Tips to prevent voltage droop from tripping low voltage detection circuits

There are many battery operated devices such as cell phones, hand-held two-way radios, and portable GPS’s that have low voltage detection circuits. These circuits are designed to prevent the device from trying to operate at battery voltages that are below a safe value for reliable operation of the internal circuitry. Voltage supplied by a battery located very close to the circuitry drawing current from it remains fairly rigid even when the device draws pulses of current which is often the case. However, during testing of the device, a power supply is frequently used to power the device instead of the battery. Voltage supplied by a power supply, typically located quite a distance from the circuitry drawing current, will often momentarily droop each time a positive edge of current is drawn. This momentary voltage droop can cause an undesired trip of the low voltage detection circuit in the device interrupting the test.

Here are some tips to reduce voltage droop caused by fast current changes on the output voltage of a power supply.

·         Shorten wires from power supply to DUT (device under test)

Wires have resistance (R) and inductance (L), both of which develop voltage across them when a current pulse flows through the wire. Shortening the length of wire will reduce the voltage drop developed across the R and L, reducing the droop at the DUT.

·         Use larger diameter wire from the power supply to the DUT

Larger diameter wire will have lower R, reducing the voltage developed across it when current flows

·         Install multiple runs of the same wire in parallel

Parallel wires will have lower R and lower L, again reducing the voltage developed across them when current flows

·         Lower the inductance of the wire

o   Tightly twist the plus and minus power supply output wires together

Never allow the power supply plus and minus output wires to become separated. This will substantially increase the inductance, increasing voltage drop with current, especially if the current changes quickly (V = L * dI/dt). Simply placing the wires next to each other is much better than letting them fall freely, but twisting them together is highly recommended over tightly coupling them without twisting. See example results shown below.

o   Add multiple wires in parallel

As mentioned earlier, adding multiple wires in parallel reduces inductance. The best method to use here is to twist pairs of plus and minus wires together, and then run each twisted set separately to the DUT (bundling the twisted sets together is not as effective as keeping the sets separated).

o   Use a low inductance cable

Some cables are designed specifically to have low inductance. Goertz wire is one example. Also, Temp-Flex makes low inductance cable. These types of wire can drastically lower the inductance in the path between your power supply and your DUT, greatly reducing voltage droop that occurs with current transients. However, these cables tend to be expensive.

·         Eliminate connectors

Remove as many connectors as possible between the power supply output and DUT. When current flows through a connector, voltage is dropped across the connection points.

·         Use a power supply with a low output impedance

Some power supply vendors publish output impedance graphs. Try to use one with the lowest output impedance possible. Current pulses drawn from a power supply with lower output impedance will drop less voltage than a power supply with higher output impedance.

·         Add low ESR capacitors at the power supply output

You can reduce the effective output impedance of your power supply by adding a low ESR (equivalent series resistance) cap right at the output of your power supply. Many power supplies already have output caps and fairly low output impedance, so this will help only if the caps you choose actually help to lower the overall output impedance.

·         Add low ESR capacitors right at the DUT

When current is demanded by the DUT, having a local cap right at the DUT to provide the current will greatly reduce the voltage drop on the wire running to the DUT. This is because the required current comes from the cap and does not have to flow through the wire where it would drop voltage. It is important to choose caps with low ESR. Otherwise, when the current flows out of the cap, the voltage will again droop due to the current dropping voltage across the ESR.

If you are having trouble with voltage droop due to fast current changes, each of the above tips will help to contribute to reducing the droop. If the droop is large, it is unlikely you will be able to use just one technique from above to fix it. Most likely, you will have to implement many if not all of the methods above to get the best performance possible from your test setup.

Below is a simple example showing measured droop differences when using three different wiring techniques: free falling wire, loosely coupled wire, and twisted wire. An Agilent N6751A power supply with 10 feet of 10AWG wire running between it and an Agilent 6063B electronic load was used. The N6751A was set for 5 V with a current limit of 5 A, and the load was set to switch between 1 A and 3 A with a rise time of about 10 us. Remote sense was used on the power supply, sensing at the load input. A current probe captured the current (lower waveforms) and the voltage droop was measured (upper waveforms) at the load input which was at the end of the 10 feet of wire.

