Ten Fundamentals You Need to Know About Your DC Power Supply

Keysight Technologies Ten Fundamentals You Need to Know About Your DC Power Supply

Application Note

Introduction Understanding how measurement tools operate can provide insight into how to improve testing methods. With modern performance and safety features in power supplies, the lexibility exists to create test setups that are simpler and more effective. Use these 10 fundamentals about your power supply to take advantage of these features. Contents

1 Program Your Power Supply Correctly to Operate in Constant Voltage or Constant Current Mode / 3

2 Use Remote Sense to Regulate Voltage at Your Load / 4

3 Use Your Power Supply to Measure DUT Current / 5

4 Connect Power Supply Outputs in Series or Parallel for More Power / 6

5 Minimize Noise From Your Power Supply to Your DUT / 7

6 Safeguard Your DUT Using Built-in Power Supply Protection Features / 8

7 Use Output Relays to Physically Disconnect Your DUT / 9

8 Capture Dynamic Waveforms Using a Power Supply’s Built-in Digitizer / 10

9 Create Time-Varying Voltages Using Power Supply List Mode / 11

10 Tips for Rack-Mounting Your Power Supply / 12

Glossary / 13

Resources / 14

2 DC Power Supply Fundamentals Program your Power Supply Correctly to Operate 1 in Constant Voltage or Constant Current Mode

The output of a power supply can operate current limit setting. This operating mode Possible causes of UNR include: in either constant voltage (CV) mode or is known as constant current (CC). In –The power supply has an constant current (CC) mode depending on Figure 1, the power supply will operate on internal fault. the voltage setting, current limit setting, the vertical line IS with Vout = IS x R L. and load resistance. In most circum- –The AC input line voltage is stances, a power supply output operates Unregulated State below the specified range. in either CV or CC mode. However, there If a power supply is unable to regulate –The load resistance is R , are some unusual circumstances that will C its output voltage or output current, the the value at which the output cause the power supply to enter a third output will become unregulated and indi- will crossover from CV to CC, mode called unregulated (UNR) mode. cate unregulated (UNR) mode. Neither or CC to CV (see Figure 1). Understanding these three modes will the voltage nor the current will be at the make it easier to correctly program your –There is another source of power corresponding set point and the values power supply. connected across your power supply at which they settle are unpredictable. output, such as when outputs are While UNR mode can occur for a variety Constant Voltage used in parallel. of reasons, it is not very common. A power supply will operate in constant –The output is transitioning from CV voltage (CV) mode provided the load to CC, or CC to CV. This transition can does not require more current than the cause a momentary UNR. current limit setting. Based on Ohm’s law, V = I x R, maintaining a constant voltage CV operating line Vout while changing the load resistance RL = RC =Vs / I s requires the current to increase or decrease. As long as the current draw V Iout = Vs/R L is less than the current limit s RL = Load resistance setting, the power supply regulates the RC = Critical (crossover) resistance output at the voltage setting. In Figure 1, RL > RC V = Voltage setting the power supply will operate on the S I S = Current setting horizontal line Vs with Iout = Vs/R L. Constant Current CC operating line

If the load resistance decreases, such RL < RC as when a DUT component fails, and the load resistance, R L, is less than R C, where R is the ratio of the power C I I supply voltage setting to the current s out limit setting, the power supply will Figure 1: Power supply output characteristic regulate the current instead. Again, Ohm’s law dictates a change in voltage if the current stays constant at the

