Part 2: Highly Integrated Solution for AC Adapters: Experimental Results

Jan. 1, 2010
A prototype of the highly-integrated boundary-conduction mode (BCM) PFC and quasi-resonant (QR) PWM combo controller exhibits efficiencies in the 90% range and harmonic distortion in the 3.75% to 14% range, depending on the ac input voltage and frequency.

A prototype of the highly-integrated boundary-conduction mode (BCM) PFC and quasi-resonant (QR) PWM combo controller exhibits efficiencies in the 90% range and harmonic distortion in the 3.75% to 14% range, depending on the ac input voltage and frequency.

The experimental platform for the BCM/QR converter was a 19-V/90-W, universal input-voltage notebook adapter. Figs. 1 and 2 show the PFC switching waveform around the peak and valley of input voltage, V g.

When the inductor current of the PFC choke discharges to zero, the inductor and parasitic capacitor of the PFC MOSFETs both resonate with each other. Then, the drain-to-source voltage, VDS, of the PFC MOSFET starts to swing down, as does the voltage signal on the zero-current detection pin, F2.

Once the voltage is lower than the threshold voltage, the PFC gate will turn on. Around the peak of vg, the switching cycle begins at the valley of VDS. In this condition, the MOSFET turns on with zero current switching (ZCS). Around the valley of Vg, as shown in Fig. 2, each of the switching cycles begin after the inhibit time.

If VDS has not swung back, then the MOSFET also turns on with zero voltage switching (ZVS). The power factor and THD test results shown in Table 1 are less than the limit of IEC1000-3-2 Class D with great margin.

The switching behavior of the PWM MOSFET from full load to no load is shown in Figs. 3 to 6. At full-load conditions, t OFF-MIN is about 8 ms with the appropriate transformer design, and the PWM MOSFET will turn on at the first valley (Fig. 3). As the output loading is decreased, tOFF-MIN is extended internally.

During tOFF-MIN, every valley signal is ignored. After tOFF-min is passed, the PWM MOSFET turns on as the valley is detected. As is shown in Figs. 4 and 5, the valley of the PWM switch is extended as output loading is decreasing. Even the 14th valley can be detected at light-load condition.

Fig. 6 shows the PWM burst-mode operation at no-load condition. The switching frequency between each group of the PWM gate is about 60 Hz.

In each group, the switching frequency is decided by the level of VFB, which is about 28 kHz, so the audible noise is extremely low. With extended-valley switching and burst-mode operation, the controller performs low-power consumption and low switching loss at no-load and light-load condition.

The test result is in Table 2. Table 3 shows the efficiency from 22.5% to 100% full load and from 90 to 264 Vac. Efficiency is in excess of 87% in each loading point.

This new combo solution (BCM PFC+QR PWM) has a simple and compact configuration. This converter can achieve high efficiency at high input voltages.

Besides very low switching loss, the power savings from this converter is lower than a traditional PFC+PWM converter because of the two-level PFC and deep extended valley detection. When looking at efficiency, the rectifier shows that this converter can achieve roughly a 1.5% to 2% improvement.

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