You can see the voltage droop was reduced as the wires became better coupled, lowering their inductance. The droop measured 1.7 V with the wires free falling. This droop was reduced significantly to 0.84 V with loosely coupled wire. Further reduction in droop was observed when the wires were twisted: the droop measured 0.69 V.

Many of the concepts presented here are explored further in a paper co-authored several years ago by one of our other Watt’s Up? blog authors, Ed Brorein. Here is a link to that paper: http://www.home.agilent.com/upload/cmc_upload/All/EPSG083914.pdf

The Joys of Owning a DC Power Supply

Back in 2000, when I was a young man and had just begun working for Agilent., Gary came up to me and told me that there was a brand new power supply that was going to be scrapped and that I should take it.  The power supply was a 6024A 200 W autoranger that was a very old design, even at that time.  I took a look and it had analog meters and no keypad!  I questioned Gary on why I would want it and he said, “Trust me it will come in handy”.

Here it is:

Gary, being older and wiser than I was at the time, was right.  I had just started working at Agilent and I was still living with my parents so I put it in the basement.  When I moved, I took it with me to my apartment.  Since then there have been several occasions where it came in very handy. 

The first is that there is always an occasional battery that needs charging so I can hook up the 6024A and charge my batteries (we highly recommend putting a blocking diode in series with the output of the supply to protect your battery). It is controlled so there is no danger to the batteries and it totally works like a champ.

The biggest use I have had for it has been troubleshooting the various electronic gadgets that I own.  The first time I used it for this purpose was to troubleshoot my wireless router.  One day it just stopped working.  Instead of throwing it out (wireless G routers were pretty expensive at that point) I figured that I would use my power supply to determine whether it was a problem with the wall adaptor for the router or the router itself.  I bought some cables from an electronics store, grabbed my soldering iron, and hooked my router up to my DC Power Supply.  I set the voltage and the current limit and the router came to life.  All I need to do was to get a new adapter and I was good to go. 

A similar thing happened with an internet streaming box that I had connected to my TV.  All of a sudden it stopped working.  I called their tech support (a terrible experience) they told me that I had to purchase a new box.  Since I had nothing to lose, I chopped up the DC adaptor and hooked it up to my 6024A.  I wish that I took a picture.  I had the box connected to my TV being powered by my 6024A.  The box came to life and I did not need to spend 100 bucks on a new one

Not everyone is as lucky as I am to work for Agilent and get a free HP supply but if you can get your hands on any kind of programmable DC power supply I recommend taking it.  I am glad that I have my power supply and I keep it on my bench, always ready to use.

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Types of current limits for over-current protection on DC power supplies

On a previous posting “The difference between constant current and current limit in DC power supplies”, I discussed what differentiates a DC power supply having a constant current operation in comparison to having strictly a current limit for over-current protection. In that post I had depicted one very conventional current limit behavior. However there is actually quite a variety of current limits incorporated in different DC power supplies, depending on the intended end-use of the power supply.

Fold-back Current Limit

The output characteristic of a constant voltage (CV) power supply utilizing fold-back current limiting is depicted in Figure 1. Fold-back current limiting is sometimes used to provide a higher level of protection for DUTs where excess current and power dissipation can cause damage to a DUT that has gone into an overload condition. This is accomplished by reducing both the current and voltage as the DUT goes further into overload. The short circuit current will typically be 20% to 50% of the maximum current level. A reasonable margin between the crossover current point and required maximum rated DUT current needs to be established in order to prevent false over-current tripping conditions. Due to the fold-back nature, and depending on the loading nature of the DUT, the operating point could drop down towards the short-circuit operating point once the crossover point is reached/exceeded. This would require powering the DUT down and up again in order to get back to the CV operating region.

Figure 1: Output characteristic of a CV power supply with fold-back current limiting

In addition to providing over-current protection for the DUT, fold-back current limiting is often employed in fixed output linear DC power supplies as a means for reducing worst case dissipation in the power supply itself. Under short circuit conditions the voltage normally appearing across the DUT instead appears across the power supply’s internal series linear regulator, requiring it to dissipate considerably more power than it has to under normal operating conditions. By employing fold-back current limiting the power dissipation on the series-linear regulator is greatly reduced under overload conditions, reducing the size and cost of the series-linear regulator for a given output power rating of the DC linear power supply.

