STCMB1 datasheet(31/59 Pages) STMICROELECTRONICS

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STCMB1

Application information

The calculated voltage VHB_Stop will have to be guaranteed by the bulk capacitors after the

hold-up time and will be the minimum voltage at which the resonant HB converter will work,

and therefore it will be dimensioned according to that minimum voltage.

Since the PFC stage is typically designed at the minimum input voltage (and maximum

output load), a DC brown-out event could occur at heavy loads, when the input voltage is

between the minimum input voltage and the AC brown-out threshold. In fact, in this

condition, the PFC is limiting the power delivered and its output voltage could decrease up

to the DC brown-out threshold at which the LLC stage is turned off: VOUT will drop and VCC

will be discharged. As VCC reaches VCCon, the HV start-up is activated, VCC recharged

and operations restarted as VCC reaches VCCon.

To avoid the occurrence of a DC brown-out event before the AC brown-out, at the full load,

the PFC stage has to be designed at the ACBO threshold rather than at the minimum input

voltage.

7.17

HB: timeshift control basic concept

The TSC methodology consists in controlling the amount of time TSH elapsing from a zero

crossing of the tank current to the next toggling of the half-bridge, as shown in Figure 16.

The edge of the square wave voltage commanded at the end of TSH has the opposite sense

with respect to that of the current zero crossing that initiated TSH. This ensures that the tank

current lags behind the applied square wave voltage and, therefore, that the MOSFETs of

the half-bridge work with soft-switching. Figure 16 shows, for example,

a negative-going edge of the square wave voltage commanded after a positive-going zero

crossing of the tank current (highlighted in blue).

Figure 16. Concept of timeshift control and control variable (TSH) range

Denoting with the phase shift of the tank current with respect to the applied square wave

voltage, Equation 18 relates the timeshift to operating frequency fsw:

Equation 18

Under no load conditions, the operating frequency reaches the maximum value fswmax, the

switching period is Tswmin = 1/fswmax and the tank current is in quadrature to the voltage

(

 = / 2; Figure 16, left-hand side), so it is TSH = Tswmin / 4. On the opposite side, at the

boundary with the capacitive mode operation, when the current and voltage are in phase

s w

S H

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