Buck-Boost Converters for Portable Systems Michael Day and Bill Johns

Transcription

1 Buck-Boost Converters for Portable Systems Michael Day and Bill Johns ABSTRACT This topic presents several solutions to a typical problem encountered by many designers of portable power how to produce 3.3 V from a single-cell Li-ion battery. The advantages and disadvantages of each solution are provided along with measured data on overall battery runtime. This data helps the designer select the best overall solution for specific system requirements. This topic also provides a detailed comparison of the Texas Instruments (TI) fully integrated TPS63000 buck-boost converter and other buckboost solutions. The efficiency, overall ease of use, and operation in transition mode when the converter switches from buck to boost mode are discussed. I. DESIGN CHALLENGE Fig. 1 graphically depicts the design challenge of producing 3.3 V from a Li-ion battery. The voltage-discharge profile of a typical Li-ion battery starts at 4.2 V when the battery is fully charged and no current is flowing. When time, t, is less than 0 minutes, the battery is unloaded and has no current flow. At t = 0, a load is applied to the battery, and its voltage drops due to its internal impedance and protection circuitry. The battery voltage drops gradually until it reaches about 3.4 V, where it starts to drop rapidly as it nears the end of its discharge cycle. The Li-ion battery s discharge curve presents the designer with two challenges: how to generate a regulated output voltage over the full discharge curve, and how to fully utilize the battery s stored energy. When the battery s discharge curve is both above and below the power supply s required output voltage, the system must be capable of both buck (step-down) and boost (step-up) conversion. Figs. 2 and 3 show that the discharge curves of dual-cell alkaline and NiMH batteries present the same problem for system voltages between 1.8 and 3.0 V. This design issue is not new, and designers have solved it using many solutions over the years. These include cascaded buck and boost converters, single low-dropout regulators (LDOs), Voltage V Time min Fig mA discharge curve of dual-cell NiMH battery. Battery Voltage V Discharge Time min Fig mA discharge curve of 1650-mAh Li-ion battery. Voltage V Time min Fig mA discharge curve of dual-cell alkaline battery.

2 single buck converters, single-ended primaryinductance converters (SEPICs), and buck-boost converters. The authors discuss some of the more popular solutions, compare advantages and disadvantages, and then show how improvements in integrated power-converter technology provide superior performance that eliminates the disadvantages of classical buck-boost converters. A. Solution 1: Cascaded Buck and Boost Converter Fig. 4 shows a popular solution, cascaded buck and boost converters that are separate and discrete. A boost converter can follow a buck converter, or vice versa. Most designers choose to have the boost converter follow the buck converter. The buck converter operates directly from the battery voltage and generates an intermediate voltage such as 1.8 V. A separate boost converter operates from the intermediate voltage to generate the regulated 3.3-V output. This architecture has several advantages, such as providing the intermediate voltage, which may already be needed in the system. It also utilizes 100% of the battery s capacity when the buck converter operates down to the battery s end-ofdischarge voltage. A disadvantage of this solution is the two stages of power conversion. The effective powerconversion efficiency is the product of the individual buck and boost converters efficiencies. The typical efficiency of buck and boost converters operating at these voltage levels is 90%; so the effective 3.3-V conversion efficiency is 90% 90% = 81%, which is relatively low for a switching converter. Additional disadvantages of this architecture are the higher cost associated with two separate converters, and the increased parts count and solution size that are prohibitive in small, portable products. B. Solution 2: Single Buck Converter A single buck converter is a solution that many designers overlook. Although this topology has not gained much widespread use, it has clear benefits that should not be ignored. Designers typically dismiss this solution when they realize it cannot provide a regulated output voltage over the battery s full discharge curve. When a buck converter s input voltage drops below its programmed output voltage, the output voltage falls out of regulation. Despite this drawback, this solution is very efficient, approaching 96% in many cases. Although the single buck converter cannot utilize 100% of the battery s energy, the energy it does use is converted at a very high efficiency. This keeps the overall system runtime comparable to other solutions with lower efficiency. This topology s solution size is smaller and less expensive compared to the cascaded buck and boost solution. To understand this topology s limitation, consider what happens when the battery voltage drops close to the converter s output voltage. Many buck converters enter a 100% duty-cycle mode in this condition. The converter stops switching and passes the input voltage directly through to the output. Fig. 5 shows that in 100% duty-cycle mode, the power metal-oxidesemiconductor field-effect transistor (MOSFET) turns on fully and behaves like a resistor with a value equal to the MOSFET s on resistance, R DS(on). The catch diode or, in the case of a synchronous converter, the synchronous MOSFET turns off and becomes an open circuit. The output voltage equals the input 1.8-V 1.8-V Buck Converter 3.3-V Boost Converter 3.3-V 100% Duty-Cycle Mode MOSFET/ R DS(on) Inductor DCR Fig. 4. Cascaded buck and boost converter. Fig. 5. Buck converter and 100% duty-cycle mode.

