Isolated POL
In a departure from most conventional non-isolated POLs, Vicor’s Picor Division has introduced the PI3106, an isolated POL converter with a high-efficiency, soft-switching power architecture. This topology allows the PI3106 to operate in distributed power architectures without requiring isolated power inputs (Fig. 4).
The PI3106’s 16-V to 50-V dc input range and 334-W/in.3 power density enable it to deliver ~50 W (12 VOUT at 4.2 A). Housed in a space-saving 0.87- by 0.65- by 0.265-in. surface-mount (SMT) package, it reduces space by about 50% compared to traditional converter topologies.
A 900-kHz switching frequency allows the PI3106 to use small input and output filter components, further reducing the total system size and cost. Its output voltage is sensed and fed back to an internal controller using a proprietary isolated magnetic feedback scheme that allows high bandwidth and good common-mode noise immunity. It requires no external feedback compensation.
The PI3106 feature set includes output voltage trim capability, output overvoltage protection, adjustable soft-start, overcurrent protection with auto-restart, and over and under input voltage lockout. Also, its temperature monitoring and protection function provides an analog voltage proportional to the die temperature as shutdown and alarm capabilities.
Another type of dc-dc converter developed by CUI Inc. employs the Solus Power Topology, which combines a single-ended primary-inductor converter (SEPIC) with a buck converter to form a SEPIC-fed buck converter. Increased efficiency is accomplished by reducing both the conduction and switching losses at several critical points within the converter circuit.
The loss reduction is so significant that it can increase the output current by 40% for a given power supply package size. Conversely, the loss reduction enables increased efficiency by several percent for a given output current and package size compared to the traditional buck topology.
The Solus topology includes one magnetic component, one control switch, and two commutation switches that are optimally PWM-controlled. The magnetic component consists of four inductively coupled inductors wound on the same core. This translates to a level simplicity on par with a traditional buck converter.
Also, Solus reduces I2R losses by channeling the operating currents into multiple paths. When the input current enters the converter, the topology branches that input into multiple paths, with each path carrying a lower current. This reduces conduction losses to a level that is significantly less than the losses within a standard buck converter.
Multi-current paths also reduce the voltage stresses on components by nearly 50%. With lower applied currents and voltages, the Solus topology reduces high-side MOSFET turn-on losses by better than 75% compared to the traditional buck.
The first product based on the Solus topology is CUI’s NQB series isolated dc-dc quarter brick (Fig. 5). This 720-W intermediate bus converter will initially support an input range of 36 to 60 V with a 12-V output and efficiency greater than 96%.
Brick Converters
Brick converters are a defined physical size, but their efficiency can depend on whether power MOSFETs, gallium-nitride (GaN) transistors, or silicon-carbide (SiC) transistors are in the output stage (see the table). All eighth bricks will have very similar maximum power loss numbers, between 12 and 14 W, at full power as the amount of heat that can be removed from the module is purely size dependent.
A typical eighth-brick converter that is 90% efficient at full load will have a maximum output power of no more than 125 W (assuming 14-W loss). Improving the efficiency by just 2% increases the available output power to 160 W, a 28% improvement.
It’s possible to reduce the power loss in the magnetic components (up to a point) by increasing their operating frequency. However, this isn’t normally done because the increase in frequency-dependent semiconductor losses outweighs that potential improvement. Usually, the operating frequency is reduced to maximize the magnetic structure size within the brick’s physical size constraints.
It’s difficult to compare an isolated brick converter using power MOSFETs with one using a GaN FET. Even when limiting the comparison to a regulated 12-V output for an eighth brick, there are still variations between commercial designs. The resultant structure, layout, and topology may differ at the same power level. Efficiency achieved in a specific brick converter, as good as it may be, can be improved easily simply by allowing the converter to increase in size.
Johan Strydom and others at Efficient Power Conversion (EPC) have designed and built a 48- to 12-V enhancement-mode GaN (eGaN) FET-based eighth-brick converter. It uses a phase-shifted full-bridge converter with a full-bridge synchronous rectifier topology.
EPC’s goal was to show that, due to their relatively small device size, a significant number of eGaN FETs can be used within the restrictive eighth-brick size limitations. The choice of transformer turns ratio (6:3) meant that, at 75 VIN, the secondary side winding voltage would be 38 V, which would be too close for 40-V devices. Therefore, 100-V devices were used on both the primary the secondary sides of the output stage.
Strydom said the eGaN FET-based converter that was developed isn’t necessarily an optimal solution. Its design goal was to deliberately push the operating frequency much higher than current commercial systems to show that eGaN devices can produce a more efficient power supply design.
The prototype eGaN FET-based fully regulated eighth-brick converter was compared with a similar MOSFET-based converter. The eGaN FET version showed improved efficiency and 15% more output power at a 33% higher switching frequency.
Design Support
The design of dc-dc converters can get an assist from Maxim’s MAX15301 controller IC, which uses digital power technology to automatically compensate the converter (Fig. 6). It accomplishes this by constructing an internal mathematical model of the supply, including its external components.
The result is a switching power supply design that achieves excellent dynamic performance with guaranteed stability. Furthermore, this power supply model optimizes a converter’s efficiency across a wide range of operating conditions.
This IC relies on mixed-signal design techniques to control the power system efficiently and precisely. It does not require any software to configure or initialize the device. The MAX15301 can regulate and perform power management tasks without any programming. Using standard PMBus commands, its functions can be monitored and optimized through an SMBus interface, resulting in ease of design and flexibility.
You could also use Powervation’s PV3012 controller IC for a dc-dc converter design. This dual-phase digital synchronous buck controller IC for POL design applications is PMBus- and DOSA-compliant and provides features to improve a converter design’s efficiency and reliability.
The PV3012’s Auto-Control technology offers real-time adaptive loop compensation for converter designs. This patented digital control loop technology optimizes the tradeoffs between dynamic performance and system stability on a cycle-by-cycle basis without requiring any noise injection or other drawbacks of part-time measurement techniques. This is a key advantage for converter designs that drive imprecise or variable loads.
Auto-Control also compensates for the power supply component parameter drift that occurs over temperature and time, as well as for tolerances seen in production or component level variation. It relieves power supply designers of the burden of compensation and plant characterization, while reducing total design iterations as well. Furthermore, Auto-Control readily enables efficiency maximization mode changes such as phase add/drop and light-load modes, again, without sacrificing transient performance and cost.
Powervation joined forces with Murata Power Solutions to co-develop a reference board for the Murata Power Solutions 45-A Power Block. Murata’s CEB019 digital dc-dc converter uses Powervation’s PV3012 to overcome issues related to variations in external components, temperature, and user layouts (Fig. 7).