Most commercially available power supplies have output voltages ranging from 3V3 to 54VDC. However, there are many end applications for power supplies where a higher output voltage is required. Given the lack of availability of standard higher output voltage supplies, it is not unusual for the required system voltage to be generated from multiple standard units in series or by connecting the outputs of a multiple output supply in series.
The usual limitation for the maximum output voltage of a power supply is the requirement to meet the safe voltage levels required by the Low Voltage Directive and defined by the relevant safety standards. A ubiquitous standard such as 60950-1 will define a safe voltage level as a maximum of 60VDC, termed as Safety Extra Low Voltage (SELV), meaning the maximum output voltage usually available from a power supply range is 54VDC allowing headroom for over-voltage protection circuits to work below the SELV limit. Provided the voltage meets the SELV and energy limits then it can be present on user-accessible parts.
It is, of course, possible to obtain safety approval on power supplies with an output(s) greater than 60VDC as evidenced by some of XP’s flexible standard platforms. In this case, the safety report details the precautions to be taken in the end application to prevent user access to the non-SELV parts. These precautions are detailed in the engineering considerations section of the safety report.
In modular power supplies, where a chassis has a combination of output modules fitted to create multiple outputs, higher power on a single rail by placing modules in parallel or higher voltage by placing modules in series, some manufacturers may be reluctant to configure modules in series to obtain a voltage above SELV limits because the safety approvals assume all outputs are SELV. In this instance, the equipment designer could connect the outputs in series and cater for the higher voltage in the end equipment approvals. XP’s modular fleXPower range allows series connections above 60VDC with functional isolation from output to earth.
Higher output voltages can also be achieved by connecting the outputs of two or more standard SELV power supplies in series. In this case, the safety approvals for the end application must consider the resultant hazardous voltage.
One other consideration for series connection of SELV supplies, whether modular or standalone, is the voltage rating of the common mode filter capacitors connected from output to ground. The output voltage with respect to ground increases for each power supply, the higher up in the series chain it is, and eventually, the filter capacitor’s voltage rating may be reached or exceeded resulting in lower overall reliability of the supply. For the same reason, basic isolation between output and ground for the lower voltage of the single output may become only functional or operational isolation for the outputs in serial at the higher voltage. The application designer must be aware of this and take this into account when considering the safety of the end system.