3 voltage minus a voltage drop across the converter. This voltage drop is a function of the power MOSFET s R DS(on), the output inductor s DC resistance (DCR), and the load current, I OUT. The converter voltage drop in 100% duty-cycle mode sets the minimum allowable battery voltage required to keep the output voltage in regulation. This minimum battery voltage is calculated as V where (nom) is the nominal 3.3-V setpoint, R DS(on) is the power MOSFET s on resistance, DCR is the output inductor s DC resistance, (Tol) is the minimum allowable outputvoltage tolerance, and I OUT is the converter s 3.3-V output current. Using a typical TPS62020 design, an appropriately sized inductor, and a maximum 3% drop yields V = V BAT( min) OUT( nom) BAT( min). 100% 3% = 33V 100 = V. 100% V 100 OUT(Tol) ( DS( on) ) OUT, R DCR I ( Ω Ω) 500 ma When the battery voltage drops to V BAT(min), the system must shut down so it does not run with the 3.3-V rail below its minimum tolerance, which could corrupt the data. With this topology, the system shuts down even though the battery still contains anywhere from 5 to 15% of its rated capacity. The actual unused capacity depends on many factors, including component resistances, load currents, battery age, and ambient temperature. Depending on the desired battery shutdown voltage, the buck-only topology provides superior efficiency with a reasonable cost and board area. C. Solution 3: LDO Another solution that doesn t get much widespread use is the low-dropout regulator (LDO). An LDO typically provides the smallest solution size and is usually the cheapest. Many designers dismiss the LDO due to its low efficiency, but close examination of an LDO s efficiency equation shows a respectable efficiency for this application: PWROUT VOUT IOUT V Eff = = = PWR V I V IN when I OUT I IN. Since the voltage of the fully charged Li-ion battery starts at 4.2 V, the LDO s efficiency starts at 3.3 V/4.2 V = 78% and increases as the battery voltage drops. The average discharge voltage is approximately 3.7 V, so the LDO s average efficiency is 3.3 V/3.7 V = 89.2%. Like the single buck converter, the LDO is not capable of utilizing the entire battery capacity because it maintains regulation only when its input voltage is higher than the output voltage. When the input voltage drops near the output voltage, the LDO enters dropout. Please see Reference [1] for more information on this subject. As an example, if the LDO has a dropout voltage of 0.15 V, the 3.3-V output voltage starts to drop when the battery voltage falls below 3.3 V 0.15 V = 3.45 V. Even with this drawback, the LDO s small size and low price make it an attractive solution with the right system requirements. D. Solution 4: SEPIC The single-ended primary-inductance converter (SEPIC) provides a buck and boost function from a single circuit. This converter topology resembles a flyback converter that uses two inductors and a flying capacitor to provide a regulated output voltage from input voltages that are above and below the output voltage. Like the cascaded buck and boost topology, the SEPIC is capable of utilizing the battery s full capacity but also suffers from relatively low efficiency. Many designers do not use a SEPIC due to its solution size, cost, and complexity of design. IN IN OUT IN,

