AN4026 datasheet(18/40 Pages) STMICROELECTRONICS

18 / 40 page

Functional check

AN4026

18/40

Doc ID 022603 Rev 1

In Figure 13 and Figure 14 the same images are captured during light load operation: note

that the half bridge transition is slower because of the lower switched current. In this case

the L6699 increases the deadtime maintaining the correct zero voltage switching operation

of the circuit. This feature allows the maximization of the transformer magnetizing

inductance, therefore obtaining good light load efficiency and also keeping correct operation

by the HB.

In Figure 15 some signals at the L6699 pins are measured. Note that the signal on pin ISEN

(#6) matches the instantaneous current flowing in the transformer primary side. Contrary to

the former resonant controllers such as the L6599A and others, requiring an integration of

current signal, the L6699 integrates the anti-capacitive mode protection it needs by sensing

the instantaneous value of the current, in order to check the phase between the voltage and

current. For the same reason, the time constant of the typical RC filter placed between the

sensing resistor and the IC pin must be maximum in the range of 200/250 ns. A significant

higher value would affect the correct anti-capacitive mode protection because of the phase

lag of current signal fed into the L6699.

Figure 11.

HB transition at full load - rising

edge

Figure 12.

HB transition at full load - falling

edge

CH1: HB voltage

CH3: HVG

CH2: LVG

CH4: res. tank current

CH1: HB voltage

CH3: HVG

CH2: LVG

CH4: res. tank current

Figure 13.

HB transition at 0.25 A - rising edge Figure 14.

HB transition at 0.25 A - falling edge

CH1: HB voltage

CH3: HVG

CH2: LVG

CH4: res. tank current

CH1: HB voltage

CH3: HVG

CH2: LVG

CH4: res. tank current

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