Why a power supply with data bus?

Let us think of applications that demand high power from the supply, but only for a comparably short duty cycle. Thus the average power required is significantly lower. Such applications could be 3D printers, laser cutting or a robotic arm that needs to be moved quickly but most of the time be held in position. Developers of such devices might be tempted to recommend a power supply that is capable of permanently delivering the highest power peak to be expected ('worst case load current'). However, dimensioning this way would result in a quite expensive power supply which is not optimal in terms of efficiency and - to a certain extent - could involve operational dangers. Since, in the event of a fault, the excessive current that is deliverable to the load, may be high enough to start a fire.

So what's more obvious than to choose a somewhat smaller power supply that can yet deliver high power peak for a defined short time, but also limits the current in the event of a continuous fault. The type of electrical load determines, under what conditions which power is required. The developer has to go for a consumer-oriented approach, which enables him to freely program behaviour of the power supply. When it comes to such requirements, a digital interface is usually preferred over analogue controls. This is especially true, when monitoring functions are also integrated in the power supply unit. When does which current flow? Is it still plausible or does the load behave abnormally? With the appropriate intelligence implemented in the power supply, and a suitable data bus available, a very simple and reliable monitoring of all loads connected to the power supply unit can be achieved. The PMBus^® (Power Management Bus), which is built into many modern power supplies, has established itself for all these tasks.

The Power Management Bus (PMBus^®) is an open standard for a digital energy management protocol: simple, robust and extensible. About 40 PMBus member companies adopt, promote and improve this standard.

PMBus properties

The PMBus allows to set operating parameters of one or more power supplies. For example, a power supply unit can be switched off completely when it is not needed. Or the output voltage and current can be dynamically adjusted. A programmable fan power supply connector can be used to make a normal fan intelligent: Fan activity and fan speed are automatically adjusted to the output power requirements, at the same time minimizing acoustic noise and extending service life of the fan. All these functions can be realized with a standard PMBus, which also has additional benefit of providing the I2C protocol, that almost every microcontroller can connect with.

Just as important as the PM control's command set are its monitoring capabilities. The PMBus provides real-time status data such as current input and output currents, voltages and power, critical component temperature and detailed fault diagnostics. From this information, the application can predict impending overload or overtemperature conditions even before they occur. If the internal temperature rises due to insufficient cooling, then the power supply does not simply reach its limits and shut down, but also issues a warning before critical thresholds are reached.

But the PMBus not only allows simple monitoring and control, the bus is also directly connected to the internal microcontroller in the power supply, which controls the digital feedback loop. Thus, not only can the operating parameters be set and monitored, but also transient response, slew rates and compensation matrices can be modified. This allows operation of the power supply to be matched to the actual load behaviour of the application, or dynamically adjusted 'on-the-fly'. In short, the standard power supply becomes a custom power supply by adding a PMBus interface.

The RACM1200-V

One power supply, that hat all the features mentioned above, is the RECOM's RACM1200-V. This series defines a new standard for compactness in the AC/DC power supply class, designed for reliable fanless operation to support longterm system availability. The baseplate-cooled design helps direct heat flow away from the application and enables continuous output power up to 1,000W, with short term output power up to 1,200W. With an optional system fan connected to the fan output, this power supply can provide 1,200W continuously.

A wide range of output voltage settings and a combination of constant voltage/power and settings for hiccup-mode make the product quite versatile - especially for supporting intrinsic safety of the application. Various analogue control and monitoring functions are accessible via the connector, as are the two auxiliary outputs; a programmable 5 V to 12 V system fan output and an isolated, "always-on", standby output of 5 V/1 A are available to power housekeeping functions.

The RACM1200-V series can be limited to inherently fail-safe settings through intelligent, controlled and fault-limiting features in project-dependent firmware setting. In the standard version, an internal status can be read out through two signal lines and the indicator LED:


In addition to analogue status and control pins, the PMBus version RACM1200-V/PMB offers a digital interface by which customers can make direct adjustments to the power supply. RECOM also offers a command line interface utility for programming the power supply. On request, the device can also be pre-programmed with a desired firmware setting.

Both versions achieve peak efficiency up to 95% and in addition the device meets eco-design requirements in standby mode. The RACM1200 series has worldwide safety approvals for medical (2MOPP, BF), industrial and ITE standards, as well as electromagnetic compatibility testing for Class A immunity and Class B emissions. All these features make the product one of the easiest to integrate modular power supply solutions in the industry.