4 C IN L1 C1 L2 D1 C OUT R L D Fig. 6. SEPIC circuit. Fig. 6 shows the SEPIC circuit topology and Fig 7a shows current flow for the following discussion. During the power switch s on time, is on and current builds up in the two inductors. L1 charges like a boost converter while L2 charges though the flying capacitor, C1. During this time, D1 is off and the output capacitor, C OUT, supplies the load current. During the power switch s off time, D1 is on, and stored energy from L1 and L2 is transferred to the load and output capacitor through the flying capacitor. Also, similar to the flyback converter, the SEPIC transfers current to the load during the power switch s off time. Close examination of the current waveforms in Fig. 7b reveals that the SEPIC s switching currents are significantly higher than those of a classical buck converter. These higher currents reduce overall efficiency; however, the SEPIC s switching currents are lower than those of a classical buck-boost converter, which makes the SEPIC slightly more efficient than a classical buck-boost converter. E. Solution 5: Classical Buck-Boost Converter Fig. 8 shows the classical buck-boost converter that uses four switches and a single inductor for power conversion. Many designers consider using this solution to generate 3.3 V from a Li-ion battery. It is a relatively simple design capable of regulating the 3.3-V output voltage from a Li-ion battery s full discharge curve. The main disadvantage is the four switches, which increase switching losses and reduce overall efficiency. The two additional switches increase the board area and cost of discrete power-supply designs. 1-D Fig. 7a. Current flow in SEPIC circuit. 0 ma 0 ma D 1-D VOUT IL1-A = IL1-B = IOUT V 08. BAT Fig. 7b. SEPIC converter waveforms. Fig. 8. Classical buck-boost converter. Voltage Current Diode Current L1 Current L2 Current

5 Fig. 9. Classical buck-boost switching and current flow. Fig. 9 shows the two switching states of the buck-boost converter. One pair of switches is used to charge the inductor, and a second pair is then used to transfer the inductor current to the output capacitor. This switching topology has relatively high root-mean-square (RMS) switching currents that increase switching and conduction losses, resulting in lower efficiencies than those of a stand-alone buck or boost converter. The worstcase situation occurs when the battery output is near the 3.3-V operating point. When =, the duty cycle is 50%, which results in a peak inductor current equal to twice the output current. The inductor current flows through all four switches during each switching cycle, resulting in higher switching losses and reduced efficiency. F. Solution 6: Modified Buck-Boost Converters Not all buck-boost converters suffer from lower efficiency. Fig. 10 shows a modified buckboost converter that overcomes the deficiencies of a classical buck-boost converter. The separate buck and boost control circuitry allows the converter to operate in buck mode when the input voltage is above the output voltage, and in boost mode when the input voltage is below the output voltage. In buck or boost mode, only two switches operate at any given time, which reduces switching losses. The RMS currents are identical to those of a stand-alone buck or boost converter and are lower than those of the classical buckboost converter, further reducing switching and Buck Control Boost Control Fig. 10. Buck-boost power stage. conduction losses. The modified buck-boost converter is a good solution for producing 3.3 V from a Li-ion battery because it utilizes 100% of the battery capacity at a fairly high efficiency. Comparison of Modified Buck-Boost Converters Buck-boost converters in portable applications have been around for a long time. Silicon and packaging technologies have advanced to the point where it is feasible to integrate the four MOSFET switches into a small package with a suitable control loop. Several integrated buckboost converters are available, each with different control topologies and operating characteristics. The buck-boost power stage shown in Fig. 10 can be operated in three distinct modes, depending on the input-to-output voltage ratio: buck mode, buck-boost mode, and boost mode. A specific IC s mode of operation is a function of the input-tooutput voltage ratio and the IC s control topology. Although different buck-boost solutions have the same power-stage topology, they have vastly different control circuitry, of which there are three main types: the classical buck-boost converter, a buck-boost control topology that operates only two MOSFETs per switching cycle, and a buckboost control topology that eliminates the transition region between buck and boost modes. The first type, the classical buck-boost converter, operates all four MOSFETs during each switching cycle and generates the classical buckboost waveforms. Careful analysis of these waveforms reveals that the RMS current through the inductor and MOSFETs is significantly higher than that of a standard buck or boost converter, resulting in increased conduction and switching losses. Operating all four switches simultaneously also increases gate-drive losses, which can significantly lower efficiency at lower output currents.

