FAQs from Rohde & Schwarz

Serial and Parallel Channel operation of Power Supply Units (PSUs) in general


My PSUs have isolated channels for one or two quadrant operation. There are several applications I would like to use it for. Unfortunately the manual doesn't give a complete answer to my questions.

  • Increase the output voltage
  • Increase the output current
  • Increase the output power by using additional PSUs (Power Supply Units)
  • Peltier Element (negative and positive voltage to apply - having two channels in use with antiparallel connection)

Can you please also let me know the wiring of the sensing lines?


1.) Increase the output voltage (Serial Mode)

The following interconnection adds the individual output voltages. The same current flows through all outputs. The current limits for the outputs wired in series must be set to the identical value. If one of the outputs exceeds the current limit, the total voltage collapses. It is recommended that you set all voltages to the same value to distribute the loads evenly.

Activate all connected channels, especially, when you have connected a load with low resistance. Otherwise, the protective diodes of a deactivated channel can be overloaded and damage the instrument.

Another important point to look at is the maximal voltage to ground- The voltage resulting from the series connection must not exceed this value.

Serial and Parallel Channel operation of Power Supply Units (PSUs) in general

2.) Increase the output current (Parallel Mode)

If it is necessary to increase the total current, the power supply outputs must be wired in parallel. The output voltages for the individual outputs must be set to the same voltage value as precisely as possible. For slight voltage differences, it is common in this operating mode to first charge a voltage output up to the current limit; the other voltage output provides the remaining current.

The maximum total current is the sum of the individual currents of all sources connected in parallel. For power supplies that are connected in parallel, it is possible that compensating currents flow within the power supplies. The use of power supplies by other manufacturers, which are potentially not overload proof, can cause destruction of these units as currents can be distributed unevenly.

Serial and Parallel Channel operation of Power Supply Units (PSUs) in general

Generally, the channel with the highest output voltage provides the highest current. When this channel reaches the current limit, the next channel connected in parallel supplies the remaining current.

Note, that the voltages in the individual channels differ slightly due to tolerances. By increasing the voltage slightly, you can distribute the load more evenly. If the voltage for a channel is to be increased by some mV, for instance (by a set of identical cables), the current is initially provided by this channel.

We also recommend, that you use the current limit settings to distribute the load to multiple channels evenly. Distribute the total current proportionately to the current limits of the connected channels. This procedure also distributes the power loss more evenly, thus improving the heat dissipation and protecting the semiconductors from damage.

3.) To be considered in general when increasing the output power by using additional Channels / PSUs

You have to take care about the points described for serial and parallel mode and in addition about the following topics:

  • The maximal voltage to ground must not exceed the lowest value of all PSUs being in use.
  • Some PSUs have backside connectors for lower connection resistance – prefer to use these connectors.
  • Always use cables with the highest possible cross-section in order to keep the conductor resistance as low as possible.
  • When using more than one PSU it is important to keep in mind, that it is not possible to switch on the outputs at the same time. In particular, if the output current has to be increased and the device under test draws full current immediately, an adequate external switch must be used that only switches the device under test after all the PSUs involved are ready for operation.

4.) Special Case negative and positive voltage to apply to the DUT (e.g. for Peltier Element):

Unfortunately, it is not possible to connect the outputs in anti-parallel - even if one of the outputs is "switched off", unwanted effects arise that can destroy the outputs.One possibility, however, would be to interconnect the outputs via a separate relay in such a way that there is galvanic separation between the channels that differ in polarity. That would then be a double changer switch.

5.) Wiring of the sense ports

If you want to use the Sense lines, these must be routed to the live channels with identical cable routing.
It is important that no current may flow over the sense lines.