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Using Optimal Component-Layout And Circuit-Design Techniques, This Device Eliminates The Need For Bulky Heatsinking.

Patrick Mannion

When it comes to power converters, the goal continues to be increasing efficiency to reduce both power consumption and the amount of heat in and around the device. To this end, many advances have been made--both in the components chosen and the methods employed to package them--that have given rise to dc-dc converters with typical efficiencies between 80 and 90% at output power levels of 5 V at 10 A.

But the move to sub-5-V outputs--with current requirements starting to exceed 30 A--has made it an almost Herculean effort to provide tightly regulated outputs over the required operating-temperature range. Combined with the ever-present user mantra of "smaller, lighter, less expensive, and more reliable supplies," it becomes apparent that every aspect of a dc-dc converter's design must be optimized for it to have any chance of meeting application demands.

It's within this context that SynQor, Hudson, Mass., introduces its PowerQor series of open-frame, isolated, dc-dc converters (Fig. 1). Using synchronous rectification (MOSFETs), instead of the typical Schottky diodes, the series' introductory 3.3-V version claims an efficiency of 90% with a 48-V input and a 3.3-V output--at 30 A. And it supplies full power over an ambient-temperature range of -40 to +55°C without the need for bulky heatsinks, which add cost and processing steps and often preclude the use of standard converters in space-constrained applications. Using suitable derating, 100°C operating temperatures can be reached. With a footprint of 2.3 by 2.4 in., a height of 0.4 in., an industry-standard pinout, and a weight of 53 g, the PowerQor converter has many advantages over the standard "brick" designs currently dominating the market.

1. The PowerQor is an open-frame, isolated, dc-dc converter that uses synchronous rectification and optimum thermal design to deliver 3.3 V at 30 A with an efficiency of 90%, thereby eliminating the need for heatsinking.

Since about the mid-'80s, these bricks have come in three standard sizes: full, half, and quarter brick. Typically, these converters comprise a pc board for the control circuitry and a metal substrate for the power circuit. Thermally conductive potting material facilitates the transfer of heat away from the board.

While these devices have been quite successful, the power components require an explicit thermal connection to the metal baseplate which, if compromised, could result in device failure. As a result, certain companies have taken to 100% X-ray inspection of all such connections. While this goes a long way toward ensuring reliability, it also adds to the cost--and there's always the chance of error. In addition, the heavy potting material adds unnecessary weight to a component already fighting for compatibility with surface-mount techniques.

Spread The Heat Around

In SynQor's open-frame approach, explicit thermal connections are not required. In addition, instead of replacing the Schottky diodes typically used in such designs with two large MOSFETs, the company uses six smaller, less expensive ones. As a result, dissipated heat is spread around the board. This leads to a greater flow of heat to ambient air, which is further aided by the board's turbulence-causing uneven surface. This turbulence greatly contributes to heat transfer, giving a thermal resistance of 3.9°C/W between the converter and ambient air blowing at a rate of 300 linear ft./min. (LFM). The laminar flow of a typically even baseplate design achieves just 4.3°C/W.

2. Even at the full rated current of 30 A, with an airflow of 300 linear ft./min. (LFM) and an ambient temperature of 55°C, the PowerQor keeps the maximum junction temperature within the 125°C specified (a). The board temperature also stays within specification at 115°C (b).

The end result is a converter with the temperature versus load-current curves shown, for both the semiconductor junction and pc board (Fig. 2a and b). The maximum output-power derating curves, in comparison to ambient-air temperature for airflow rates of zero through 400 LFM, also are shown (Fig. 3). The curves demonstrate the maximum output power the converter can deliver while keeping its hottest semiconductor-junction temperature below 125°C and the hottest area of the pc board below 115°C. A key point to remember is that the open-frame design means that the direction of airflow is not important. This is in contrast to heatsink-based designs, where the airflow must be parallel to the direction of the fins.

3. The maximum output-power derating curves for airflow rates of 0 to 400 LFM show that the full rated power is available right up to 55°C at 300 LFM. Almost 60°C can be reached at 400 LFM.

Patented Circuit Design

Besides the optimal thermal layout of components, the device's ability to deliver full rated power at ambient temperatures exceeding 55°C also can be attributed to a circuit design that endows the device with its efficiency rating of 90%. While the particulars of the circuit are awaiting a patent, available details indicate that some variation of a forward-converter topology is used with soft switching. The switching takes advantage of resonant rings to nearly losslessly recover the energy associated with the parasitic inductors and capacitors. This is a technique commonly used in such devices.

The first stage is a buck converter, followed by a transformer stage to provide the functions of input/output isolation and voltage step-down. The isolation is rated at 2000 V, 10 M. Both stages switch at a fixed frequency of 200 kHz. The six low-loss synchronous rectifiers are then employed to rectify the transformer's output. While synchronous rectifiers have been around for a long time, the last three years have seen their on-resistance drop by a factor of five. Combined with falling costs and lower load voltages, this makes them an appealing alternative to lossy Schottky diodes for low-voltage outputs at high currents.

Thanks to the proprietary circuit design, low-loss components, and special thermal consideration given to the layout, the device achieves its efficiency rating of 90%, a good 5 to 10% above that of such devices available to date. This additional 10% means that the devices dissipate half the power, thereby eliminating the need for a heatsink and its associated cost, bulk, and process steps. A graph of efficiency versus load current for minimum, nominal, and maximum input voltages with 25°C air flowing at 300 LFM is shown (Fig. 4). As predicted, the efficiency remains at 90% right up to the rated current of 30 A.

4. This graph of efficiency versus load current for nominal, minimum, and maximum rated input voltages at 25°C ambient and 300 LFM shows the device performing at the predicted efficiency of 90%.

Control And Protection

A slew of control and protection features add to the PowerQor's efficiency and thermal-management capabilities. Chief among the control inputs are an on/off input for sequencing and power reduction, positive and negative remote sense for accurate load-voltage maintenance, and an output-voltage trim (+10%, -20%). Protection features include input undervoltage lockout, output current limiting to 110% of the rated output current, output overvoltage limiting, and input overvoltage shutdown. The converter also comes with a temperature sensor located in the center of the pc board. The sensor sends a signal to shut down the converter under abnormal temperature conditions. Once the converter has cooled, the sensor turns back on--without having to recycle the input power.

Key Specifications

The PowerQor operates over an input-voltage range of 36 to 75 V, with a maximum input current of 3.5 A (at 100% load). From this, it generates a nominal output voltage of 3.30 V with a minimum value of 3.27 V, a maximum value of 3.33 V, and a typical line and load regulation of ±2 mV. The output-voltage ripple and noise is typically 15 mV p-p, with a maximum value of 30 mV. The output current ranges from 0 to 30 A. Other parameters include an output settling time of 300 µs to within 1%, an off-converter input current of 1.6 to 3 mA, and an inrush-current transient rating of 0.001 I²t.

Price And Availability

The PowerQor is available now for $121 each in 1000-unit quantities. Delivery is about four weeks.

SynQor Corp., 188 Central, Hudson, MA 01749; Renee Catalano (978) 567-9596; fax (978) 567-9697; http://www.synqor.com.

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