Fit and forget: a 1200W AC/DC that quietly gets on with its job!

The AC/DC design team takes a collective deep breath, picks up their pencils (or, more likely, styli these days), and starts weighing up the perhaps thousands of design decisions that must be made. These often conflict with each other and range across disciplines, from circuit theory to safety, EMC, magnetics, mechanics, cooling, aesthetics, standards compliance, and more. Modern designs at this power level invariably have a digital element with embedded firmware, making programming skills another element to incorporate. It is urgently required, and marketing may change the specifications, so project management is not the least of the relevant disciplines needed.

Having hopefully generated some sympathy for the AC/DC product designers, we will now examine the RECOM RACM1200-V series (Figure 1), as an example of where it has all come together successfully, and how the product has been improved even further through recent upgrades. This AC/DC operates from universal 80-264VAC and provides a single 24V, 36V, or 48V output with a wide adjustment range at 1kW continuous and 95% typical efficiency.

The size achieved for the RACM1200-V ticks the first box with overall dimensions of 228 x 96.2 x 40mm (9 x 3.8 x 1.57 inches for the metrically challenged). This represents one of the highest power density ratings on the market and fits within a standard 1U rack height (1U = 1.75 inches). AC input and DC output are at opposite ends for convenient connections and an efficient internal layout.

However, the size must always be considered in relation to the cooling necessary for the claimed rated output. In the typical applications for these products, an internal fan is undesirable for maintenance and acoustic noise reasons, so the product is rated for stand-alone, cold-wall, and forced-air cooling. Although we are talking about typical 1kW applications, there is a hint in the product part number: 1200W is available as a peak power rating for defined periods or continuously with a modest 2.5m/s airflow, all with a maximum baseplate temperature of 85°C.

There are several trade-offs in achieving a particular power output: ambient and cold wall temperatures, airflow, AC input voltage, and duration of peak power, and RECOM has helpfully given a comprehensive guide to what is possible in the product datasheet. One parameter often missing from specifications is the allowed repetition rate of a peak power rating - the cooldown period necessary so that temperatures do not “ratchet up” to high levels with multiple consecutive peak load events. RECOM provide the guidance in this regards, as shown in Figure 2.

The RACM1200-V was created with versatility in mind, and the thermal data supplied allows the user to extract maximum power from the product under specific conditions, thereby avoiding the need for a more expensive, larger, higher-rated AC/DC. Stress to the product is avoided, though, with a comprehensive protection scheme that includes precision over-current sensing, a constant power mode for small overloads, then constant current, and eventually a “hiccup” style shutdown, cycling four times before latch-off (Figure 3). At the same time, over-temperature and output over-voltage protection are in operation.

Yet more flexibility is built into the product, with protection levels adjustable through the built-in PMBus^™ interface. Even the over-temperature protection has levels, with a “hard” and immediate shutdown from the sensing of primary switches and a “softer” shutdown from the sensing of output components, with the power supply reacting by reducing output power initially and setting a warning flag before eventual safe switch-off if the temperature continues to rise.

The firmware for RACM1200-V’s digital intelligence has been significantly upgraded, allowing the product to effectively monitor power demand in real-time and match it with its capabilities, given external conditions of AC mains voltage and temperature measured at multiple locations. The effect is to provide users with peace of mind that the product cannot be damaged or overstressed, which could have reduced its lifespan. To additionally help with this, a fan drive output is provided for the end system, and its speed and status can be monitored via a provided “tacho” signal. With temperature control of the fan speed, dissipation is reduced, and acoustic noise is minimized.

The PMBus provides additional monitoring and control so that the RACM1200-V can be integrated into a SCADA (Supervisory Control and Data Acquisition) or intelligent power management system. However, fast and secure analog control is not forgotten; remote sense and on/off are provided along with an ACOK signal to warn after the AC mains fail but before hold-up time is exhausted. A PSU GOOD signal is available to show that the output is within specification, and an “always on” 5V standby output at 1A is provided with a useful 1.5kV isolation from the main output. The fan drive output already mentioned can be preset to be off or between 5V and 12V with a 1A rating. To add some visual excitement, an RGB LED signals seven different PSU states with a combination of continuous, flashing, and alternating colors:


The product’s high degree of precision control makes it suitable for a wide variety of load types. For example, motors with a high inrush would benefit from the inbuilt constant-current characteristic. Battery charging is another application; lead-acid batteries are still widely useed and are charged with constant current and voltage. This can be accommodated within the control functionality of the RACM1200-V.

If you cannot get enough of the product, up to three can be paralleled to increase the power rating. Sharing is done via the “droop” method to ensure that each provides an equal current contribution.