3 DC Power Supply Fundamentals Use Remote Sense to Regulate 2 Voltage at Your Load

Ideally, lead connections from your power As a general rule, for every 3-gauge –Do not twist or bundle sense leads supply to your load have no resistance. increase in your copper wire, the resis- together with load leads. In reality, lead resistance increases tance doubles. Since you must select the –Prevent an open circuit at the sense with lead length and wire gauge. The proper gauge wire to satisfy the current terminals, as they are part resulting effect when a supply delivers requirements of the load, remote sense of the output’s feedback path. current through the wire may decrease at the load will improve voltage regula- the voltage at the load. To compensate, tion without shortening lead length or - - Keysight Technologies, Inc. uses use remote sensing to correct for these decreasing wire gauge. internal sense protect resistors. voltage drops. These resistors prevent the output When you connect the remote sense ter- voltage from rising more than a Typically, a power supply is shipped minals to the load, the internal feedback few percent if the sense leads from the factory with the sense leads amplifier sees the voltage directly at the inadvertently open. connected locally at the output terminals. load, rather than at the output terminals. However, for setups with long load leads Since the control loop senses the voltage –Most supplies can compensate only or for complex setups with relays and directly at the load, the supply will keep for a maximum load lead drop of connectors, the voltage at the output the load voltage constant, regardless a few volts. terminals will not accurately represent of voltage drops caused by load lead To implement remote sense (Figure 3): the voltage at the load. (Figure 2) gauge, load lead length, output relays, or connectors. 1. Disconnect the sense terminal Depending on the gauge and length of connections from the main outputs. wire, the resistivity of your load connec- Remember the following when you use 2. Connect each sense terminal to tions could cause a much lower voltage remote sense: the proper polarity load contact. at your load than you want. High current –Use two-wire twisted shielded cable situations, for example, will invariably 3. If necessary, set the power supply for your sense leads. lead to significant voltage drops even to remote sense mode or 4-wire mode. with short load leads. Consider the –Connect the sense lead cable’s shield resistances of different gauges of to ground on only one end of the cable. copper wire in the following table:

Wire gauge Resistance Figure 2: +S (AWG) (mΩ / ft) The effects of 6 feet + --- 22 16.1 of 14 AWG leads with +OUT Power 0.015 Ω + 20 10.2 sense connected to supply lead resistance the output terminals. programmed 5.0 V 10 A Load 4.7 V for 5 V --- 18 6.39 A 0.3 V drop develops --- --OUT + 16 4.02 over the leads (0.15 V per lead). 0.015 Ω 14 2.53 --S lead resistance 12 1.59 +OUT and --OUT 10 0.999

Table 1: Resistance in mΩ per foot +S for different wire gauges +OUT + --- Figure 3: Power 0.015 Ω Using remote sense supply lead resistance + 5.3 V to compensate for load programmed 10 A Load 5.0 V for 5 V --- lead voltage drop --- --OUT + 0.015 Ω lead resistance --S +OUT and --OUT

4 DC Power Supply Fundamentals Use Your Power Supply 3 to Measure DUT Current

You can obtain an accurate DUT current In addition to these concerns, installing Power supply measurement specifica- measurement with an ammeter, a a current shunt requires you to break tions account for the errors that affect current shunt, or the built-in readback your circuit in order to connect the shunt an external shunt. Therefore, your power on your power supply. Ultimately, you in series. A current shunt installed in a supply readback may already be accurate should select one method over another rack-mount system may even require enough for most current measurement after considering the advantages and complex connections involving relays applications, particularly for currents disadvantages of each. More often than and switches. between 10% and 100% of the rated not, the current readback on your power output current of the supply. supply can provide you with the accuracy Built-in current readback you need for a measurement. Choose built-in current readback when You can avoid the difficulties involved you will benefit from these attributes: Ammeter with connecting current shunts by using a power supply’s built-in readback. –Reduction in connection A common way to measure DUT current Current readback on a power supply uses equipment – no need for relays, is to use a bench DMM set in ammeter an internal shunt, selected to comple- switching, and wiring mode. While an ammeter has the benefit ment the output rating of the supply. You –Simplicity of use of a specified accuracy, you must break do not need to disconnect the DUT or the circuit to insert the ammeter. A DMM connect a DMM. –Power supply provides readings also has a limit on the maximum current directly in amps Consider the level of measurement accu- you can measure, typically several amps. –Circuit disconnect not required racy you can expect with a high-quality External current shunt/DMM power supply (Table 2). –Specified accuracy – accuracy values already account for shunt errors You can also make current measurements Power supply –Synchronized measurements – with shunts. With a current shunt, current readback accuracy you can conveniently select the most readback measurements can be Output Typical triggered to start with other appropriate shunt resistor to match your current level accuracy current range. Your accuracy is based on power-related events 100% 0.1% to 0.5% the DMM’s voltage measurement accu- of rated output racy and the precision of the shunt. While this method can produce highly accurate 10% 0.5% to 1% of rated output results, certain errors can adversely affect your measurements. You must pay 1% Near 10% attention to these commonly overlooked of rated output complications: Table 2: Relative accuracy of power – Thermal EMF – dissimilar metals supply current readback cause thermocouple voltages to develop – Shunt miscalibration – accurate readings require calibration of the shunt resistance – Self-heating effects – higher temperature from current flow can cause shunt resistance to change

