Modern electronic equipment requires careful design of its power architecture to optimize cost, efficiency, and size. In a product, it is usual to see one or more AC/DC power supplies followed by DC/DC isolated bus converters and then switching regulators as Point-of-Load converters. There might also be isolated DC/DCs to provide auxiliary voltage rails of different polarities and combinations, often to power isolated communications or sensor interfaces.

Procuring all these different converters from different sources is a risk; however, there is no guarantee of compatibility between parts. Additionally, if the system does not work as expected, different suppliers may claim that the other suppliers are to blame. An example of this is an EMC fail report where individual parts may comply with the standards on paper under set conditions, but when coupled with other converters under real-life conditions, statutory limits for conducted or radiated emissions are exceeded. Other compatibility issues can arise from instability introduced when driving one switching converter from another. Differences in product data sheet measurement methods, such as temperature rise, can also lead to confusion. For example, definitions of ambient temperature and where it is measured can vary between suppliers.

The solution to all this is to use RECOM for all your power conversion needs. We will support you from initial system design and selection from our vast array of products, from debugging to assistance with EMC qualification of your end product.

Having a single system power supply that generates all of the voltages needed in a product is now an outdated concept due to the increasingly tight specifications for critical power rails and the need to maximize efficiency while minimizing cost and size. This has pushed system designers towards the Distributed Power Architecture (DPA) approach, where typically an optimized AC/DC converter with integrated power factor correction generates a common bus voltage, perhaps 12V or 48V, or even 400V in data centers, allowing power to be distributed at lower currents with fewer consequent resistive losses. Downstream DC/DC converters may drop the voltage to a lower-level bus and then Point of Load (PoL) converters provide high current rails close to the loads, for best static and dynamic regulation. The DPA approach optimizes efficiency, spreads the heat load and is very flexible, allowing relatively easy upgrades, re-configuration, and isolation of different parts of a system. Costs can be lower, as standard modules can be utilized with their specifications chosen to meet the exact requirements.

On-board filters that make an individual part compliant with an EMI specification may combine with filters in other products to even make overall EMI worse, due to unintended resonances and consequent voltage ‘peaking’.

A particularly insidious interaction is when a regulated switched mode converter is fed from the output of another. For constant efficiency and output load, input power to a regulated switching converter is constant, so if the input voltage increases, the current decreases, appearing as a negative resistance. An upstream regulated converter will be characterized for a typical positive resistive and perhaps capacitive load, but may become unstable with a negative resistance load. Similarly, a positive voltage transient from the upstream converter will cause a reducing current in the following converter, which can cause the control loop of the first converter to push the voltage still higher, amplifying the original effect, perhaps to damaging levels.

Parts interacting in these ways are not necessarily faulty, but the individual suppliers may be reluctant to try to fix a problem if they see that other suppliers are involved. For this reason, it is advisable to procure parts from one trusted source who will take responsibility for all of the supplied parts working together as intended. It is not guaranteed that will never be a compatibility problem with every combination of product, but a supplier such as RECOM has the resources to assist with the initial system specification, selection of appropriate modules, and identification and resolution of any difficulties. For example, RECOM offers a range of filter components that have been evaluated with power conversion products that can be assembled to evaluation boards to check for overall system EMI performance. EMC test facilities are also available for RECOM customers to debug system issues and do pre-compliance testing.