One factor contributing to the low parts count is the converter's unique integration of microcontroller and pulse-width modulation (PWM) controller functions. The microcontroller is Microchip's 8-bit PIC16C782, which contains an analog section for the PWM controller.
The low parts count also benefits the converter's mechanical design by allowing room for four hefty M3 inserts. These provide a rugged mechanical attachment of the converter to the customer's pc board. Al-though the Pisces II is designed to operate without external heatsinking, the inclusion of M3 inserts gives designers the option of attaching a baseplate and heatsink to the converter to reduce thermal impedance further. That enables operation at higher output currents or higher ambient temperatures. The company offers baseplates and heatsinks of various heights as options.
In contrast, some other open-frame quarter bricks don't allow the customer to attach an external heatsink. While through-hole packaging is standard for the Pisces II, surface-mount packaging is offered as an option—a feature not available on most competing quarter bricks.
Because dc losses are the main determinant of efficiency in Galaxy Power's converter design, the company strove to boost efficiency by adding MOSFETs and by reducing the dc resistance associated with the pc-board interconnect. In particular, the company aimed to eliminate solder joints, which don't add appreciable resistance on their own, but introduce resistance to the pc board where the copper foil trace necks down to the solder joint.
The goal of eliminating solder joints led to a key innovation, the development of a planar-type "split" output inductor. Based on a custom core design with a nonstandard geometry, the output inductor employs just a single turn of winding. As a result, the current passes through this winding without jumping layers. This eliminates losses associated with interlayer vias.
In general, a one-turn inductor has about one-fourth the resistance of a two-turn inductor, which is used in some converter designs. So, the inductor design was a key step in achieving 60 A in a quarter-brick form factor.
The total resistance on the dc-dc converter's secondary side—from the output pins through the inductor and around the transformer—is nearly 0.001 Ω excluding the MOSFET resistance. To make this approach possible, the company developed a technique for sharing a ground reference between the converter's synchronous-FET drive and the output voltage regulation circuit. This lets the converter pass external control signals through the inductor along with the power on the return side. As a result, the control signals are present on the output referenced to that ground. But they also are usable "inside" the inductor and referenced to that ground. The company has applied for a patent on this control technique.
In addition, the design employs a "split" transformer winding whereby the secondary is positioned on either side of a balanced primary to reduce leakage inductance and lower the ac resistance. The use of a balanced winding design also lowers capacitance in the transformer, producing low common-mode noise.
Moreover, the design contains an innovative primary side circuit that combines snubber and bias functions and captures the snubber energy for the control circuit. Galaxy Power is seeking a patent on this innovation too.
Other features include a two-stage input filter, remote sense, constant switching frequency, and protection against overtemperature, overvoltage, overcurrent, and over-or-under input voltage. Because these features are controlled by firmware and not hardware, the circuit behavior can be changed to suit customer needs without modifying the pc board.
In addition to Pisces II's initial 1.2- to 2.5-V, 60-A outputs and the 3.3-V, 50-A output, the company plans to introduce higher-voltage models, such as a 5-V output at 30 A and a 7-V output at 20 A. Galaxy Power also expects to develop versions with 24-V input. In these models, the 60-A rating will be lowered to 50 A, the 50-A rating to 40 A, and the 30 A to 25 A with no 7-V output offered.
Price & Availability
Pisces II will be sampling in September with production quantities expected early in the fourth quarter. The converter will cost $85 at OEM quantities.
Galaxy Power Inc., 155 Flanders Rd., Westborough, MA 01581; (508) 870-9775; www.galaxypwr.com.