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Switched-capacitor DC-DC converters and coil DC-DC converters: a comparison

Manufacturers are constantly pushing for longer battery operating time in mobile phones, MP3 players and other portable devices, so 'smart' power management technology is increasingly widely deployed. DC-DC converters play an important role in smart power management. The drive to prolong battery life means that efficiency is often considered the most important factor in DC-DC converter specifications, but in today's mobile consumer electronics devices, size is also of critical importance.

Today, the designer has to choose from two fundamentally different types of switched converter: inductive (coil) converters and switched-capacitor (SC) converters. While coil converters are widely used, switched-capacitor converters are still regarded as exotic, and are found in few designs. This is surprising, as switched-capacitor converters offer a significant size advantage in mobile applications. This article will show why.

Principles of converter operation

For the sake of simplicity, this article will focus on DC-DC converters operating in boost mode.

The coil converter operates in a two-phase mode, as most switched converters do. During the first phase, the battery is in parallel to the inductor (see Figure 1). The current through the inductor IL increases at a rate expressed by Vg /L until the phase ends or a pre-programmed maximum current is reached.

During the second phase, the inductor is in series with the battery and the output capacitor. The inductor current continues to flow in the same direction, and so the voltage across the capacitor is the sum of the battery voltage and the inductor voltage.

The current through the coil now decreases at a rate expressed by “(Vg -Vout )/L while it is charging the output capacitor.

Referring to Figure 1, it is clear that a regulator circuit is required to control the duty cycle D (the ratio of phase 1 to phase 2), in order to keep a constant pre-programmed output voltage. Figure 1 also shows that a coil converter needs two external components: an inductor at the input terminal, and a capacitor at the output terminal.

Fig. 1: the two phases of a coil converter's operation

A switched-capacitor converter does not need a coil. Instead it uses a so-called flying capacitor to transfer energy from the input to the output terminal. For ease of comparison with coil converters, we will examine here a switched-capacitor converter in two-phase mode, although multi-phase operation is also possible.

In contrast to the coil converter, the switched-capacitor converter uses a fixed duty cycle D of 0.5. During phase 1, the flying capacitor is put in parallel to the battery voltage supply (see Figure 2). The flying capacitor is charged by the battery supply voltage. During phase 2, the fully charged flying capacitor is put in series to the battery voltage, so that the voltage across the output capacitor is the sum of the battery voltage and the flying capacitor's voltage. This arrangement is put in parallel to the output capacitor, which is then charged.

In order to keep a pre-programmed constant output voltage, a regulator circuit controls the amount of charge transferred to the output capacitor.

Fig. 2: switched-capacitor converter operating in two-phase mode

Efficiency comparison

The efficiency characteristics of each type of converter are best illustrated by reference to an example ” in this case, a typical mobile device application. This application would require a switched converter with a 2.7-5V input voltage range, a regulated output of 5V, and a maximum load current of 50mA.

As mentioned above, the coil converter regulates its output voltage by adjusting the duty cycle D . D is defined as the ratio of the duration of phase 1 tON and the switching period tON fs = tON /Ts . As long as the converter operates in continuous conduction mode (CCM), the voltage conversion ratio M is equal to the duty cycle D , so M = D in CCM. In discontinuous conduction mode, the voltage conversion ratio becomes load-dependent and the effect of the duty cycle becomes non-linear.

In both operating modes, however, the output voltage can be regulated directly by adjusting the duty cycle D of the coil converter. For this reason, the efficiency of a coil converter is almost constant over the entire input (battery) voltage range (see Figure 3a).

On the other hand, the switched-capacitor converter needs to multiply the input voltage by its voltage conversion factor. In our example, these factors are 1.5 and 2, referring to an arrangement using two flying capacitors. (This dual-capacitor configuration common in switched-capacitor converters provides for more efficient operation.)

These factors are fixed by topology and are set automatically by the device. To power the mobile device properly, the system must choose the multiplication factor that gives a result equal to or larger than the required output voltage. If the voltage generated is higher than the required output voltage, the amount of charge transferred to the output capacitor is limited by a regulator circuit.

