Why Does Over Current Protect (OCP) have a Programmable Delay Value in the First Place?

Since I am on a roll about over current protect (OCP), having just completed a two-part posting “Why does the response time of OCP vary on the power supply I am using and what can I do about it?” (Review part 1) (Review part 2) there is yet another aspect about OCP that is worth bringing up at this time. And that is “why does OCP have a programmable delay value in the first place?” This actually came up in a discussion with a colleague here after having read my part posting.

It may seem a bit ironic that OCP has a programmable delay in that in my posting on OCP I shared ideas on how one can minimize the response time delay encountered. But this is not contradictory. One may very well want to minimize it, eliminating extra delay being encountered, but not necessarily eliminate it altogether. As can be seen in my previous postings, I had programmed the OCP delay time to 5 ms.

The programmable OCP delay does serve a purpose, and that is to prevent false OCP trips. Adding some delay time prevents these false trips.  For someone who knows the root cause of false OCP tripping they might be half right. There are actually been two main causes of false OCP trips which are prevented by adding some delay time.

The original problem with OCP was that it would be falsely tripped when output voltage settings were changed on the power supply, due to capacitive loading at the test fixture or within the DUT. This is especially prominent with inrush current when first bringing up the voltage to power the DUT. An OCP delay prevents false triggering under these conditions. To correct the false tripping the delay would be invoked when output programming changes were made. As one example, the OCP delay description in our manual for our 663x series power supplies states:

“This command sets the time between the programming of an output change that produces a constant

current condition (CC) and the recording of that condition by the Operation Status Condition register. The

delay prevents the momentary changes in status that can occur during reprogramming from being

registered as events by the status subsystem. Since the constant current condition is used to trigger

overcurrent protection (OCP), this command also delays OCP.”

Under this situation the momentary overcurrent is induced by the power supply. Although not nearly as much as in issue in practice, momentary overcurrents can also be DUT-induced as well. This is the second situation that can cause a false tripping of the OCP. The DUT may be independently turned on after the bias voltage has already been on and draw a surge of current. Or the DUT may change mode of operation and draw a temporary surge of current.  If the OCP delay is invoked only by an output programming change it does not have any effect in these situations.

On later generation products, such as our N6700, N6900, and N7900 series, the user also has the ability to programmatically select between having the OCP delay activate from either an output change, or from going into CC condition. This gives the user a way to remain consistent with original operation or have OCP delay effective for momentary DUT-induced overload currents as well!

Why does the response time of OCP vary on the power supply I am using and what can I do about it? Part 1

In a previous posting of mine “Providing effective protection of your DUT against over voltage damage during test”(click here to review), an important consideration for effective protection was to factor in the response time of the over voltage protect (OVP) system. Due to the nature of over voltage damage, the OVP must be reasonably fast. The response time can typically be just a few tens of microseconds for a reasonably fast OVP system on a higher performance system power supply to hundreds of microseconds on a more basic performance system power supply. This response time usually does not vary greatly with the amount of over voltage being experienced.

Just as with voltage, system power supplies usually incorporate over current protect (OCP) systems as well. But unlike over voltage damage, which is almost instantaneous once that threshold is reached, over current takes more time to cause damage. It also varies in some proportion to the current level; lower currents taking a lot longer to cause damage. The I²t rating of an electrical fuse is one example that illustrates this effect.

Correspondingly, like OVP, power supply OCP systems also have a response time. And also like OVP, the test engineer needs to take this response time into consideration for effective protection of the DUT.  However, unlike OVP, the response time of an OCP system is quite a bit different. The response time of an OCP system is illustrated in Figure 1.

Figure 1: Example OCP system response time vs. overdrive level

Here in Figure 1 the response time of the OCP system of a Keysight N7951A 20V, 50A power supply was characterized using the companion 14585A software. It compares response times of 6A and 12A loading when the current limit is set to 5A. Including the programmed OCP delay time of 5 milliseconds it was found that the actual total response time was 7 milliseconds for 12A loading and 113 milliseconds for 6A loading.

