# General Electronics Measurement

5 Posts authored by: CS

# Does My Power Supply Have Multiple Range Output?

Posted by CS May 24, 2018

During the last couple of decades of technology evolution, we’ve seen how electrical and electronic design platforms have product cycles shortened to three to five years. Measurement instruments need to catch up to fast-moving innovation and technology advancements, or they will quickly become redundant.

Mobile device chargers have evolved from an output of 5 V to variable voltage outputs of 5 V, 9 V, or even 12 V for fast charging. For high-power application, electrification of vehicles no longer uses 12 V or 42 V. Voltages now range from tens of volts to power air-conditioning systems and other car electronics to a few hundred volts to drive the powertrain. These demands require a power supply that is equipped with multiple output ranges.

In this blog, we are going to discuss the types of power supply output ranges available in the market and why they are important. Let’s start the discussion by understanding a power supply characteristic.

## Power supply output characteristic

A power supply output characteristic shows the borders of an area containing all valid voltage and current combinations for that particular output. Any voltage-current combination that is inside the output characteristic is a valid operating point for that power supply.

There are three main types of power supply output characteristics: rectangular, multiple-range, and auto ranging. The rectangular output characteristic is the most common.

## Rectangular output characteristic

It’s not surprising to see a rectangle shape power supply output characteristic on a voltage-current graph (see Figure 1). Maximum power is produced at a single point coincident with the maximum voltage and maximum current values. For example, a 20 V, 5 A, 100 W power supply has a rectangular output characteristic. The voltage can be set to any value from 0 to 20 V, and the current can be set to any value from 0 to 5 A. Since 20 V x 5 A = 100 W, there is a singular maximum power point that occurs at the maximum voltage and current settings.

Figure 1. Rectangular output characteristic.

## Multiple-range output characteristic

When shown on a voltage-current graph, a multiple-range output characteristic looks like several overlapping rectangular output characteristics. Consequently, its maximum power point occurs at multiple voltage-current combinations. Figure 2 shows an example of a multiple-range output characteristic with two ranges, also known as a dual-range output characteristic. A power supply with this type of output characteristic has extended output range capabilities when compared to a power supply with a rectangular output characteristic. It can cover more voltage-current combinations without the additional expense, size, and weight of a power supply of higher power. So, even though you can set voltages up to Vmax and currents up to Imax, the combination Vmax/Imax is not a valid operating point. That point is beyond the power capability of the power supply, and it is outside the operating characteristic.

Figure 2. Dual-range output characteristic.

## Autorange output characteristic

When shown on a voltage-current graph, an autoranging output characteristic looks like an infinite number of overlapping rectangular output characteristics. A constant power curve (V = P / I = K / I, a hyperbola) connects Pmax occurring at (I1, Vmax) with Pmax occurring at (Imax, V1). See Figure 3.

Figure 3. Autoranging output characteristics.

An autoranger is a power supply that has an autoranging output characteristic. While an autoranger can produce voltage Vmax and current Imax, it cannot produce them at the same time. For example, Keysight N6755A has maximum ratings of 20 V, 50 A, 500 W. You can tell it does not have a rectangular output characteristic since Vmax x Imax (= 1000 W) is not equal to Pmax (500 W). So, you can’t get 20 V and 50 A out at the same time. You can’t tell just from the ratings if the output characteristic is multiple-range or autoranging, but a quick look at the documentation reveals that the N6755A is an autoranger. Figure 4 shows its output characteristic.

Figure 4. N6755A output characteristic.

For applications that require a large range of output voltages and currents without a corresponding increase in power, an autoranger is a great choice. Here are some example applications where using an autoranger provides an advantage:

• The device under test (DUT) requires a wide range of input voltages and currents, all at roughly the same power level. For example, at maximum power out, a DC/DC converter with a nominal input voltage of 24 V consumes a relatively constant power even though its input voltage can vary from 14 V to 40 V.

During testing, this wide range of input voltages creates a correspondingly wide range of input currents even though the power is not changing much.

