AC-DC Converter Control that Achieves Both Power Factor Correction and High EfficiencyTwo New Control Methods to Achieve Both PFC and High Efficiency

－So the BM1C101F aims at achieving both power factor correction and high efficiency through a PFC controller on/off function and a new control method to switch the PFC output voltage. To begin with, could you explain the “function enabling internal PFC controller on/off control at a preset voltage”?

First, in order to help the reader conceptualize a circuit as a power supply, we look at an example of an application circuit. This conversion circuit has a quasi-resonant design, and is a self-excitation type flyback converter that uses the voltage resonance of the transformer primary windings as inductor and a resonance capacitor. The power for turning PFC on and off, which is one important feature, utilizes detection of the QR_CS terminal current limiter level, and can easily be set, taking into consideration the resistor at the P_OFFSET terminal.

The principle of efficiency improvement through the PFC on/off function is comparatively simple. By halting PFC switching operation during light loading or while in standby mode, the power consumption of the PFC circuit is reduced, and the tendency to lower efficiency is minimized. In particular, in a load range in which there is no need for PFC, such as for example applications in Europe, if parameters are set such that PFC is turned off at power levels of 75 W or below, the power supply efficiency can be improved.

－How much of an improvement in efficiency can be expected?

It all depends on the circuit, but as one example, please have a look at the graph of efficiency in evaluations of a 100 W-class power supply. It is clear that high efficiency is maintained even under light loading. In an example of evaluation of standby power, the standby power was 85 mW or lower at AC 100 V and 190 mW or lower at AC 230 V, satisfying the Energy Star 6.0 requirement of 210 mW or less.

－So what is the other method, the “new control method able to switch the PFC output”?

As you know, the voltages of commercial AC power supplies differ from country to country. A universal power supply that can be used with all of these accepts a wide range of input voltages, but the output voltages of conventional PFC controllers were fixed. Hence when, for example, the PFC output voltage is 400 V, but the input AC power supply is 230 V and 100 V, switching losses are larger for the 100 V input due to the higher step-up ratio, and efficiency is worsened. The new control method of the BM1C101F prevents such drops in efficiency by automatically switching the PFC output voltage to a voltage suited to the AC input voltage. This diagram illustrates how the PFC output voltage is switched according to the input voltage.

－About how much is efficiency improved using this function?

For a 100 W-class power supply, the efficiency at AC 100 V input can be improved by around 2% compared with a case in which the PFC output is fixed.

As an example, this is a comparison of efficiency. The data is for cases in which the Energy Star 6.0 standard, which could not have been satisfied otherwise, was met using this function.

－Are there other features as well?

As an IC for power supplies, the BM1C101F uses a high-efficiency, low-noise quasi-resonant circuit. This design uses a control method that features low EMI through soft switching. Further, a burst function in PFM operation is incorporated, so that high efficiency is maintained even under light loading. Apart from this, most of the protection functions included in power supply controller ICs in recent years are also incorporated. Because this is a controller type IC, the MOSFET is added externally, and there is a high degree of freedom in design.

Also important is the fact that, by integrating a PFC controller and a quasi-resonant controller, the number of components of the shared design can be reduced, so that the number of components is decreased by about 20% compared with a case in which the circuits are designed individually, making further miniaturization possible. In 100 W-class apparatuses, of which large-screen LCD TVs are representative, using a wall adapter instead of on-board power supplies is proceeding apace in order to reduce the time and number of processes required in design and evaluation, and in the interest of ease of repair. Compared with conventional circuit configurations that use individual power supply ICs and PFC controllers, this approach, with a smaller number of components, lends itself to further wall adapter miniaturization.

－In the circuit we saw above, it seems that a transformer and insulating components are necessary. Can customers expect to receive support in selection of such components, circuit design, and the like?

For optimal power supply circuit design, in addition to IC selection, choosing the optimal peripheral components and board layout are of course essential. ROHM not only develops and sells ICs, but also offers a specialized staff to support customer design efforts. They are capable of providing various suggestions and support, and await your inquiries.

－Could you please give a summary.

We have used the example of the BM1C101F, a controller IC for AC-DC converters that integrates a PFC controller and a quasi-resonant controller, to explain how high efficiency in standby mode can be maintained even when a power factor correction circuit is added. The BM1C101F adopts a function for turning the PFC controller on and off at a preset power and a new control method to automatically switch the PFC output voltage to a voltage suited to the AC input voltage, in order to simultaneously achieve power factor improvement and high efficiency under light loading. As a result, Energy Star 6.0 requirements can be met while conforming to the regulations relating to high-frequency currents of many countries. Moreover, the number of components can be reduced by 20% or so, contributing to smaller sizes for 100 W-class AC adapters, for example.