AN2644 datasheet(46/64 Pages) STMICROELECTRONICS

46 / 64 page

Conclusion

AN2644

46/64

Conclusion

The operation of the LLC resonant converter has been examined in detail and its most

important properties have been deduced by inspection of its salient waveforms under

different operating conditions. To summarize, the most significant merits of this topology are:

●

Soft-switching of all semiconductor devices: ZVS (zero-voltage switching) at turn-on for

the MOSFETs and ZCS (zero-current switching) at both turn-on and turn-off for the

secondary rectifiers. The first property results from a correct design of the resonant

tank. The second one is a natural feature of the topology.

●

Ability to accommodate an extremely broad load range, including zero load, with an

acceptable frequency variation. Also this property results from a correct design of the

resonant tank.

●

Magnetic integration, which allows the combination of different magnetic devices into a

single physical device.

●

Smooth waveforms: the current is piecewise sinusoidal with no steep edges. The

voltage, although a square wave, does not have very high dv/dt edges. EMI emissions

are considerably low and filtering requirements are relatively loose.

●

As a result of all the above merits, high-efficiency, high switching frequency capability,

high power density are typical characteristics of the converters based on this topology.

The most significant facts concerning the design of an LLC resonant converter are:

●

The quantities that determine whether the converter operates at resonance, above

resonance or below resonance are essentially the input and the output voltages and

the transformer's turn ratio. There is just a second-order dependence on the load

current due to the variation of the voltage drop across parasitic elements such as

winding resistance, rectifier drop, etc.

●

Operation at resonance looks like the preferred operating point, where load regulation

is ideally zero, where tank current is maximally sinusoidal and where CCM operation

minimizes peak tank current for a given power throughput. Whenever possible (e.g.

when the LLC converter is powered by a PFC preregulator) it seems a good design

strategy to design the converter to work at resonance under nominal conditions, and

use below resonance operation to handle mains voltage dips and above resonance

operation to handle light load or transient overshoots of the input voltage.

●

The ability to operate under no-load conditions and to ensure ZVS operation depend

essentially on the transformer's magnetizing inductance. Its value has to be traded off

against the switching losses at full load operation and the input consumption at no-load.

●

Avoiding capacitive mode operation is a must. It is a too risky operating mode and any

design procedure should not leave this fundamental aspect out of consideration.

●

Overcurrent and short circuit protection must be provided. They will not only have to

prevent excessive currents from flowing in both the resonant tank, the transformer and

the output rectifiers but also avoid entering capacitive mode operation.

●

Soft-start is highly recommended. At startup there is a situation very similar to a short

circuit and, if not properly controlled, potentially destructive currents might flow in the

half-bridge leg and the resonant tank.

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