Specifying board-mounted dc/dc converters

The specification of board-mounted dc-dc converters is an important and detailed process. Correctly implemented, it results in a cost-effective solution that meets all the needs of the application. Incorrectly specified dc/dc converters can result in a more costly device than necessary and/or a device that is not suited for the application. This FAQ will review the major specifications for board-mounted dc/dc converters. FAQs two and three will take deeper dives into thermal management and electromagnetic compatibility considerations, respectively and the fourth and final FAQ will review reliability and availability factors when specifying board-mounted dc/dc converters.

Efficiency is often considered to be the most important specification for board-mounted dc/dc converters. It has a major impact on many aspects of system design. Even in high-efficiency designs, efficiency improvements have a significant impact. Consider an 80% efficient dc/dc converter compared with a 95% efficient design. The 80% efficient converter dissipates 20% of the input power as heat; the 95% efficient design dissipates 5%, four-times less. That difference impacts many aspects of system design:

• Operating temperatures can be lowered, or the system power density can be increased at the same operating temperature
• The physical size of the system can be reduced
• System cost will be lower as a result of smaller heatsinks, or even the elimination of heatsinks altogether
• Reliability can increase
• For ac-powered systems, the front-end ac/dc power supply will be smaller and less costly
• Battery-operated systems can employ smaller batteries or offer longer runtimes for a given charge level
• Energy costs and environmental impact for the system will be less

Efficiency can be specified in several ways, as a typical value (very common), as a guaranteed minimum value, at various input voltage levels, at various output power levels, and so on. And, efficiency is generally not flat across the range being considered. In the case of output power versus efficiency, it is important to consider the shape of the efficiency curve and match it with the system’s expected operation levels to maximize efficiency under real-world operating conditions.

No load power consumption can be an important specification in many applications, especially battery-powered devices. It is related to the power consumption of the switch mode circuitry and is a limiting factor for overall efficiency.

Output regulation

Rated output current is a straightforward specification. Some dc/dc converters also specify a minimum load. Depending on the converter, operation below the minimum load will negatively impact voltage regulation but should not damage the converter. Output voltage is a more complex parameter to specify. Two factors that provide a starting point for specifying output voltage are the nominal value or “set point” and the variations from that nominal value as a function of various independent parameters such as changes in the output load, changes in the input line voltage, and changes in operating temperature.

An example of a setpoint specification is ±1% at nominal input voltage, full load, and 25°C. Line and load regulation are typically specified as either a percentage or an absolute range, for example, ±0.1% or ±5mV. Temperature regulation is typically specified “per degree C,” such as ±0.01%/°C or parts per million (PPM) as in PPM/°C. Some dc/dc converter suppliers provide a single specification for “total regulation” for all possible variations rather than providing each of the individual specifications outlined above. For voltages below 3Vdc, output voltage regulation may be more important to specify in detail.

In typical applications, the line input voltage and operating temperature tend to change relatively less during system operation compared with the output load level. As a result, load regulation is the more critical specification. In addition, there is dynamic voltage regulation (sometimes called transient response) resulting from step-function changes in the output load.

Dynamic output voltage regulation

For many systems, dynamic regulation can be more critical than static voltage regulation. When specifying dynamic regulation, it is necessary to quantify the absolute change in the load, the rate of change, the definition of “recovery,” and the time to arrive at recovery. For example: “25% to 75% load change with a dI/dt of 0.1A/µs, with a 3% maximum deviation and recovery to 1% of the setpoint within 200ms.” The output voltage will decrease during an increasing current transition and increase during a decreasing current transition.

Dynamic response is as much a system design concern as it is a power supply design consideration. The impedances and decoupling design of the power distribution network have a major impact on dynamic regulation. For board-mounted dc/dc converters, dynamic regulation can be particularly important when powering large digital ICs such as FPGAs and microprocessors.

The output of switch-mode dc/dc converters contains both low-frequency (ripple) and high-frequency (noise) components, which are typically specified in mV peak-to-peak from 0 to 20 or 50 MHz. A typical specification for ripple and noise would be 75mV peak-to-peak for a 5V output.  The frequency of the ripple is related to the switching frequency of the converter. Noise is more variable and results from ringing in parasitic inductances caused by the high di/dt inherent in the operation of switch-mode converters. Noise occurs in bursts during the switching transitions and appears superimposed on the lower frequency ripple. A detailed review of electromagnetic compatibility considerations when using board-mounted dc/dc converters is the focus of the third FAQ in this series.

Protection Functions

Over current protection is intended to protect the converter from faults in the system, such as short circuits. There are three common methods to implement current limiting protection, maximum current limit, foldback current limit, and hiccup current limit. In maximum current limiting, load current is limited to a set not to exceed a maximum value. When that value is reached, the output voltage drops. Power dissipation in the dc/dc converter is usually higher in the current limiting stage than in normal operation. Foldback current limiting reduces the output current when a fault is detected. That results in a lower maximum power dissipation compared with maximum current limit. However, foldback current limit may provide less current at startup. As a result, the output rises slower, or the converter may not start up if the load current during start-up is larger than the foldback current limit supports.

When the current-sense circuit sees an over-current condition in hiccup current limiting, the dc/dc converter is shut down for a set period of time and then tries to start up again. If the over-load condition has been removed, the converter will start up and operate normally; otherwise, the controller will see another over-current condition and shut down, repeating the cycle. Hiccup operation eliminates the drawbacks of the other two over current protection methods. However, it is more complicated since it requires a timing circuit.

Overvoltage conditions on the output resulting from failure in the converter are usually clamped at a specific level. The clamping device typically fails in a shorted condition, preventing damage to the host system. Some dc/dc converters have an undervoltage lockout that shuts down the converter in a low input voltage. The converter is operated in “brownout mode” where the output power is limited to prevent excessive input current flows.

General Specifications

Numerous additional specifications may be important in specific applications, such as PMBus communications capability for converter configuration and monitoring. Remote on/off capability to control the power up and down sequencing of multiple converters or safety reasons. Remote sense capability can be important for some applications.

Most board-mounted dc/dc converters are non-isolated buck converters. Still, there are occasions when an isolated converter is needed, and the level of isolation voltage needs to be specified. The isolation capacitance can also be important; it measures the parasitic coupling between the transformer primary and secondary windings in an isolated converter.

This FAQ has provided an overview of several of the important specifications for board-mounted dc/dc converters. The next FAQ will take a look at thermal management considerations when using board-mounted dc/dc converters.

References:

DC-DC converter applications, RECOM
DC-DC converter data sheet terms and specifications, Calex
Selecting a power supply for system application, Helios Power Solutions