Increasing complexity and performance of the latest FPGA’s, processors and ASICS and associated memory demands are resulting in ever increasing system power needs while real estate available to place power solutions continues to shrink. This increasing power density also elevates the importance of system thermal management.
By Mike Michael Althar, Jian Wang Yin and Steve Rivet, Intersil Corporation
In response to these trends, new flexible high density power modules are being developed that provide advanced electrical and thermal performance in an easy to use integrated package to meet these challenges.
As system performance increases drive increased power requirements, and time to revenue pressures push designers to cut development schedules, the need for full functioned, quick to implement power solutions has exploded. Power modules are providing those solutions and with the flexibility and performance system designers need to get maximum power out of minimum space. Power module options range from simple co-packs of controllers, FETs and a capacitor or inductor to full power solutions which include all of the IC components as well as virtually all of the discrete and magnetic components required to implement a full power supply in a single package; these devices are available both in open frame or fully encapsulated module form factors. Modules are also available with either analog or digital control loop; analog modules provide extreme ease of use while digital modules such as the Intersil ZL9117 can provide added benefits of telemetry, auto-compensation, and in-situ programmability. This article will focus on the latest in high power, simple to use analog power modules such as the 30 amp ISL8225 newly introduced by Intersil Corporation.
One of the greatest challenges inherent in high power (greater than 100W) module design is thermal management. Footprint and ultimate power rating, especially at high ambient temperature, are dictated by the module’s electrical and mechanical design. This challenge is addressed by the design of the controller, inductor, and switching FETs, which contribute to high efficiency and low power loss, and the use of thermally efficient packages that can move the heat out of the module. In a highly integrated device such as a power module, the electrical and mechanical design is highly interdependent.
Inductor design is a key piece of the electrical/mechanical design task. Commercially available inductors need to be large to achieve high overall efficiency and maintain reasonable operating temperature. However, many applications have limited space with high ambient temperature and need a very small, low loss inductor; if forced to use an undersized inductor due to space limitations, the designer has to live with limited output current, heat crowding/hot spots and/or poor efficiency.
ISL8225M addresses these issues by utilizing a 3-D stackable inductor structure. In this structure, a large inductor, almost the as large as the entire module footprint can be used; this inductor is installed over the other components. This technique doubles the available area for placing other components on the substrate vs. a side by side mounting method, at the expense of a growth in package height. The larger inductor can have a very small Direct Current Resistance (DCR), which reduces the circuit conduction loss. Additionally, a large inductance can be used to reduce the current ripple, thus reducing the inductor core loss. The MOSFET switching loss can be decreased as well since the switching frequency can be lowered with the large inductance. The 3-D structure can reduce the overall power loss in the power supply and achieve a solution with high efficiency, higher power density and better thermal performance.
ISL8225M is a flexible device, which can be used in dual output mode, or a high-current single output. In the dual output mode, the ISL8225 provides independent outputs to supply two separate voltages with no cross-talk. The ISL8225M 2-phase inductor is a proprietary design utilizing non coupled windings on a single core. In the single output mode, the novel 2-phase inductor can partially cancel the magnetic flux of the two windings, thus reducing the inductor core loss and improving efficiency. Figure 1 shows the efficiency advantage of this structure vs. windings on separate cores with the same DCR. The design of the ISL8225 inductor reduces both the footprint and power loss vs. traditional design.
Many power modules on the market today use materials such as FR4 insulating laminates for mounting the module components. While these insulating laminates / LGA form factors provide ease of routing of signal traces of the components within the module, their ability to dissipate heat is limited due to the high thermal resistivity of the FR4 laminate and the limited conduction area of the copper vias that run from the components to the external pads of the package. Conversely the leading high power modules such as those offered by Intersil utilize a QFN style leadframe where the power devices are mounted directly to a copper leadframe, offering both a low thermal resistivity and very large conduction surface area to allow for efficient heat transfer. The difference in these packaging technologies is obvious when comparing high ambient temperature derating characteristics of the devices manufactured with each construction method. The improved thermal efficiency inherent to packages utilizing the copper leadframe package enables these devices to operate at rated output power levels to a much higher ambient temperature without derating versus similar laminate-based modules.
