Instrument power supply applications
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Instrument power supply applications

Posted Date: 2024-01-19

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

DC power supply overview

Figure 1 shows the basic circuit block diagram of a DC power supply. Transformers electrically isolate the AC line from the rest of the circuit. Transformers also step down or step up the voltage of the AC line depending on the DC output voltage required from the power supply. The rectifier circuit block converts the AC voltage from the transformer into a unipolar AC voltage. Next, the filter block converts the unipolar AC power into an imperfect DC voltage with ripple effects. The regulator regulates the output voltage to the desired level and adds further filtering so that a constant DC voltage is output.

 

Deviation from ideal output

The DC power output does not always provide a user programmed output. Based on component tolerances, the manufacturer will define the accuracy of the DC output. Manufacturers can also specify a temperature coefficient to increase output tolerance. Another reason for the DC output to be lower than programmed is that the internal resistance of the various components in the power supply will drop more voltage under high current loads. Manufacturers specify this effect as load regulation, which is the percentage error in full-scale voltage. To fully determine the output accuracy of a DC power supply, add the load regulation error to the output accuracy.

The output of a DC power supply also produces noise. All electronic components related to electron motion and atomic collisions have inherent noise. Conditions such as AC lines, EMI, and stray currents on ground wires can also create noise in the power supply output. No matter how well designed, the output of a DC power supply will generate noise.

Power topology

There are two types of power supply topologies: linear and switch-mode designs. Linear designs allow power to flow continuously through the circuit. Their designs have the advantages of low noise and low complexity, but are not very efficient. Linear power supplies have an efficiency of less than 60%. Switch-mode power supplies can achieve efficiencies of up to 90%, but are more complex and have higher output noise. The higher noise is caused by the transistors, which act as switches that cycle power on and off at kHz rates. Switch-mode power supplies benefit from the use of smaller, lighter transformers than comparably powered linear power supplies. Although both topologies are suitable for low-wattage power supplies, switch-mode power supplies are almost always used for designs with power levels above 500 W.

DC power version

Most power supplies are unipolar instruments operating in the quadrant (positive V and I). Bipolar output power supplies operate in Quadrant I and Quadrant IV. The output voltage can be positive or negative, while the current is always positive. The third type of power supply can operate in Quadrant I and Quadrant II. This type of power supply is called a bidirectional power supply. In Quadrant I, the power source is a DC voltage source. In Quadrant II, the power supply draws current and operates as an electronic load. Therefore, a bidirectional 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 courtesy of Hakata Electric Power Systems

Control DC output

In addition to 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 detection circuit monitors the output voltage and feeds it back to the error/power amplifier. The error/power amplifier increases or decreases its output depending on the polarity of the voltage difference across the amplifier's inputs.


Figure 3a. Output adjustment stage.Image courtesy of Hakata Electric Power Systems
Figure 3b: Output regulation stage with local and remote sensing.Image courtesy of Hakata Electric Power Systems
Figure 3. Power supply output stage showing voltage control (output filtering not shown)

When the load draws a small amount of current, it is sufficient to monitor the voltage at the output terminals of the DC power supply. At small load currents, the voltage drop across the leads is negligible. However, for large load currents, the voltage drop across the leads can be large and the voltage applied to the load is lower than the programmed output voltage:

 V L o a d = V S up p ly ? 2 ? V L e a d ? _ _

If the power supply is designed with a 4-wire connection, with two wires supplying power to the load and two wires sensing the load voltage, the power supply will correct for the lower voltage at the load. Figure 3b shows the 4-wire connection of the load.

The output voltage sampling circuit has a high input impedance; therefore, it consumes negligible current. Since the voltage drop on the sense line is negligible, 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 increases its output voltage by (2cdot V_{Lead}) to compensate for the voltage drop in the source lead. This feature, called remote sensing, ensures that the output of the load is the desired voltage. Using a 4-wire connection ensures a more accurate load voltage. 2 ? VLead_ _

Output properties options

Power supplies can use different methods to deliver power to loads. A typical power supply has a rectangular output IV characteristic. The output of the power supply can be any set of voltage and current values ​​within the rectangle. The thick blue line in Figure 4 illustrates a DC power supply with rectangular IV output characteristics. The second transfer method is called autoranging. DC power supplies with auto-adjusting IV output characteristics have a combination of rectangular and hyperbolic outputs. The autoranging feature can provide a wider range of load currents and sometimes even a wider voltage range than comparable power supplies with rectangular outputs. The black and red curves shown in Figure 4 are examples of autoranging output characteristics.


Figure 4. Comparison of rectangular output characteristics and auto-ranging output characteristics.Image courtesy of Hakata Electric Power Systems


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