Make The Right Designer Decisions In Choosing DC-DC Converters

Jan. 27, 2011
Board-mounted bricks have functionally improved in efficiency and power density, and they’re now available in a variety of sizes that span half bricks to 1/32nd units.

ULS series


Common DC-DC converter

Design engineers are spoiled for choice when it comes to selecting dc-dc converters. Board-mounted bricks have functionally improved in efficiency and power density, and they’re now available in a variety of sizes that span half bricks to 1/32nd units (see the table). So, today’s designers have many technical decisions to make when they’re selecting power bricks. 

Output Power
A best-in-class 1/16th brick delivers 66 W, while half bricks are typically capable of output power levels up to 450 W (Fig. 1). The available power from each standard brick size has increased steadily since these products first appeared about 25 years ago. We’ll get 1 kW from a half brick in the foreseeable future as technology drives a 2% to 3% improvement in efficiency. Advances in component technologies and materials, particularly for magnetic components, have driven these improvements.

When choosing the most appropriate power rating, consider both the duty cycle that the load imposes on the converter and the wider operating environment, particularly with respect to ambient temperature.

Datasheet ratings will normally refer to continuous operation at an ambient temperature of 25°C. If you only need full power some of the time, you may be able to manage with a smaller dc-dc converter. This not only will cost less, it also may operate nearer its full load rating, at which point it will be more efficient.

As a rule, it’s better to carry out development work using a converter with power to spare and then scale back to a more appropriate power rating for the final converter when the application requirements are fully understood. This approach also means you’re unlikely to find there’s not enough space for the converter you need when your design is finalised.

Efficient Operation
Typically, dc-dc converters are most efficient when operating near their full rated load, and the larger the difference between input and output voltage, the less efficient the conversion process is. You should remember to check the efficiency curve at lower loads and to account for the operating environment when estimating how much heat you’re going to have to remove from your equipment.

Today’s best-in-class isolated dc-dc converters are achieving efficiencies in the 80% to 97.5% range. Don’t underestimate the value of a 1% difference in efficiency. If a converter is 91% efficient, rather than 90%, that 1% difference equates to 10% less heat you need to remove from your product.

Check the datasheet to determine whether power is specified as a convection-cooled or forced-air cooled rating. Isolated dc-dc converters are less efficient than non-isolated types, simply because there are inevitably some losses in the isolation components.

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Keeping Cool
Power rating, power density, efficiency, and the operating environment all affect a system’s thermal management requirements. You can make much smaller systems if you add a fan for forced-air cooling. However, the addition of such electromechanical components reduces the predicted mean time between failures (MTBF) because they wear out.

Adding a fan also adds cost, noise, and a maintenance task, so it’s better to avoid doing so unless size constraints make it mandatory. Running the system at a higher temperature is always an option. But, as a rule of thumb, every 10°C rise in ambient temperature will halve the life of the component. Conduction cooling via a base plate is often desirable, where the converter is designed for this approach (Fig. 2).

Conduction Cooling
A base plate is typically used to add an element of conduction cooling to a convection-cooled system, where little or no airflow is present. Some designers chassis-mount the base plate on the dc-dc converter to the equipment enclosure or to a cold plate. Others attach an additional heatsink to the base plate to increase the heat dissipation.

A combination of base plate and forced-air cooling may also be used to achieve the highest possible output. Ultimately, the temperature of the converter is the limiting factor with respect to its maximum output power. Of course, today’s dc-dc converters nearly all have integral protection circuits so they shut down with overtemperature (or other fault conditions), but it’s not something that’s desirable in day-to-day operation.

Handling The Load
It’s relatively rare for an application to present the dc-dc brick with a nice, non-reactive, steady-state load. Converter efficiency at low load is becoming an ever-more important consideration in the drive to improve energy efficiency and reduce carbon-dioxide emissions.

Take data centres as an application example. Power systems might operate at 20% of their maximum load for 80% of the time, the power requirements varying dynamically with data throughput. Converters that are relatively efficient at low load may provide better overall performance than converters with marginally higher maximum efficiency at full load.

The dynamic response of the converter may also be important in some applications, i.e., how quickly it can respond to changes in load. This is expressed as the slew rate (di/dt). The amplitude of any load step also has to be considered, together with the maximum voltage excursion that the load can handle. External capacitors are often used to provide short-term response, providing recovery time for the converter.

Input Voltage Range
If you choose dc-dc converters with a wide input voltage range, you may be able to use the same product in more applications. You’ll then have the advantage of buying higher volumes of each converter at lower prices, and you’ll benefit from lower inventory management costs.

Converters with a 2:1 input voltage range were once the standard. Now, products with a 4:1 input range offer much greater application flexibility. For example, some Murata Power Solutions products will operate with input voltages from 9 to 36 V or from 18 to 75 V dc.

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Output Voltage Trimming
The option of output trimming adds application flexibility to a dc-dc converter. It is often possible to trim output rails by ±10% or so, without any change in the available output current. If you are trimming to a higher voltage, bear in mind that the increased power being drawn from the converter may have implications for thermal management.

Isolation Aspects
Isolation isn’t always needed. It adds cost and reduces efficiency, so only choose an isolated converter when you need to. It’s most commonly required for compliance with the numerous safety regulations but can also be useful as a way of increasing noise immunity or providing an alternative ground reference.

For instance, telecom applications are often based on a 48-V positive ground input whilst the outputs need to have a negative ground. Medical applications are particularly stringent with respect to isolation and leakage current requirements. These regulations are set out in IEC60601. In dc-dc converters, transformers isolate inputs from outputs.

The wide variation in application environments, the breadth of choice of dc-dc converters now available, and the challenges of comparing one datasheet with another combine to make dc-dc converter selection a real challenge for system designers. Talking to your preferred suppliers early in the life of a project and ensuring an in-depth understanding of how power-supply data is presented are important in ensuring you arrive at the most cost-effective solution.


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