In a previous article, I wrote about generating negative voltages via a standard unipolar dc power supply outfitted with polarity-reversal relays (see “Turn Positive Voltages Negative With Relays”). This method works well for certain applications, namely those requiring discrete test conditions that are sometimes positive and sometimes negative.
For such applications, polarity-reversal relays offer an inexpensive way to generate the desired positive/negative voltage. However, these relays also come with three significant limitations: power interruption during polarity-reversal relay operations, the inability to provide small positive and negative voltages, and increased test execution time.
What Is A Bipolar Power Supply?
A bipolar power supply overcomes those limitations. Most importantly, it can provide both positive and negative voltages from a single pair of terminals. There are no relays to switch polarity, so a bipolar supply can move smoothly from positive, through zero, to negative voltages. It also regulates zero volts or other very small voltages. Effectively, bipolar power supplies are large dc-coupled power amplifiers. In fact, they’re occasionally known as bipolar power amplifiers.
Often, a bipolar power supply is called a four-quadrant power supply. Take, for instance, the locus of output voltages and currents for a bipolar power supply that’s plotted on a set of axes (Fig. 1). Note that a bipolar power supply can generate positive and negative voltages, as well as positive and negative currents. As a result, the power supply will operate anywhere within the four quadrants-hence, the “four-quadrant supply” moniker.
In contrast, a standard dc power supply only generates positive voltages. Thus, it’s a unipolar power supply operating in only one quadrant (with only positive voltage and positive current).
Two Quadrants Vs. Four Quadrants
Some power supplies will operate in just two quadrants (Fig. 2). They always generate positive voltages but also can source current (positive current) or sink current (negative current). Such supplies particularly suit the testing of batteries or battery-charging circuits, which involves both sourcing current (e.g., charging the battery) and sinking current (e.g., discharging the battery).
Bipolar Power-Supply Applications
Typically, a bipolar power supply provides much higher bandwidth than a regular power supply, meaning it can rapidly slew from one voltage to another. Therefore, when a test requires the generation of a fast signal, such as a narrow pulse, some engineers will opt for a bipolar power supply.
For this test, the bipolar supply operates only in quadrant 1 (positive voltage, positive current), but the desired characteristic is the bipolar’s speed. While a common unipolar dc source might be able to create a 100-ms pulse width, and a high-performance unipolar dc source might be able to create a 1-ms pulse width, a bipolar power supply often achieves sub-millisecond pulse widths.
Because bipolar power supplies produce positive and negative voltages and currents, they’re the ideal choice for testing magnetic and inductive devices, such as motors, inductors, magnets, coils, and magnetic sensors. They’re also well suited for generating signals that swing between positive and negative voltages to simulate the output of sensors.
Moreover, bipolar power supplies can be used to test batteries. Voltage never goes negative for battery testing, which means there’s only need for two of the bipolar supply’s four quadrants.
Another application involves the testing of solar cells. When illuminated, a solar cell becomes a power source. Thus, it requires an electronic load to absorb the output energy from the solar panel.
For a solar-cell test, the bipolar power supply can absorb current and act as an electronic load in quadrant 2 (where voltage is positive and current is negative). However, another important test concerns the measurement of the solar cell’s dark current (Fig. 3). In this test, no light is placed on the solar cell and a reverse (negative) voltage is placed across the panel. Current will flow into the panel, which allows for the evaluation of the solar cell’s internal resistance and diode characteristics.
With a bipolar supply, the test can run in the forward direction with the solar panel producing power and the bipolar supply acting as an electronic load. Subsequently, the dark current test can be performed, with the bipolar supply producing negative voltage and forcing the solar panel into a reverse-bias condition.
The bipolar power supply appears to be the ideal power supply because it can provide any voltage from positive to zero to negative. However, it does manifest several limitations. First, due to their design complexity, these supplies are typically much more expensive than their unipolar dc power-supply counterparts.
Second, standard unipolar dc power supplies can adequately handle most dc power applications, and so most power-supply manufacturers don’t offer a wide variety of bipolar power supplies. Thus, finding the right bipolar supply can be difficult. As such, bipolar power supplies tend to be available only as specialty products (e.g., very high precision source measure units at low-power or large, high-power sources).
Third, most bipolar power supplies are linear. Consequently, they’re quite large and heavy, especially at high power.