ROHM's latest AC/DC power supply technology achieves both improved power factor and high efficiency

In the development of electronic equipment, the efficiency of power supply has become an important theme year by year. In addition, not only Japan, which is facing the problem of power energy, but also power companies related to power generation and transmission around the world, the popularization and high efficiency of power factor improvement equipment are also top priorities. Here we introduce AC/DC power supply technology that achieves both power factor improvement when the equipment is working and high efficiency when in standby mode.

1. Power factor and power factor correction circuit (PFC: Power factor correction)

Power factor refers to whether the power produced by the power company is transmitted to the electronic equipment without loss; efficiency refers to whether the power is transmitted to the electronic equipment without loss.

When the phase difference between the voltage and current of the AC power is φ, the power factor is calculated according to the formula Power Factor = COSφ. When there is no phase difference between the voltage and current, that is, when it is a sine wave, the power factor is 1.

Simply put, when there is a pure resistive load, there is no phase delay between the voltage and current waveforms, so the power factor is 1 (Figure 1).

[Figure 1] Waveform and circuit example when the power factor is 1

However, in modern electronic devices, switching power supplies are widely used, and capacitors are generally used to smooth the input AC voltage (called capacitor input type rectification and filtering). Through this filtering capacitor load, the input AC voltage will only flow when it is higher than the filter capacitor voltage, so the conduction angle becomes smaller and the current waveform becomes a non-sinusoidal current containing high-frequency components (Figure 2).

[Figure 2] Waveform and circuit example when high frequency current is used

Therefore, even if the same power is consumed, a large instantaneous current will flow on the power supply side (for example, when the power factor is 0.5, the peak current is twice as high as when the power factor is 1). The power company has to spend a huge amount of money on additional power generation and countermeasures for equipment damage accidents for this non-sinusoidal current with high-frequency components.

To prevent these problems, countries around the world have implemented high-frequency current limits on devices with power above a certain level, and this is reflected in domestic regulations and enforcement in each country. One of the means to meet these limits is to use a power factor correction circuit (PFC) to change the input current waveform to a near-sine wave, thereby suppressing high-frequency current.

As means for improving such power factor, a passive method using a passive element (inductor) and an active method using a power device on and off are generally used.

The passive method has a simple circuit structure, but it is difficult to meet a wider input voltage range and is difficult to miniaturize. In contrast, the active method can meet a wider input voltage range and is conducive to miniaturization (Figure 3).

[Figure 3] Comparison of current waveforms before and after power factor improvement

From the perspective of efficiency, this active power factor correction circuit (PFC) has a reduced efficiency due to its own power consumption, which is particularly noticeable in modern electronic devices with standby mode.

2. Achieve both power factor improvement circuit and high efficiency

ROHM has developed an AC/DC converter IC (BM1C001F) that realizes both a power factor improvement circuit and high efficiency with a built-in PFC control function. This product is equipped with a function to turn on/off the power factor improvement circuit (PFC) controller at any power and a new PFC output control method. These technologies not only significantly reduce standby power consumption, but also help meet the levels specified by the international standard Energy Star 6.0. In addition, by integrating the power factor improvement circuit (PFC) controller and the quasi-resonant circuit (QR) controller, the number of components can be reduced by 20% compared to the past, which helps to achieve miniaturization of the power supply.

(1) The PFC controller ON/OFF setting function improves conversion efficiency at light loads and reduces power consumption during standby (Figure 4)

[Figure 4] PFC controller ON/OFF setting function diagram

・Monitoring the load power on the secondary side and controlling the PFC controller on/off based on the power, especially helping to expand the load range where PFC is not required

(below 75W) power conversion efficiency.

・When this IC is used in a 100W class power supply using our evaluation board, the standby power consumption is 85mW or less at AC100V and 190mW or less at AC230V, which meets the requirement of 210mW or less stipulated in Energy Star 6.0 (established by the U.S. Environmental Protection Agency).

(2) ROHM's unique new PFC output control method achieves higher efficiency for AC input power supplies in all countries around the world (Figure 5)

[Figure 5] New PFC output control method diagram

・The AC power input range varies from country to country, and the output voltage of conventional PFC ICs is set to be constant. Therefore, when the boost voltage is relatively large (for example, when the PF output is 400V at AC100V input), the switching loss increases and the efficiency decreases. This product is equipped with a new PFC output control method to output a PFC output voltage that matches the AC input voltage, thereby suppressing the decrease in efficiency of the power factor correction circuit (PFC).

For example, in a 100W class power supply, by comparing the efficiency at AC100V input, it is estimated that the conversion efficiency can be improved by about 2% compared to the case where the PFC output is fixed.

(3) Using quasi-resonant circuits that are conducive to high efficiency and low noise

This method allows for greater freedom in power supply design because the switching MOSFET and current detection resistor, which contribute to soft switching and low EMI, are external. In addition, the built-in pulse function achieves high efficiency under light loads.

(4) The power factor correction circuit (PFC) controller and quasi-resonant circuit (QR) are integrated into one package, greatly reducing the number of parts

By integrating the components into one package, the number of components in the common design can be reduced by approximately 20% compared to the case where each component is independent.

3. Excellent power product development system and perfect service support system

When designing a power supply circuit, a good IC is not enough to make a good power supply. In order to create the best power supply circuit, in addition to IC selection, you also need

The selection of passive components such as capacitors, coils and transformer winding design, as well as PCB bottom film (Artwork) and many other design tips. Therefore, choosing a manufacturer that can support its application design is as important as choosing an IC.

ROHM not only develops and sells LSI products, but also has a dedicated team to support customer design. We can provide the best power supply design solution in accordance with the user's required specifications (output voltage, output current, etc.).

They will provide comprehensive circuit design support to customers together with AC/DC converters, DC/DC converters and discrete components such as MOSFETs, diodes, resistors, etc.