Beyond Size and Power Density: What Matters in Industrial DC/DC Integration

For decades, AC transformers with rectifiers and linear regulators were used to generate DC voltages. They were bulky and, with efficiencies below 60%, generated a significant amount of waste heat.

The technology to convert voltages using switched-mode power supplies was already known at the beginning of the 20th century. The ignition circuit of a gasoline engine, invented in 1910, is a flyback converter with maximum switching frequencies of a few hundred Hertz (figure 1).

Figure 1: 4-cylinder gasoline engine ignition circuit, a simple flyback converter.

Technological advances in the 1950s and 1960s enabled switching frequencies of over 50kHz, and switched-mode power supplies entered the market. These were 75% smaller than AC transformers and achieved efficiencies greater than 80%. From the 1970s onwards, switched-mode power supplies were first used in measurement devices and computers and later in industrial and domestic applications.

Different topologies, advanced power semiconductors, and controller chips, combined with switching frequencies of several hundred kHz, enabled further size reductions and efficiency improvements. Today's solutions require only a fraction of the space and have been part of the general trend toward smaller electronic devices. The size and power density (W/cm³ or W/inch³) of DC/DC converter modules have become an important feature. But are these parameters the only decision criteria?

More than “just” size and power density

What finally counts are the overall size, reliability, and maximum ambient temperatures of a total power solution, which includes the DC/DC converters, cooling elements, input EMIR filters, protection circuits, and output capacitors. In this article, we will look at an existing design using P-DUKE’s FED60W, a 2 x 1 inch DC/DC module delivering 12V/60W, and then show how two new products from P-DUKE enable designers to either increase power to 100W within the same size or reduce the overall size by shrinking power demands to 50W.

Cooling and power derating

Every conversion process generates some heat, and the higher the efficiency of the device, the lower the losses calculated by the formulas

According to the datasheet, the FED60W has an efficiency of 92% when delivering 12V/60W and losses are

The overtemperature of the module and maximum ambient temperature can be calculated by using the following formulas:

Where
TRise is the temperature rise on the module's case that was generated from the power dissipation.
Rth is the thermal impedance of the case to ambient.
PLoss is the power dissipation generated during the conversion process of the power module.

Although values for Rth can be found in the datasheet, it is much easier to determine maximum ambient temperatures by using the power derating curves for different cooling configurations in the datasheets (Figures 2 and 3).

Figure 2: power derating curve of the FED60W module (no heatsink, no PCB)

Explanation: With 100 LFM airflow, the FED60W converter without a heatsink can deliver 100% of its rated power up to 68°C ambient temperature. At 80°C, maximum power is reduced to 67% or 40W. To increase power, additional airflow or a heatsink is required.

Figure 3: Power derating curves for different heatsinks (natural convection, no PCB)

Important notes

Thermal resistance depends on the heatsink's size and shape, as well as the velocity and direction of airflow in the application. When mounted on a PCB, some of the heat generated by a DC/DC module is dissipated to the PCB. The graphs in Figures 2 and 3 are based on the module without a PCB. Due to this heat dissipation to the PCB, the thermal resistance of a 2 x 1 inch module mounted on a PCB is reduced by around 25–35%. For new products, many manufacturers use this configuration for derating curves as it better matches real applications. Exact definitions of thermal resistance can be quite complex; therefore, it is recommended to measure temperatures in the actual application.

With 100 LFM airflow and a 0.5” heatsink, the module can deliver full power at 85°C ambient temperature, a typical value for industrial applications. This brings us back to the topic of this article: What is the real size and power density of this 60W solution? Table 1 shows the values in W/inch³, commonly used to demonstrate power density:

Table 1: power density of the 60W solution with FED60W and heatsink

Due to the necessary heatsink in this real-life application, the overall power density is reduced by 56% as the volume of the solution includes the converter (44%) and the heatsink (56%). If a supplier offers a smaller module but with the same losses, the heatsink size and overall power density don’t change. When reducing the footprint of a module, the contact areas between the heatsink and PCB are smaller, resulting in less heat transfer and higher Rth (Figure 4).

Figure 4: Heatsink size and heat transfer area can be a dominating factor for the overall power density

What options does a customer have if they need 100W power in the same size or want to significantly shrink the overall size by reducing power demands to 50W? This is where P-DUKE’s new products come into play, as they can deliver more power in the industry-standard packages shown in Figure 5.

