AN2644 datasheet(39/64 Pages) STMICROELECTRONICS

39 / 64 page

AN2644

The LLC resonant half-bridge converter

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values but also ZVS from being lost. In principle, its setpoint must be such that the

condition R>Rcrit is always fulfilled.

Limiting the minimum operating frequency of the converter is not always effective. It

prevents losing ZVS under overload or short circuit only if this minimum value is above

the resonant frequency fR1. Thereby, relying on just limiting the minimum operating

frequency would force giving up the below-resonance operating region and its

interesting properties, severely limiting the usability range of the converter.

Furthermore, the problem of having too high circulating current would be still unsolved.

Converters are often specified to have peak load demands that are considerably higher

than the maximum continuous power, and whose duration is such that it cannot be

averaged by the output capacitor bank. While these peak loads may be thermally

irrelevant, from the electrical standpoint they must be considered as steady-state. OCP

circuits must not be triggered and the converter must be designed so that the condition

R>Rcrit is not violated under these transient conditions as well.

The inspection of the waveforms under heavy load conditions shows that, unlike PWM-

controlled converters, the peak current in a switching cycle is not reached at the end of the

conduction time of either MOSFET. This suggests that the usual cycle-by-cycle current

limitation so widely used in PWM-controlled converters is not applicable to LLC resonant

converters.

The simplest and also most immediate action to take in response to the detection of an

overload or short circuit condition is to increase the operating frequency. It is advantageous

to push the frequency well above the resonance frequency fR1, so that the converter

definitely operates in the inductive region, and ZVS is maintained, with the input current kept

under control by the inductive reactance of the tank circuit.

However, this frequency rise is not typically sufficient to effectively limit the short-circuit

output current at safe values. In fact, on one hand, the input impedance of the tank circuit in

the inductive region is essentially proportional to frequency. Since there are practical limits

on the maximum operating frequency (it rarely exceeds 3-4 times fR1), the short circuit tank

current can still be considerably large. On the other hand, as the output voltage drops

because of the action of the OCP circuits, the voltage reflected across Lp becomes smaller

and smaller. Then, less and less current flows through Lp and the transformer tends to

transfer all the primary current to the output.

As a consequence, the short circuit output current can be still much higher than the nominal

full-load current and the resulting stress, especially for the secondary rectifiers, might be

unacceptable. In addition to current limiting it is therefore advisable to provide some timed

shutdown protection that either forces an intermittent operation of the converter to

drastically reduce the average value of the output current or latches it off if the OCP circuits

are active for more than some time.

2.6

Converter's startup

Like in PWM-controlled converters, startup is quite a critical moment that needs to be

properly handled in LLC resonant converter as well. When the converter is first switched on

(or also while it is recovering after a protection shutdown) the energy flow should be

progressively increased to allow a slow buildup in output current and voltage. This is

commonly known as "soft-start". Doing otherwise, high and potentially destructive currents

might be drawn from the input source and through the power devices in an attempt to

charge the output capacitors and bring the output voltage to the regulated value.

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