• There are a variety of different DUTs of similar power consumption but different voltage and current requirements. Again, different DC/DC converters in the same power family can have nominal input voltages of 12 V, 24 V, or 48 V, resulting in input voltages as low as 9 V (requires a large current) and as high as 72 V (requires a small current). The large voltage and current are both needed, but not at the same time.

• A known change is coming for the DC input requirements without a corresponding change in input power. For example, the input voltage on automotive accessories could be changing from 12 V nominal to 42 V nominal, but the input power requirements will not necessarily change.

• Extra margin on input voltage and current is needed, especially if future test changes are anticipated, but the details are not presently known.

# 3 Things You Need to Know about Remote Sensing

Posted by CS Apr 27, 2018

Have you ever set your power supply output voltage to a value and found the voltage at your load was lower than you expected? Many of us have experienced that outcome, and that’s because remote sensing needs to be part of the setup. In this article we are going to share with you three things you need to know about remote sensing to help you get a value you can trust.

Figure 1 shows the power supply setup using remote sensing. The remote sense terminals are connected to the load at the points where you want the 5 V setting to be regulated. In this case, the power supply regulates 5 V at the load by adjusting its output voltage to 5.3 V to make up for the drops in the load leads. It does this by using the voltage across the sense leads as part of the feedback loop inside the power supply to adjust the voltage on the output terminals.
The purpose of the power supply is to keep the sense lead voltage constant at the setting; the power supply changes the output terminal voltage based on the sense terminal voltage. The input impedance of the sense terminals is high enough to prevent any significant current flow into the sense terminals – this makes any voltage drop on the sense leads themselves negligible.

Figure 1: Using remote sense to compensate for load lead voltage drop.

# 2. Use sense leads for overvoltage protection (OVP)

One of our military customers providing DC power to a very expensive device during test asked about the availability of a special option on one of our power supplies. They wanted the option that changed the location of the overvoltage protection (OVP) sensing terminals from the output terminals of the power supply to the sense terminals of the power supply. Since the device under test (DUT) is located quite a distance away from the power supply, they are using remote sensing to regulate the power supply voltage right at the device under test. And since the DUT is very expensive and sensitive to excessive voltage, it’s important to protect the input of the DUT from excessive voltage as measured right at the DUT input terminals.

The power supply used, Keysight N6752A installed in an N6700C mainframe, normally uses the output terminals as the sensing location for the overvoltage protection. OVP is used to prevent excessive voltage from being applied to sensitive devices. If the voltage at the output terminals exceeds the OVP setting, the output of the power supply shuts down.

Since this customer is very interested in preventing excessive voltage from being applied to the expensive DUT, sensing for an overvoltage condition right at the DUT is important. For the N6752A, Keysight offers a special option (J01) that adds the ability to perform OVP sensing with the sense leads. See Figure 2. with the J01 option added to the N6752A, the customer’s DUT is protected against excessive voltage.

Figure 2: OVP sensing is done right at the DUT using sense lead terminals in addition to the output terminals.

You may be wondering why the standard OVP would sense at the output terminals instead of at the sense terminals. Probably the biggest reason for sensing at the output terminals is because that approach provides more reliable protection than sensing at the sense leads even though it is less accurate. The output terminals are the power-producing terminals.

If the sense leads become inadvertently shorted, the voltage at the output terminals would rise uncontrolled beyond the maximum rated output of the power supply. This uncontrolled high voltage could easily damage any device connected to the power supply’s output leads. So, sensing for an overvoltage condition at the output terminals makes sense. It may not be the most accurate way to protect the DUT, but it is the most reliable given all of the things that can go wrong, such as a wiring error or an internal fault in the power supply.

# 3. Remote sensing can affect load regulation performance

The voltage load effect specification tells you the maximum amount you can expect the output voltage to change when you change the load current. In addition to the voltage load effect specification, some power supplies have an additional statement in the remote sensing capabilities section about changes to the voltage load effect spec when using remote sensing. These changes are sometimes referred to as load regulation degradation.

