Analysis and comparison of power topology solutions for portable systems

"Smaller size, lower cost, and higher efficiency" are the market requirements for the new generation of portable devices, but it is difficult for design engineers to optimize these three requirements independently. The optimization solution must be based on the overall system requirements and comprehensively consider factors such as size, cost, and work efficiency. For design engineers, there are many choices for power supply topologies, including buck converters, low-dropout regulators (LDOs), buck/boost converters, etc., but they each have their own advantages and disadvantages, and a trade-off should be made when selecting.

This article will discuss the various power topologies, especially the pros and cons when converting Li-Ion battery voltage to a 3.3V rail, the supply voltage for most portable devices. It will also describe different applications for buck/boost converters and explain why buck/boost converter solutions need to be “tailor-made.”

As can be seen in Figure 1, the design of converting the voltage of a lithium-ion battery to a 3.3V voltage rail is challenging. When fully charged, the typical lithium-ion battery discharge curve starts at 4.2V. The starting point on the x-axis is "-5 minutes", which corresponds to the open circuit voltage when the battery is fully charged. At "0 minutes", the battery is connected to the load and the voltage begins to drop due to the internal impedance and protection circuit. The battery voltage slowly drops to about 3.4V, and then the voltage begins to drop rapidly because the discharge cycle is nearing the end. To fully utilize the stored energy in the battery, the 3.3V voltage rail needs to use a step-down converter for most of the discharge cycle and a boost converter for the rest of the discharge cycle.

Figure 1: 1650mA-hr 18650 lithium-ion battery discharge curve.

The problem of how to effectively generate a 3.3V rail from lithium-ion battery voltage has been around for a long time, and there are many solutions. This article discusses several common solutions, including cascaded buck and boost, buck/boost, buck, and LDO power supply topologies, and discusses the pros and cons of each design, as well as measuring and comparing system operation time.

Cascaded Buck and Boost Converter Solution

The cascaded buck and boost converter consists of two independent and separate converters, the buck converter and the boost converter. The buck converter regulates the voltage at a medium voltage (such as 1.8V), while the boost converter increases the medium voltage to 3.3V. Since it can utilize 100% of the battery power, this architecture is very suitable for systems that require lower voltage rails. However, due to the two-stage conversion mechanism, this is not the best solution from an efficiency perspective.

The effective power conversion efficiency is the product of the buck regulator efficiency and the boost regulator efficiency. Working under the above voltage conditions, the typical efficiency values ​​of the buck and boost converters are both 90%, so the effective power conversion efficiency of the 3.3V converter is 90%×90%=81%. Since this architecture includes two independent converters, the number of components and the system volume are increased, which is not only difficult to use in small portable products, but also increases the cost.

Standalone Buck Converter Solution

A buck converter is also an option to convert the Li-Ion battery voltage to 3.3V, but this solution is often overlooked and not widely used. Designers often dismiss this solution after observing the battery discharge curve (Figure 1). This is because the buck regulator cannot generate a 3.3V rail as shown in the fully discharged battery curve (Figure 1). When the input voltage of the buck converter drops close to the output voltage, many buck converters enter 100% duty cycle mode. Under this condition, the converter stops converting and directly outputs the input voltage. In 100% duty cycle mode, the output voltage is equal to the input voltage minus the voltage drop of the converter. This voltage drop is determined by the (MOSFET on-resistance, the DC resistance of the output inductor, and the load current, which sets the minimum battery voltage that remains in regulation. Assuming the system considers the 3.3V rail to be in regulation after a 5% drop, the following equation can be used to calculate the minimum battery voltage at which the system can operate.

Vbattery_min=Vout_nom×0.95+(Rdson+RL)×Iout(1)

Where: Vout_nom is the rated value of 3.3V, Rdson is the on-resistance of the power MOSFET, RL is the output inductor dc resistance, and Iout is the output current of the converter at 3.3V.

When the battery voltage drops to Vbattery_min, the system must shut down when it is below the minimum tolerance to avoid running on the 3.3V rail and corrupting data. The system may shut down even if the battery still has 5-15% energy left. How much battery energy is left before the system shuts down depends on many factors such as component resistance, load current, battery age, and ambient temperature.

Most designers would forgo using a standalone buck topology for this reason, but a closer look at the actual system runtime shows that the standard buck/boost, cascaded buck, and boost topologies are much less efficient than a standalone buck converter. While these topologies make full use of the battery charge, they are much less efficient than a buck converter. In many cases, a standalone buck converter will run longer than either of the other two topologies. Until 2005, a fully integrated buck converter was considered the best choice for generating the 3.3V rail.

Low Dropout Regulator Solutions

Another less common solution is LDO. Similar to the "single buck" solution, LDO cannot fully utilize the entire battery power because it can only regulate the voltage when the input voltage is greater than the sum of the output voltage and the LDO voltage drop. If the LDO voltage drop is 0.15V, the 3.3V output voltage begins to drop when the battery voltage is lower than 3.3V+0.15V=3.45V. The battery power that cannot be fully utilized due to this solution may be much greater than that of the single voltage drop solution. Despite this disadvantage, LDO also has advantages in certain environments.

LDO solutions are usually the smallest in size, so they are an ideal choice when the main system has strict space requirements. LDO solutions are usually the lowest in cost, so they are very suitable for low-cost applications. Many design engineers give up using LDO solutions due to their low efficiency, but after careful study, it can be found that the efficiency in this application is still good:

When the fully charged Li-ion battery starts at 4.2V, the initial efficiency of the LDO is 78%, and its efficiency increases as the battery voltage decreases.

Buck/Boost Converter Solutions

Buck-boost topology is widely used. This topology combines all the advantages of the other solutions mentioned above. As the name suggests, this topology has both buck and boost functions, so it can use 100% of the battery power.