Fold-forward Current Limit

A variety of loading devices, such as electric motors, DC-DC converters, and large capacitive loads can draw large peak currents at startup. Because of this they can often be better suited for being powered by a DC power supply that has a fold-forward current limit characteristic, as depicted in Figure 2. With fold-forward current limiting after exceeding the crossover current limit the current level instead continues to increase while the voltage drops while the loading increases.

Figure 2: Output characteristic of a CV power supply with fold-forward current limiting

As one example of where fold-forward current limiting is a benefit, it can help a motor start under load which otherwise would not start under other current-limits. Indeed, with fold-back current limiting, a motor may not and then it would remain stalled, due to the reduced current.

Special Purpose Current Limits

Unlike the previous current limit schemes which are widely standard practice, there is a number of other current limit circuits used, often tailored for more application-specific purposes. One example of this is the current limiting employed in our 66300 series DC sources for powering mobile phones and other battery powered mobile wireless devices. Its output characteristic is depicted in Figure 3.

Figure 3: Agilent 66300 Series DC source output characteristics

We refer to this power supply series as battery emulator DC sources. One reason why is they are 2-quadrant DC sources.  Like a rechargeable battery, they need to be able to source current when powering the mobile device and then sink current when the mobile device is in its charging mode.  In Figure 3 there are actually two separate current limits; one for sourcing current and another for sinking current. Each has different and distinctive characteristics for specific purposes.

Many battery powered mobile wireless devices draw power and current in short, high peak bursts, especially when transmitting. To better accommodate these short, high peaks, the 66300 series DC sources have a time-limited peak current limit that is of sufficient duration to support these high peaks. They also have a programmable constant current level that will over-ride the peak current limit when the average current value of the pulsed current drain reaches this programmed level. With this approach a higher peak power mobile device can be powered from a smaller DC power source.

Just like an electronic load, when the 66300 series DC source is sinking current the limiting factor is how much power it is able to dissipate. Instead of using a fixed current limit, it uses a fold-forward characteristic current limit (although folding forward in the negative direction!). This is not done for reasons that a fold-forward current limit that was just discussed is used; it is done so higher charging currents at lower voltage levels can be accommodated, taking advantage of the available power that can be dissipated. Again, this provides the user with greater capability in comparison to using a fixed-value limit.

Other types of current limits exist for other specific reasons so it is helpful to be aware that not all current limits are the same when selecting a DC power supply for a particular application!

Reference: Agilent Technologies DC Power Supply Handbook, application note AN-90B, part number 5952-4020 “Click here to access”

The difference between constant current and current limit in DC power supplies

Constant Voltage/Constant Current (CC/CV) Power Supplies

In most of our discussions in “Watt’s Up?” on current limiting we have primarily talked about power supplies as having a constant current (CC) output characteristic. This is what is found in many lab and industrial system power supplies, including most of the power supplies provided by us. Even though the terms often get used interchangeably, there is actually a distinction between constant current and current limit. To help explain this distinction, Figure 1 illustrates the output characteristics of a constant voltage/constant current (CV/CC) power supply.

Figure 1: Operating locus of a CC/CV power supply

Five operating points are depicted in Figure 1:

1. With no load (i.e. infinite load resistance): Iout = 0 and Vout = Vset
2. With a load resistance of RL > Vset/Iset: Iout = Vset/RL and Vout = Vset
3. With a load resistance of RL = Vset/Iset: Iout = Iset and Vout = Vset
4. With a load resistance of RL < Vset/Iset: Iout = Iset and Vout = Iset*RL
5. With a short circuit (i.e. zero load resistance): Iout = Iset and Vout = 0

The advantage of a CV/CC power supply is it can be used as either a voltage source or a current source, providing reasonable performance in either mode. The point at which RL = Vset/Iset is the mode crossover point where the power supply transitions between CV and CC operation. For a CV/CC power supply there is a sharp transition between CV and CC operation. Note that for an ideal CV/CC power supply the CV slope is zero (horizontal), indicating zero output resistance for CV operation while the CC slope is infinite (vertical), indicating infinite output resistance for CC operation. Note that this is at DC. How close the slope of each mode is to ideal is what determines quality of load regulation for each.  To achieve good performance for both CV and CC modes requires carefully designed and more complex control loops for each mode. More details about using a power supply as a current source is provided in an earlier posting here, entitled: “Can a standard DC power supply be used as a current source?”