Lack of heat sink/airflow requirements allow flexibility in placing solutions where needed or where space is available instead of where the air flow is available. Conversely, having airflow available allows these modules to operate in either a higher ambient temperature environment or at a higher maximum operating power.
An added benefit of the QFN style packaging over the LGA form factor and footprint is that all of the signal leads come out to the edge of the package vs the “hidden” land patterns under the device with an LGA. The benefit of lead access at the edge of the package is it enables both visual inspection of all solder joints, removing the need for x-ray inspection of the joints while also providing test probe access for initial system analysis & board debug.
Encapsulated modules are significantly more mechanically rugged than open frame modules and can be handled with auto pick and place equipment; in many cases this can eliminate a manual placement step needed with through hole open frame modules.
Flexibility to utilize similar power solutions without having to implement a major design overhaul can significantly reduce a system designer’s development time. Cascading multiple modules can provide higher power but does it come at significant design complexity? The latest generation controllers that are utilized in the ISL8225M allow for cascading of up to 6 modules for a total of 180A output with minimal system design changes. Clock synchronization between devices is automatically handled by tying the clockout (CLKOUT) of the master device to the sync pins of the slave devices; the integrated controller handles clock synchronization and automatically adjusts the phase relationship between outputs to produce fully interleaved multiphase operation. With two phases per module the auto phase shift can handle up to 12 phases (6 modules) with all being offset by 30 degrees; this significantly reduces output ripple and instantaneous power loads on the input supply. Connecting the current share (ishare) pins of each of the modules also accomplishes automatic current sharing via a proprietary algorithm resulting in all of the cascaded modules output current being within +/-10% of each other, minimizing transient spikes during load transitions and maximizing thermal uniformity across the board. The result of this balance can be shown in fig 3 on a three module evaluation board running at 100A total load with the modules all operating within 3 degrees of each other.
The uniform heat dissipation and minimal external components allows for modules to be placed in relatively close proximity to each other. This is accomplished with no additional circuitry to manage current sharing, clock synchronization or phase shifting!
ISL8225M utilizes a voltage-mode loop to regulate the output voltage, with a special current loop to balance the current between phases and achieve accurate current sharing. Unlike most voltage-mode controlled power supplies, the current loop of ISL8225M provides the flexibility to parallel the outputs in different combinations by connecting the ISHARE pins of each module together. The ISL8225M can be easily programmed to realize phase interleaving, reducing input/output ripple and filter requirements. In figure 2, a 2-module design is shown to highlight the flexibility available to system designers. The CLK signal of the 2nd module can synchronize to the 90° shifted CLK signal from the 1st module; the internal 2 phases of each ISL8225M module operate with 180° phases shift. In this configuration, all four phases are evenly interleaved. ISL8225M also provides the flexibility to combine the outputs to supply more current for certain rails, to meet a wide variety of design requirements. Two ISL8225M can provide 4 rails with each at 15A or 1 rail at 60A; two ISL8225M modules can also be programmed to provide 3 rails (30A, 15A, 15A), 2 rails (30A, 30A), or 2 rails (45A, 15A). The ISL8225M makes the design the complicated system power supplies very straightforward.
The flexibility, electrical and thermal performance, and high level of integration all contribute to ease of use, enabling board designer who are not power experts to design dense, high performance power supplies. Intersil also provides several design tools to support board designers using our power modules. iSim is Intersil’s design simulation tool that is available to be run from the www.intersil.com web site, or that can be downloaded and run on customers’ PCs. The iSim tool also includes an extremely simple auto design interface that enable board designers to generate a schematic, BOM, and simulations by specifying only Vin, Vout, and Iout. Evaluation board that demonstrate typical single output, dual output, and three cascaded module operation are also available; schematic, BOM, and layout files for each of these boards can be download and can be used as reference designs.
In summary, power modules can provide a dense, easy to use solution for high performance power conversion needs. High levels of integration, and a suite of easy to use design tools dramatically increase the rates of first pass power design success, and enable quick board designs and fast time to revenue.