AN2644 Datasheet, PDF (16/64 Pages) STMicroelectronics – An introduction to LLC resonant

The LLC resonant half-bridge converter
AN2644
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 I_R 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
Dead-time
Q1 ON
Q2 OFF
Q1 OFF
Q2 OFF
Q1 OFF
Q2 ON
t₀ – t₁ Q2’ body diode conduction time;
coinciding with dead-time
t₁ – t₂ Q1’s body diode recovery time
Q1 has soft turn-off
V_HB
Node HB voltage
I_R
I(Q1)
I(Q2)
Tank circuit’s current
is negative
t₀
Q2 is hard-switched
Resonant capacitor voltage
High dv/dt
Q1’s body diode
is recovered
t₁ t₂
Current is circulating
through Q1’s body diode
Vc = Resonant capacitor voltage
V_HB = Node HB voltage
I_R = Tank circuit’s current
I(Lp) = Lp (magnetizing) current
I(Q1) = MOSFET Q1 current
Then, the converter must be operated in the region where the input impedance is inductive
(the inductive region), that is, for frequencies f > f_R1 or in the range f_R2 < f < f_R1 provided the
load resistance R is such that R> R_crit. 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:
1. 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);
2. 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.
3. low noise and EMI generation, which minimizes filtering requirements and makes this
converter extremely attractive in noise-sensitive applications.
4. all of the above-mentioned adverse effects of capacitive mode, which not only impair
efficiency but also jeopardize the converter, are prevented.
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