Thermal performance
Amid the growing demand for power solutions, the consolidation of suppliers, and the restructuring of the telecom, datacom, and computer markets, dc-dc converters have undergone a period of technological innovation. Today’s dc-dc converter, which is the heart of the modular power supply, offers greater output voltage flexibility, higher power density, more effective current sharing, and more thermal and noise management options than ever.
OUTPUT VOLTAGE FLEXIBILITY
Most dc-dc converter manufacturers offer a wide variety of standard output voltages. Some systems or applications, however, need voltages outside the standard offering.
Many converters offer a trim range (or program range) to the standard dc-dc converter. This trim range varies by manufacturer, but typical trim ranges include ±10%, –25% to +10%, and –90% to +10%. If a converter has a set point of 48 V and a trim range of ±10% the output can be adjusted from 43.2 to 52.8 V. If another converter with the same set point has a trim range of –90% to +10%, the output can be adjusted from 4.8 to 52.8 V. In many cases, however, care must be taken not to trim the module above its overvoltage protection set point. When trimming the module down, the power is diminished because the maximum current is fixed.
In addition to creating non-standard output voltages, the trim range of the converter can be used to overcome voltage drops in the system and provide design flexibility in various applications. If a system has a wide range of variability within the same application such as a battery charger, the trim range of the dc-dc converter can be used to create a universal supply that can operate over a wide range of output voltages. The charger could be designed to charge a 12-V battery system as well as a 24- or 48-V battery system.
This would minimize the number of models that would have to be stocked. The same concept could be used if the supply was powering a varying string of LEDs. A wide output voltage range is very useful when creating a controlled current source as in the applications mentioned above.
The output of the dc-dc converters (Fig. 1) can be trimmed using fixed resistor methods or dynamically with variable resistors or external voltage sources. Fixed and variable resistor trimming methods use very few components, and the values can be determined easily by calculation or by using online calculators provided by the converter manufacturer.
Trimming with an external voltage source introduces more design issues to consider. The converter will have a maximum rate of change on the output voltage. The response time of the converter, the ripple current through the output capacitors, and converter stability are a few items that will limit the rate of change of the output voltage. Some converters are limited to a 30-Hz rate of change for the output voltage.
HIGH POWER DENSITY
As systems become smaller and more “feature” dense, the demands on the power supply grow. The supply has to produce enough power for the system, but the real estate allocated for the supply is shrinking. A smaller supply reduces both board size and system cost. Taking advantage of higher operating frequencies, advanced magnetic design, and silicon integration, dc-dc converters have contributed to more power-dense components, with more than 1000 W per cubic inch.
The operating frequency of the dc-dc converters is also higher, approaching several megahertz. This higher frequency allows the magnetic components that are used in the power train, the “muscle” of the converter, to become smaller. Within dc-dc converters, the power train takes up over 75% of the total board real estate. Additionally, the higher operating frequency of the converter allows for advancements in magnetic design.
The typical magnetic design would consist of a bobbin, a magnet wire, and a core made of a ferrite material. The use of planar magnetics eliminates the need for magnetic wire and bobbins. Printed-circuit board (PCB) traces replace the magnet wire, and the core is attached through the board, eliminating the bobbin. In cases where bobbins and magnetic wire are still used, the magnetic shapes and materials are configured to handle greater flux densities and produce fewer losses.
The control circuitry of the converter, which can be considered the “brains,” also has gone through development to increase power densities. In early dc-dc converters, the control circuitry used up to 50% of the board real estate within the converter. Now with the use of silicon integration and custom ASICs, the control circuitry can be condensed, providing more room for the power train. The ASIC now controls the power train and can also provide more converter features for the designer while using less real estate.
CURRENT SHARING
Power supplies built with modular power components such as dc-dc converters and modular input and output filters offer a high level of design flexibility, and the supply design cycle is shortened over discrete designs. In many cases, multiple converters are needed to provide enough system power or if redundancy is needed in the system.
In such cases, the dc-dc converters have to work together and share the load for expanded power or share current for redundancy. When sharing for redundancy, each converter can handle the entire load. But when they’re power sharing, the converters will run at lower temperatures and will typically increase the mean time before failure (MTBF) of the power system.
If the outputs of the dc-dc converters are just placed in parallel, the converter that has the higher set point will source the entire load current and the other will sit idle. Current sharing in a redundant system will also reduce the transient response when one converter has to transition to handle the full load.
There are two primary methods of current sharing, external current sharing and internal current sharing, where the converter provides the current-sharing control. In external current sharing, a control circuit measures the current delivered by each of the parallel converters and then adjusts the outputs of the converters to match their current delivery. This implies that the converter must have a means of adjusting the output voltage. External current sharing uses valuable board space and can become complicated when paralleling more than two converters.
To ease the designer’s task of current sharing, dc-dc converters are incorporating internal current-sharing methods (Fig. 2). The sharing methods are becoming as easy as connecting two pins on the converters. In many cases, care must be taken with routing the current-sharing signal to avoid miscommunication between the dc-dc converters.
Some single-pin current-sharing methods also allow for board-to-board current sharing. This is good when power supplies need to be hot-swapped in the application. The single-wire current-sharing technique is very advantageous to the power designer because it uses minimal space and components. It also allows current sharing for redundancy and power expansion.
THERMAL MANAGEMENT
Since the dc-dc converters aren’t 100% efficient, components within the converter will generate heat. They also must be cooled below their maximum operating temperatures. Within the converter, the major heat-generating components are either attached to the converter’s external base plate or attached to other heat-dissipating surfaces within the converter, which is the case with open-frame converters.
The typical maximum temperatures are 100°C with converters that have base plates and an 85°C ambient temperature with open-frame converters in a given air flow. Both constructions offer a variety of cooling options, but in either case, the higher the efficiency, the easier the thermal management.
Many open-frame converters are designed so any airflow direction will provide the same thermal results. Adding airflow or mounting the supply in the airflow supplied by the system can cool the open-frame converters. Some converters also have a thermal path down to the PCB. This allows the designer to use the copper within the PCB to help cool the major heat generating components.
Also, dc-dc converters with base plates provide a wider variety of cooling options. Many converter manufacturers offer heatsinks that can be placed on the converter. These heatsinks have been characterized for their thermal performance.
Available charts provide thermal impedance numbers and thermal performance (Fig. 3) in given ambient temperatures at various power levels. The base plate also can be mounted to a larger heat-dissipating surface such as system chassis or a large heatsink that can cool multiple dc-dc converters. Fans can work in conjunction with heatsinks to aid in cooling.