AN2644 datasheet(38/64 Pages) STMICROELECTRONICS

38 / 64 page

The LLC resonant half-bridge converter

AN2644

38/64

intuitive that the impedance of the Ls-Cr resonant tank plays a key role. Note that this

impedance becomes zero at f = fR1. Two cases will be considered:

The converter always works above resonance (some means is provided to bottom-limit

the operating frequency at a value fm > fR1). The control loop, in the attempt to keep the

output voltage regulated, will reduce the switching frequency to the minimum value fm.

In this case the converter still operates in the inductive region (CCM submode) and

ZVS is ensured. However, the impedance of the Ls-Cr tank circuit can be very low and

then the current Iin can reach very high values, the closer the minimum frequency to

fR1 is, the lower the impedance and the higher the current will be.

The converter can operate below resonance (some means is provided to bottom-limit

the operating frequency at a value fm such that fR2 < fm < fR1). Again, the control loop,

in the attempt to keep the output voltage regulated, will reduce the switching frequency

to the minimum value fm. In this case, the input current will be limited by the impedance

of the Ls-Cr tank circuit at a value that is as lower as fm is lower. However the capacitive

region is entered and ZVS is lost, with all the consequent troubles previously

mentioned. The waveforms will be similar to those in Figure 22.

It is interesting to see what happens under short circuit if the converter is operating at

resonance (f = fR1). Tank circuit's impedance is zero, then the short circuit is directly

connected across the input source and the current Iin is theoretically unlimited.

Theoretically, switching frequency should not change (the system is working in the load-

independent point). In real-world operation, Iin, though reaching very high values, will be

limited by the parasitic resistance of the short-circuit mesh, the ESR of Cr and by

transformer's nonidealities. Additionally, the drop across the secondary rectifiers does not

reflect a true short circuit on the primary side. Because of the parasitic resistances, the

output voltage will drop and the control loop will react pushing the operating frequency to the

minimum fm, which, realistically, must be lower than fR1 if operation at resonance is allowed.

In the end this will fall into case 2.

Still with reference to case 2, it is worth remembering that the capacitive region is entered

and ZVS for MOSFETs is lost as

, well before short circuit.

From the above analysis it is possible to draw the following conclusions:

Two major issues arise under overload or short circuit conditions: very high current

levels, and capacitive mode operation (with loss of ZVS, etc.). This makes the operation

of the LLC resonant converter under overload or short circuit inherently unsafe if no

overcurrent protection (OCP) is used, just like in conventional PWM-controlled

converters. OCP will have to prevent not only the tank current from exceeding unsafe

Figure 24.

Circuit's equivalent schematic

under: no-load conditions

Figure 25.

Circuit's equivalent schematic

under: short-circuit conditions

Vin

a:1

Vout

a·Vout

Vin

a:1

Vout

a·Vout

Vin

Iin

Vin

Iin

crit

Zo∞

⋅

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