Buck/Boost DC/DC Regulator Solutions with Up to 96% Efficiency

Buck and boost DC/DC converters solve the unidirectional voltage conversion problem, they always regulate the voltage below or above the input supply, respectively. However, if the input supply varies above, equal to, or below the output voltage (i.e., bidirectional variation), how to accurately regulate the voltage? People have been using different and more complex DC/DC topologies to solve this problem.

For example, many battery backup systems use a flyback topology in which the battery voltage varies depending on the charge state and charge capacity. Buck/boost topologies are also useful solutions in systems with poor input supply regulation, in which the input supply may rise above or fall below the desired output voltage value.

From Flyback, SEPIC, Synchronous to Micromodule Regulator Systems

An early approach to designing a DC/DC regulator that can regulate a fixed output regardless of input voltage relative values ​​(whether VIN is above or below VOUT) is to use a flyback topology. Flybacks have been used for many years because they are familiar to most analog circuit engineers and are fairly simple to design, in addition to the fact that engineers have a good understanding of magnetic components and MOSFET operation. However, implementing a flyback regulator for some DC/DC conversions may require a special transformer design, and flyback regulators are often too efficient for most applications today.

This leads to another approach: SEPIC.

SEPIC is short for Single-Ended Primary Inductor Converter. Despite its rather complicated name, the basic elements of a SEPIC design include a DC/DC boost converter IC and a coupled inductor or transformer. High-power SEPIC designs also require external power MOSFETs, which are often large because they must withstand high voltage transients and provide low RDS(ON). The coupled inductor is often not an off-the-shelf inductor, and how well it is made has a significant impact on the performance of the power supply design. In addition, SEPIC operates at 67% to 86% efficiency, which is affected by many factors such as conversion ratio, magnetic component selection, capacitors, MOSFETs, etc., in a way that is similar to the efficiency and factors of the flyback topology. Layout also has a critical impact on stability and thermal management.

For applications that require a high-power buck/boost converter but cannot accept flyback topologies and SEPIC due to low efficiency and thermal issues, a solution using a single-inductor 4-switch buck/boost controller has been available for several years. Its circuit and architecture can easily achieve 90% ~ 95% efficiency at 60W (12V, 5A) output. This is very good news for system designers who are troubled by heat dissipation. This solution meets the power requirements of most applications and provides power at very high efficiency, while the transition between different operating modes from buck to boost or from boost to buck can be seamless. Using 4 MOSFETs, an off-the-shelf uncoupled inductor, a synchronous DC/DC controller (LTC3780 from Linear Technology) and compensation circuits, plus good layout knowledge, designers can achieve a small and efficient design. Figure 1 compares the SEPIC and this buck/boost synchronous solution.

A Simple Buck/Boost Solution

Compared to a flyback or SEPIC topology, a synchronous 4-switch buck/boost solution allows designers to deliver more power while also generating less heat. Magnetic components are also more readily available, with no special inductors or transformer windings required. However, while some may be familiar with this circuit, which requires about 24 components, others may not have analog design expertise or need to complete a complex system design on a tight schedule, and the last thing they want is to rack their brains over the power supply portion of the project.

Not long ago, Linear Technology found a way to integrate 90% of the LTC3780 buck/boost circuit into a 15mm × 15mm × 2.8mm, 1.5g land grid array (LGA) package that looks like a surface mount integrated circuit. It only requires an off-the-shelf inductor, a resistor to set the output voltage, a sense resistor, and some bulk input and output capacitors. The MOSFETs, compensation circuitry, and complex DC/DC controller are all integrated into the protective plastic molded package. Figure 2 compares this DC/DC micromodule (mModule) buck/boost circuit with a discrete LTC3780 design.

The LTM4605 and LTM4607 buck/boost micromodule regulators are built entirely from DC/DC controllers and MOSFETs designed using Linear Technology chip design technology. Because all chips are designed and manufactured by Linear Technology, the micromodule is optimized for high- power

buck/boost operation. The MOSFET gate charge, RDS(ON), VDS and the powerful driver of the DC/DC controller together achieve amazing performance (Figure 3). Figure 3 shows a 60W design, but only 2W is consumed here. This design can be put into the most restricted space. The layout is also very simple and there is no possibility of error. The layout is provided in the data sheet (www.linear.com..cn/micromodule). The high-power version of these two devices is the LTM4607, which has a rated input voltage of 4.5V to 36V, a rated output voltage of 0.8V to 24V, and can provide a rated power of up to 160W.

Conclusion
Analog switching power supply design requires knowledge of physics, such as the effects of layout, ground planes, PCB vias, soldering, and thermal management, in addition to the math required to select component values. Linear Technology's high power and high efficiency buck/boost solutions that require minimal power design effort can alleviate many of the worries and concerns of digital circuit designers.