When you are choosing a power supply, if you want the output voltage to be well regulated at your load, be sure to consider all the specifications that will affect the voltage. Be aware that as your load current changes, the voltage can change as described by the load effect spec. Additionally, if you use remote sensing, the load effect could be more pronounced as described in the remote sensing capability section (or elsewhere). Be sure to choose a power supply that is fully specified so you are not surprised by these effects when they occur.

# Summary

Remote sense is used to regulate the set voltage at the DUT, compensating for any loss in your leads. Using remote sense will have an impact on regulation performance, which should be considered along with the benefit of compensating for the voltage drop in your leads. Overvoltage protection at the power supply outputs should be used in conjunction with remote sense to protect the DUT.

# Safeguard your Power-sensitive DUTs from an Over Power Condition

Posted by CS Apr 20, 2018

As the world continues to trend toward increased energy savings and green energy sources, more and more heavy machinery and vehicles are becoming electrified. Mechanical combustion engines are being replaced by electric motors as part of this technology trend. As these demands accelerate, higher expectations for reliable and safe power are spurring engineers to put all their brain power into coming up with the most efficient product designs. The last thing that R&D engineers want is to mess with the power supply reliability and create a potential safety issue.

## Over Voltage and Over Current Protection

Today’s system DC power supplies incorporate a variety of features to protect both the device under test (DUT) as well as the power supply itself from damage due to a fault condition or setting mishap. Over voltage protect (OVP) and over current protect (OCP) are two core protection features that are found on most system DC power supplies to help protect against power-related damage. But is that all that you need to know? In this blog, we will also discuss power protection on your devices that do not operate on fixed voltage and current levels.

Over voltage protect helps ensure the DUT is protected against power-related damage in the event the voltage rises above an acceptable range of operation. As over voltage damage is almost instantaneous, the OVP level is set at reasonable margin below this level to be effective; yet it is set suitably higher than the maximum expected DUT operating voltage so transient voltages do not cause false tripping. Causes of over voltage conditions are often external to the DUT.

Over current protect helps ensure the DUT is protected against power-related damage in the event it fails in some fashion, causing excess current, such as an internal short or some other type of failure. The DUT can also draw excess current by consuming excess power due to overloading or from an internal problem that causes inefficient operation and excessive internal power dissipation.

OVP and OCP are depicted in Figure 1 below in an example DUT that operates at a set voltage level of about 48V and uses about 450W of power. In this case the OVP and OCP levels are set at around 10% higher to safeguard the DUT.

Figure 1. OVP and OCP settings to safeguard an example DUT.

## Over Power Protection

However, not all DUTs operate over a limited range, as depicted in Figure 1. Consider, for example, that many (if not most) DC-to-DC converters operate over a wide voltage range while using relatively constant power. Similarly, many devices incorporate DC-to-DC converters to give them an extended range of input voltage operation. To illustrate with an example (see Figure 2), consider a DC-to-DC converter that operates from 24 to 48 volts and runs at 225 W. DC-to-DC converters operate very efficiently, so they dissipate a small amount of power and the rest is transferred to the load. If there is a problem with the DC-to-DC converter that causes it to run inefficiently, it could be quickly damaged due to overheating. While the fixed OCP level depicted here will also adequately protect it for over power at 24 volts, you can see that it does not work well to protect the DUT for over power at higher voltage levels.

Figure 2. Example DC-to-DC converter input V and I operating range.

A preferable alternative would be to have an over power protection limit, as depicted in Figure 3. This would provide an adequate safeguard regardless of the input voltage setting.

Figure 3. Example DC-to-DC converter input V and I operating range with over power protect.