6 Buck Control > : Buck Mode and Switching, On, Off < : Boost Mode and Switching, On, Off Fig. 11. Buck-boost switch configuration. The second buck-boost control topology is newer and reduces losses by operating only two MOSFETs per switching cycle. This control topology operates in three distinct modes: buck, boost, and the transition mode between the two (see Fig. 11). When is greater than, the converter operates in buck mode by opening and closing. and switch to form a classical buck converter. When is less than, the converter operates in boost mode by opening and closing. and switch to form a classical boost converter. In the past, many designers had a problem when the input voltage approached the transition region. The transition region is the operational point where the converter transitions from buck mode to boost mode in the case of a falling input voltage, or where the converter transitions from boost mode to buck mode in the case of a rising input voltage. The transition point creates discontinuous boundaries in the power stage s operating point between buck and boost modes. It also creates discontinuities in the feedback control loop. Unable to cope with Boost Control these issues, IC designers opted to insert the classical buck-boost mode of operation into the transition point. As already discussed, this operating mode is less than ideal due to the fourswitch operation and reduced efficiency. Unfortunately, the transition region falls near the battery voltage where most of the energy is available. Therefore, converters using this control topology operate in the inefficient buck-boost mode for much of the battery s discharge time. The third buck-boost control topology provides a significant improvement in performance and efficiency by eliminating the transition region between buck and boost modes. TI s TPS63000 buck-boost converter contains an advanced control topology that eliminates the traditional buck-boost issues while also eliminating the transition-region issues. In all operating conditions, the TPS63000 operates only two switches per switching cycle and stays in either buck mode or boost mode. This results in reduced power losses and maintains high efficiency across the full battery-discharge curve. Fig. 12 shows that, unlike some solutions, the TPS63000 integrates all compensation circuitry and requires only three external components for operation, which minimizes the solution size. L1 Fig. 12. Typical TPS63000 buck-boost application requires only three external components. L2 VINA FB 10 µf 10 µf EN PS/SYNC GND 2.2 µh PGND 3.3 V

7 (2 V/div) (2 V/div) (1 V/div) (10 mv/div) Inductor Buck Node (5 V/div) Inductor Buck Node (5 V/div) Inductor Boost Node (5 V/div) Time - 20 ms/div Time - 1 µs/div Fig. 13. TPS63000 transition from buck to boost mode. Fig. 13 shows the TPS63000 switching waveforms as the input voltage drops through the transition region. The waveforms clearly show the transition from buck mode to boost mode as the input voltage drops from 4.2 V down to 1.7 V. With above, the supply is in buck mode and and are switching. In buck mode, the left side of the inductor (L1 in Fig. 12) is switching, and the right side (L2 in Fig. 12) is fixed at the output voltage. Fig. 14 shows the buck node, L1, switching between and ground while the boost node, L2, is connected to. The L1 switching waveform is identical to that of a buck converter. When drops below, the converter transitions to boost mode and and are switching. Fig. 15 shows the converter switching in boost mode with the right side of the inductor, L2, switching between and ground, and the left side of the inductor, L1, fixed at the input voltage. Fig. 16 shows the switching waveforms with equal to. During this transition mode, the TPS63000 switches back and forth between buck and boost modes. Even though the converter switches between buck mode and boost mode every other cycle, only two of the four power switches operate during each switching cycle. The switching between buck and boost modes ensures a well-regulated output voltage during this transition region. Fig. 14. TPS63000 buck mode with = 3.7 V and ripple shown at 10 mv/div (AC-coupled probe). (1 V/div) Inductor Buck Node (5 V/div) Time - 1 µs/div (10 mv/div) Fig. 15. TPS63000 boost mode with = 3 V and ripple shown at 10 mv/div (AC-coupled probe). (1 V/div) Inductor Buck Node (5 V/div) Time - 1 µs/div (10 mv/div) Fig. 16. TPS63000 buck and boost modes with = and ripple shown at 10 mv/div (AC-coupled probe).

