DC-DC converter integrated circuits/modules have become not only the heart of various power electronic devices, but also the key to the high-efficiency, low-power, safe and reliable operation and automated control of various power electronic devices and systems.
Thanks to numerous technological advancements, an increasing number of suppliers are offering power modules. Now is the time to leverage the next generation of power modules. The process of selecting a power module is crucial; designers need to choose the optimal solution in terms of value (performance and size) and cost-effectiveness.
The demand for higher-density circuit boards and smaller system sizes is driving the need for smaller DC/DC solutions as power modules evolve. Power module technology took many years to reach the mass market. In the early days, designing power converters was extremely difficult. They were designed entirely with discrete and leaded components, or simply taken from transformer taps. The reality was that designing power converters was considered a black art. Experts in this field with a deep understanding of all aspects of power conversion were rare. Design cycles often took over a year due to stability issues caused by high di/dt or dv/dt, component failures, EMI problems, and inappropriate or constrained layouts that increased radiated EMI, requiring multiple design iterations.
Later, companies like Unitrode developed PWM controllers, and power transistor suppliers began offering MOSFET technology to replace bipolar transistors. Switching frequencies increased to approximately 100 kHz, and surface-mount components became mainstream. With improvements in process technology, packaging, and MOSFET technology, DC/DC regulators with integrated controllers and power switches emerged. This further reduced board space and increased power density (see Figure 1).
Today, only a handful of semiconductor suppliers offer integrated DC/DC solutions in a single package. Inductors and passive components, aside from the controller and power switches, are now integrated into the package. To reduce the module package size, inductor size must be significantly reduced while still providing good performance. This can be achieved by increasing the switching frequency, allowing the use of smaller inductors with lower inductance, which also reduces the inductor's DC resistance. The trade-off is increased controller and MOSFET switching losses.
The good news is that semiconductor processes and MOSFET technology have improved significantly over the years, reducing the impact of higher switching frequencies. Smaller geometries enable improved performance and allow for smaller silicon dimensions. Newer MOSFETs with improved quality factors help optimize the trade-off between switching and conduction losses, resulting in higher efficiency and allowing for smaller package sizes with sufficient power dissipation. Furthermore, with decreasing thermal resistance, packaging technology has evolved to the point where a few watts of power can be achieved in small packages. Additionally, passive power components (capacitors, diodes, resistors) are reducing their footprint to save space. All these technological improvements have enabled integrated power modules to achieve the power density levels currently available (Figure 2).
Why use a power module instead of a discrete approach?
Besides taking up less space than discrete solutions, power modules offer many other advantages. While discrete methods can achieve the highest efficiency, if saving board space is your primary requirement, you can meet density requirements by compromising a few percentage points in efficiency. For example, Micrel power modules achieve 90% efficiency and are highly efficient under light load conditions thanks to HyperLight Load™ technology.
Discrete power converters require more space and careful component placement. Optimizing layout and routing can be very challenging. AC current loops are larger and more susceptible to radiated EMI issues because these loops act like antennas. Integration of critical power components within the module minimizes loop size; simply placing the input and output capacitors close to the IC and connecting them to GND is easily achieved. For most customers, meeting CISPR22, CLASS B, or EN55022 requirements is essential. Figure 3 shows the performance of these new modules.
Additional performance considerations for power supply design include load transients and thermal management. Load transients are a function of control loop architecture, switching frequency, and output filter size. Thermal issues relate to the operating ambient temperature and the ability to remove heat from power components. This is one of the main issues for power modules, as most heat is dissipated within the package. Smaller footprint and smaller contact area with the circuit board are desirable (for QFN packages). Therefore, to realize the advantages of using small power module solutions, a high-performance package with low thermal resistance (especially junction-to-board) and a high-efficiency regulator are essential (Figure 4).
Meeting challenging system requirements
Fully integrated, the latest power module solutions offer end users flexibility, such as setting current limits, frequency, and output voltage using external resistors. This allows the same module to be tuned for different output voltages in the system. The ability to adjust current limits allows for optimal overcurrent protection. The ability to adjust frequency resolves the efficiency-transient tradeoff. Furthermore, since many components are contained within the power module, layout and routing are easier. Depending on the power module supplier, the size and shape of exposed thermal pads may vary. Some suppliers offer pad locations that are very easy to use with QFN packages, while others use LGA/BGA packages, which may be more difficult and expensive to assemble.
When designing distributed power systems for industrial or medical applications, a small, high-voltage solution with a wide operating input and output voltage range is ideal. The MIC28304, in a small 12 x 12 x 3 mm package, offers a 70 V/3 A module, reducing PCB requirements by over 60% compared to discrete solutions. External components allow for flexible setting of the current limiting frequency (if a specific switching frequency needs to be avoided) and the output voltage (programmable from 0.8 V to 24 V). The device delivers impressive efficiency levels under both bright and full load conditions in the HyperLight Load version. Finally, this power module complies with EMI CISPR 22 Class B specifications.
The requirements differ when designing enterprise infrastructure, data communications, FPGA power supplies, or distributed 12V bus applications. These applications typically demand higher current, higher efficiency, and smaller size because board space is extremely valuable. Point-of-load (POW) locations often need to be close to the processor. For such applications, the MIC452xx series devices offer the smallest form factor and the highest power density. The control architecture used is optimized for fast loop response, thus requiring a smaller output capacitor. Meanwhile, the wide input voltage range of 5V to 24V allows for the use of the same series to provide either a 5V or 12V common bus rail, reducing the number of eligible and necessary components in inventory. Again, the individual device offers high flexibility and a minimal number of external components, enabling a very small form factor. These devices are packaged in a QFN package, making layout easier compared to LGA solutions, with all components placed on top (Figure 5). If an even smaller solution is required, some passive components can be placed on the bottom.