The way the buck/boost converter is deployed determines its extremely high conversion efficiency. For example, the Texas Instruments (TI) fully integrated buck/boost converter TPS63000 achieves a conversion efficiency of about 95% when dropping from 3.6V to 3.3V. High conversion rate means that the battery power can be fully utilized to achieve the longest running time. Compared with the number of components and volume of buck solutions, fully integrated buck/boost converters that integrate power switches, compensation components, and feedback circuits are not at a disadvantage, and the external components only require input capacitors, output capacitors, and inductors. Highly integrated single-chip IC solutions help reduce overall system costs.

The buck/boost power stage is shown in Figure 2. This topology consists of a buck power stage with two power switches and a boost power stage with two power switches, which are connected through a power inductor. These switches can operate in three different modes: buck/boost mode, buck mode, and boost mode. A specific IC operating mode has a specific input-to-output voltage ratio and IC control topology.

Figure 2: The buck-boost power stage consists of a buck power stage with 2 power switches and a boost power stage with 2 power switches.

Buck-boost converters are not all the same

The need for buck/boost converters in portable applications has been around for a long time, but the size and efficiency requirements are usually very stringent. Only recently has semiconductor packaging technology advanced to the point where it is possible to integrate four MOSFET switches and the corresponding control loop into a small package.

Although different buck/boost solutions have the same power stage topology, the control circuits vary greatly. Three standard buck/boost converters are available. The first one has all four MOSFET switches active during each switching cycle. This mode of operation produces the standard buck/boost waveform. A closer look at these waveforms shows that the effective current (RMS) through the inductor and MOSFET is much higher than that of a standard buck or boost converter. This results in increased conduction and switching losses for standard buck/boost converters. Running the four switches synchronously also increases gate drive losses, which dramatically reduces efficiency at low output currents.

The second new buck/boost control method operates only two MOSFETs in each switching cycle, thereby reducing losses. As can be seen from Figure 2, this control scheme can operate in three different modes. When Vin is greater than Vout, the converter turns on Q4 and turns off Q3, and then uses Q1 and Q2 as a standard buck converter; when Vin is less than Vout, the control circuit turns on Q2 and turns off Q1, and then uses Q3 and Q4 as a standard boost converter. However, this control mode will cause some operation and control problems in the transition region between buck and boost modes. To solve these problems, the standard buck/boost mode can be used during the transition process. Because all four switches are in operation in the standard buck/boost operating mode, these control problems can be solved. However, the increase in switching losses and RMS current causes a sharp drop in efficiency in the transition region, and this efficiency drop region is close to the battery voltage (most of the battery power is provided at this time), so in most areas of the battery discharge curve, the converter operates in an inefficient buck/boost mode.

The third buck/boost control mode eliminates the transition area between buck and boost modes, so performance and efficiency are significantly improved. TI's TPS63000 buck/boost converter includes an advanced control topology that solves the problems faced by standard buck/boost converters. Regardless of the operating mode, the TPS63000 has only two switches in operation in each switching cycle, which not only reduces power consumption but also maintains high efficiency during the battery's full discharge curve. Unlike some solutions, the TPS63000 integrates all compensation circuits and only requires three external components to operate, minimizing product size.

Figure 3 shows the discharge curves of the lithium-ion battery voltage down to 3.3V versus run time for four solutions. These solutions include cascaded buck and boost converters, separate buck converters, LDO converters, and TPS63000 buck/boost converters. A fully charged 18650 lithium-ion battery with 1650mAHr capacity is used in the figure. The load current is 500mA, and the system shuts down when the 3.3V rail voltage drops 5% below the initial set value. The same battery is required to avoid data deviation due to battery capacity differences. As expected, the LDO has a shorter run time of only 190 minutes, while the buck/boost converter has the longest run time of 203 minutes, and the cascaded buck/boost solution has the shortest run time of only 175 minutes. Table 1 shows the key area comparison of the discharge curves of the real system.

Other factors to consider

The data in Figure 3 is measured under constant DC load conditions, which is a common practice for performance testing, but it is different from actual applications. To maximize the operating time of portable applications, the load should be connected only when needed and disconnected when not needed. Displays, processors, and power amplifiers are the main sources of significant transient current on the system battery, and their load fluctuations will cause the voltage on the battery bus to drop due to the internal source resistance of the battery, protection circuits, and distributed bus impedance. If these load fluctuations occur at the end of the discharge cycle, the battery voltage can drop below 3.3V. If a buck or LDO solution is used, it may cause the system to shut down prematurely, while a buck/boost solution will survive the transient and continue to operate, thereby extending the system operating time.

Load transient currents that are not noticeable during lab testing are noticeable in real applications because the internal impedance of a lithium-ion battery doubles after 150 charge/discharge cycles and doubles when operating at 0°C to 25°C. Figure 4 shows the bus voltage of a lithium battery operating under load transient current conditions. The buck and buck/boost converters have a constant load current of 250mA, which loads the battery bus with a transient current of 500mA. The buck converter output drops to a point where it cannot be regulated, causing the system to shut down. The TPS63000 buck/boost converter can survive the transient and operate normally without a change in output voltage.

Conclusion

There are many design options for converting Li-ion battery voltage to 3.3V, and design engineers can choose the best solution based on the specific requirements of the system. Buck/boost converters are suitable for most systems because they have the longest operating time, smallest size, and relatively low cost, making them the best overall solution for most portable applications.

When selecting a buck/boost converter, it is important to understand that the characteristics of various buck/boost converters are not the same, and it is important to pay attention to factors such as operating mode, efficiency throughout the battery operation phase, and overall solution size.