8 II. SOLUTION COMPARISONS Fig. 17 shows a side-by-side comparison of the battery-discharge curves and runtimes for four solutions generating 3.3 V from a Li-ion battery. These solutions are the cascaded buck and boost converter, the LDO, the single buck converter, and the TPS63000 buck-boost converter. The setup uses a fully charged Li-ion battery with 1650-mAh capacity. The load current is set at 500 ma, and system shutdown is defined as the point where the 3.3-V rail drops 5% below the initial setpoint. Each setup uses the same battery to eliminate variations in data due to differing battery capacities. As anticipated, the cascaded buck and boost, TPS62040 and TPS61031, achieve the shortest runtime with only 175 minutes. The LDO, TPS73633, has the nextshortest runtime at 188 minutes. The buck converter, TPS62040, operates only 2 minutes longer than the LDO. The TPS63000 buckboost converter achieves the longest runtime with 203 minutes. Table 1 shows a comparison between several key areas of concern for these four solutions. III. OTHER CONSIDERATIONS The data in Fig. 17 are taken with a constant DC load. This is typical of bench testing but not of real applications. To maximize runtime in portable applications, loads are switched on only as long as required, then switched off. Displays, processors, and power amplifiers are examples of loads that produce significant transients on the system battery. Their load steps result in voltage drops on the battery bus because of the battery s internal source Voltage - V Voltage - V Voltage - V Voltage - V Cascaded Buck and Boost TPS62040 and TPS61031 V BAT min LDO TPS V BAT min Buck TPS V BAT min Buck-Boost TPS V BAT min Time - min Fig. 17. Runtimes to generate 3.3 V from a 1650-mAh Li-ion battery with a 500-mA load. TABLE 1. COMPARISON OF THE FOUR SOLUTIONS IN FIG. 17. Topology Size Cost Efficiency/Runtime Cascaded Buck and Boost Large High Low LDO Small Low Medium Buck Medium Medium Medium Buck-Boost Medium Medium High

9 resistance, protection circuitry, and distribution bus impedance. When these load steps occur near the end of the discharge cycle, they can pull the battery voltage below 3.3 V. With the single-buck and LDO solutions, this results in early system shutdown. The buck-boost solution continues to operate through these transients, thereby extending the system s operating time. Load transients that appear insignificant in lab testing get much worse under real-world conditions. A Li-ion battery s internal resistance doubles with only 150 charge/discharge cycles. Internal resistance also doubles when the battery is operated at 0ºC versus 25ºC. Fig. 18 compares how the TPS62040 buck converter and the TPS63000 buck-boost converter perform with a load transient on the battery. When the partially discharged battery is lightly loaded, its output voltage is 3.4 V, which is high enough for the buck converter to operate normally. When the battery s load current increases, the battery voltage drops to 3.3 V, which is too low for the buck converter to maintain regulation. The buck converter enters 100% duty-cycle mode and its output drops out of regulation, which could cause a system shutdown. The TPS63000 buck-boost converter operates through the transients with no change in output voltage. The TPS63000 s ability to regulate its output voltage through system-level transient loads is critical to the product s runtime and data integrity. IV. CONCLUSION Many solutions are available to provide a 3.3-V bus from a Li-ion battery, but there is no single solution that optimizes all system-level requirements. All the circuits analyzed in this topic have advantages and disadvantages. The TPS63000 integrated buck-boost converter s modified control topology provides the best compromise between runtime, battery utilization, and solution size and cost. This topology maintains high efficiency over the full battery-discharge curve. Its high switching frequency and fully integrated power FETs and compensation circuitry minimize solution size and design time. Lightly Loaded Battery Voltage = 3.4 V (500 mv/div) Heavily Loaded Battery Voltage = 3.3 V Buck Output Voltage = 3.3 V (500 mv/div) TPS63000 Output Voltage = 3.3 V (500 mv/div) Battery Current = 600 ma to 1150 ma (500 ma/div) Fig. 18. Performance of buck versus buck-boost converter with pulsed load on Li-ion battery.

10 V. REFERENCES [1] Understanding LDO Dropout, Application Note, TI Literature No. SLVA207 [2] TPS63000 Datasheet, TI Literature No. SLVS520 [3] TPS62040 Datasheet, TI Literature No. SLVS463 [4] TPS61031 Datasheet, TI Literature No. SLUS534 [5] TPS73633 Datasheet, TI Literature No. SBVS038