5 DC Power Supply Fundamentals Connect Power Supply Outputs in Series 4 or Parallel for More Power

You can connect two or more power the voltage value of the CC outputs. The supply outputs in series to get more CC outputs supply the output current to voltage, or connect outputs in parallel which they have been set and drop their to get more current. output voltage until they match the voltage of the CV unit, which supplies only When you connect outputs in series for enough current to fulfill the total load higher voltage, observe the following demand. precautions: To sense the voltage directly at the load, –Never exceed the floating voltage rat- use remote sense with your series or ing (output terminal isolation) parallel setup. For some power supplies, of any of the outputs you must deliberately set each output –Never subject any of the power supply for “remote sense,” sometimes called outputs to a reverse voltage “4-wire mode.” +S –Only connect outputs that have Power Using remote sense with supply +OUT identical voltage and current ratings --OUT series connections: output in series --S When you use remote sense in a series + Set each power supply output indepen- Load configuration, wire the remote sense -- dently so that the voltages sum to the +S terminals on each output in series and total desired value. To do this, first set Power connect them to the load as shown in supply +OUT each output to the maximum desired --OUT current limit the load can safely handle. Figure 4. output --S Next, set the voltage of each output to Using remote sense with sum to the total desired voltage. For example, if you are using two outputs, set parallel connections: Figure 4: Series connection with remote sense each to one half the total desired voltage. When you use remote sense in a parallel If you are using three outputs, set each to configuration, wire the remote sense one third the total desired voltage. terminals on each output in parallel and +S Power When you connect outputs in parallel connect them to the load as shown in +OUT + supply Load for higher current, observe the following Figure 5. output --OUT -- precautions: -- To simplify the settings for paralleled S –One output must operate in constant outputs, some power supplies support an voltage (CV) mode and the other(s) advanced feature called “output group- +S in constant current (CC) mode ing.” Up to four identical outputs can be Power +OUT “grouped,” enabling you to control all supply –The output load must draw enough --OUT grouped outputs as if they were a single, output current to keep the CC output(s) higher-current output. --S in CC mode –Only connect outputs that have Figure 5: Parallel connection with remote sense identical voltage and current ratings in parallel

Set the current limit of all outputs equally such that they sum to the total desired current limit value. Set the voltage of the CV output to a value slightly lower than

6 DC Power Supply Fundamentals Minimize Noise From Your Power Supply 5 to Your DUT

If your DUT is sensitive to noise on its Shield supply-to-DUT The ground loop current can produce DC power input, you will want to do connections voltage on the cabling that appears as everything you can to minimize noise on noise to your DUT. the input. Here are three simple steps The connections between your supply In addition to proper shielding, balancing you can take. and DUT can be susceptible to noise your cable impedance can preserve the pick-up. Different types of interference low noise profile of your power supply. Choose a power supply that include inductive coupling, capacitive has low noise coupling, and radio frequency interfer- Balance output-to-ground ence. There are a number of ways to To minimize noise, start at your source. reduce noise, but the most effective impedance Since filtering noise from your power is to ensure your load and sense connec- Common-mode noise is noise that is supply can be difficult, you want to select tions use shielded two-wire cables. generated when common-mode current a power supply that has very low noise to When you use shielded cable, make flows from inside a power supply to begin with. Choosing a linearly regulated sure to connect the shield to earth earth ground and produces voltage on power supply can accomplish this; how- ground at only one end. For example, impedances to ground, including cable ever, linear power supplies can be large connect the shield on the power supply impedance. To minimize the effect of and can generate large amounts of heat. end to earth ground, as shown in common-mode current, equalize the Instead, consider choosing a switching- Figure 6. Neglecting to connect the impedance to ground from the plus regulated power supply. Modern switch- shield on either end can increase and minus output terminals on the mode power supply technology has capacitive pick-up. power supply. You should also equalize improved to the point where the noise on the impedance from the DUT plus and the output can be comparable to that of a Do not connect the shield to ground at minus input terminals to ground. Use a linear supply. A comparison of noise on a both ends because ground loop currents common-mode choke in series with the typical linear supply versus a performance can occur. Figure 7 shows a ground output leads and a shunt capacitor switching supply is shown in Table 3. loop current that developed because from each lead to ground to accomplish of the difference in potential between this task. the supply ground and the DUT ground.

