[Guide]Buck converters have a history of over a century. Without them, it would be impossible to imagine what modern Electronic circuits would become. This article explains how bulky electromechanical step-down converter components have evolved into miniature PCB mounting components capable of handling hundreds of watts of output power.
The buck converter converts the power supply voltage to a lower output voltage. Their basic components are shown in Figure 1. First, the switch SW1 is closed, so that current flows into the coil L1. This causes the current to continue to rise until the switch SW1 is opened and the switch SW2 is closed. This will cause a change in current. Capacitor C1 acts as an integrator; therefore, the final output voltage is a function of current and the on-time of switches SW1 and SW2.
S1 and S2 were actually mechanical switches at first, and were soon replaced by silicon devices. S1 has a transistor and S2 has a diode.
Circuits change with technological progress
For many years, the industry has integrated as many components as possible into the circuit to reduce cost and size. It is a breakthrough to be able to integrate the main switch S1 directly into the controller IC, while the coil and diode are still external. In order to further improve efficiency, the newer step-down conversion models then use MOSFET devices to implement two switches (SW1 and SW2), thereby achieving a switching frequency of up to 2MHz.
The goal now is to go one step further towards miniaturization. As the switching frequency continues to increase, the size of the coil structure can now be reduced. The magnitude of the current drops, which affects the size of the basic capacitor. The use of advanced capacitors with less internal heat generation has further promoted this progress.
Smaller size and higher efficiency
The current goal is to reduce the design size while further improving efficiency. In order to achieve this, the cyclic switching power supply circuit must be minimized, and the components on the Z axis must be stacked on top of each other. It can be easily achieved by using lead frame flip chip (FCOL) packaging technology, which connects the controller IC (with integrated power transistor) directly and vertically to the lead frame with an SMD that is also directly mounted on the lead frame. Chokes (Figure 2).
This structure enables fully automated production of highly compact buck converter modules, shortens the connection of self-shielding inductors, and therefore also has a positive impact on electromagnetic compatibility. Products manufactured in this way can also be molded to create a leadless QFN package with MSL3 level (quad flat no lead) and provide comprehensive environmental protection. One example is the Recom RPX series (Figure 3), which has a 2.5A output that can be adjusted between 1.2V and 6V, and the package size is only 4.5mm x 4mm x 2mm, requiring only external input and output capacitors.
These step-down converter modules are complete solutions that can be installed on the user’s PCB using standard SMD assembly and furnace soldering processes. Recom recently launched two RPX series power modules based on FCOL technology. RPX-1.0 and RPX-1.5 modules can be shipped in an ultra-compact QFN package with a size of only 3mm x 5mm x 1.6mm, and can reach a power supply voltage of 1.5A at 36V DC.
In the past few decades, buck converters have made significant developments. Innovations in capacitors, inductors, controller ICs, and packaging technologies have enabled all components to be integrated into increasingly miniaturized packages, and power densities continue to increase. By using innovative 3D power packaging technology for isolated and non-isolated converters, the goal of manufacturing low-power DC/DC converters similar to ICs has been achieved to a large extent. It is expected that performance and power density will be further improved in the future. On the other hand, a fully equipped buck converter module has a size similar to ordinary SMT components and will also be used in the final application.