Author:
Tom Tillman, Director of Marketing, TDK-Lambda Americas
Date
04/20/2023
PDF
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Figure 1: 1000W Brick on board solution
AC-DC and DC-DC power modules became a popular method of creating non-standard or custom AC-DC and DC-DC power supplies when they were introduced approximately 30 years ago. Compared to the newer products now available on the market, they were large in size and had relatively low efficiencies. It became a relatively quick and easy way for mainstream power supply manufacturers, custom product designers and in-house power supply engineers to create a power supply that met their exact needs.
Newer products
This “brick on board” trend has slowed a little in recent years with most of the top manufacturers focusing research on higher volume, higher efficiency and lower cost standard products. The market for multiple output power supplies with a traditional output combination of+5V, +/-12V and 24V has seen a trend towards single output 12V products. These 12V products use lower cost chip-based regulators to generate other voltages.
Recently, new, refreshed power module designs are now being released by multiple manufacturers. The older products do tie up valuable engineering resources for sourcing alternative components. The older model lower efficiencies are also becoming somewhat intolerable. The earlier generation 1500W rated, 360V output TDK-Lambda PF-A AC-DC power module had an efficiency of 95% and generated 79W in losses. The recently launched PF1500B-360 (Figure 2) though has an 96.5% efficiency and will only dissipate 54W. With lower losses it has been possible to reduce the size from 19.5sq” to just 11sq”, thus consuming less area on the PCB.
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Figure 2: 96.5% efficient PF1500B-360
Higher power DC-DC modules are also being launched, such as the TDK-Lambda 1200W rated full brick converter.
Note the underside of the modules are aluminum, which helps to dissipate waste heat from the module to either a heatsink or cold plate.
AC-DC and DC-DC power module design
A simplified block diagram (Figure 3) shows an AC-DC power module driving three isolated DC-DC converters.
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Figure 3: AC-DC power module block diagram
The AC input is supplied to an AC-DC converter module - also known as a PFC (Power Factor Correction) module. The 360Vdc output is distributed to wide range isolated DC-DC converters capable of operating from ~200 to 420Vdc. These provide power for the low voltage loads (3.3Vdc to 48Vdc) and non-isolated DC-DC converters. Note, the PFC converter is non-isolated, the input to output (primary to secondary) isolation is performed by the DC-DC converter.
For single output designs there are now a number of new AC-DC power modules available like the PFH500F (Figure 4). These have the necessary 3,000Vac primary to secondary isolation for industrial use.
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Figure 4: PFH500F 500W rated single output AC-DC power module
The advantages of using AC-DC and DC-DC power modules
Many power modules are encapsulated for enhanced ruggedization and typically are only 0.5” (12.7mm) tall. This gives them a very low profile making them suitable for applications requiring high shock and vibration. A conventional power supply in the rotating gantry of a CT (Computed Tomography) machine will receive high g-forces, potentially causing mechanical failure. Large, but portable RF (Radio Frequency) equipment used by defense forces and first responder teams may receive shock and vibration during transit to remote areas.
The low profile is also a benefit to up-scale, high resolution video signage, allowing free standing slimline displays.
The operating temperature range is often very wide, from -40 to +100oC. This makes them suitable for outdoor applications in sealed enclosures.
The ability to conduction-cool the modules through the baseplate, enables their use in sealed out-door enclosures or for environments with conductive contaminants that can cause early field failures. Mounting power modules on to a cold plate allows the use of water cooling, removing the need for large and noisy fans. High levels of ambient noise can cause operator tiredness and fatigue with the possibility of hearing loss over long periods of time.
Higher power systems
High volume semiconductor testing requires large amounts of power, in the 10kWs order of magnitude. Manufacturers often use water cooling to remove waste heat from the test heads, rather than forced air cooling. 48V input DC-DC converters fed by high wattage AC-DC power supplies are a common solution. This adds a degree of complexity to the design as a 10kW load will require the AC-DC power supply to provide 48V 200A. This requires large copper busbars from the power supplies to the DC-DC converters.
Using DC-DC converters capable of operating from higher voltage in the order of 360 to 380Vdc drops that 200A current down to a manageable 28A. Wire cables and connectors can handle this current level and provide easier access for servicing. The TPF45000-385 45kW 19” rack mount power supply is one example having a 385V output, well within the 200 to 425Vdc input range of DC-DC power modules.
Many countries are testing and implementing HVDC (High Voltage Direct Current), supplying machinery and LED lighting capable of operating from 380 to 400Vdc. This removes the rectification stage of a power supply, saving energy. The higher voltage can be used in conjunction with solar panels, wind generated power and rechargeable batteries to provide a lower cost, reliable source of power.
Designing with power modules
Although it is more expensive looking from the bill of material aspect to use power modules for a custom design, the cost of developing a power from the ground up should be factored in. Power modules are a known entity having been tested over their full input range, from zero to full load and over their entire temperature range. Importantly, they are already safety certified, reducing the amount of time and effort required for the end design.
Supporting a custom design will require multiple component sources from the initial production to the decline of the program. Magnetics may be subject to minimum ordering requirement. Safety certification may require upgrades, even for the widely used IEC 62368-1 standard. In a number of years that has transitioned from edition 1 to edition 3 with edition 4 being considered. There are also a host of environment reports required, in addition to CE and UKCA marking, namely RoHS, REACH, Conflict Minerals disclosure and electronic waste recycling.
Application tips
Purchase or get a sample of an evaluation board if possible. That will assist you with the size and layout of the printed wiring board.
For EMI (Electromagnetic Interference) compliance, use a high quality, multiple layer pcb. Ground planes can then be established, dramatically reducing the size of the input filtering.
The input filter will require special safety certified capacitors, not just a regular ceramic capacitor (even through the voltage rating looks okay). X capacitors (metallized film) are positioned across the Line and Neutral, Y capacitors are connected between the L/N and earth ground.
Some AC input power modules use a ceramic inrush resistor with a thermal cutoff inside. Ask the manufacturer for recommendations. Not every wire wound resistor is suitable.
Position electrolytic capacitors away from hot areas. For every 10oC rise in case temperature, the field life halves.
Position the polyester input ripple capacitors close to the power module.
Summary
When considering a power module design, it is important to find a manufacturer that can support you face to face with local Field Application Engineers in addition to having Technical Support in your time zone and the engineering resources available to review your power system design.
Always check to see when the product was introduced to the market. A search of the manufacturer’s press releases on their website can determine this. A look at some of the older products may give an indication of how long the provider keeps their production in manufacture.
