually considerably higher than in the capacitive region, then these

power losses may easily overheat Q1 and Q2. Finally, note in Figure 14 that I(Lp) is exactly

superimposed on IR, then the secondary side of the transformer is open and CHB is

resonating with the total inductance Ls+Lp.

Figure 14.

Bridge leg transitions under no-load conditions

From what we have seen we can conclude that the conditions in order for the half-bridge

switches to achieve ZVS are:

Under heavy load conditions, as one switch turns off, the tank current must have the

same sign as the impressed voltage and be large enough so that both the rail-to-rail

transition of the node HB is completed and the current itself does not reverse before the

end of the deadtime, when the other switch turns on.

With no-load, the tank current at the moment one switch turns off (which has definitely

the same sign as the impressed voltage) must be large enough to complete the node

HB transition within the deadtime, before the other switch turns on.

Both conditions can be translated into specifying a minimum current value IRmin that needs

to be switched when either power MOSFET turns off. In general, different IRmin values are

needed to ensure ZVS at heavy load and at no-load. One can simply pick the greater one to

ensure ZVS under any operating condition by design. On the other hand, this minimum

required amount of current is to the detriment of efficiency.

At light or no-load a significant current must be kept circulating in the tank circuit, just to

maintain ZVS, in spite of the current delivered to the load that is close to zero or zero. Using

ac-analysis terminology, a certain amount of reactive energy is required even with no active

energy.

Finally, also at heavy load the value of IRmin to be specified is the result of a trade-off. In fact,

its value is directly related to the turn-off losses of both Q1 and Q2. The higher the switched

current is, the larger the switching loss due to voltage-current overlap will be.

The discussion on the switching mechanism has been focused on the primary-side

switches, and the conditions in order for them to achieve soft-switching (ZVS at turn-on,

precisely) have been found. One important merit of the LLC resonant converter is that also

the rectifiers on the secondary side are soft-switched. They feature zero-current switching

(ZCS) at both turn-on and turn-off. In fact, at turn-on the initial current is always zero and

ramps up with a relatively low di/dt, so that forward recovery does not come into play. At

turn-off they become reverse biased when their forward current is already zero, so that their

I(Lp)

Q2 is hard switched

Q1 ON

Q2 OFF

Q1 OFF

Q2 ON

Q1 OFF

Q2 OFF

VHB = Node HB voltage

IR = Tank circuit’s current

I(Lp) = Lp (magnetizing) current

I(Lp)

Q2 is hard switched

I(Lp)

Q2 is hard switched

Q1 ON

Q2 OFF

Q1 OFF

Q2 ON

Q1 OFF

Q2 OFF

VHB = Node HB voltage

IR = Tank circuit’s current

I(Lp) = Lp (magnetizing) current

AN2644 Datasheet(PDF) 19 Page - STMicroelectronics

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