This is quite different than the response time of an OVP system. Even if the OCP delay time was set to zero, the response is still on the order of milliseconds instead of microseconds for the OVP system. And when the amount of overdrive is small, as is the case for the 6A loading, providing just 1A of overdrive, the total response time is much greater. Why is that?

Unlike the OVP system, which operates totally independent of the voltage limit control system, the OCP system is triggered off the current limit control system. Thus the total response time includes the response time of the current limit as well. The behavior of a current limit is quite different than a simple “go/no go” threshold detector as well. A limit system, or circuit, needs to regulate the power supply’s output at a certain level, making it a feedback control system. Because of this stability of this system is important, both with crossing over from constant voltage operation as well as maintaining a stable output current after crossing over. This leads to the slower and overdrive dependent response characteristics that are typical of current limit systems.

So what can be done about the slower response of an OCP system? Well, early on in this posting I talked about the nature of over current damage. Generally over current damage is much slower by nature and the over drive dependent response time is in keeping with time dependent nature of over current damage. The important thing is understand what the OCP response characteristic is like and what amount of over current your DUT is able to sustain, and you should be able to make effective use of the over current protection capabilities of your system power supply.

If however you are still looking how you might further improve on OCP response speed, look for my follow up to this in my next posting!

Creating a "bumping" auto-restarting over current protect on the N6900A/N7900A Advanced Power System

The two main features in system power supplies that have traditionally protected DUTs from too much current are the current limit and the over current protect (OCP). When a device, for any of a number of reasons, attempts to draw too much current, the current limit takes control of the power supply’s output, limiting the level of current to a safe level. An example of current limit taking control of a power supply output is shown in Figure 1.

Figure 1: Current limit protecting a DUT against excess current.

For those devices that cannot tolerate a sustained current at the current limit level, the over current protect can be set and activated to work with the current limit and shut down the power supply output after a specified delay time. This will protect a DUT against sustained current at the limit.  An example of an OCP shutting down a power supply output for greater protection against excess current is shown in Figure 2.

Figure 2: OCP protecting a DUT against excess current

We have talked about the current limit and OCP in previous posts. For more details on how the OCP works, it is worth reviewing “What is a power supply’s over current protect (OCP) and how does it work?” (Click here to review)

Sometimes it is desirable to have something that is in between the two extremes of current limit and OCP.  One middle-ground is a fold-back current limit, which cuts back on the current as the overload increases. More details about a fold-back current limit are described in a previous posting “Types of current limits for over-current protection on DC power supplies” (Click here to review). One thing about a fold-back current limit is the DUT and power supply will not be able to recover back into constant voltage (CV) operation unless the DUT is able to cut way back on its current demand.

Another type of current limit behavior that operates between regular current limit and OCP is one that shuts down the output, like OCP, but only temporarily. After a set period of time it will power up the output of the power supply again. If the DUT is still in overload, the power supply will shut down again. However, if the DUT’s overload condition has gone away, it will be able to restart under full power. In this way the DUT is protected against continuous current and at the same time it the power supply is not shut down and requiring intervention from an operator.

While this type of current limit is not normally a feature of a system DC power supply, it is possible to implement this functionality in the N6900A/N7900A Advanced Power System (APS) using its expression signal routing feature. This is a programmable logic system that is used to configure custom controls and triggers that run within the APS. Here the expression signal routing was used to create an auto-restarting current shutdown protect in the example shown in Figure 3.

Figure 3: Custom auto-restarting current shutdown protect configured for N6900A/N7900A APS

A custom control was created in the expression signal routing that triggers the output transient system to run if the current limit is exceeded for longer than 0.3 seconds. A list transient was programmed into the APS unit to have its output go to zero volts for 10 seconds and then return to the original voltage setting each time it is triggered. In this way the output would pulse back on for 0.3 seconds and then shut back down for another 10 seconds if the overload was not cleared. The custom trigger signal was graphically created and downloaded into the APS unit using the N7906A software utility, as shown in Figure 4.

Figure 4: Creating custom trigger for auto-restarting current shutdown protect on APS

Current limit and over current protect (OCP) are fairly standard in most all system DC power supplies for protecting your DUT against excess current. There are not a lot of other choices beyond this without resorting to custom hardware. One more option now available is to make use of programmable signal routing like that in the N6900A/N7900A APS. With a little ingenuity specialized controls like a auto-restarting current shutdown protect can be created through some simple programming.