Constant Voltage/Current Limiting Power Supplies

In comparison a constant voltage/current limiting (CV/CL) power supplies are intended to be used only as a voltage source while providing over-current protection for the DUT, as well as protection for the power supply itself. Figure 2 depicts typical output characteristics of a CV/CL power supply.

Figure 2: Operating locus of a CV/CL power supply

In CV/CL power supplies the current limit may be a fixed maximum value or it may be settable. In comparison to Figure 1 CV operation is still the same. However, what is found at the current limit cross-over point there is loss of voltage regulation where the voltage starts falling off. Unlike true CC operation in a CV/CC power supply, CL operation does not typically have as sharply a defined cross-over point and once in CL it may not be tightly regulated between the cross-over and short circuit points. The reason for this is CL control circuits are usually more basic in nature in comparison to a true CC control loop. CL is meant for over-current protection only, not CC operation.  For this reason the correct use of CL is to set its value a bit higher than the maximum current required by the DUT. This assures good voltage regulation for the full range of normal loading. You may find many of the more basic bench power supplies have CV/CL operation and may not be useful as current sources as a result.

Reference: Agilent Technologies DC Power Supply Handbook, application note AN-90B, part number 5952-4020

Happy New Year! May it be a “powerful” one!!

I just want to take this opportunity to thank all of our readers for taking an interest in the Watt’s Up? blog posts. Power-related topics have been a part of our professional careers (and personal lives) for decades and we are both thrilled and honored to be able to share some of what we have learned over the years with you, our readers.

During 2012, we have seen our readership grow by more than 5 times that of 2011! And we hope to see that growth continue during 2013. To make that happen, we would like to hear from you, our readers, about what power-related topics are of greatest interest to you for 2013. Please comment below and we will be happy to post about any power-related issues you bring up!  

Finally, on behalf of Ed Brorein, Matt Carolan, and myself, and all of us at Agilent Technologies, Happy New Year! May 2013 be a “powerful” year for you all!!

Two-quadrant power supplies are better than one!

Back in October, I posted an explanation about what was a bipolar (four-quadrant) power supply (see post here: http://powersupplyblog.tm.agilent.com/2012/10/what-is-bipolar-four-quadrant-power.html). That post covered two-quadrant supplies as well. Last week, while in Lorton, Virginia, I had an opportunity to meet with some of our U.S. Army customers  - engineers working at Fort Belvoir. Many of the engineers worked in the Counter Measures Research Laboratory (CMRL). While they are very careful to not reveal any details about the specifics of the work they do, one of the engineers shared a story with me about two-quadrant operation that is worth repeating.

The story was told while I was providing a demonstration of one of our power supplies, the N6705B DC Power Analyzer (see Figure 1). I was explaining to a group of engineers that some of the 34 power modules that can be installed in the N6705B are two-quadrant power supplies: they can source current and also sink current at one voltage polarity. Other power modules are four-quadrant power supplies: they can source and sink current, and provide positive or negative voltage. This explanation inspired one of the engineers to tell the group that the N6705B helped him solve a problem!

A battery operated device (he did not mention what it was) came into his lab because it was not functioning properly: it had some type of intermittent problem. In an attempt to reproduce the problem, he removed the battery and connected the device’s power input terminals to a power supply on his lab bench. But even after running the device for long periods of time and through all of its operating modes, he was unable to reproduce the intermittent problem.

