An alternative to isolated DC-DC converters

Traditionally when several voltages (5V to 24V) are required in a system, either a multiple output power supply is used or a single output “bulk” supply with isolated DC-DC converters.  For voltages lower than 5V (0.6 to 3.3V) the electronics industry has migrated to using multiple non-isolated DC-DC converters, often referred to Point of Load or POLs to drive FPGAs powered from a bus voltage between 5V to 12V.

With low power (typically less than 300W) dual, triple or quad requirements in the standard voltages of 5V, 12V, 15V and 24V, a single AC-DC power supply is used.  These are cost effective and readily available.

For medium power requirements (350W to 1500W), often the choice is to use a modular power supply like TDK-Lambda’s NV, Vega or Alpha series.  As the term “modular” implies, they are put together using pre-assembled modules and are available with short lead-times.  All the outputs are conveniently put into one package.

TDK-Lambda’s Vega series

Another choice is to use a single output AC-DC power supply with board mount isolated DC-DC converters to produce additional outputs.  These readily available converters range from around 10W to 60W, can accept input voltages of 12V, 24V or 48V and supply single, dual or triple outputs, with output voltages of 3.3V, 5V, 12V and 15V.

TDK-Lambda’s CCG series of 25mm x 25mm 30W isolated DC-DC converters

When the requirement is for a higher power (100W or greater) second, third or fourth output, the DC-DC converter choice becomes more limited and because of the power involved, heat dissipation is harder to manage.  Cost can also become an issue.  Utilizing technology developed from the low voltage output Point of Load non-isolated converters, higher output voltage non-isolated converters are now being considered.

Without the constraints of input to output isolation, high performance “buck” (step-down) converters with very high efficiencies can be achieved.  With less waste heat, package sizes can be minimized and costs reduced.

TDK-Lambda’s i6A24014A033V, for example has the following specifications:

Input voltage:    +9 to 40Vdc

Output range:    +3.3 to 24Vdc

Output power:   Up to 250W

Output current:  Up to 14A

Efficiency:         Up to 98%

Package size:     33mm x 23mm

As a note, these types of (step-down) buck converters cannot supply a voltage higher than the input.

Although these types of converters have no input to output isolation, the AC-DC power supply will have, in accordance with the safety standards IEC 60950 / 60601.

Below is a typical application using the i6A:

Power Guy

Power Supply Filter Capacitor Values Can Change with the Applied Voltage

I read an article in one of the publications we advertise in recently where an Engineer had designed a timing circuit, but when he tested it, the frequency was too high.  He rechecked his calculations and found out from the capacitor datasheet that the value of the particular multilayer ceramic capacitor he had chosen, changes with the applied voltage.

I did not pay much attention to it, being more involved with power supplies, until I was talking with one of our TDK-Lambda Engineers regarding non isolated POL (Point of Load) converters, and he gave me the same warning about the value selection of the filter capacitors.  This is covered in the ceramic capacitor datasheets under DC Bias characteristics.

POL converters rely on quite large ceramic capacitors on the input and output to reduce the effect of the fast transient currents drawn by FPGAs, (which can cause the output voltage to deviate), with values sometimes approaching 2,000uF.

The concern our Engineer had was that our customers might not know about this.  Intrigued I decided to investigate.  Below is the DC-Bias Characteristic for a 22uF 16V multilayer ceramic capacitor.

If a capacitor value of 22uF was recommended by the application note for filtering the 12V input voltage on a non-isolated converter and the user picked this particular part, in actuality, the real capacitance would be closer to 12uF.

Upon testing the filtering, the user might complain that the application note was incorrect, whereas in fact the capacitor datasheet had not been interpreted correctly.

Something to bear in mind!

 

Power Guy

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