RMS Peak-to- noise peak noise Shield Power +S DUT Linearly supply regulated power ~ 500 μV ~ 4 mV --S supply + DC input Switching- -- regulated ~ 750 μV ~ 5 mV +OUT power supply --OUT

Table 3: A comparison of power Shield supply noise for linearly regulated versus switching-regulated supplies Figure 6: Shield is connected to earth ground only on one end of cable

Selecting a supply with low RMS and Power DUT supply peak-to-peak output voltage noise Shield specifications is an excellent start, but +S + DC input you can also minimize the noise with --S -- proper attention to the lead connections to your DUT. Ground loop current ﬂows in shield

Earth ground 1 ∆Vground Earth ground 2 Figure 7: Shield connected improperly (at both ends) results in ground loop current

7 DC Power Supply Fundamentals Safeguard Your DUT Using Built-in 6 Power Supply Protection Features

Most DC power supplies have features Over-current protection (OCP) OCP shuts off the output to prevent that protect sensitive DUTs and circuitry excessive current flow to the DUT. When from exposure to potentially damaging Most power supplies have an output you enable OCP, if the supply enters voltage or current. When the DUT trips voltage setting and a current limit setting. CC mode, a protection will trip and turn a protection circuit in the power supply, The current limit setting determines off the output. In effect, OCP turns the the protection circuit turns off the output the value in amps at which the power current limit setting into a trip value in and displays a notification. Two common supply will prevent excessive current amps. Set your current limit low enough protection features are over-voltage and from flowing. This constant current (CC) to protect your DUT from excessive cur- over-current protection. mode regulates the output current at rent, but high enough to prevent nuisance the current limit but will not turn off the tripping due to normal fluctuations in When you design your test, it is impor- output. Instead, the voltage decreases the output current that can occur during tant to understand these protection below the voltage setting and the power output transient conditions, such as features to protect your DUT. supply continues to produce current at during an output voltage change. When a the current limit setting in CC mode. power supply is shipped from the factory, Over-voltage protection (OVP) OCP is turned off. OVP is a value set in volts designed to protect your DUT from excessive voltage. When the power supply output voltage exceeds your OVP setting, the protection will trip and turn off the output.

OVP is always enabled. When power supplies are shipped from the factory, OVP is typically set well above the maximum rated output of the power supply. Set your OVP trip voltage low enough to protect your DUT from excessive voltage, but high enough to prevent nuisance tripping from normal fluctuations in the output voltage. Fluctuations can occur during output transient conditions, such as load current changes.

CAUTION: On most power supplies, OVP responds to the voltage at the output terminals, not the sense terminals. When using remote sense, program Figure 8: Power supply front panel showing over-voltage protection, over-current your OVP trip voltage high enough to protection, constant voltage, and constant current mode account for load lead voltage drops.

OVP circuits can respond to an over-voltage condition in microseconds, however, the output voltage itself will take longer to go down. The time for the output to go down depends on the down-programming capabilities of the power supply and the load that is connected to the output. Some power supplies have a silicon-controlled rectifier (SCR) across the output that fires when the OVP trips, which brings the voltage down much faster.

