What size and type of output wires should I use?

There are two main considerations for sizing DC wiring from the output of a power supply to its load. They are ampacity (fancy term for the number of Amps) and voltage-drop (remember ohms law: V = I x R). Ampacity refers to a safe current carrying level as specified by safety organizations such as Underwriters Laboratories and the National Fire Prevention Association, which publishes the National Electric Code (NEC).

AWG stands for American Wire Gauge and defines the diameter and cross sectional area of the wire. The smaller the AWG number, the larger the diameter, cross-sectional area, and current carrying capacity of the wire. Always use insulated wires with solid or stranded pure copper conductors (do not use aluminum or copper-clad steel wires). The voltage-drop is simply the amount of voltage lost in a length of wire due to the resistance of the conductor.

DC wires may be sized for either ampacity or voltage drop depending on the wire length and conductor heating. In general, ampacity considerations will drive wire selection for short wire lengths (less than 50 feet) and voltage drop will drive wire selection for longer lengths (greater than 50 feet). Note: If you are using the Remote Sense feature of the power supply, remember to stay within the maximum voltage drop across the cables that the Remote Sense is designed to compensate for, which can range from 0.3V to 1.0V (check the power supply’s user-manual for details).

The National Electric Code table 310.16 provides ampacity values for various sizes, bundles, and insulation temperature rated wires. ALWAYS FOLLOW THE NEC RULES, LOCAL CODES, AND YOUR COMPANY’S PRACTICES WHEN SELECTING DC WIRING.

Table 1 shows the MINIMUM recommended wire sizes for different load currents. The use of larger diameter wires (with a smaller AWG number) would reduce the voltage drop (and heat generated) across the wires. The current ratings in Table 1 are based upon using 90° C rated insulated wire. If using a lower temperature rated insulated wire (e.g., 60° C), the wire diameter would need to be larger. Refer to the following web site for more information about wire gauges: http://en.wikipedia.org/wiki/American_wire_gauge .

For example, per Table 1 below, a load current of 200 Amps would require a minimum of two # 2 AWG wires connected in parallel for each of the output connections (one pair or wires for the positive (+) and one pair for negative (-) output connections to the load). Again, larger diameter wires would decrease the voltage drops across these wires.

Types of Distributed Power Architectures

Compact DC-DC converters have made their way into millions of electronic products and systems. The vast majority of these depend upon an AC front-end-box to convert the AC power source into a DC voltage from which the converters operate. In addition, international regulations have mandated that these front-end-boxes include Power Factor & Harmonic Correction (PFHC) to maximize the available power from the power grid.

Traditional Distributed Power Solutions

Traditional designs that employ distributed power architecture place DC-DC converters on PC boards very close to the point-of-load to maximize system speeds and efficiencies. To power the DC-DC converters, the required AC-DC power supply with PFHC is typically mounted somewhere in the system’s enclosure, external to the main pc-board (Figure 1).



This technique is quite reasonable for most applications. However, when it comes to equipment that must be mounted outdoors and occupy the smallest possible volume, there are now improved power products available.

Improved Power Distribution Methods

Typical medium power (400-700 watts) PCB mounted DC-DC converters are packaged in “full brick” sizes (e.g., 2.4” W x 4.6” L x 0.5” H). A number of major manufacturers of DC-DC converters have seen the need for, and are now providing AC input PFHC front ends in brick-formats that are PCB mountable near to the DC-DC converter(s). This has the advantage of placing all the power components on the same pc-board thus reducing the end products size and eliminating the power interconnect wires (Figure 2).


These AC-DC w/PFHC front-end bricks require some external components (capacitors, resistors, etc.), but the space required for these items is small in comparison to the elimination of the external “metal boxed AC front end”. And, these external components can be robotically inserted during the production of the pc-board. An added benefit of utilizing these brick packages is that they can be cooled without fans, by means of heat sinks or cold plates (e.g., mounting the brick bases against the system’s metal enclosure).

The Latest AC-DC Power “Brick” Solutions

Power Supply manufacturers have not stopped developing smaller and better power solutions. In fact, in recent times the AC/PFHC brick mentioned above has been merged with a DC-DC converter to form the ultimate power solution; an AC/PFHC/DC integrated brick. These 2-in-1 devices accept wide range 85 to 265 VAC inputs, correct the power factor, and provide the DC output(s) to the system. All this is accomplished within the same size constraints of a single “full brick” package measuring only 2.4” W x 4.6” L x 0.5” H, thus providing a 50% board space savings (Figure 3).

These integrated 2-in-1 pcb-mounted Power Bricks are ideal for Distributed Power Architectures where POL (Point of Load) Converters are needed. Since the 2-in-1 Power Bricks provide the conversion from AC to DC (with PFHC) along with the needed isolation, and the Intermediate Bus Voltage, the use of multiple low-cost, non-isolated POL converters becomes quite practical (Figure 4).

Recent advances in components and power design technologies have made these new
2-in-1 pcb-mount power bricks possible. In order to increase power densities, special Permalloy cores have been developed and employed in the inductors. New substrates and innovative transformer winding techniques have facilitated component height compressions and improved thermal management. And, of course, advances in integrated and hybrid circuits have contributed greatly to this next generation of power products.

Applications of 2-in-1 AC-DC Power Bricks

These new “2-in-1” AC-DC power bricks are ideal for many outdoor and indoor applications including:

• Custom Power Supplies
• PCB Mounted Bulk Power for Multiple DC-DC or POL Converters
• Large LED & Liquid Crystal Displays
• Traffic Information, Control, & Signaling Equipment
• Toll Devices
• Pico & Cell Phone Repeaters
• WiFi, Telecom Sub-Stations
• Underwater Surveying Devices
• Automatic Pass-Reading-Devices for FastTrac Car Lanes
• Oil Pumping & Pipeline Monitoring Devices
• Security Systems

New 2-in-1 AC-DC Power Bricks

Lambda, a unit of TDK Corp., is currently one of the manufacturers of a new range of integrated “single-brick” AC-DC power bricks. These “2-in-1” pcb-mount devices are so innovative, they have seven patents pending.