One of his colleagues suggested he try connecting the device to a two-quadrant power supply installed in the N6705B they owned. The original power supply he was using was a one-quadrant supply – it could source power, but could not absorb power. The battery that normally powers the device can source and sink (absorb) power, so perhaps a power supply that more closely mimicked the behavior of the battery could help uncover the problem. Well, this worked! With the device connected to the two-quadrant power supply in the N6705B, the intermittent problem showed up again proving that it was related to the battery being able to source and sink power – a power supply with similar characteristics was needed. Apparently, the device has a mode in which it momentarily forces current back out of the battery input terminals. That current is normally absorbed by the battery. And during that time, this intermittent problem must show up. During test, a single-quadrant power supply is unable to absorb the power and therefore does not reveal the problem. A two-quadrant power supply can sink the momentary current, and the problem was back, enabling the engineer to track it down and eliminate it! See Figure 2 for an example of the output characteristic of a two-quadrant power supply.

This example demonstrates the importance of choosing a power supply with the right output characteristics for your test. When testing a device or circuit with a power supply, the closer that power supply’s behavior is to the actual power used with the device or circuit, the more you will reveal about the actual performance of your device or circuit.  There are applications in which a two-quadrant power supply will better replicate a battery’s behavior than a single-quadrant power supply, even if you don’t expect the battery to absorb power during test. One CMRL engineer experienced this firsthand.

More on power supply current source-to-sink crossover characteristics

On my earlier posting “Power supply current source-to-sink crossover characteristics” I showed what the effects on the output voltage of a unipolar two-quadrant-power supply were, resulting from the output current on the power supply transitioning between sourcing and sinking. In that example scenario, the power supply was maintaining a constant output voltage and the transitioning between sourcing and sinking current was dictated by the external device connected to and being powered by the power supply. This is perhaps the most common scenario one will encounter that will drive the power supply between sourcing current and sinking current.

Other scenarios do exist that will drive a unipolar two-quadrant power supply to transition between sourcing and sinking output current. One scenario is nearly identical to the earlier posting. However, instead of the device transitioning its voltage between being less and greater than the power supply powering it, the power supply instead transitions its voltage between being less and greater than the active device being normally powered.  A set up for evaluating this scenario on an Agilent N6781A two-quadrant DC source is depicted in Figure 1.

Figure 1: Evaluating current source-to-sink crossover on an N6781A operating in constant voltage

In this scenario having the DC source operating as a voltage source and transitioning between 1.5 and 4.5 volts causes the current to transition between -0.75 and +0.75A.  The voltage and current waveforms captured on an oscilloscope are shown in Figure 2.

Figure 2: Voltage and current waveforms for the set up in Figure 1

The waveforms in Figure 2 are as what should be expected. The actual transition points are where the current waveform passes through zero on the rising and falling edge. An expanded view to the current source-to-sink transition is shown in Figure 3.

Figure 3: Expanded voltage and current waveforms for the set up in Figure 1

As can be seen the voltage ramp transitions smoothly at the threshold point, or zero crossing point, of the current waveform. The reason being is that the DC is maintaining its operation as a voltage source. Its voltage feedback loop is always in control.

Yet one more scenario that will drive a unipolar two-quadrant source to transition between sourcing and sinking current is operate it as a current source and program is current setting between positive and negative values. In this case the device under test that was used is a voltage source.  One real-world example is cycling a rechargeable battery by alternately applying charging and discharging currents to it. The set up for evaluating this scenario, again using an N6781A two-quadrant DC source is depicted in Figure 4.

Figure 4: Evaluating current source-to-sink crossover on an N6781A operating in constant current

For Figure 4 the N6781A was set to operate in constant current and programmed to alternately transition between -0.75A and +0.75A current settings. The resulting voltage and current waveforms are shown in Figure 5.

Figure 5: Voltage and current waveforms for the set up in Figure 4

The waveforms in Figure 5 are as what should be expected. The actual transition points are where the current waveform passes through zero on the rising and falling edge. An expanded view to the current source-to-sink transition is shown in Figure 6.

Figure 6: Expanded voltage and current waveforms for the set up in Figure 4

As the N6781A is operating in current priority the interest is in how well it controls its current while transitioning through the zero-crossing point. As observed in Figure 6 it transitions smoothly through the zero-crossing point. The voltage performance is determined by the DUT, not the N6781A, as the N6781A is operating in constant current.

So what was found here is, for a unipolar two-quadrant DC source, transitioning between sourcing and sinking current should generally be virtually seamless as, under normal circumstances, should remain in either constant voltage or constant current during the entire transition.