8 DC Power Supply Fundamentals Use Output Relays to Physically Disconnect 7 your DUT

Although you may expect your power In critical applications where you require opening and closing of the external relays supply output to be completely open a complete disconnect between the with other power-related events. when you set an “output off” state, this power supply output and your DUT, check may not be the case. When set to “off,” with your power supply vendor to see When available, built-in output discon- the output impedance will vary from if an output relay option exists that will nect relays provide these advantages model to model and may depend upon the provide a complete disconnect. If this over external relays: options installed in the power supply. The configuration is not available, you may –Less complexity “output off” state will typically set the have to provide your own external output - - Less wiring output voltage and output current to zero disconnect relays. and disable the internal power-generating - - No external relay control circuitry The downsides of an external relay con- circuitry. However, these settings do not –Consumes less space guarantee that no current will flow into figuration are the added cost and com- or out of your DUT, as would be the case plexity to your test setup and the extra –Better built-in synchronization of relay if the output terminals were physically space required. You will need to provide open/close with other power-related disconnected from your DUT. the relays, connect wires from the power events supply output to the relays, and install a –Relays open upon fault conditions When the power supply output is “off” means to control the relays. You may also such as over-voltage and over-current but not completely open, your DUT test find it more difficult to synchronize the could be adversely affected for a number of reasons: Inside power supply –Your DUT contains a source of DC power that is connected directly + across the power supply output. + –Your DUT contains a source of DC power that is connected across the DUT output in a reverse-polarity configura- -- tion. -- –Your DUT is sensitive to extra capacitive loading. Reverse RFI/ESD Output Internal protection ﬁlters capacitor output –Your DUT produces a changing voltage diode relays across the power supply output. Figure 9: An example of a power supply with internal relays located right at the output terminals. With the relays open, your DUT Some power supply models have an is completely disconnected. internal output relay option that can completely disconnect the power supply output from your DUT. The relay in Inside power supply Figure 9 opens when you use an “output off” setting and stops all current flow + to the DUT. But even with a relay option installed, certain models may still have + output capacitors or capacitively coupled DUT networks connected from the output terminals to chassis ground because of the -- location of the relays, so your DUT will -- still be connected to these components (see Figure 10). Reverse Internal RFI/ESD Output protection output ﬁlters capacitor diode relays Figure 10: An example of a power supply with internal relays located inboard of some output components. With the relays open, these components remain connected to your DUT.

9 DC Power Supply Fundamentals Capture Dynamic Waveforms Using a 8 Power Supply’s Built-in Digitizer

While most power supplies can measure When two parameters are set, the Figure 12 shows a sample waveform DUT steady-state voltage and current, remaining parameter will be determined obtained on a cell phone’s current draw some power supplies can also measure by the following equation: using a power supply output digitizer and dynamic voltage and current. These sup- device characterization software (this is Acquisition time = Time interval x plies feature a built-in digitizer. not an oscilloscope display). (Number of samples --1) Traditionally, digitizers are used for data When you use device characterization In a similar manner, a power supply’s acquisition to capture and store analog software, the captured data is displayed built-in digitizer can be configured to signals. Like an oscilloscope, which uses graphically in the time domain much like trigger and capture power supply output a digitizer to display the analog signal an oscilloscope displays a signal. The voltage or current waveforms. The present on one of its inputs, a power idle, receive, and transmit current states supply’s digitizer will store a buffer of supply’s built-in digitizer captures the are discernible from the waveform. Of readings with the waveform data points. dynamic voltage and current waveforms course, you can analyze digitized data in You can retrieve the data and use any produced on its output. ways other than using device character- standard software for analysis. You also ization software. Basic digitizer operation can use your own customized program or available device characterization soft- You can use a bus interface such as USB, Figure 11 shows a digitizer converting ware to easily visualize the results in the LAN, or GPIB to capture and retrieve an analog waveform into a set of data time domain (oscilloscope-like view or digitized waveform information. Retrieved points. Upon a trigger, the digitizer takes data-logger view) or perform a statistical data can be returned either as a scalar measurement samples and stores them analysis. value, with the power supply calculating in a buffer. a single number averaged from the data An example digitizer (as it does for the front panel display), When you make a digitizing measure- application or as an array of values. You can even ment, you can set two of the following acquire pre- and post-trigger data by three parameters: If you use your power supply in place of a changing the trigger offset to capture battery, you can capture dynamic informa- – Time interval – time between samples waveforms such as peak current draw tion about the current flowing into your during a DC inrush current test. – Number of samples – total number DUT, allowing you to better understand of samples you want to take the current drain on your DUT batteries. Consequently, you can make appropriate – Acquisition time – total time during design adjustments to optimize your DUT which you want to take samples power management during the DUT’s various modes of operation.

