By EA Elektro-Automatik

At its core, a power supply is an electrical device that supplies electric power, a combination of voltage and current, to an electrical load, such as a computer, appliance, consumer electronics, or a battery charger, to name a few. Since the primary use of a power supply is to convert electric power from a source, like the utility grid, to the correct voltage, current, and frequency to power the load, it is also called an electric power converter. The power supply is built into the equipment in some loads, like desktop computers. In contrast, power supplies for motors and appliances are often separate, standalone units.

In some applications, a single power supply may not be sufficient to provide the power required by the load. For example, the required voltage may be much higher than the power supply provides, or the current requirement for the load may be much higher than the capability of a single power supply unit.

Series connected DC power supplies

In those applications where the power required is much higher than a single power supply can provide, the user can connect multiple power supplies in series or parallel, depending upon the requirement. Connecting multiple power supplies in parallel will increase the current and power while the voltage remains constant. Conversely, connecting them in series adds the voltages of the individual supplies, resulting in a higher total output voltage while the current remains the same.

This is represented in the equation below.

V_out = V₁ + V₂ + V₃ +………………

V_out is the output voltage, and V₁, V₂, V₃…. are the individual power supplies connected in series. However, there are some dos and don’ts that the user must follow before connecting the power supplies in series. For instance, series connection is only allowed with supplies of the same kind and manufacturer model, i.e., power supplies with similar voltage, current, and output impedance ratings. But, most importantly, the current rating should match.

Furthermore, series operation of two or more power supplies can be realized only up to the output isolation rating of any one power supply to obtain a higher voltage than that available from a single power supply. In short, the six critical tips suggested by manufacturers for series connections are:

EVs: paralleling application examples

Now, where would the user need such series connections? There are applications like electric vehicles (EVs) where manufacturers are moving from 400 V batteries to 800 V. Some key factors motivating car makers to adopt higher voltages are increased EV efficiency, faster charging, and weight reduction of other components. Analyst data shows that by increasing the battery voltage to 800 V, the current required to power motors and other devices in the EV is much lower than that required to power 400 V, enabling thinner cables and smaller electronic components.

As a result, it reduces the vehicle’s weight and minimizes energy losses to heat, thereby increasing overall efficiency and battery range. Plus, this helps the battery to charge much faster, a significant factor attracting electric car buyers. Currently, some ten car OEMs, including Tesla (model Cyber Truck), have adopted 800 V architecture. And this number is growing fast.

Multi-voltage DC power supplies

Before embarking on a project, ensure the power supplies have similar ratings before connecting them in series. Two similar power supplies must be connected in series to generate 800 V supply out of 400 V units (Figure 1). 500 V units come closest to 400 V required, such as a programmable bidirectional DC power supply, and its output can be set to 400 V, as shown in Figure 1.

Now, to make a serial connection, connect the negative terminal from one power supply unit to the positive terminal of the other power supply. These output terminals are available at the product’s back panel, as shown in Figure 2. The numbers indicate the function of the connectors as described below.

1. Slot for interfaces
2. Master-Auxiliary-Bus Interface to set up a system for parallel connection
3. Share Bus Interface to set up a system for parallel connection
4. Output voltage Remote Sense input terminal
5. Output terminal, copper busbar
6. Mains input terminal
7. Ethernet Interface
8. USB Interface
9. Connector (DB15 female) for isolated analog program, monitor, and other functions
10. Grounding connection screw

Extra measures must be taken if a bidirectional power supply is used as a load. We offer the Serial Connection Box for these applications.

Other examples require creating plus(+) and minus(-) supplies from a single + power supply, such as in aircraft applications. For instance, aircraft use +/-270 Vdc to create 540 Vdc (Boeing 787). models in series with the middle point grounded, allowing the user to create a +270 Vdc reference and a -270 Vdc reference and providing 60kW of power (Figure 4). For safety and isolation reasons, some restrictions are recommended by the manufacturer, which are explained in the product datasheet.

Connecting power supplies in parallel operation

 The solution connects two or more power supplies in parallel for applications requiring higher power and current than a single power supply. However, before connecting the supplies in parallel, the user must ensure that the load current is evenly shared between the power supplies. For that, the user must ensure that identical power supplies are used and identical cables or conductors are used to connect the DUT’s + and – output connectors. Furthermore, the same wiring length and thickness of cables are used to connect the power supply units to the load. As a result, the voltage drop across the wiring cables is identical, ensuring identical voltage across power supplies and equal distribution of current from each power supply in this operation.

In practice, many power supply manufacturers rate their products, both switch-mode and linear power supplies, as compliant for paralleling. Ideally, for a perfect current share, the various supplies employed in this configuration must present identical output impedances and the output voltage set-up as close as possible. As time passes, this scenario is not guaranteed due to normal dispersion of the output parameters and the impact of aging, including temperature.

For that, makers have incorporated a technique called Master-Auxiliary with a dedicated load-share bus. In this mode, when multiple power supply models of the same series are connected via the Master-Auxiliary bus and Share bus, the entire multi-device system behaves as a single device. With the Master-Auxiliary bus, system data such as total power and total current are collected and shown on the Master device. Warnings and alarms for the auxiliary devices are shown clearly on the display. The Share bus provides an equal load distribution to the individual device.

In short, there are three steps to paralleling the company’s programmable bidirectional DC power supplies:

1. Physically parallel the +/- outputs
2. Connect the Share bus to ensure units are sharing current equally
3. Connect the master/auxiliary bus to ensure communication between the units
4. Configure each unit as master or auxiliary in the settings menu and set the bias and termination resistors as shown below.