AN2644 datasheet(14/64 Pages) STMICROELECTRONICS

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The LLC resonant half-bridge converter

AN2644

14/64

Figure 9.

Detail of Q1 ON-OFF and Q2 OFF-ON transitions with soft-switching for

There is an additional positive side effect in turning on Q2 with zero drain-to-source voltage.

It is the absence of the Miller effect, normally present in power MOSFETs at turn-on when

hard-switched. In fact, as the drain-to-source voltage is already zero when the gate is

supplied, the drain-to-gate capacitance Cgd cannot "steal" the charge provided to the gate.

The so-called "Miller plateau", the flat portion in the gate voltage waveform, as well as the

associated gate charge, is missing here and less driving energy is therefore required. Note

that this property provides a method to check if the converter is running with soft-switching

or not by looking at the gate waveform of Q2 (which is more convenient because it is source-

grounded), as shown in Figure 10 and 11.

With similar reasoning it is possible to understand that the same ZVS mechanism occurs to

Q1 when it turns on if IR is flowing out of the resonant tank circuit (negative current).

In the end we can conclude that, if the tank current at the instant of half-bridge transitions

has the same sign as the impressed voltage, both switches will be "soft-switched" at turn-on,

i.e. turned on with zero voltage across them (ZVS). It is intuitive that this sign coincidence

0 –t1

Q1 current fall time;

voltage-current overlap for Q1

0 –t2

Node HB transition time

2 –t3

Q2’ body diode conduction time

Q1 ON

Q2 OFF

Q1 OFF

Q2 ON

Dead-time

Q1 OFF

Q2 OFF

VHB

Tank circuit’s current

is positive

I(Q1

)

Q1’s current

falls to zero

t0 t1

Magnetizing current equals

tank circuit’s current

Vc = Resonant capacitor voltage

VHB = Node HB voltage

IR = Tank circuit’s current

I(Lp) = Lp (magnetizing) current

I(Q1) = MOSFET Q1 current

I(CHB)

I(CHB) = CHB current

HB node’s parasitic capa-

citance discharge current

Q2 is switched on with essentially

zero drain-to-source voltage: ZVS!

Low turn-off losses

0 –t1

Q1 current fall time;

voltage-current overlap for Q1

0 –t2

Node HB transition time

2 –t3

Q2’ body diode conduction time

Q1 ON

Q2 OFF

Q1 OFF

Q2 ON

Dead-time

Q1 OFF

Q2 OFF

VHB

Tank circuit’s current

is positive

I(Q1

)

Q1’s current

falls to zero

t0 t1

Magnetizing current equals

tank circuit’s current

Vc = Resonant capacitor voltage

VHB = Node HB voltage

IR = Tank circuit’s current

I(Lp) = Lp (magnetizing) current

I(Q1) = MOSFET Q1 current

I(CHB)

I(CHB) = CHB current

HB node’s parasitic capa-

citance discharge current

Q2 is switched on with essentially

zero drain-to-source voltage: ZVS!

Low turn-off losses

Figure 10.

Q2 gate voltage at turn-on: with

soft-switching

Figure 11.

Q2 gate voltage at turn-on: with

hard-switching (no ZVS)

HB node is falling down

here; current due to Cgd

Miller effect

Current injection through

Cgd due to hard-switching

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