Trigger Measurement occurs sample (point)

Time interval between samples Acquisition time = Time interval x (number of samples --1)

Figure 11: A digitizer converts an analog waveform into data points by sampling.

Figure 12: Device characterization software uses a power supply’s built-in digitizer to capture data showing a cell phone’s current draw from the power supply.

10 DC Power Supply Fundamentals Create Time-Varying Voltages 9 Using Power Supply List Mode

Typically, power supplies are used to bias – Voltage waveform test – a test where Using list mode circuits that require a constant voltage. measurements are taken while the However, more advanced applications DUT is exposed to a stimulus voltage You can use list mode for performing may require a time-varying voltage (or waveform. a voltage waveform test on automotive current). Modern power supplies can electronic systems. During the startup easily manage both using list mode to In both cases, the stimulus involves of an internal combustion engine, also address the time-varying applications. creating a sequence of voltage steps. known as a cold-crank, battery voltage The first has multiple levels of steady- levels drop considerably as enormous What is list mode? state voltages and the second has a amounts of current are drawn by the continuously varying voltage profile. The electric starter motor (see Figure 14). The Normally, you can program a PC to two tests are commonly used for DUT battery voltage then plateaus once the change the voltages on a power supply design verification. Be aware that DC engine is turning and hits a final level as output for discrete periods of time. In this power supplies are limited in bandwidth the electric starter turns off. way, your program controls the transi- and typically can generate voltage wave- tions between voltages to allow you to forms only at frequencies up to tens of You can enter the simplified sequence in test your DUT at different volt ages. kilohertz. Also, most power supplies are Table 4 into a list to perform ECU design validation testing on an automotive List mode lets you generate these voltage unipolar devices that create only positive electronic system. (Simulate transitions sequences and synchronize them to voltages. between voltage levels with additional internal or external signals without tying steps.) This test ensures that the auto- up the computer. You set individually motive electronics have adequate power programmed steps of voltage (or current), transient immunity. Use list mode in this and an associated step duration. After manner when you need to apply a time- setting the duration for each step, you Dwell Trigger time varying voltage to your DUT. trigger the list to execute directly on the power supply. You may set the power supply to move on to the next step based on dwell times or triggers. A list can be Step # 0 1 2 3 4 5 0 1 2 3 4 5 programmed to repeat once or multiple Iteration 1 Iteration 2 times (see Figure 13). Repeat count = 2 To create a list, set the following: Figure 13: A list is a sequence of individually programmed voltage – One or more voltage or current steps – (or current) steps initiated with a trigger. defined voltage or current values – Dwell times – duration associated with each voltage or current step Vﬁnal

– Repeat count – the number of times Supply voltage you want the list to repeat Vplateau Two uses of list mode for testing Vlow

The list mode on a power supply can be 300 ms 500 ms 400 ms Time an effective tool for running two types of tests: Figure 14: An automotive cold-crank profile represented with list steps – Voltage sequence test – a test where measurements are taken while the Step Voltage level Voltage value Dwell time DUT is exposed to discrete stimulus 0 Vlow 8 V 300 ms voltage values. 1 Vplateau 12 V 500 ms 2 Vfinal 14 V 400 ms

Table 4: A simple list used to simulate the automotive crank voltage profile in Figure 14

