AN4027 datasheet(20/41 Pages) STMICROELECTRONICS

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Functional check

AN4027

20/41

DocID022604 Rev 3

optimize the design of the LLC converter and prevent capacitive mode operation that may

occur because of operation at too low input voltage.

The DELAY pin (#2) is zero, as it must be during normal operation, because it works during

the overcurrent protection operation. The DIS pin (#8) is used for open loop protection and

therefore, even in this case, its voltage is at ground level.

In Figure 17 the pin voltages relevant to the control part of the L6699 are reported: the

RFmin pin (#4) is a 2 V (typ.) reference voltage of the oscillator, the switching frequency is

proportional to the current flowing out from the pin. CSS pin (#1) voltage is the same value

as pin #4 because it is connected to the latter via a resistor (R44), determining the soft-start

frequency. A capacitor (C18) is also connected between the CSS pin and ground, to set the

soft-start time. At the beginning of L6699 operation the voltage on the CSS pin is at ground

level because C18 is discharged, then the CSS pin (#1) voltage increases according to the

time constant till the RFmin voltage level is reached. The STBY pin (#5) senses the

optocoupler voltage; once the voltage decreases to 1.25 V, both gate drivers stop switching

and the circuit works in burst mode. The CF pin (#3) is the controller oscillator; its ramp

speed is proportional to the current flowing out from the RFmin pin (#4). The CF signal must

be clean and undistorted to obtain correct symmetry by the half bridge current, and

therefore care must be taken in the layout of the PCB.

4.1

Startup

The waveforms relevant to the board startup at 90 Vac and full load have been captured in

Figure 18. Note that the output voltage reaches the nominal value approximately 800 ms

after plug-in. The L6563H, HV PFC controller, has an embedded high-voltage startup

charging the Vcc capacitor by a constant current, ensuring a constant wake-up time. This

can be seen by comparing Figure 18 with Figure 19, relevant to a startup at 265 Vac and no

load, the output voltage rises at the nominal level in the same time. In both conditions the

output voltage has no overshoot or dips.

In Figure 20 the salient waveforms in the resonant tank during start up of the LLC are

reported. In Figure 21 the detail of waveforms at the beginning of operation shows that the

resonant circuit is working correctly in zero voltage switching operation from the initial

cycles. In the L6699 a new startup procedure, called “safe-start”, has been implemented in

Figure 18. Startup at 90 Vac - full load

Figure 19. Startup at 265 Vac - no load

CH1: C9 bulk voltage CH2: GD L6563H

CH3: +12 Vout

CH4: VCC L6563H

AM11411v1

CH1: C9 bulk voltage CH2: GD L6563H

CH3: +12 Vout

CH4: VCC L6563H

AM11412v1

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