Buck-boost converters change with the times

Tag：Car Voltage Regulator

Buck-boost converter topologies fit into a wide range of applications. Whether you are charging a battery from a battery, powering a string of LEDs, or running a handheld device from a single cell, the buck-boost topology can provide an important weapon in your arsenal of design tricks. Whether you need low cost, high efficiency, or low noise, some version of buck-boost topology can solve the problem. And, if your buck-boost design works for multiple products, you can save yourself the considerable effort of designing separate power supplies for each load voltage. However, as with any type of design, the buck-boost-converter brings its share of design challenges.

For example, consider one common application for buck-boost converters: battery-to-battery charging, such as using a car battery to charge a 10.8V NiMH (nickel-metal-hydride) battery (Figure 1a). At first blush, you might think that you could use a low-dropout linear regulator for this task because the regulator’s 10.8V voltage is close to the 12V lead-acid battery’s voltage. If the car is running, however, the battery’s charging voltage is 13.75 to 14.2V, indicating that you might need to use a switching regulator to prevent power loss. You might still think that a simple buck regulator should do the job. However, NiMH batteries receive their charge from a constant current, so their cell voltages rise to 1.4 to 1.6V per cell. Thus, for a nine-cell, 10.8V pack, the charge-termination voltage must reach 12.6V. A modern synchronous-buck regulator that can deliver power with 100-mV drop might still do the job, but this approach assumes that the car is running. In a real-world application for test equipment that diagnoses cars, however, you must assume that some cars won’t start. A lead-acid automobile battery charges at 13 to 14V, but the no-load voltage is 12V. Clearly, you cannot charge a nine-cell NiMH battery to its 12.6V termination with a 12V source and a buck regulator.

The automotive-test-equipment application may be esoteric, but system designers face a far more common problem: how to power a 3.3V handheld electronic system from one lithium-ion cell (Figure 1b). Consider a handheld computer that uses Windows. Its digital electronics, including memory, must operate from a 3.3V power supply, and one lithium-ion cell delivers 3 to 3.7V of power, so it may be tempting to just operate 3.3V ICs at 3V. However, digital processes are less forgiving than analog when it comes to power-supply-voltage range—to the point that some manufacturers refuse to characterize chips at 3V.

Another approach employs two lithium-ion cells; this method has several disadvantages, however. First, consider that a battery is a more problematic power source than a cell. You must worry about reliability: If either cell fails in an open circuit, the system loses power. If either cell short circuits, the internal fusible link blows—and let’s hope it blows before a fire breaks out. In any case, after a short circuit, your product cannot function. Just as troublesome is the problem of balancing the cells’ charge. Because batteries are metal-plating devices, you charge them by plating lead, lithium, or nickel from the cathode to the anode. When you discharge the battery, the metal or metal ions discharge from the anode to the cathode. Another problem occurs when you recharge the battery: If one cell in a string accepts less charge, it limits the pack’s output. With two lithium-ion cells, this approach would limit the charge voltage to 8.4V. But this approach does not ensure that exactly 4.2V exists across each cell. To ensure that amount of voltage, you must implement complex and expensive charge-balancing circuits that charge and discharge each cell at the optimum voltage. For these reasons, most modern handheld products use a single cell. Because lithium-ion cells output 3 to 3.7V, handheld devices requiring 3.3V are appropriate applications for buck-boost converters.

Other broad applications for buck-boost converters are automotive-LED drivers (Figure 1c). They share the same battery-voltage-range issues as the automotive-test equipment. Indeed, even more important restrictions exist for automotive use. When the car is starting, the battery may sag to 8V as it cranks the starting motor. A charger circuit for automotive-test equipment would have to function for a longer time than it takes a car to start. If the power converter is operating a string of brake lights, however, you would not want the output of the circuit to drop out due to input-voltage swings. Buck-boost architectures can handle those cold-cranking periods, as well as a 40V transient from a clamped-load-dump event.

A similar application is driving LED-flash units in a cell phone (Figure 1d). The forward voltage of the LED may be higher or lower than that of a single-cell lithium-ion cell. A buck-boost topology ensures that the flash LED receives the same current no matter what the state of the battery and no matter what process variations of the LED change its forward voltage. “Look at a cell-phone camera where an LED is used in a flash application, perhaps one where you can drive a 0.5A through the LED,” says Sam Nork, a design manager at Linear Technology. “Under those conditions, the forward-voltage drop of the LED is around 3.6V. Depending on temperatures, part variations, and battery conditions, that [situation] is a classic case where you would like a buck-boost converter to get the best performance.” The same design benefits apply to LED flashlights that use lithium-ion cells for power.

Although you might think of buck-boost converters when dealing with a widely variable input voltage, they also work well in applications in which the output voltage varies due to component variations. Rohit Tirumala, staff application engineer at Supertex, points out that some general-lighting applications use an inexpensive 24V “brick” supply. Although the input voltage is fairly regulated, the output voltage across an LED string can vary widely from part to part. “Because of the LED-voltage variation, the string of LEDs might require a buck or a boost,” he says. “For example, each LED can vary by as much as 1V. The forward voltage can be 3 to 4V, so a six-LED string might require 18 to 24V.”

Brian Wengreen, product-marketing manager at Analog Devices, points out that Panasonic and other lithium-ion-battery manufactures are creating modified battery chemistries that produce more energy as the battery discharges from 3 to 2.5V. “A cell phone or camera that operates from a single [lithium-ion] cell may have a zoom lens or some sort of actuator that requires a steady voltage that provides torque to a mechanical system,” he says. These camera manufacturers use buck-boost converters in this case because they can wring that last bit of energy from the battery.