Batteries & Fuel Cells

Seeing the wood

despite the trees

Emir Serdarevic looks at how to choose the right non- inverted conversion method for battery-powered systems

E

ngineers designing battery- powered devices can choose from a variety of types of power

converter for producing a regulated supply from an input voltage that can at different times be higher or lower than the required output voltage. While the basic operation of each

type of converter might be easily understood, it is not obvious, without a comparison of the relevant performance data, which type performs best in which conditions. Where buck-boost conversion is used

The mostly widely used power management devices are DC/DC boost converters (where the output voltage must be higher than the input) and DC/DC buck converters or Low Drop Output (LDO) converters (where the output voltage must be lower than the input). But in battery-powered devices, a

simple buck or boost converter is often unsuitable, because the input voltage from the battery fluctuates across a range, depending on how much charge remains in the battery, and on external conditions such as temperature. Lithium-ion batteries operate at

between 2.7V and 4.2V, and dual-cell alkaline (NiCd or NiMH) batteries have a 1.6V-3.4V range. Typical application circuits require a stable supply bus of 2.8V, 3V or 3.3V. Producing such a stable output from a battery input is clearly, then, only possible with buck- boost conversion.

Desirable attributes

The ideal buck-boost converter would be both tiny and perfectly efficient. The challenge for the design engineer is to get as close to this ideal as possible in the real world. High efficiency extends the battery’s

operating time between charges. Small size enables the converter to be accommodated in small devices or space-constrained circuit boards. Smaller ICs and boards are also cheaper than equivalent but larger ICs and boards, so small size also helps to reduce cost.

Topologies

One method of building a buck-boost converter is to connect a boost converter in serial with an LDO. In such a scheme, the boost converter is configured to produce an output voltage equal to the top of the battery’s

20 April 2010

Figure 2: Schematic of buck converter

The most commonly used topology,

however, is the cascaded buck-boost DC/DC converter. This device type (see Figure 3) is the result of combining the topology of the buck converter (see Figure 2) with that of the boost converter. Buck-boost conversion is

accomplished by the operation of four internal switches. The switches on the left side of the inductor in Figure 3 are responsible for buck conversion, and the switches on the right side of the

Components in Electronics

voltage range. This output from the boost converter becomes the input to the LDO, which scales it down to the target supply voltage for the application. This inelegant solution is both

inefficient – because the input undergoes two conversions rather than one – and expensive, because it requires two ICs. Better techniques are available.

inductor for boost conversion. It is clear from a comparison of Figures 1 and 3 that the cascaded buck-boost converter is more complicated, which means that the device will be larger. At the same time, in the cascaded buck- boost converter the current must always flow through two switches in serial, whereas in the boost converter with buck capability it flows through just one – and switches dissipate energy.

Figure 1: Schematic of boost converter (implemented with buck capability)

One such is the boost converter with

buck capability. This is implemented simply, through the operation of just two internal switches (see Figure 1). The drawback of this topology is that, in order to achieve down-conversion, the device simply dissipates energy until the required lower voltage is reached. This energy dissipation is essentially wasteful, and compromises the efficiency of the device. On the other hand, the current in

the coil always flows through only one of the two switches at any one time, and the energy losses in boost mode are small, if the switches have a low resistance value.

Comparisons in the real world

The AS1331 is a cascaded buck-boost converter. Average efficiency across the input voltage range is around 85%, and efficiency peaks in the middle of the range at 90%. In addition, efficiency is stable over the input voltage range and across the range of load currents. Overall, the device is less efficient

than a simple buck or boost converter, and this is because the coil current flows through two switches in serial. The AS1331 is produced in a 3x3mm TDFN package. The AS1337 is a boost converter with

buck capability. Efficiency in boost mode is very high, peaking at 97% when the input and output voltages are the same. But this device also betrays the

weakness of this topology, because efficiency droops noticeably in buck mode, to less than 70%. However, at 1.7mm2, the die size of

the AS1337 is much smaller than the AS1331’s, because of the device’s much simpler structure. Like the AS1331 it is produced in a 3x3mm TDFN package.

So which converter type?

For applications in which the battery’s voltage is generally higher than the output voltage, the cascaded buck- boost converter is the right choice because of its stable high efficiency across the whole input voltage range,

in both buck and boost mode. A typical example of such an application would be a device requiring a 3V supply bus from lithium-ion batteries with a voltage range of 2.7V-4.2V. If, however, the battery’s voltage is

lower than the required output for more than 50% of the device’s operating time, the boost converter with buck capability will usually be the right choice. A device such as the AS1337 is also smaller and cheaper than the cascaded buck/boost equivalent. A suitable application example for

this converter type is a device requiring a 3V supply bus from dual-cell alkaline batteries with a voltage range of 1.6V- 3.4V. Here, the device will operate in low-efficiency buck mode for only around 10%-20% of the time; overall efficiency will be approximately 90%. In this case, the cheaper solution is also the more efficient.

Austriamicrosystems

| www.austriamicrosystems.com

Emir Serdarevic is an Analogue Design Engineer (Standard Linear Products), austriamicrosystems

Figure 3: Cascaded buck-boost converter

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