AN2644 datasheet(16/64 Pages) STMICROELECTRONICS

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The LLC resonant half-bridge converter

AN2644

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subject to its di/dt, which may damage any control IC coupled to the half-bridge leg, not to

mention the big EMI generation.

Similarly, it is possible to show that the same series of adverse events will happen to Q1 and

Q2, with exchanged roles, when Q2 is turned off if IR is flowing into the resonant tank circuit

(positive current).

The obvious conclusion is that, if the tank current and the impressed voltage at the instant of

half-bridge transitions have opposite signs, both switches will be hard-switched and the

reverse recovery of their body diodes will be invoked, with all the resulting negative effects. It

is intuitive that this sign opposition occurs if the tank current leads the impressed voltage,

which is typical of capacitors and then occurs if the resonant tank input impedance is

capacitive. This kind of operation is often termed "capacitive mode" and the frequency range

where tank current leads the impressed voltage is called the "capacitive region".

Figure 12.

Q1 ON-OFF and Q2 OFF-ON transitions with hard switching for Q2 and

recovery for DQ1

Then, the converter must be operated in the region where the input impedance is inductive

(the inductive region), that is, for frequencies f > fR1 or in the range fR2 < f < fR1 provided the

load resistance R is such that R> Rcrit. This is a necessary condition in order for Q1 and Q2

to achieve ZVS, which is evidently a crucial point for the good operation of the LLC resonant

half-bridge.

To summarize, ZVS brings the following benefits:

low switching losses: either high efficiency can be achieved if the half-bridge is

operated at a not too high switching frequency (for example < 100 kHz) or high

switching frequency operation is possible with a still acceptably high efficiency

(definitely out of reach with a hard-switched converter);

reduction of the energy needed to drive Q1 and Q2, thanks to the absence of Miller

effect at turn-on. Not only is turn-on speed unimportant because there is no voltage-

current overlap but also gate charge is reduced, then a small source capability is

required from the gate drivers.

low noise and EMI generation, which minimizes filtering requirements and makes this

converter extremely attractive in noise-sensitive applications.

all of the above-mentioned adverse effects of capacitive mode, which not only impair

efficiency but also jeopardize the converter, are prevented.

Q1 ON

Q2 OFF

Q1 OFF

Q2 ON

Dead-time

Q1 OFF

Q2 OFF

High dv/dt

VHB

Tank circuit’s current

is negative

Q1’s body diode

is recovered

0 –t1

Q2’ body diode conduction time;

coinciding with dead-time

1 –t2

Q1’s body diode recovery time

I(Q1)

Resonant capacitor voltage

Node HB voltage

Vc = Resonant capacitor voltage

VHB = Node HB voltage

IR = Tank circuit’s current

I(Lp) = Lp (magnetizing) current

I(Q1) = MOSFET Q1 current

Current is circulating

through Q1’s body diode

I(Q2)

Q2 is hard-switched

Q1 has soft turn-off

Q1 ON

Q2 OFF

Q1 OFF

Q2 ON

Dead-time

Q1 OFF

Q2 OFF

High dv/dt

VHB

Tank circuit’s current

is negative

Q1’s body diode

is recovered

0 –t1

Q2’ body diode conduction time;

coinciding with dead-time

1 –t2

Q1’s body diode recovery time

I(Q1)

Resonant capacitor voltage

Node HB voltage

Vc = Resonant capacitor voltage

VHB = Node HB voltage

IR = Tank circuit’s current

I(Lp) = Lp (magnetizing) current

I(Q1) = MOSFET Q1 current

Current is circulating

through Q1’s body diode

I(Q2)

Q2 is hard-switched

Q1 has soft turn-off

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