Some of the salient features of Lambda’s single-brick AC-DC PFE Series power modules include:

• Operates from Universal 85 to 265VAC, 47-63Hz Input
• Power Factor & Harmonic Correction Meets EN61000-3-2
• Low Profile, Single-Brick Footprint
• High Power Density (up to 129W/in3) & Efficiency (up to 90%)
• Regulated and Isolated DC Outputs with Wide Operating Temperatures (at baseplate)

• PFE500-12: 12VDC Output, 400 Watts, -40 to +85°C
• PFE500-28: 24 to 28VDC Output, 500 Watts, -40 to +100°C
• PFE500-48: 48VDC Output, 500 Watts, -40 to +100°C
• PFE700-48: 51VDC Output (semi-regulated), 714 Watts, -40 to +85°C

• ±20% Output Voltage Adjustment Range
• Over Voltage/Current/Temperature Protection
• Approved to UL/CSA/EN60950-1, CE Marked, & RoHS Compliant
• Optional Heatsinks & Evaluation Kits Available

Linear vs. Switch-mode Power Supplies

The Power Guy blog focuses on modern switch-mode power supplies and converters. However, to provide the newbie (newcomer) with some background information, we have included the following discussion.

Introduction
Linear power supplies were the mainstay of power conversion until the late 1970’s when the first commercial switch-mode became available. Now apart from very low power wall mount linear power supplies used for powering consumer items like cell phones and toys, switch-mode power supplies are dominant.

What are the differences and how do they work?
Linear power supplies have a bulky steel or iron laminated transformer. It provides a safety barrier between for the high voltage AC input and the low voltage DC output. The transformer also reduces and the AC input from typically 115V or 230VAC to a much lower voltage, perhaps around 30VAC. The lower voltage AC is then rectified by two or four diodes and smoothed into low voltage DC by large electrolytic capacitors. That low voltage DC is then regulated into the output voltage by dropping the difference in voltage across a transistor or IC (the shunt regulator).

Switch-mode supplies are a lot more complicated. The 115V or 230VAC voltage is rectified and smoothed by diodes and capacitors resulting in a high voltage DC. That DC is then converted into a safe, low voltage, high frequency (typically switching at 200kHz to 500kHz) voltage using a much smaller ferrite transformer and FETs or transistors. That voltage is then converted into the DC output voltage of choice by another set of diodes, capacitors and inductors. Corrections to the output voltage due to load or input changes are achieved by adjusting the pulse width of the high frequency waveform.

Comparisons of both technologies
Size: - A 50W linear power supply is typically 3 x 5 x 5.5”, whereas a 50W switch-mode can be as small as 3 x 5 x 1”. That’s a size reduction of 80%.

Weight: - A 50W linear weighs 4lbs; a corresponding switcher is 0.62 or less. As the power level increases, so does the weight. I personally remember a two-man lift needed for a 1000W linear.

Input Voltage Range: - A linear has a very limited input range requiring that the transformer taps be changed between different countries. Normally on the specification you will see 100/120/220/230/240VAC. This is because when the input voltage drops more than 10%, the DC voltage to the shunt regulator drops too low & the power supply cannot deliver the required output voltage. At input voltages greater than 10%, too much voltage is delivered to the regulator resulting in over heating. If a piece of equipment is tested in the US and shipped to Europe, or even to Mexico in some cases, the transformer “taps” have to be manually changed. Forget to set the taps? The power supply will most certainly blow the fuse, or may well be damaged.

Most switch-mode supplies can operate anywhere in the world (85 to 264VAC), from industrial areas in Japan to the outback of Australia without any adjustment. The switch-mode supply is also able to withstand small losses of AC power in the range of 10-20 milliseconds without affecting the outputs. A linear will not. No one will care if the AC goes missing for 1/100th of a second when charging your cell phone, it will take 100 of these interruptions to delay the charge by one second. However, having your computerized equipment shutdown or reboot 100 times a day will cause a great deal of heartburn.

Efficiency: - A linear power supply because of its design will normally operate at around 60% efficiency for 24V outputs, whereas a switch-mode is normally 80% or more. Efficiency is a measure of how much energy the power supply wastes. This has to be removed with fans or heatsinks from the system. For a 100W output linear, that waste would be 67W. A 100W switch-mode would be just 25W. Therefore, 67W – 25W = 42W is the extra power lost by a linear supply. Doesn’t sound much, but don’t try touching a 40W light bulb. If the equipment were running 24 hours a day, then the extra losses would be 367kW hours, at the current average cost of $0.10 per kW hour; that’s an extra $37 a year for a power supply that costs around $80.

As a quick note, in Europe, they are trying to limit those losses of all power supplies used by consumers particularly when operating in the “Off” mode (as many products are left plugged in 24 hours a day). Imagine 250 million power supplies eating up a couple watts. That equates to the output of a whole power station.

About Power Topics

The purpose of this blog is to provide end-users of AC-DC Power Supplies and DC-DC Converters with useful information regarding product applications, helpful hints, news, comments, and answers to your questions. My focus is on modern switch-mode power products in the range of 1 to 3,000-watts.

My goal is to provide OEM designers, who select power products for their end-products, and purchasing agents, who buy these devices, with a valuable resource to assist you in making decisions involving power solutions for your next and/or existing products.

I hope you find this website and blog informative and useful. You are invited to contact me with your questions and comments.

Thank You,
Power Guy