60-A Quarter-Brick DC-DC Converter Keeps Its Cool

Aug. 19, 2002
High efficiency extends the power and current available from this standard package style, while easing thermal management.

With the migration from Schottky Rectifiers to Synchronous Rectification a few years ago, dc-dc converters achieved dramatic improvements in efficiency. As an example, consider a 30-A telecom-style dc-dc converter designed for the popular 3.3-V supply voltage. When this type of converter moved from Schottky-based rectification to synchronous rectification using MOSFETs, efficiency rose from 80% to about 86%. Meanwhile, the converter's power losses fell from 25 to 16 W, a 36% decrease.

Since power-supply manufacturers adopted synchronous rectification on a wide scale, the gains in efficiency have been more modest. Unfortunately, even these improvements in efficiency have been offset by the shift among users to lower output voltages, which tends to hurt efficiency.

That's because the converter's power loss doesn't drop proportionately to the output voltage, although the output power does. So as electronic systems continue to incorporate 2.5-, 1.8-, 1.5-, and even 1.2-V supplies, dc-dc converter vendors are being challenged to improve their power conversion efficiencies and meet demands for greater power and current density in their dc-dc converters.

A dc-dc converter developed by Galaxy Power is making great strides toward this goal. It provides a leap ahead in efficiency that's on par with what was achieved by the switch to synchronous rectifiers. The Pisces II is a quarter-brick dc-dc converter, rated for 60 A of output at 1.2, 1.5, 1.8, or 2.5 V (see the table). Additionally, it's offered with a 3.3-V output that's rated for 50 A. The converter features an open-frame construction and an industry-standard pinout and footprint, and it operates from the usual 48-V nominal input. Package height is 0.4 in.

The 60-A rating on the Pisces II places it well above the current crop of quarter bricks, which are typically rated for 30 or 40 A of output. Although one competitive unit is rated as high as 45 A, its high power losses require significant derating of the device over temperature, limiting its usability in applications with high ambients.

While the high current rating is significant, even more important is the manner in which it was achieved—by greatly boosting the converter's efficiency. For example, at a 2.5-V output, efficiency ranges from about 91.5% at full load to 94% at half load (Fig. 1). In contrast, the closest competitor with respect to efficiency is a 40-A quarter brick that achieves efficiencies ranging from 88% at full load to 91% at half load.

As can be seen in the graph, power efficiency varies according to input voltage with performance decreasing as the input voltage is raised. The input voltage values shown in the graph (40 and 60 V) indicate the input voltage range over which the dc-dc converter may be expected to operate continuously in telecom applications, rather than the full 36- to 75-V range for which the device must be specified.

The impact of several percentage points of gain in efficiency can be seen more clearly when stated in terms of power losses. At 2.5-V and 40-A output, the Pisces II dissipates about 7 W of power versus a 13.5-W loss for the 40-A quarter brick previously mentioned (Fig. 2). So at the 40-A output, Galaxy Power's quarter brick cuts power losses (or heat generation) by 48%. Pisces II also achieves similar percentage reductions in power losses at output voltages other than 2.5 V. That 48% improvement is better than the reduction in losses associated with the switch to synchronous rectification at 3.3 V.

Higher efficiency translates directly into more usable output power and current for a given ambient temperature and airflow. As a result, the Pisces II requires less cooling for a given output power and suffers less derating over-temperature than existing quarter bricks. When employing forced air cooling at 200 LFM, the converter can deliver its full rated current at ambient temperatures of as high as 60°C. If the output is reduced to 40 A, the converter will operate at up to 50°C with no moving air.

To achieve its high efficiency, Galaxy Power's quarter brick uses a total of 14 MOSFETs, four on the primary side and 10 on the secondary. By increasing the number of power semiconductors over what's typically seen in a quarter-brick dc-dc converter, the company lowered the total on-resistance of these components. Also, generated losses are spread over more devices, resulting in a lower temperature rise for the power components. In Galaxy Power's converter design, on-resistance dominates over switching losses because it em-ploys zero-voltage switching on the secondary with a relatively low switching frequency.

Consequently, the efficiency of the MOSFETs largely depends on their on-resistance. So adding more MOSFETs boosts efficiency, while reducing their temperature rise, which expands their operating temperature range.

Adding MOSFETs to the converter's pc board is made possible by a design that limits the converter's overall parts count and frees up board space. The Pisces II pc board contains approximately 115 components. Competitive units may have anywhere from 50% to 150% more parts.

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.


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