Since an over power level setting is not a feature that is commonly found in system DC power supplies, this would then mean having to change the OCP level for each voltage setting change, which may not be convenient, desirable, or in some cases, practical to do. However, in the Keysight N6900A and N7900A advance power system DC power supplies, it is possible to continually sense the output power level in the configurable smart triggering system. This can then be used to create a logical expression to use the output power level to trigger an output protect shutdown. The N7906A software utility was used to graphically configure this logical expression, and then it was downloaded it into the advance power system DC power supply, as shown in Figure 4. Since the smart triggering system operates at hardware speeds within the instrument, it is fast-responding, an important consideration for implementing protection mechanisms.

Figure 4. N7906A software utility graphically configuring an over power protect shutdown.

A glitch delay was also added to prevent false triggers due to temporary peaks of power being drawn by the DUT during transient events. While the output power level is being used here to trigger a fault shutdown, it could just as easily be used to trigger a variety of other actions.

## Summary

We have discussed that advance system power supplies can provide over voltage and over current protection as well as protection for over power conditions. For more information on protection against power related damage, download the Protect Against Power-related DUT Damage During Test application note.

# 3 Traps to Avoid When Making Resistance Measurements

Posted by CS Feb 14, 2018

A resistance meter normally works by sending a small, precise current through the resistance to be measured. Then it measures the voltage drop. Once the meter knows the current and voltage, it applies Ohm’s law to derive resistance. Ohm’s law says that resistance is voltage divided by current, or R = V/I.

For example, if there is a 10 mA (0.01 A) current going through a resistor and there is a voltage drop of 1 V over the resistor, then the resistor is R=V/I = 1 V / 0.01 A = 100 ohms.

Different test conditions may have different impacts on resistance measurements. In this article, we will discuss some common mistakes that result in resistance measurement error and ways to counter them.

Trap 1. Impact of Temperature on Resistance

From the R=V/I equation, you might think that making an accurate resistance measurement on a material sample is trivial, but in reality, this may not be true. The reason for this is that the resistivity of all materials varies with temperature. When you attempt to measure a sample’s resistance, you inevitably heat it up to some extent. This is referred to as the Joule self-heating effect.

Joule self-heating makes resistance measurements a tricky balance between two factors:

1. To keep the resistor from heating up and the resistance value from changing, you need to keep the current (= power) low.
2. Small currents mean that we need to measure smaller voltages, which in-turn requires a higher voltage measurement resolution.
 V = I x R(T)    Resistance depends on temperature!

How Much Power Can I Apply to a Structure?

After understanding the effect of temperature on the resistance measurement, how do you establish the relationship between temperature and resistance? We just learned that temperature change is directly proportional to the power applied to the DUT. We also know that Power = Voltage x Current. The expression of the voltage across a resistor in terms of applied power and resistance is shown in the equation below.

To determine the maximum power we can apply to a structure without changing its resistance, we need to know something about its thermal characteristics. Let’s look at an example of copper. We know that the resistance of copper changes by about 0.35% for every degree Celsius change in temperature. For a 10 mm by 10 mm sample and resistance tolerance of 0.1%, we can see that maximum allowable power is about 0.04 mW:

Plugging this back into the top equation, we see that this amount of power creates a voltage change of approximately one microvolt, which tells us roughly how much voltage measurement resolution the instrumentation needs to have.

 Need 1 mV of voltage measurement resolution!

Trap 2. Thermo EMF in Resistance Measurement

Another factor to consider when making any type of measurement (not just resistance measurements) is thermo electromotive force (or EMF). Thermo EMF is a transient voltage pulse that is generated when a reed relay switch opens or closes. Since virtually all SMUs employ reed relays, thermo EMF effects are something that you need to consider when making sensitive low-level measurements.

The picture shown in Figure 1 is of a commercial grade relay chart. It is NOT characteristic of the relays used in SMUs, which are specially designed to minimize EMF. We can see thermo-EMF is generated over the time period when a relay is operating. This EMF can have a significant impact on low resistance measurements; it will distort the resistance value measured.

Figure 1. Thermo-EMF example of general reed relay.