This in turn produces power losses in the device and leads to the efficiency plot shown in Figure 3c. The large step evident at 3.7V arises from an automatic switch of operating mode, a feature that improves the average efficiency over the supply range.

Fig. 3a: coil converter efficiency v input voltage

Fig. 3b: coil converter v load current

Fig. 3c: switched-capacitor converter efficiency v input voltage

Fig. 3d: switched-capacitor converter v load current

Examining the efficiency-versus-load-current plots, it is obvious that both systems run into problems at low load currents (see Figure 3b,d). Here the quiescent losses and switching losses are large in comparison to the output power. Coil converters use pulse-skip modes or burst modes to try to overcome this loss of efficiency, while switched-capacitor converters reduce their switching frequency depending on the load current.

The different board area requirements of coil and SC converters

It is at higher load currents, around 500mA, that switched-capacitor converters really have an advantage over coil converters. Inductors suitable for handling these relatively high currents are large. For this reason, switched-capacitor converters are ideal for mobile fun-lighting, flash-lighting and torch applications.

Indeed, the issue of PCB footprint is a critical element in a designer's decision about which converter topology to use. The challenge is not just to reduce PCB area, but also the profile (height) of the components on the board. When the two topologies are examined in detail, it becomes clear that the switched-capacitor has a marked advantage in this respect over the coil converter.

Coil converters need three components for proper operation: CIN , COUT , and L. Assuming an additional input decoupling capacitor, switched-capacitor converters need (in dual-mode operation) four components: CIN , COUT , CFLY1 , CFLY2 .

Normally, a coil has a larger footprint than a capacitor. In addition, the high magnetic flux of the coil imposes a requirement for interference counter-measures in the PCB design.

It is true that, since switched-capacitor converters need more external components their packages have a higher pin count and a larger area. But the larger package of the switched-capacitor converter is outweighed by the huge area occupied by the coil.

In total, the PCB area requirement of a switched-capacitor converter will generally be between 30% and 50% smaller than the area requirement of a coil converter.

Switched-capacitor converters are not just smaller: they are lower as well. In the example described above, the inductor requires an inductance of around 2µH. Industry-standard shielded inductors are 1.5mm high and have a footprint of more than 9mm2 . Shielded inductors have become the industry standard for use with switched-converters due to EMI concerns.

Assuming the coil converter is in a TSOT-23 6L package, while the switched-capacitor converter package is a TDFN 10L, the switched-capacitor converter will be 0.8mm high, and its SMD 0603 capacitors will be 0.87mm high. The coil converter arrangement, by contrast, is nearly twice as thick.

To counter this disadvantage, the designer could use a processed multilayer inductor with a component height of around 1mm for a 2µH device. But these components typically occupy a larger area than the shielded coil described above, and the inductor has a lower Q-factor, which harms the performance of the converter as a whole.

Conclusion

Coil converters are today used almost universally to boost battery outputs in small consumer devices, because they offer excellent conversion efficiency. Parts such as the AS1322 or AS1329 coil converters from austriamicrosystems, for instance, offer efficiency of 95% and deliver 150mA (AS1322) or 160mA (AS1329) at 3.3V from a single AA cell. Both parts are available in a 6-pin TSOT-23 package.

While switched-capacitor converters offer, across the input voltage range, far lower efficiency than coil converters, they provide a significant advantage in terms of board space requirement and profile. For consumer electronics and other applications that attach a premium to sleek design and small form factors, this size advantage could prove more important than the few percentage points of efficiency that are sacrificed through use of a switched-capacitor converter. A device such as the AS1301 boost converter offers up to 92% efficiency and delivers a 5V output from a 2.7-5.25V input voltage. The related AS1302 offers up to 90% efficiency and a 5V output from a 2.9-5.15V input. Both devices are available in a TDFN (3 x 3 x 0.8mm) or WL-CSP package.

Author Profile: Andreas Hartberger is a Senior Design Engineer within the Standard Linear Products group at austriamicrosystems AG.

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