Why are DC/DC regulator efficiency indicators misleading?

Many people think that the voltage conversion efficiency of a DC/DC switching regulator is the last performance item to be measured. The efficiency number is a measured value, listed in the data sheet, and when the system designer is trying to choose a solution from several manufacturers, the efficiency curves are drawn for a range of voltages and currents and compared. In order to achieve high efficiency (the word is relative, but let's assume that high efficiency means efficiency numbers above 85%), analog chip designers and analog application engineers will very carefully try every trick in the book, such as adjusting the switching frequency of the power switch and changing the size of its gate drive. Surprisingly, the chip or IC alone cannot guarantee that a circuit will achieve the highest efficiency. The choice of external components does have a great impact on an IC and can make a very good IC perform mediocrely. The choice of inductor, capacitors and PCB layout, as well as the skills of the system designer, are all critical factors in designing a high-efficiency DC/DC switching point-of-load regulator. However, with the "high efficiency" number, the thermal management business is not over.

Judging the thermal performance of a point-of-load DC/DC regulator based solely on its conversion efficiency is like judging the speed of a car based on the size of its engine. Using a 12-cylinder Lamborghini engine to drive a dump truck might give people an exaggerated idea of ​​its speed, but the laws of aerodynamics completely deny such a truck the possibility of participating in a Formula One race. Similarly, if a DC/DC step-down regulator with 90% efficiency and 3.5W of heat is packaged in a very attractive and tiny package with a 22oC/Wj-a thermal resistance, the thermal management challenges will make this DC/DC regulator almost impractical and often too expensive to use: 3.5WX22oC/W at an ambient temperature of 40oC produces a junction temperature of about 117oC. Of course, there are several ways to remove heat from the package, such as using fans, increasing the PCB copper area, adding heat sinks, etc. In short, these remedies increase the complexity of the design, increase the cost, and require more space to dissipate the heat.

The desire for power and its control

The battle to control heat dissipation and improve power distribution efficiency is intensifying. Optimal control and reliability of digital devices and infrastructure largely depends on the performance of DC/DC converters used as distributed DC power supplies for FPGAs, ASICs, transceivers and memory modules, as well as RF amplifiers and sensors. In addition to electrical performance such as regulation accuracy or transient response, thermal performance has become an increasingly critical factor in the selection of DC/DC regulators.

Scalable and modular DC/DC regulator solutions

The 72W solution (see Figure 1) relies on accurate current sharing and low thermal resistance values ​​of four µModule™ regulators to evenly dissipate heat over a compact surface area to prevent hot spots. Each DC/DC µModule regulator is a complete power supply with inductor, MOSFETs, and DC/DC controller circuitry in a package with IC-like form factors. Each regulator can deliver 12A (or more if paralleled) from a wide input range of 4.5V to 20V, making it a versatile and scalable solution. Parallel system design involves more than just copying and pasting each circuit layout. The µModule regulators occupy only 15mmx15mm of board area and are only 2.8mm high. In addition to good efficiency performance, the package also has a thermal resistance of only 15oC/Wj-a. Such a flat package allows air to flow smoothly over the entire circuit, removing the heat generated by the circuit (see Figures 2 to 5). This solution has little thermal shadowing of the surrounding components, which helps further optimize the thermal performance of the entire system.

Figure 1: A system of four DC/DC µModule regulators share current to provide a regulated 1.5V at 48A, each µModule regulator is only 2.8mm high and occupies a 15mmx15mm board area. Each µModule weighs only 1.7g and has an IC-like form factor, making it very easy to pick and place with any pick-and-place machine during board assembly.

Think beyond efficiency

Figures 2 through 5 are thermal images of the circuit board shown in Figure 1, providing temperature readings at specific locations as well as the direction and speed of air flow. Cursors 1 through 4 show an estimate of the surface temperature of each module. Cursors 5 through 7 indicate the surface temperature of the PCB. Note the temperature difference between the two inner regulators (cursors 1 and 2) and the two outer regulators (cursors 3 and 4). The outer-placed µModule regulators have large flat surfaces on the left and right that help dissipate heat, keeping the outer µModule regulators a few degrees cooler. The inner two µModule regulators have only small top and bottom flat surfaces to dissipate heat, so they are slightly hotter than the outer two.

Airflow has a big impact on the thermal balance of a system. Note the temperature difference between Figure 2 and Figure 3. In Figure 3, 200LFM of airflow is evenly directed from the bottom to the top of the demo board, and one side of the board is 20°C cooler than the other side, compared to Figure 2 with no airflow. The direction of airflow is also important. In Figure 4, airflow is flowing from right to left, pushing heat from one µModule regulator to the next, causing a stacking effect. The µModule regulator on the right, closest to the source of the airflow, is the coolest. The leftmost µModule regulator is slightly hotter due to heat escaping from the other LTM4601 µModule regulators. Figure 5 shows an extreme case of heat stacking from one µModule device to another. Each of the four µModule regulators is equipped with a BGA heat sink, and the entire board operates in a container with an ambient temperature of 75°C.

Figure 2: This thermal image of the 48A, 1.5V circuit in Figure 1 shows balanced power sharing and low temperature rise between the DC/DC μModules, even in the absence of airflow (VIN = 20V to 1.5VOUT/40A).

