Design of high-density DC/DC power supplies in space-constrained environments

Introduction

High performance power supply designs continue to demand more power in shrinking board space. Higher power density presents new challenges to power supply designers. Designs must have conversion efficiencies greater than 90% to limit power dissipation and temperature rise in the power supply. Thermal design is especially important as the space for heat dissipation is very tight due to losses in DC/DC power conversion and limited airflow. These power supplies must have excellent output ripple and transient response while limiting external capacitance to reduce the overall size of the power supply design. Power supply designers are forced to choose between designing a discrete power converter or purchasing a traditional power module solution. Discrete power supply designs and traditional power modules are both fabricated on a printed circuit board using discrete components. The key is to provide a fully integrated circuit, easy-to-use, compact power supply design, and Linear Technology's LTM4600 micromodule power supply provides a solution for space-constrained power supply designs. This high performance point-of-load (POL) micromodule solves the space constraints without sacrificing temperature or electrical characteristics. We compare this solution to the compact (POL) regulator design challenge with discrete power converters and traditional power modules.

How to Allocate Space for Power Designs

This high power (POL) regulator is a good example of a space-constrained power design. On large system boards, power supplies are often placed in close proximity to a microprocessor, FPGA, or ASIC to provide the necessary power. Large digital devices may require currents ranging from a few amps to over 100 amps. A large system board often requires several of these point-of-load supplies, so allocating space for each power design becomes an issue. In addition, the back of the system board is often restricted in height and is not generally suitable for power designs. Discrete power converters often utilize both sides of the system board for compact designs, and traditional power module designs are limited to the top of the system board due to their inherent height. Traditional power module designs are generally strategically placed on the system board to avoid blocking the airflow required by other integrated circuits. This often results in performance degradation because the location of the load is considered first when considering the location of the power regulator. The LTM4600 micromodule can be mounted on the surface or back of the system board very close to the point of load.

Figure 1 shows the difference between a double-sided discrete POL design and an integrated circuit module POL design. Discrete designs offer the flexibility of being mounted on a system board or as a traditional power module, but they require more PCB space than IC module designs. Discrete designs do not use board space efficiently on the surface or back of a system board. Discrete designs also require many components and careful board layout, which requires careful selection and sourcing of components, as well as certain design time and skills. Traditional power modules have the same disadvantages as discrete designs. The difference is that traditional power modules place discrete components on a small printed circuit board. Such devices are considered easy to purchase and use, but they require heat dissipation and significant airflow issues. In contrast, the IC module approach is very easy and requires very few external components. Such components can be mounted or soldered on the PC board like a standard integrated circuit. In addition, due to the small footprint and excellent thermal performance, the IC module design can also be easily replicated and installed in multi-power channel applications.

Either design approach must be efficient to limit power dissipation. Figure 2 shows the efficiency curve for a typical 12V to 3.3V design. Note why the efficiency is mostly above 90% across the range of output currents. This is generally true for most high-performance POL regulators, but there is a trade-off between efficiency and size. The power conversion efficiency of a point-of-load regulator is generally proportional to size and inversely proportional to switching frequency. For example, a smaller power design using smaller inductors, fewer capacitors, lower power MOSFETs, and less PCB copper traces generally results in greater power dissipation and lower efficiency because these smaller components have higher thermal resistance. Higher switching frequencies can reduce the size and value of inductors and capacitors in the design without requiring large resistors, but power MOSFETs will incur greater losses at higher switching frequencies due to the parasitic capacitance of these devices. Power designers must make many calculations to compare and select between various discrete converter designs, often requiring a trade-off between switching frequency, efficiency, and size.

Optimized Packaging Technology

IC designers have made progress in reducing parasitic capacitance and inductance using innovative packaging technology. This packaging technology, combined with leading power control, power MOSFET and inductor technology, can provide very dense power supply designs. Now, much higher switching frequencies can also be achieved without parasitic issues that will reduce efficiency. Higher frequency operation allows much less external capacitance to be used for a given voltage ripple and transient response. Figure 3 is a simplified point-of-load module circuit diagram. An advanced power control architecture, coupled with optimized power channels and packaging, can provide an excellent solution for space-constrained POL designs. The power control architecture needs to have high-frequency switching, overcurrent protection, overvoltage protection, current sharing, accurate voltage regulation and fast control loop capabilities to maintain output regulation during load transients. Discrete power regulators and traditional power modules without these improvements have certain limitations in performance and size. The LTM4600 micromodule incorporates all of these advanced technologies in a complete, integrated power solution.

The high power density of point-of-load regulators represents a significant thermal challenge in most system designs. The real issue is getting the heat out of the regulators within the system, which needs to operate over a wide temperature range, often up to 50°C and above. Figures 4 and 5 show thermal images of the two sides of a 33W discrete or conventional module point-of-load regulator. Figure 4 shows the temperature of the inductor mounted on the board surface at 20°C compared to the board temperature. The inductor does not conduct heat to the board very well, so the thermal resistance (q _JA ) of the inductor is not optimal. Figure 5 shows a thermal image of two power MOSFETs mounted on the back side of the board. The temperature of both power devices is nearly 100°C, 40°C higher than the board temperature. The eight external leads of the power MOSFET are poor thermal conductors, presenting a high thermal resistance. This is a very serious thermal problem for the back side of the board due to limited airflow. Heat dissipation is difficult for discrete designs due to the uneven height between components. Some industry-standard power modules have the same disadvantages as discrete converters because they have a similar structure. These modules use relatively low thermal resistance discrete components and standard printed circuit board materials, and the uneven component heights also make heat dissipation difficult. An ideal power module requires optimized thermal design for both the front and back sides of the device.

Figure 6 shows a thermal image of the LTM4600 µModule at 33W, as in the discrete design. The power dissipation is very similar to the discrete design, but with a smaller board footprint. The µModule’s optimized thermal package allows for consistent temperature rise. The power components are mounted on an optimized base plate inside the µModule with very low thermal resistance. The µModule’s pins are also optimized to not only facilitate power delivery but also to ensure low thermal resistance. The
topside mold compound also has low thermal resistance and temperature consistency. In a 33W application, the µModule’s temperature is only 13°C above the board’s temperature. If a small BGA heat sink is mounted on top of the LTM4600, the device’s temperature is significantly reduced. Airflow across the heat sink further reduces the temperature rise, allowing the µModule to operate at full power even with higher ambient temperatures. Due to its low profile and superior thermal performance, the LTM4600 µModule can be mounted on the backside of the board with the option of mounting the heat sink on the chassis or a board carrier with thermal pads.

The LTM4600 has unique advantages for space-critical power supply designs. The micromodule is a unique power device that integrates all the components required for high-performance power supply in a very small volume. The micromodule can be soldered like any other surface-mount integrated circuit, requiring very few external components. The micromodule uses a 15mm×15mm×2.8mm LGA package, which can increase the power to 40W with an efficiency of up to 94%. Two micromodules can even be connected in parallel to double the output power. Now that product design cycles are getting shorter and shorter in the market, the ease of use of the LTM4600 will be able to shorten the time to market.

In general, the design difficulties of high power density can be effectively solved through innovative integrated circuit and packaging technologies. The LTM4600 micromodule combines these innovative technologies to solve the problems in high power density design. The trend of modularization will continue to be popular because it is very effective in solving space and heat problems that often occur in advanced power supply designs.