Buck Converters – From Discrete Circuits to Fully Integrated Modules

A buck converter converts the input voltage to a lower output voltage, and the basic principle is shown in Figure 1. Initially, switch SW1 is turned off and current flows into coil L1. Since the coil is a differential element, the current increases steadily until switch SW1 turns on and SW2 turns off, causing the current to change. Capacitor C1 is an integrating element, so the resulting output voltage is a function of current and the conduction time of switches SW1 and SW2.

Originally S1 and S2 were real mechanical switches but were soon replaced by silicon – S1 was a transistor and S2 was a diode.

Figure 1: Buck converter basics (Image credit: Recom)

Circuits change as technology advances

Over the years, people have integrated as many devices as possible into control circuits to reduce cost and size. One of the breakthrough developments is to integrate the main switch S1 directly into the controller IC, but the coil and diode must still be mounted outside. Later, in order to further improve the efficiency, in the new version, both SW1 and SW2 switches are equipped with MOSFETs, so the switching frequency can be as high as 2MHz.

Discrete Design Asynchronous Synchronous Integrated Inductor

Figure 2: The evolution of integrated buck converters (Image credit: Recom)

Integrated coils are key to miniaturization

The move towards miniaturization needs to continue after the switch has been successfully converted to MOSFET. Due to the ever-increasing switching frequency, it is now possible to shrink the coil size. The reduction in current magnitude affects the size of the output capacitor, which can be further improved by using high-quality capacitors with lower self-heating losses.

However, the current goal is to shrink the design even more and improve efficiency. To achieve the goal, the switching path must be shortened and the device overlapped on the Z axis.

The simplest example is leadframe flip-chip (FCOL) packaging technology; the controller IC (with integrated power transistors) is connected upside down directly to the leadframe stamped grid, next to an SMD Inductor that is also directly connected to the leadframe (Figure 3) .

Figure 3: Leadframe Flip Chip Structure

This design enables fully automated production of very compact buck converter modules. The shortening of the shielded inductor connection wires also has a positive effect on the EMC performance. Products manufactured in this way can also be wrapped to form QFN (Quad Flat Package No-Lead) with MSL3 moisture sensitivity level and complete environmental protection. One example is the RECOM RPX series (Figure 4), which offers 2.5A output current and an adjustable output voltage from 1.2V to 6V in a small 4.5 x 4 x 2 mm package, requiring only external input and output capacitors.

Figure 4: Integrated chip inductor and leadframe flip-chip design of RECOM RPX buck converter POL module (Image credit: Recom)

These modules are complete solutions by themselves and can be mounted on the user’s PCB using standard SMT and reflow processes. RECOM has two other RPX series modules in FCOL packaging technology: the RPX-1.0 and RPX-1.5 series capable of delivering up to 36VDC input voltage and 1.5A output current in a 3 x 5 x 1.6mm ultra-compact QFN package.

in conclusion

Buck converters have come a long way over the decades. Innovations in capacitors, inductors, control ICs, and packaging technologies allow devices to be integrated into ever-shrinking packages at higher power densities. Today, isolated and non-isolated converters use innovative 3D power packaging technology to largely ICize low-power DC/DC converters, with further improvements in performance and power density expected in the future. And as general-purpose modules, full-featured buck converters are of the same magnitude as normal SMT devices and find their place in the end application in the same way.