Optimal Power Module Selection Criteria

Today, many telecommunications, data communications, electronic data processing, and especially wireless networking systems are powered by distributed power architectures. These complex systems require power management solutions that can monitor the power supply down to every precise parameter. To achieve this level of performance, most designs use FPGAs, microprocessors, microcontrollers, or memory blocks.

The complexity of this design has increased the burden on wireless network and wired system application engineers. Their only choice is to either invest heavily in improving the level of internal power management technology or rely on the expertise of external design companies.

Recently, a third option has emerged: point-of-load DC/DC power modules. These modules integrate most or all of the components needed for a plug-and-play solution, replacing up to 40 different components. This integration helps simplify and speed up design while reducing the size of the power management system.

The key to achieving the performance required of these modules while staying within budget and space requirements is to have a good grasp of the different technologies available.

As shown in Figure 1, most traditional general-purpose non-isolated DC/DC power modules still use single-in-line packages. These open frame solutions have made some progress in reducing design complexity, but they only use standard packaged components on printed circuit boards. They are generally low-power designs (about 300kHz) and their power density is not outstanding. Therefore, due to their size, they are difficult to become the choice for many space-constrained applications. The next generation of power modules needs to work hard to reduce size to increase design flexibility.

Figure 1 Traditional SIP open framework module

To increase the power density required by designers, power management system suppliers must increase switching frequency to reduce the size of energy storage components. However, increasing switching frequency with standard devices will lead to a decrease in efficiency, which is mainly caused by MOSFET switching losses. This situation has prompted the industry to find cost-effective ways to reduce the parasitic impedance of the MOSFET driving power path in DC/DC modules, making the finished module equivalent to the size of an integrated circuit.

Size and cost are two important considerations when evaluating solutions for a specific application. However, other factors are equally or more important to the end application. Some of these considerations are described below.

Reliability
Reliability is a major issue that all system designers need to address. Many distributed power architecture applications require years of normal operation with little to no failure. Reliability plays a major role in the total cost of ownership of the system. Reliability becomes an important issue that must be addressed in power modules due to the large number of components combined in a package, thermal fatigue caused by high power density, and failure of attached circuits.

The failure rate of electronic systems and components is in the shape of a bathtub curve (see Figure 2). The steepness and sharpness of the transition from one state to another in the curve depends on the selected components and component grades, as well as the compatibility of these components with other components in the module. For example, using a 30V MOSFET, under 20V input conditions, as long as attention is paid to the selection of the drive circuit, Schottky diode and buffer circuit, the DC/DC module can meet the expected requirements.

Figure 2 Life cycle failure rate

Thermal fatigue in power modules is caused by low power conversion efficiency and limited space for heat dissipation. This situation will eventually increase the temperature, thereby shortening the product life. To reduce the impact of temperature on mean time between failures (MTBF), system designers should consider heat dissipation, airflow and module power loss derating curves, as shown in Figure 3.

Figure 3 Typical power loss derating curve

Another phenomenon that can lead to serious failures is the temperature rise caused by cracked solder joints. If the module is subjected to mechanical shock or multiple temperature cycles, the solder joints can easily crack and eventually separate from the substrate, resulting in increased resistance and increased temperature stress. This situation can occur repeatedly until the wire breaks, causing a fatal failure.

Electrothermal Performance
Balancing performance, reliability and economy is a major challenge facing system designers when selecting the best module. The lack of standardized test conditions and measurements, especially in terms of key parameters published in data sheets such as power, efficiency and transient response, further increases the difficulty of module selection.

When making an efficiency comparison, it is important to consider the input voltage, output voltage, and current for the efficiency comparison. Transient response is another parameter that needs to be considered for an effective comparison. It is important to ensure that the input and output voltages are the same, the output capacitors have the same value or similar parameters (ESR, ESL, etc.). Finally, the size and magnitude of the transient current step change are the same.

In many applications, power modules need to work in harsh environments. When comparing module efficacy, we should not only care about the electrical performance at 25°C, but also consider the system ambient temperature, airflow and module heat dissipation method. For example, Intersil's ISL820xM series with QFN package optimizes the thermal conductivity of the PCB, and the large area of ​​copper foil at the bottom of the module helps to improve the overall efficacy level.

In short, new, higher power density products will become the future choice of the non-isolated load point DC/DC converter market. The ISL8201M module launched by Intersil integrates most of the components required to form a DC/DC converter, including PWM controller, MOSFET and inductor. The input voltage is 1~20V and the current is 10A. Its switching frequency is higher than that of traditional SIP DC/DC modules. It uses a small 15mm×15mm×3.5mm QFN package, eliminating MOSFET packages and combined package devices (see Figure 2). The ISL8201M is the first product in this module series, and modules with smaller size and further improved performance are under development.

Figure 4 ISL8201M efficiency curve (Vin=12V)

The ISL8201M is very good in terms of power efficiency. At the same time, the excellent heat dissipation performance of the QFN package facilitates compact structure design without the need for a heat sink. These features enable the ISL8201M to achieve a power density of almost 200W/in3, which is about four times that of traditional open frame modules.