Now let’s take a look at how to perform a modified Kelvin measurement that can eliminate the effects of thermo EMF, as well as the effects of any offset voltages in your circuitry. Figure 2 shows a picture of a standard Kelvin measurement on a resistor R with the EMF and offset voltages modeled as voltage sources.

Figure 2. Modified Kelvin resistance measurement.

First, set up one SMU as a current source and source current through the resistor you want to measure. Then use another measurement resource (either a voltmeter or an additional SMU) to measure the voltage across the resistor. After calculating the resistance, reverse the current flow and repeat the measurement. Then take the two resistance values that you have measured and average them.

If you check this by going through the KVL and KCL equations for this circuit, you will see that by measuring twice with both positive and negative current, the EMF and offset voltages cancel out. Of course, when making this measurement, you also need to make sure that you do not apply too much power to the resistor so that thermal effects do not alter its resistance value.

Trap 3. Floating vs. Grounded Measurements

In electrical circuits, voltage is always measured between two points: a point of high potential and a point of low or zero potential.

The term “reference point” denotes the point of low potential because it is the point to which the voltage is referenced. A floating measurement is a differential measurement that is not referenced to ground (zero potential). It can be a concern if anyone is mistakenly making a floating measurement while expecting a ground measurement.

Let’s examine the counter measures to address this concern. As you can see from Figure 3 below, the configuration using a Keysight B2980A electrometer for these two cases is quite different.

If you are floating your DUT with respect to earth ground (such as in the top left of Figure 3), you can measure the resistance between the high terminal and the low terminal. Parasitic resistances and capacitances may provide a “sneak path” to ground on the low side. You can mitigate measurement errors by connecting the negative terminal of Vs source to the low terminal. In this way, the ammeter and the DUT low terminal have a “common” reference point.

The bottom left shows the circuit diagram that corresponds to a floating device measurement. The test device is connected between the VS positive output and the Ammeter input. Since the Ammeter measures very low currents and is very noise-sensitive, it should be measured close to ground potential in order to shield the test device for better measurement results.

On the top right, you can see the case where the DUT is grounded. Since the low side is grounded, the applied test voltage and the current measurement must both occur at the DUT's high side terminal. The bottom right shows the circuit diagram that corresponds to a grounded device measurement. In this configuration, the Ammeter is connected to the VS positive output because the device is grounded on one side.

Neither one of these configurations is necessarily “better” than the other, and you can obtain good high resistance measurement results using either setup.

Figure 3. Floating vs. grounded measurement.

Conclusion

# How to make IV measurements with a source measure unit (SMU)

Posted by CS Jan 26, 2018

## Why Perform Current-Voltage (IV) Measurements?

IV measurements obtain the current vs. voltage or resistance characteristics by providing a voltage/current stimulus and measuring current/voltage reaction. This is a basic electrical measurement and a fundamental way to understand the characteristics of various materials and devices under test (DUT).

Figure 1 shows the IV curves of some common electrical components. In the first graph, we see a linear relationship between voltage and current, so we immediately know that we are looking at a resistor. The graph in the center shows an exponential relationship between voltage and current. This probably means we are looking at a diode (or something that exhibits diode-like behavior). Finally, the graph on the far-right indicates that we are looking at some transistor curve. The characteristic of IV curves are able to provide us with immediate insight into a component, making them crucial in science and engineering.

Figure 1. IV curves of common electrical components

So, how do you make quick and accurate IV measurements? The most common tool for measuring IV curves is a source measure unit (SMU).

## What is a Source Measure Unit (SMU)?

An SMU combines the capabilities of a current source, voltage source, current meter and voltage meter into a single unit. This gives it the ability to evaluate the IV characteristics of devices across all four measurement quadrants without the need for any additional equipment. You get substantial cost and space savings compared to having multiple instruments.

Besides being able to output and measure voltage or current very accurately, SMUs also possess a compliance feature that allows a limit to be placed on the voltage or current output to prevent device damage.

As Figure 2 shows, a source measure unit packs an amazing amount of measurement capability into a very small package. SMUs can act as ideal 4-quadrant sources, meaning they will always try to maintain their programmed current or voltage until they reach the limit of their output power or a user-defined limit.

