Tustin, Calif.—Embracing a fresh new approach to front-end rectifier design, Cherokee International proclaims its newly released CAR2500 as a breakthrough for powering next generation, high-density intermediate and distributed power architectures. Indeed, the CAR2500, combined with existing DC/DC converter offerings, claims unsurpassed performance for platforms that will ultimately extend from 0.5U to 3U—a design paradigm that OEMs would do well to take a closer look at, says Mike Wagner, vice president of marketing.
"We've put in an effort to doubling the power density every two years or so," he says. It's at the core of the company's multi-pronged effort to establish a building-block approach to secure higher power capability with higher density and efficiency, all without sacrificing time-to-market. Indeed, the company introduced the CAR 548 in 2002, a 1U, 500-watt product providing 10 watts per cubic inch, and the CAR1248 in 2004, the company's 1U, 48/-54/12 volt 1200-watt supply (19 watts per cubic inch). Cherokee's CAR2500 provides about 500 watts more power in a 1U than any previously announced front-end. "So now we're offering the CAR2500 platform at a power density of 27 watts per cubic inch. And if you can imagine you'd fit four of those units in a shelf, it's about 10,000 watts, or 7,500 watts in an N+1 redundant system, a lot of power in a 1U space," says Wagner.
But achieving the design goals has been anything but automatic, he stresses. To that end, the company pursues a "part analog, part digital" approach to maximize the flexibility of its front-end design (i.e., micro controller-based, and analog feedback loop). It's the best way to go, says Wagner, given the lack of a cost-effective paradigm to optimize the magnetics of a true digital supply and the questionable advantages of using a digital feedback loop.
Cherokee's real-estate objective cuts to the central issue (although perhaps not recognized by the casual observer): Improving the power budget. Saving system space is a strategy that certainly makes room for various system infrastructure including communications boardware, and being able to do more via system software. But more than that, it makes for maximizing front-end efficiency, where losses in power server farms, for instance, will otherwise be significant. On the other hand, how do you maximize capacity, density, and efficiency at the same time?
"There were two main design challenges for this product: Maximizing efficiency/reducing losses at each stage of the design, and optimizing the thermal design in a very small volume ensuring that we offer a reliable design to market," said Wagner. Cherokee deftly approached the problem by examining power losses at every possible point to arrive at a series of solution steps that, together, fit like a jigsaw puzzle with none of the pieces gone. Starting from the input, the company developed a proprietary PFC stage design based on zero voltage switching (ZVS) technology, with the technique delivering a 1 percent boost in overall efficiency. The second challenge was selecting a suitable topology and finding how to apply it to this particular design. Cherokee selected a ZVS full bridge coupled with a patent pending circuit used to recover energy normally dissipated on the secondary side of the circuit, thus securing an additional 1 percent gain in efficiency. "Both contribute to achieving an efficiency of 92 percent at 48 VDC and 91 percent at 12 VDC for a 27W/in3 design without sacrificing holdup time, a game played by some in our industry," notes Wagner.
Gaining additional leverage, the design also optimizes the switching efficiency of the topology to minimize magnetics and switching and conduction losses, all while reducing the physical size of the front-end's transformers and output choke by about 35 percent. That in turn minimizes core and copper losses. The end result is another 1 percent gain in efficiency. Contributing to higher efficiency is the use of a special clamp circuit (patent pending) used on the secondary side that works in conjunction with Schottky diodes in the 48 and -54 volt supplies. On the other hand, the company's 12-volt version uses synchronous rectification to minimize losses on the secondary side. Both schemes are designed to recover more than half the energy dissipated during the turn-off time. Ultimately, this energy-recovery technique boosts overall efficiency by another 2 percent.
Thermal management in such a small space also becomes as important as maximizing efficiency, a problem that's essentially four times the challenge when it comes to supporting 10 KW (versus 2,500 watts) in a 1U shelf. Component size and physical placement becomes an even greater issue to ensure maximum air flow through the unit to optimize thermal management strategies. Perhaps the most difficult challenge is thermal management for the 12-volt unit, explains Wagner, where peak current flow in the output stage exceeds 200 amps. That represents a power-loss factor (i.e., I2R losses) of four compared to a supply with a 48-volt output. To that end, the proprietary ZVS full bridge is implemented with a innovative "current doubler" design in the secondary that incorporates multiple secondaries in parallel. This technique, which allows use of common cost-effective materials, secures a uniform distribution of current. The resulting scalable topology, explains Wagner, facilitates balanced distribution of power transfer to the secondary circuit (implying automatic load sharing); in addition, heat losses are uniformly distributed (no hot spots, supported by thermal simulations). Indeed, says Wagner, competitors' supplies run 25°C hotter.
So, the end result, he explains, results in much higher power capability than competing products for a given input, yet cuts down on the cost of custom magnetics and still works at significantly higher efficiency. Despite the increased power density, the front-end's hold-up time parameter, adds Wagner, is not compromised in achieving the efficiency results, and the front-end offers the end user sufficient hold-up for applications that do not support battery backup.
The company's next step, perhaps a year away, includes expansion of its high-density, high-efficiency approach across the full spectrum from 0.5U up through 3U designs as well as developing supplies with vastly greater capabilities for dynamic power management.