Believe in the Benefits of Bidirectional Power Supplies

Power supplies are among the most essential test equipment units, serving as energy sources for many measurements. An earlier blog, “Perusing Specs for Power Supplies and Electronic Loads”, provided guidance for choosing separate power supplies and electronic loads. But power supplies and electronic loads are also available as a single unit, integrated as a bidirectional power supply with many benefits working together. Also known as regenerative power supplies, bidirectional power supplies can be programmed to evaluate many different electronic energy systems, such as rechargeable batteries in electric vehicles (EVs) and hybrid EVs (HEVs). They are also useful for testing the energy circuits and systems in unmanned aerial vehicles (UAVs), unmanned ground vehicles (UGVs), and many other drones with electric-powered drives.

A bidirectional power supply typically incorporates an AC-to-DC converter, DC-to-DC-converter, and a regenerative electronic load in a one rack-mount unit housing. A traditional electronic load converts energy channeled to it into heat. At high power levels, large heatsinks are needed for proper thermal management. In contrast, the regenerative electronic load in a bidirectional power supply can return most of the electronic energy sent to it back to the source, such as a power grid or battery. It can perform what is known as two-quadrant power testing, working as both a power source and electronic load or sink. By returning power instead of converting it to heat, a bidirectional DC power supply achieves higher efficiency and can handle higher power levels with smaller size and weight than conventional separate power supply and electronic load.

Some commercial bidirectional DC power supplies can operate with 90% or better efficiency, meaning that only 10% of test power is lost as heat or wasted energy. This also typically translates into lower operating costs for a bidirectional power supply than for separate DC power supply and load at a given power level. Bidirectional power supplies can be programmed for different modes of operation, including constant voltage (CV), constant current (CC), constant power (CP), and constant resistance (CR). By automatically adjusting the other parameters, a parameter of interest is kept constant.

A bidirectional DC power supply makes it possible to evaluate how changes on supply voltage, current, or power affect the performance of a DUT. For any mobile electronic equipment running on battery power, for example, a rechargeable battery with low state of charge (SOC) may suffer a drop in voltage as its amount of stored energy decreases. A bidirectional DC power supply is often used in CV or CC mode to simulate a rechargeable battery for a DUT, to better understand how a device or circuit will operate when its voltage fluctuates or drops below its nominal operating current or power level. The use of the CV or CC mode also makes it possible to perform testing on a rechargeable battery without danger of overcharging or overheating the battery under test. The regenerative load within the bidirectional DC power supply allows power not used by the DUT to be returned to its source for high operating efficiency. With its bidirectional capabilities a regenerative DC power supply simplifies testing of such components as motor inverters, batteries, bidirectional on-board chargers (BOBCs), and bidirectional DC-to-DC converters without having to switch connections when testing the reverse current as with a conventional DC power supply.

Surveying Supplies

As an example of power testing versatility with high efficiency, EA-PSB 10000 30 kW programmable bidirectional DC power supplies from Elektro-Automatik save the weight and size of a separate power supply and electronic load by integrating them, along with a versatile function generator, in a single 4U-high, 19-in. rack-mount enclosure. Units in the series are available with maximum voltage ratings from 60 to 2000 VDC, maximum current ratings from 40 to 1000 A, and as much as 30 kW power per unit. An example from this series that we carry is the EA PSB10080-1000, which offers 30 kW of power, and a voltage of 80V. For ease of worldwide connection to a power grid, the bidirectional DC power supplies contain an active power factor correction (PFC) circuit and designed for three-phase connections to power source between 380 and 480 VAC and 208-VAC models available for the U.S. For those preferring on-unit control of settings, these units provide rotary knobs for adjustment of voltage, current, power, and resistance as well as a 5-in.-diameter color thin-film-transistor (TFT) touch-sensitive display.

In an even smaller housing, the 62000D series of programmable bidirectional DC power supplies from Chroma each fit within a single 3U-high, 19-in.-wide rack-mount enclosure. The series includes units with maximum voltage ratings from 100 to 1800 VDC, maximum current from 40 to 540 A, and maximum power from 6 to 18 kW, they can be used as a regenerative load or a power source and automatically switch from a source to a load upon detecting a voltage exceeding a programmed level. An example from this series that we carry is the Chroma 62180D-1800, which offers wide input power range of three-phase 200Vac to 480Vac input. Multiple units can be interconnected in parallel and series arrangements (with master/slave control) to reach output test levels as high as 180 kW. These programmable bidirectional DC power supplies provide typical source and load (sink) efficiency of better than 91% for lower-power models; for most models, voltage can be set with 10-mV resolution and current with 10-mA resolution. Voltage is regulated within ±0.01% of full scale (FS) for most models while current is held within ±0.05% FS for most models. The units feature almost instant changeover from supply to load characteristics, with typical transient response time of 1.5 ms and support flexible slew-rate control of voltage and current with built-in automatic sequencing functions. Remote computer control is extremely flexible, with interconnections for USB, GPIB, LAN/LXI, APG, and CAN buses.

The RP79000 Series of regenerative power systems from Keysight Technologies are bidirectional power supplies well equipped with safety mechanisms to protect the DUT and the operator. These compact, rack-mount units are available for 200/208 VAC or 400/480 VAC input power and maximum ratings of 2000 VDC, 800 A, and 20 kW. They can sink as much as 100% of the rated current for extended periods of time with no degradation in output power characteristics when making the transition from source to load. Noise and ripple as less than 200 mA in CC mode for all models. An example from this series that we carry is the Keysight RP7935A, which offers a voltage range of 0 to 80V and a current range of 0 to 250A.

For all these example units, measurement ranges are wide in support of many test applications, from electric motor drives and batteries to charging circuits and energy storage systems. For instance, the bidirectional DC power supplies can program resistance to simulate a battery’s SOC. All the supplies include computer interfaces for connection to an external computer and software. To meet the challenges of a particular application, users are invited to contact Axiom Test Equipment (www.axiomtest.com) to find their best match.