Figure 2. Simplified equivalent circuit (2-wire measurements) in an SMU

In addition, while sourcing current or voltage, SMUs can simultaneously measure both current and voltage. Figure 2 represents a single SMU channel, and this channel can exist as a standalone product or as one SMU channel within a mainframe that supports multiple SMU channels.

Notice that the “Low Force” side of the SMU can be connected to chassis ground via a relay. In fact, the default boot-up state of most SMUs has them configured with their low side grounded. However, it is sometimes convenient to float the “Low Force” side connection (for example when making differential measurements). In those cases, keep in mind that disconnecting the low side from ground is possible.

## Why Use an SMU for IV Measurements?

Without an SMU, here are some common test challenges that you may face when performing IV measurements:

• Difficulty in controlling and synchronizing multiple instruments
• Complex cabling and setup
• Difficulty in obtaining accurate, reliable measurements

When using multiple instruments simultaneously for an IV measurement, it is not easy to obtain good performance and accuracy due to the cabling and grounding with various instruments. In contrast, an SMU typically provides superior measurement performance that goes down to sub pA and sub µV resolutions.

SMUs integrate many capabilities into a single channel. It is possible to source voltage and measure both the sourced voltage and current (or source current and measure both the sourced current and voltage). But usually, either only voltage is sourced while current is measured, or only current is sourced while voltage is measured. Hence, the most common SMU use models are shown here (also in Figure 3):

• VSIM – Source voltage and measure current
• ISVM – Source current and measure voltage

Figure 3. Two of the most common modes of operation for an SMU

For why would you need to measure a source if the SMU will always hold its sourced value? When you specify a sourced value, you also must specify some limit on the measure unit value. And if for some reason the measured unit hits its limit, the value you expect to source may not actually be what you set. Below is a simple example to explain this (Figure 4).

Figure 4. IV curve of a sweep voltage

Suppose we are sweeping the voltage applied to a DUT (sourced value) and measuring the current flowing through the DUT. If the IV curve of the DUT hits the set current limit before the applied voltage reaches its stop value, then the applied voltage will remain at the voltage level for the rest of the sweep. In fact, all voltage and current data points beyond the limit value will be the same. However, unless you tell the SMU to measure its applied voltage, you will not be able to see it happening. If you are only measuring current, then all you will see is that the current does not change for the last part of the sweep.

All SMUs have some sort of indicator to tell you that a measurement has hit the limit. However, if you do not see this indicator, then reviewing the measured data is the only other way to catch this.

## Source Measure Units Make IV Measurements Easier

So, what are reasons that make an SMU the preferred choice for IV measurements – as opposed to using multiple discrete instruments?

It ultimately boils down to two things:

• Convenience, and
• Form factor.

A good example of this is shown in Figure 5, which shows measurement of current and voltage at the inputs and outputs of a four-terminal DUT. Performing these operations with discrete sources and meters is complicated and time-consuming, and requires many manual connection changes to modify a measurement setup.

In contrast, the same measurements can be performed using a 2-channel SMU with a very 'clean' setup. One channel can be connected to the device input, and the other channel can be connected to the device output. All the necessary connections to measure different current and voltage parameters can be performed without the need to modify any physical connections.

Figure 5. IV measurement setup comparison: using multiple instruments (left) vs. using a 2-channel SMU (right)

## Summary

IV measurements are an important part of testing as they provide unique insights into your device under tests. Source measure units (SMUs) are recommended instruments for IV measurements as they are able to provide better measurements, easy to set-up and operate, and take up minimal bench space – which makes them a cost effective investment.

If you'd like to know more about how to make accurate resistance measurements – which is one of the more challenging areas of measurement science – download this Application Note: Resistance Measurements Using the B2900A Series of SMUs. You will learn how you can overcome measurement issues such as residual test lead resistance, thermal electromotive force and leakage currents in the measurement path.

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