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2.3 Adoption of active filter

<< Worldwide range of input voltage >>

Japan has a special situation of supply mains. In Japan, many supply mains at household is AC 100V. Nominal voltage varies across the world. Table 2.5 shows nominal voltage of supply mains in the world.

Table 2.5 Nominal voltage of the world
Table 2.5 Nominal voltage of the world

As shown in table 2.5 nominal voltage varies in many countries, but classified mainly into ranges. One is from 110V to 125V, the other is from 220V to 250V. Nominal voltage usually has 10% of tolerance. If power supply corresponds to 115V and 230V, it covers nominal voltage of 110V to 125V and 220V to 230V. This is why many power supplies are equipped with selection switch of 115V/230V (Photo 2.8). It is ideal to prepare for suitable input power supply in individual country, but very expensive. So many power supplies carry selection switch.

Photo 2.8 Selection switch of 115V/230V
Photo 2.8
Selection switch of 115V/230V

The selection switch is turned to 115V for use in Japan. The nominal voltage has 10% of tolerance. The lower limit is:

115×0.9 = 103.5V

Generally, many power supplies operate at approx. 90V, but some do not cover 100V of Japan's nominal voltage. Furthermore, the nominal voltage in Japan also has 10% of tolerance.
The lower limit is:

100×0.9 = 90V

When power supplies with voltage selection switch are used in Japan, a slight voltage drop is likely to bring a big problem to them.

Given this factor, active filter is mounted to boost dc voltage once (Except some models). (Figure 2.4)

Figure 2.4 Active filter concept (Primary circuit)
Figure 2.4 Active filter concept (Primary circuit)

With active filter, it does not matter, for example, if input voltage is 90V or 240V. The dc voltage is boosted up to approx. 350V to always stabilize secondary voltage which is supplied to PC. That is why our products can be used in Japan, in Europe, or anywhere in the world without selecting input voltage.

<< Harmonic current countermeasures >>

Active filter also has anther effect to limit, so called "Electric pollution," harmonic current as well as corres-ponding to input voltage worldwidely. Take a look at Figure 2.5. The waveform of both voltage and current is sinusoidal when active filter is built in. On the contrary, without active filter, voltage waveform is always sine, but current waveform becomes non-linear and discrete. This means, if power frequency is, for example 50Hz, harmonic current of odd frequency like 150Hz, 250Hz, 350Hz, and so, flows. This does not cause problems at general household for sure, but causes big problems at power stations and transmission facilities. Actually, harmonic current generated by air conditioning, lighting, and OA equipments in office buildings have caused abnormal heat and malfunctions at substation installations.

Figure 2.5 Harmonic current suppression by active filter
Figure 2.5 Harmonic current suppression by active filter

As the standard to regulate harmonic current, IEC61000-3-2 is applied to electrical and electronic equipment of which input current is up to and including 16A per phase. The equipment is classified into 4 classes as below.
Class A:Balanced three-phase equipment and all other equipments that do not belong to other Classes.
Class B:Handheld power tool
Class C:Lighting equipment
Class D:Equipments with up to 600W of input power shown below.
* Personal computer and monitor
* TV set

For ATX power supply, Class D is applied.

<< Power factor correction >>

Furthermore, Power Factor (PF) is improved by active filter which makes current waveform sinusoidal.
Power factor is the ratio of superficial power (apparent power) to actually consumed power (active power), and described as follows,

[Power factor]=[Active power]/[Apparent power]

When power factor is low, input current of power supply increases t result in poor utilization of power distribution installation eventually.