Figure 3: Thermal image of four LTM4601s in parallel with 200LFM bottom-to-top airflow (20VIN to 1.5VOUT/40A).

Figure 4: Thermal image of four LTM4601s in parallel (12VIN/to 1.0VOUT/40A) in a container at 50°C ambient with 400LFM right-to-left airflow

Figure 5: Thermal image of four LTM4601s in parallel (12VIN to 1.0VOUT/40A) with a BGA heat sink in a container at 75°C ambient with 400LFM right-to-left airflow

How environmentally friendly is your system?

Here is another example of a 3.3Vin system that requires high load current up to 15A. The LTM4611 is packaged in a thermally enhanced LGA (land grid array) package, which offers attractive high efficiency with a small land pattern (only 15mmx15mm) and small physical volume (only 4.32mm in height and occupies only 1 cubic centimeter of space). Figure 6 shows the efficiency of the LTM4611 for various input and output voltage combinations. In addition to high efficiency, the power loss curve of the LTM4611 is relatively flat for a given input voltage condition, which makes it easy to thermally design and reuse the LTM4611 in subsequent products, even when the rail voltage becomes lower due to IC die shrinkage.

In an increasing number of applications, reducing power consumption at light loads is as important, if not more important, than reducing power consumption at heavy loads. It is becoming increasingly common for digital devices to be intentionally designed to operate in lower power states whenever possible and whenever practical (in terms of energy savings), and only draw peak power (full load) intermittently. Figure 6 shows the efficiency benefits that can be achieved by operating in PSM and Burst Mode at lighter load currents (<3A).

Figure 6: Efficiency of the ultra-low VIN15ADC/DC uModule regulator LTM4611

Thermally enhanced packaging

The device’s LGA package allows for heat dissipation from both the top and bottom, facilitating the use of a metal chassis or BGA heat sink. This form factor facilitates excellent heat dissipation with or without airflow. Figure 7 shows an infrared (IR) thermal image of the top of the LTM4611, tested on a bench with no airflow, showing a power loss of 3.5W while converting a 5V input to a 1.5V/15A output. The hottest surface temperature was approximately 65°C.

Figure 7: Top thermal image of the LTM4611 regulator outputting 1.5V/15A from 5Vin. Power dissipation is 3.5W. Tested on a bench with no airflow, the surface temperature hotspot is 65°C.

In contrast to Figure 7, Figure 8 shows an IR thermal image of the top of another LTM4611, when tested on a bench with no airflow, showing a power loss of only 3.2W and converting a 1.8V input to a 1.5V/15A output. The location of the hotspot (but not the size of the hotspot) has changed slightly from what was seen when operating with a 5V input.

Figure 8: Technical video at 1.8VIN, 1.5VOUT/15A output load, 3.2W power loss, 0LFM airflow, 65oC surface temperature (URL: http://video.linear.com.cn/55)

Thermal Performance TechClip

For many DC/DC μModule regulators, to demonstrate thermal performance, a quick 45-second video tech clip is an extremely useful way to understand the thermal characteristics of the device, in addition to providing efficiency specifications and output power derating curves. Figure 8 shows an example tech clip for the LTM4611. An infrared camera was used to record the surface temperature of the LTM4611 as it heated up during operation. Pay special attention to the vectors used to measure the surface temperature of the LTM4611 (labeled 1 and 2). The ambient temperature was measured at 31.5oC.

Blue represents the lowest temperature, and yellow indicates hotter areas. Note that when looking at color to determine temperature, the color spectrum (blue to yellow to white) indicates the change in temperature, not the absolute value. For example, yellow may correspond to 70oC in one case, but 110oC in other test conditions. Therefore, in addition to color, pay special attention to the value of the temperature. Color can be used to quickly determine hot and cold areas, but when it comes to temperature values, it is always necessary to read the value of the fine line.

The technical clip in Figure 8 was tested at 1.8Vin and 1.5V out (very low dropout switching regulation) and a very high 15A load current. No linear regulator can provide low dropout at 15A. Despite the calculated efficiency of 83% (=1.5/1.8V), a linear regulator can easily dissipate 4.5W at 15A. The LTM4611 has a negligible power loss of only 3.2W at 0LFM and a hotspot temperature of only 65oC. These numbers allow the system designer to build a very compact circuit because thermal constraints are minimized. There is less reliance on heat sinks, fans and large PCB copper areas. If you have 45 seconds left, watch this short video.

in conclusion

If the efficiency of the DC/DC regulator is high or acceptable, then study the package thermal resistance. Try to understand the thermal (temperature) performance of the product under different operating conditions. In a DC/DC switching regulator, for example, the thermal and efficiency values ​​are different depending on the ratio of VIN to VOUT. Of course, also consider the ambient temperature and airflow. While doing this, also be aware that high efficiency conversion values ​​can be misleading when thermal management is a concern. A wise idea is to simply calculate the power dissipation to determine the approximate junction temperature. Study the extensive thermal data provided by the manufacturer, such as thermal images and derating curves. A high-quality DC/DC regulator solution (especially in a modular format) should convince you of its performance with relevant data, images, and perhaps a video clip.