AN2644 datasheet(13/64 Pages) STMICROELECTRONICS

13 / 64 page

AN2644

The LLC resonant half-bridge converter

13/64

Figure 8.

Power MOSFET totem-pole network driving a resonant tank circuit in a

half-bridge converter

As previously stated, there is no overlap between the conduction of Q1 and Q2.

Additionally, a deadtime TD between the transitions from one state to the other of either

switch, where they both are open, is intentionally inserted. It is intended that when Q1 is

closed and Q2 is open, the voltage applied to the resonant tank circuit is positive. Similarly

we will define as negative the voltage applied to the resonant tank circuit when Q1 is open

and Q2 is closed. Consistently with two-port circuits sign convention, the input current to the

resonant tank, IR, will be positive if entering the circuit, negative otherwise.

Let us assume Q1 closed and Q2 open. It is then IR = I(Q1). Despite that the voltage applied

to the circuit is positive (VHB = Vin), IR can flow in either direction since we are in presence of

reactive elements. Let us suppose that IR is entering the tank circuit (positive current) in the

instant t0 when Q1 opens, and refer to the timing diagram of Figure 9.

The current through Q1 falls quickly and becomes zero at t = t1. Q2 is still open and IR must

keep on flowing almost unchanged because of the inductance of the resonant tank that acts

as a current flywheel. The electrical charge necessary to sustain IR will come initially from

CHB, initially charged at Vin, which will be now discharged. Provided IR(t1) is large enough,

the voltage of the node HB will then fall at a certain rate until t = t2, when its voltage

becomes negative and the body diode of Q2, DQ2, becomes forward biased, thus clamping

the voltage at a diode forward drop VF below ground. IR will go on flowing through DQ2 for

the remaining part of the deadtime TD until t = t3, when Q2 turns on and its RDS(on) shunts

DQ2. When this occurs, the voltage across Q2 is -VF, a value negligible as compared to the

input voltage Vin. In the end, this is what is called zero-voltage switching (ZVS): the turn-on

transition of Q2 is done with negligible dissipation due to voltage-current overlap and with

CHB already discharged, there will be no significant capacitive loss either. Note, however,

that there will be nonnegligible power dissipation associated to Q1's turn-off because there

will be some voltage-current overlap during the time interval t0 - t1.

Resonant

Tank &

Load

Vin

Node

DQ1

DQ2

Coss1

Coss2

CStray

Resonant

Tank &

Load

Vin

Node

DQ1

DQ2

CHB

Resonant

Tank &

Load

Vin

Node

DQ1

DQ2

Coss1

Coss2

CStray

Resonant

Tank &

Load

Vin

Node

DQ1

DQ2

CHB

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