The FED100W can deliver 100W in the 2 x 1 inch package. Other suppliers only achieve 60–80W or use the 67% larger footprint of the Quarter Brick Format (2.3 x 1.45 inches) for 100W converters. In the 1 x 1-inch standard package, the new LCD50W can deliver 50W of power, while products from other manufacturers only achieve 30–40W.

Figure 5: P-DUKE‘s new converters can deliver up to 67% more power in the industry packages

Key elements for achieving this improved performance are lower losses, improved heat transfer to the heatsink and PCB, and avoiding thermal hotspots in the module. Advanced semiconductors and core materials with lower losses, combined with other technologies, contributed to reduced switching losses.

With an efficiency of up to 94%, P-DUKE’s new FED100W family can deliver 12V/100W in the standard 2 x 1-inch package with only 6.38W losses at full power. By using the same heatsink and airflow, plus an appropriate PCB design, this converter is a direct replacement for the previously used FED60W but with the capability to deliver up to 100W. Table 2 shows that power density with the heatsink increased by 67%.

Table 2: With P-DUKE’s new FED100W power density increases by 67%

Another unique feature of this part is the maximum case temperature of 110°C, enabling higher ambient temperatures, smaller heatsinks, and lower airflows in a given application.

The same approach was taken when designing the new LCD50W converter, P-DUKE’s 50W module in the 1 x 1 inch package with efficiency levels of up to 92%. Modules from other manufacturers only deliver 30-40W in this package with 1-2% lower efficiency levels. Customers wanting to increase power during a redesign can get 10W more power without changing the mechanical construction. If, during a redesign, power can be reduced to 50W, the footprint can be halved by using P-DUKE’s 1 x 1 inch module.

The derating curve of this module in Figure 6 shows that when mounted on a 3” x 3” PCB, it can deliver full power up to ambient temperatures of 55°C with natural convection and no heatsink.

Figure 6: Derating curve of the LCD50W (mounted on a 3” x 3” PCB, no heatsink)

With the 0.5” heatsink and 100LFM airflow in our example application, the module can deliver 50W up to an ambient temperature of 85°C.

Figure 7 shows the efficiency curves of the FED100-24S15W (9 – 36Vin, 15Vout). At 100% load (red line in the graph), used to define deratings, it remains flat over the full input range.

Figure 7: The efficiency vs input voltage curve of the FED100-24S15W remains flat at 100% load

EMI Filter

Necessary internal and external EMI filters are another important factor for the overall power density of a solution. Using smaller capacitors and magnetics inside the module to reduce its size does not help, as more noise will be conducted to the outside, and the external EMI filter becomes larger.

The better approach is to focus on the overall EMI performance of a module by reducing noise generated by the switching processes and creating a low-impedance path for noise inside the module itself. The best way to compare data from manufacturers is by looking at reference designs to meet Class A or Class B. Typical filters and reference designs are shown in Figure 8.

Figure 8: Typical FED60W series EMI filters and suggested layouts

Values and size of the components vary by design and input voltages. As shown in Figure 9, Class A filters can be kept quite small, but the footprint of a complete Class B filter can be around 40 -50% of the converter’s footprint.

Protection circuits

Last but not least, the DC/DC converter module must be protected against over-voltages and, in many applications, against reverse polarity, too. Safety standards require fuses at the input. Figure 9 shows the complete circuitry.

Figure 9: Complete circuitry of a DC/DC converter with filters and protection circuits

All these components take up space on the circuit board and, as a whole, make up a significant proportion of the total volume.

Summary

When comparing different DC/DC converter modules, high power density is certainly a good indicator that a small size can be achieved in a power supply design. As shown in this article, various other factors like low losses, optimized cooling for the highest reliability, and components needed for filters and protection circuits also play a significant role in the overall size of any power architecture.

With the development of these new 50 and 100W converters, P-DUKE has not only achieved leading-edge power density for the converter module itself but also enables designers to increase the power of an existing design by over 25%. If less power is needed, the size of the solution can be reduced by 40% or more.

To learn more about these high-performance solutions, explore LCD50W and FED100W.