Instrumentation Power Supply Basics

This article is published by EEPower as part of an exclusive digital content partnership with Bodo’s Power Systems.

 

Power supply test instruments are no longer just DC output devices. Like the products they work with, power supplies have evolved to interface with PCs, simulate power sources such as batteries and solar cells, and execute test sequences. Individual instruments can range in power from as low as 30 W to as high as 60 kW with voltages as high as 2000 V, and current output as high as 1000 A. Power supplies are sophisticated instruments.  

 

Overview of a DC Power Supply

Figure 1 shows the basic circuit blocks of a DC power supply. The transformer galvanically isolates the AC line from the remainder of the circuitry. The transformer also steps down or up the voltage from the AC line depending on the supply’s desired maximum DC output voltage. The rectifier circuit block converts the AC voltage from the transformer to a unipolar AC voltage. Next, the filter block converts the unipolar AC into an imperfect DC voltage with a ripple effect. The regulator adjusts the output voltage to the desired level and adds further filtering so that the output is a constant DC voltage.

 

Figure 1. Generic block diagram of DC power supply. Image used courtesy of Bodo’s Power Systems [PDF]

 

Deviation From the Ideal Output

DC power supply outputs will not always deliver the exact output programmed by the user. Based on component tolerances, the manufacturer will define the accuracy of the DC output. The manufacturer may also specify a temperature coefficient to add to the output tolerance. Another reason the DC output will fall below a programmed value is that, at high current loads, more voltage is dropped across the internal resistances of various components in the power supply. Manufacturers specify this effect as load regulation, a percentage error of full-scale voltage. For a complete determination of the output accuracy of a DC power supply, add load regulation error to the output accuracy.

 

Image used courtesy of Freepik

 

The DC supply will also have noise on its output. There is noise inherent in all electronic components related to electron motion and collisions with atoms. Noise is also induced in a power supply’s output from conditions on the AC line, EMI, and stray currents on ground lines. No matter how good the design, noise will be on the DC power supply’s output.

 

Power Supply Topologies

There are two types of power supply topologies, linear and switchmode designs. Linear designs have power flowing continuously in the circuitry. Their designs have the benefit of low noise and low complexity, but they are not very efficient. Efficiencies for linear power supplies are below 60%. Switch-mode supplies can have efficiencies in the 90% range but much greater complexity and much higher output noise. Higher noise is due to transistors, used as switches that cycle power on and off at kHz rates. Switch-mode power supplies benefit from using a smaller and much lighter transformer than comparably powered linear power supplies. While either topology is suitable for low-wattage power supplies, switch-mode power supplies are almost always used to design power supplies with power levels above 500 W.

 

Versions of DC Power Supplies

Most power supplies are unipolar instruments that operate in Quadrant 1 (positive V and I). Bipolar output power supplies operate in Quadrant I and Quadrant IV. The output voltage can be either positive or negative, while the current is always positive. A third type of power supply can operate in Quadrants I and II. This type of supply is known as a bi-directional power supply. In Quadrant I, the supply is a DC voltage source. In Quadrant II, the supply absorbs current and operates as an electronic load. Thus, a bi-directional power supply combines the performance of two instruments, a DC power supply and a DC electronic load. See Figure 2.

 

Figure 2. Three types of DC power supplies. Image used courtesy of Bodo’s Power Systems [PDF]

 

Controlling the DC Output

As well as filtering the output to provide a ripple-free DC output, the regulator maintains the output voltage at the programmed level. We can model the regulator circuit as a feedback amplifier, as shown in Figure 3. The output voltage sensing circuit monitors the output voltage and feeds it back to the error/power amplifier. The error/power amplifier either raises or lowers its output based on the polarity of the voltage differential at the amplifier input.

 

Figure 3a. Output regulation stage. Image used courtesy of Bodo’s Power Systems [PDF]

 

Figure 3b: Output regulation stage with local and remote sensing. Image used courtesy of Bodo’s Power Systems [PDF]

Figure 3. Power supply output stage showing voltage control (not showing output filtering)

 

Monitoring the voltage at the DC power supply output terminals is sufficient when the load draws a small amount of current. The voltage drop across the lead wires is insignificant at small load currents. However, with large load currents, the voltage drop across the lead wires can be substantial, and the voltage applied to the load is lower than the programmed output voltage:

\[V_{Load}=V_{Supply}-2\cdot V_{Lead}\cdot\]

The supply corrects for the lower voltage at the load if the supply is designed with 4-wire connections in which two wires supply power to the load and two wires sense the load voltage. Figure 3b shows a 4-wire connection to a load.

The output voltage sensing circuit has a high input impedance; thus, it draws a negligible current. With negligible voltage drop in the sense wires, the voltage sense circuit measures the actual voltage at the load and feeds that voltage back to the power supply’s error/power amplifier. The amplifier raises its output voltage by \(2\cdot V_{Lead}\) to compensate for the voltage drop in the source leads. This feature is known as remote sensing and ensures the output at the load is the desired voltage. Using a 4-wire connection ensures greater accuracy of the voltage at the load.

 

Output Characteristic Options

Power supplies can deliver power to their load using different methods. A typical power supply will have an output I-V characteristic that is rectangular. The supply’s output can be any set of voltage and current values within the rectangle. The thick blue lines in Figure 4 illustrate a DC power supply with a rectangular I-V output characteristic. A second delivery method is known as autoranging. A DC supply with an auto-ranging I-V output characteristic has a combination of rectangular output and a hyperbolic output. The auto-ranging characteristic can supply a wider range of load current and, sometimes, a wider voltage range than a comparably powered supply with a rectangular output. The black and red curve shown in Figure 4 is an example of an auto-ranging output characteristic. The auto-ranging characteristic allows full power delivery along all points on the red curve, while the rectangular output delivers full power only at its maximum voltage and current.

 

Figure 4. Comparison of a rectangular output characteristic with an auto-ranging output characteristic. Image used courtesy of Bodo’s Power Systems [PDF]

 

Innovative DC Power Supplies

DC power supplies are essential instruments for the design and testing of all electronics. The extensive variation of electronic products has resulted in a wide range of power supply types and power supply features.

 

This article originally appeared in Bodo’s Power Systems [PDF] magazine.