The buck converter has been in use for a century and is an indispensable part of the current electronic circuits. This article describes how a crude electromechanical component has evolved into a miniature assembly that can process hundreds of watts of power.
buck converter
The buck converter converts an input voltage to a lower output voltage. Its basic elements are shown in Figure 1. Initially, the switch SW1 is closed; that is, current flows into the coil L1. Since the coil is a differentiating component, the current flow increases steadily until switch SW1 is opened and SW2 closes. This causes the current flow to change.
Capacitor C1 is an integrating component; therefore, the resulting output voltage is a function of the current and the turn-on times of switches SW1 and SW2. Originally S1 and S2 were mechanical switches, but these were quickly replaced by silicon - S1 with a transistor and S2 with a diode.
Circuits change with technological progress
Over the years, efforts were made to integrate as many components as possible into the control circuit to reduce cost and size. One breakthrough was to integrate the main switch, S1, directly into the controller IC, but the coil as well as the diode still had to be mounted externally.
Then, to further increase efficiency, in newer versions both switches (SW1 and SW2) were equipped with MOSFETs, allowing switching frequencies up to 2MHz.
Figure 2: The development of down converter integration (Image source: RECOM)Coil integration is the key to miniaturization
The amplitudes of the currents decreased, which affected the size of the output capacitor
After the changeover to MOSFETs had been successful, it was necessary to take a further step towards miniaturization. Due to the ever-increasing switching frequencies, it was possible to reduce the design of the coil. The amplitudes of the currents decreased, which affected the size of the output capacitor.
The use of higher-quality capacitors with lower self-heating losses further supported this improvement.
reduce design sizes
Currently, however, the goal is to reduce design sizes and increase efficiency even further; to do this, the switched paths must be minimized and the components must be mounted on top of each other on the Z-axis.
The simplest example is Flip Chip on Leadframe (FCOL) packaging technology, where the controller IC (with integrated power transistors) is connected upside down directly to the leadframe punch grid, next to an SMD choke that is also attached directly to the leadframe (Fig. 3).
Figure 3: Flip chip on leadframe constructioncompact step-down converter modules
This design enables the fully automated production of very compact step-down converter modules. The resulting shortened connections of the self-shielding inductor also have a positive effect on EMC behavior. Products manufactured in this way can also be over-molded, resulting in a lead-free QFN (Quad Flat No-lead) package with an MSL3 rating and full environmental protection.
An example of this is the RECOM RPX series (Figure 4) with a 2.5A output adjustable from 1.2V to 6V in a 4.5 x 4 x 2 mm small package, requiring only external input and output capacitors.
Figure 4: The POL module of the RECOM RPX buck converter with integrated chip inductor and flip chip on leadframe design (Image source: RECOM)RPX Series power modules
The RPX-1.0 and RPX-1.5 series modules can deliver high input voltages of up to 1.5A
These modules are a complete solution that can be mounted on the user's PCB using standard SMD assembly and oven soldering processes.
RECOM has recently released two more RPX Series power modules based on FCOL technology using these new packaging techniques: The RPX-1.0 and RPX-1.5 series modules can deliver high input voltages of up to 1.5A at up to 36VDC in ultra-compact 3 x 5 x 1.6mm QFN packages.
Conclusion
Buck converters have evolved significantly over many decades. Innovations from capacitors, inductors, control ICs, and packaging technologies have enabled the integration of all components into ever smaller packages with ever higher power density.
The goal of making low-power DC/DC converters IC-like is largely achieved through a combination of innovative 3D Power Packaging technologies for both isolated and non-isolated converters, with further improvements in performance and power density expected in the future.
For general use as modules, however, the fully featured step-down converters are of the same order of magnitude as normal SMT devices and will find their place in end-use applications in the same way.
