Some systems require a wide input voltage and/or programmability in their output voltage. This is where a buck-boost converter would typically be used. For a programmable or adjustable power supply, a buck-boost converter is a natural option. When the input voltage is wide, a buck-boost converter is also an excellent option when a compact solution without isolation is needed.
Wide input voltages create challenges in voltage regulation that can only be solved with buck-boost converters. The controller IC or circuit must be selected to ensure accurate regulation and power delivery throughout the input voltage range. This article will examine how to select buck-boost converter controllers such that performance specs can always be hit in your system.
Wide Input Power Sources Need Buck-Boost
The buck-boost topology can technically be used for programmable or adjustable output power supplies, but the most common and important usage of these power converters is to regulate power sources with wide input voltages. Two major examples of these sources are large battery arrays and rectified AC power with variable amplitude/frequency. In either case, we could use a flyback converter with voltage-mode control, or a buck-boost converter with voltage-mode control.
Buck-boost controllers give a more compact solution for mid-range voltages. Converting higher voltages, such as rectified AC line voltage, typically use a flyback converter. Buck-boost is more preferred in the following situations:
- Regulating a DC source with mid-range voltage
- The DC source has a current limiting feature
- The DC source could have wide swings in its output voltage
Features for Wide Input Voltage
Buck-boost regulators draw bursts of current into a switching element (MOSFETs), and they use reactive components (inductors and capacitors) to stabilize the rectified voltage being output from the switching element. The important features to examine in a buck-boost controller with wide input voltage are found in the following table:
These specs generally cover most of the operational capabilities of a buck-boost controller for a power regulator.
Current Dropout at Low V-in
When the input voltage drops too low, it is possible for the current output from a buck-boost regulator to exhibit a precipitous drop. This could happen regardless of the power output capabilities of the sourcing DC supply, and this would be accompanied by a corresponding decrease in efficiency (see below) and total power output.
The datasheet excerpt shown below illustrates what can happen when the input voltage drops too low. In this example, the x-axis voltage is shown throughout the converter’s rated input voltage range. However, the output current begins dropping even with constant target output voltage.
Example maximum current output vs. input voltage from the LTM8055 module.
This specification should be matched to the voltage range you can reliably supply from the DC source in the system. The buck-boost converter should be chosen such that the DC source’s voltage range sits within the full power output range of the buck-boost regulator IC. If not paired correctly, the regulator will not be able to provide the required power output if the input voltage drops towards the low end of your specification.
Another problem that can arise when the input voltage gets too low or too high is efficiency drop. Very broadly, buck-boost converters tend to have their highest efficiency within some intermediate input voltage range and intermediate output current/power (below full load, see above). Sometimes, the reduction from peak efficiency is small (a few percent), but this is not always the case and the efficiency quickly drop below 80-90% as shown above.
This efficiency curve shows what can happen at various loading levels when the input voltage is too high or too low. Example for the LM5118.
At the low-end, the converter needs to work harder (switch ON longer) in order to hit the target output voltage, so there are more losses during the ON state. At the high-end of the input range, the average current could be lower in buck mode due to the less-frequent switching and larger peak-to-peak modulation of the output current. Both factors may result in power conversion efficiency reduction of the circuit.
ESD and EMI
Non-isolated power systems like a basic buck-boost converter may include different levels of ESD protection. They may also have differing abilities to withstand common-mode or differential-mode noise. When selecting these components, this could be one of the most important points as deployment in the environment could expose loads to voltages on the order of 1 kV or higher. Typical ESD withstand voltages could reach into the 2 kV range or higher; isolated components could have similar ratings. Rugged systems for automotive, aerospace, infrastructure, and energy management will need to have high ratings alongside wide input voltage ranges.
Whenever you want to design a wide-input buck-boost converter from discrete components, or you need to simulate your wide input voltage regulator IC performance, use the complete set of simulation tools in PSpice from Cadence. PSpice users can access a powerful SPICE simulator as well as specialty design capabilities like model creation, graphing and analysis tools, and much more.