Analysis and comparison of power system topology options for portable systems - News - Global IC Trade Starts Here 工业

This article will explore various power supply topologies, particularly focusing on converting Li-Ion battery voltages to 3.3V voltage rails, which are commonly used in portable electronic devices. We’ll also examine the different applications of buck/boost converters and explain why buck/boost converter solutions need to be tailored specifically for each application.

As shown in Figure 1, designing a circuit to convert a Li-Ion battery voltage to a 3.3V rail presents several challenges. At full charge, a typical lithium-ion battery starts at 4.2V. The X-axis begins at “-5 minutes,” representing the open circuit voltage when the battery is fully charged. At “0 minutes,” the battery is connected to the load, causing the voltage to drop due to internal impedance and protection circuits. The voltage gradually decreases to about 3.4V, then drops sharply as the discharge cycle nears completion. To maximize the use of the battery's stored energy, the 3.3V rail needs a step-down converter for most of the discharge cycle and a boost converter for the remainder of the cycle.

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

The challenge of efficiently generating 3.3V voltage rails from lithium-ion batteries has existed for a long time, with various solutions proposed. This article discusses several common approaches, including cascaded buck and boost, buck/boost, buck, and LDO power topologies, analyzing the pros and cons of each design and comparing their system runtime performance.

Cascaded Buck and Boost Converter Solutions

The cascaded buck and boost converters consist of two separate converters: a buck converter and a boost converter. The buck converter stabilizes the voltage at a medium level (such as 1.8V), while the boost converter steps up this voltage to 3.3V. This architecture is beneficial for systems requiring lower voltage rails, as it ensures 100% battery power utilization. However, due to the two-stage conversion process, it’s not the most efficient solution.

The effective power conversion efficiency is the product of the efficiencies of the buck and boost converters. Assuming both converters have efficiencies of 90%, the overall efficiency for the 3.3V converter would be 81% (90% × 90%). Additionally, the architecture includes two independent converters, increasing the number of components and the overall system size. This makes it less suitable for small portable products and increases costs.

Independent Buck Converter Solution

Using a buck converter to convert the Li-Ion battery voltage to 3.3V is another approach, though it is often overlooked. Design engineers typically dismiss this solution after observing the battery discharge curve (as shown in Figure 1), since a buck converter cannot generate 3.3V throughout the entire discharge cycle. Many buck converters switch to 100% duty cycle mode when the input voltage approaches the output voltage. Under these conditions, the converter stops switching and outputs the input voltage directly. The output voltage equals the input voltage minus the voltage drop across the converter, which depends on the MOSFET on-resistance, the DC resistance of the output inductor, and the load current. This determines the minimum battery voltage that can remain regulated.

Assuming the system allows the 3.3V rail to drop by 5% and still remain regulated, the minimum battery voltage for system operation can be calculated using the following equation:

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

Where Vout_nom is the nominal 3.3V, Rdson is the power MOSFET on-resistance, RL is the output inductor DC resistance, and Iout is the converter's output current at 3.3V.

When the battery voltage drops to Vbattery_min, the system must shut down to prevent the 3.3V rail from dropping further and potentially damaging data. Even if the battery still has 5 to 15% of its charge remaining, the system might shut off. The exact amount of remaining battery power depends on factors like component resistance, load current, battery age, and ambient temperature.

Most engineers abandon this topology due to this limitation, but careful analysis reveals that the conversion efficiency of standard buck/boost and cascaded buck/boost topologies is actually lower than that of individual buck converters. While these topologies make better use of battery power, they are far less efficient than standalone buck converters. In many cases, standalone buck converters provide longer runtime than the other two options. Up until 2005, the fully integrated buck converter was considered the best choice for generating a 3.3V rail.

Low Dropout Regulator Solutions

Another less common solution is the LDO (low dropout regulator). Similar to the standalone buck solution, the LDO cannot fully utilize the full battery power because the input voltage remains stable only when it is higher than the sum of the output voltage and the LDO voltage drop. For example, if the LDO voltage drop is 0.15V, the 3.3V output voltage begins to drop when the battery voltage falls below 3.3V + 0.15V = 3.45V. This results in significant wasted battery power.

Despite these limitations, LDOs offer advantages in certain scenarios. They are typically the smallest in size, making them ideal for systems with strict space constraints. Their cost is usually the lowest, making them suitable for budget-conscious applications. Many engineers abandoned the LDO solution due to its inefficiency, but upon closer examination, it performs well in specific situations:

When the initial voltage of a fully charged lithium-ion battery is 4.2V, the initial efficiency of the LDO is 78%, rising as the battery voltage decreases.

In conclusion, selecting the right power supply topology depends on the specific requirements of the application, including efficiency, size, cost, and runtime. Each topology has its own set of trade-offs, and understanding these nuances is key to designing an optimal power management solution.