11 DC Power Supply Fundamentals Tips for Rack-Mounting 10 Your Power Supply

When you are planning out a test rack, Weight distribution Heat management selecting an instrumentation layout can be a challenging task. Safety, reliability, Typically, a power supply is one of the Typically, power supplies have internal and performance are among the many heaviest instruments in your test rack. cooling fans. When you mount your requirements that affect your choices. Mount the power supply near the bottom power supply in a rack, be sure to provide of the rack to lower the rack’s center of adequate spacing for the power supply’s Specifically, pay attention to these gravity and consequently lower the risk air intake and for exhaust air. Keep considerations when you put your DC of tipping the rack. (Figure 15) thermally sensitive instruments such power supply in a rack: as DMMs away from power supplies AC input power – Weight distribution because high temperatures can have an adverse effect on DMM readings. Distribute weight properly to avoid When you plan the size of your AC input rack instability line, use the maximum current rating of Magnetic interference – AC input power each instrument in your rack to ensure Provide adequate AC input power the AC line providing power to your rack LCD displays have replaced most CRT to avoid excessive current draw is adequate. Most instruments draw a displays; however, if older computers relatively constant amount of current. or oscilloscopes with CRT displays are – Heat management However, a power supply’s AC input used, be aware that they are susceptible Provide proper heat management current varies with the power supply’s to magnetic fields. Magnetic fields can to avoid excessive temperatures output loading. If you do not know the also affect the performance and accuracy – Magnetic interference maximum load you expect on the output of some instruments. For example, a Place instruments properly to minimize of the power supply, plan for the worst voltmeter’s circuitry could be susceptible magnetic interference case scenario by using the maximum to a large magnetic field produced by rated input current of the supply. a transformer, such as that inside a – Routing wires power supply. Be sure to install your DC Route wires to minimize conducted supplies away from your magnetically and radiated noise sensitive instruments, especially your DMM. Routing wires

Top heavy Since power wires can radiate electrical poorly balanced noise and both stimulus and measure- test system ment signal-carrying wires are susceptible to this noise, separate power wires from signal-carrying cables.

Front Front of rack of rack

Well balanced test system with low center of gravity

SIDE VIEW SIDE VIEW

Figure 15: To properly balance your test system, place larger, heavier instruments near the bottom.

12 Glossary

Down programming When a power supply with current sink capability is programmed to a voltage level less than that of the voltage at the output terminals, the supply will automatically begin to sink current. The downprogrammer can be thought of as an internal load across the power supply’s output terminals that helps bring the output voltage down quickly.

DUT Device under test

Linear-regulated power supply A power supply design technique consisting of placing a control element in series with the full-wave bridge rectifier and the outputs. Simplified, the control element can be thought of as a variable resistor controlled by a feedback circuit that monitors the output and adjusts the resistance accordingly to keep the output voltage constant.

RFI/ESD filters RFI (radio-frequency interference) filters prevent undesired behavior from power supply by providing a path to ground for noise current to flow. Similarly, ESD (electrostatic discharge) filters prevent damage to your power supply by providing a path to ground for static electricity to discharge.

Self-heating effects (shunt) Current flowing through a shunt resistor dissipates power (I2 x R) and heats up the shunt causing a change in the resistance value.

SCR (silicon controlled rectifier) An SCR placed across the output terminals of a DC power supply creates a direct short-circuit on the output of the supply when an over-voltage condition is detected. Also known as a “crowbar,” this over-voltage protection prevents excessive voltage from the reaching the load.

Switching-regulated power supply A power supply design technique that uses a regulating element that acts like a rapidly opened and closed switch. The duty cycle, or the ratio of time that the switches are open or closed, is controlled by a feedback circuit. This circuit monitors the output and adjusts the duty cycle to keep the output voltage constant.

Thermal EMF (thermal electromotive force) Thermoelectric voltages are generated when you make circuit connections with dissimilar metals at different temperatures. This occurs because any metal-to-metal junction forms a thermocouple, which generates voltage proportional to the junction temperature. These junctions occur anywhere you connect metal leads, such as at the DUT, at the relay, and to the multimeter.

Voltage/current rating The specified maximum voltage or current output that a power supply can produce.

13 Resources

Related Keysight Literature

10 Hints For Using Your Power Supply to Decrease Test Time 5968-6359E http://cp.literature.keysight.com/litweb/pdf/5968-6359E.pdf

10 Practical Tips You Need to Know About Your Power Products 5965-8239E http://cp.literature.keysight.com/litweb/pdf/5965-8239E.pdf

Test-System Development Guide 5989-5367EN http://cp.literature.keysight.com/litweb/pdf/5989-5367EN.pdf

14 15 | Keysight | Ten Fundamentals You Need to Know About Your DC Power Supply - Application Note

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