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Friday, October 26, 2012
What is a bipolar (four-quadrant) power supply?
To answer this question, I have to start with a basic definition of polarity conventions. Figure 1 shows a simple diagram of a power supply (a two-terminal device) with the standard polarity for voltage and current. A standard power supply typically is a source of power. To source power, current must flow out of the positive voltage terminal. Most power supplies source energy in this way by providing a positive output voltage and positive output current. This is known as a uni-polar power supply because it provides voltage with only one polarity. By convention, the “polarity” nomenclature typically refers to the polarity of the voltage (not the direction of current flow).
If current flows into the positive voltage terminal, the power supply is sinking current and is acting like an electronic load – it is absorbing and dissipating power instead of sourcing power. Most power supplies do not do this although many Agilent power supplies can sink some current to quickly pull down their output voltage when needed – this is known as a down-programmer capability – see this post for more info: http://powersupplyblog.tm.agilent.com/2012/03/if-you-need-fast-rise-and-fall-times.html.
To fully define power supply output voltage and current conventions, a Cartesian coordinate system is used. The Cartesian coordinate system simply shows two parameters on perpendicular axes. See Figure 2. By convention, the four quadrants of the coordinate system are defined as shown. Roman numerals are typically used to refer to the quadrants. For power supplies, voltage is normally shown on the vertical axis and current on the horizontal axis. This coordinate system is used to define the valid operating points for a given power supply. A graph of the boundary surrounding these valid operating points on the coordinate system is known as the power supply’s output characteristic.
As mentioned earlier, some power supplies are uni-polar (produce only a single polarity output voltage), but can source and sink current. These power supplies can operate in quadrants 1 and 2 and can therefore be called two-quadrant supplies. In quadrant 1, the power supply would be sourcing power with current flowing out of the more positive voltage terminal. In quadrant 2, the power supply would be consuming power (sinking current) with current flowing into the more positive voltage terminal.
Some power supplies can provide positive or negative voltages across their output terminals without having to switch the external wiring to the terminals. These supplies can typically operate in all four quadrants and are therefore known as four-quadrant power supplies. Another name for these is bipolar since they are able to produce either positive or negative voltage on their output terminals. In quadrants 1 and 3, a bipolar supply is sourcing power: current flows out of the more positive voltage terminal. In quadrants 2 and 4, a bipolar supply is consuming power: current flows into the more positive voltage terminal. See Figure 3.
Agilent’s N6784A is an example of a bipolar power supply. It can source or sink current and the output voltage across its output terminals can be set positive or negative. It is a 20 W Source/Measure Unit (SMU) with multiple output ranges. See Figure 4 for the output characteristic of the N6784A.
To summarize, a bipolar or four-quadrant power supply is a supply that can provide positive or negative output voltage, and can source or sink current. It can operate in any of the four quadrants of the voltage-current coordinate system.