Upcoming Seminar on Protecting Your Device against Power-Related Damage during Test

Here on “Watt’s Up?” we have provided a good number of posts about various protection features incorporated into system power supplies to protect your device against power-related damage during test. Just recently my colleague Gary posted “How Does Power Supply Over-Voltage Work?” (Click here to review) Here he reviews inner workings of different OVP implementations.  I recently posted “Safeguarding Your Power-Sensitive DUTs against an Over-Power Condition” (Click here to review) Here I go over a method to protect your DUT against excess power when other power supply features like over current protection may be less than ideal.

The reason why we frequently share power-related protection topics here is protecting your DUT is extremely important, there are a lot of different capabilities incorporated in system power supplies for this purpose, and there are a lot of practical considerations when putting them to use.  

Hopefully a number of you have found our posts on protection-related topics of help. Because this is a very important topic and there is so much more you should know about it I will be giving a live web-based seminar “Protecting Your Device against Power-Related Damage during Test” on August 20^th, just a few weeks away from today. I will be going over a number of protection-related topics which we have not yet covered here on “Watt’s Up?”.  One of my objectives is to provide a more holistic view of the many ways a system power supply is able to better safeguard against power-related damage as well as what is practical to expect when using these various capabilities incorporated in the power supply.

You can register online at the following (Click here for description and registration page) In case you are not able to attend the live event on August 20 you will be able to register and listen to seminar afterward as well, as it will be recorded.

So if protecting your device against power-related damage is important to you I hope you are able to attend the seminar!

Safeguarding your power-sensitive DUTs from an over power condition

Today’s system DC power supplies incorporate quite 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 all system DC power supplies to help protect against power-related damage.

OVP helps assure the DUT is protected against power-related damage in the event 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 is suitably higher than maximum expected DUT operating voltage so that any transient voltages do not cause false tripping. Causes of OV conditions are often external to the DUT.

OCP helps assure 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 from consuming excess power due to overloading or internal problem causing inefficient operation and excessive internal power dissipation.

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

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

However, not all DUTs operate over as limited a range as depicted in Figure 1. Consider for example many, if not most all DC to DC converters operate over a wide range of voltage 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, consider a DC to DC converter that operates from 24 to 48 volts and runs at 225W is shown in Figure 2. DC to DC converters operate very efficiency 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 causing 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, as can be seen 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 instead be to have an over power protection limit, as depicted in Figure 3. This would provide an adequate safeguard regardless of input voltage setting.

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

As 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 or desirable, or in some cases practical to do. However, in the Agilent 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 in turn be used to create a logical expression to use the output power level to trigger an output protect shutdown. This is depicted in Figure 4, using the N7906A software utility to graphically configure this logical expression and then download it into the Advance Power System DC power supply. As 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 have been just as easily used to trigger a variety of other actions as well.

Protect your DUT from over-current in more ways than one

Last month, I posted about one of our new families of products: the N6900/N7900 Series 1- and 2-kW Advanced Power System (APS) DC Power Supplies (click here). I typically like to post about more general power topics rather than focus on specific Agilent products, but this product has some really interesting features from which you can benefit. After 33 years of working on power here, there aren’t too many new products that get me excited, but this is one of them! So here is a story about an application for it.

Earlier this month, I visited one of our customers that had a device under test (DUT) whose input was sensitive to too much current. That is typically not a difficult issue to protect against using Agilent power supplies with over-current protection (OCP). Set the current limit to a value that you don’t want to exceed, turn on OCP, and the power supply output will go into protect (turn off) when the current limit value is reached. Simple enough! But this customer had an additional requirement. In addition to an OCP value as just described, he also wanted to shut down the output if the current exceeded a lower current for more than a specified amount of time. So he wanted the power supply output to go into protect (turn off) if either of the following conditions occurred on his DUT (I changed this example to protect his information):

       1.  DUT input current exceeds 6 A for any amount of time, or
       2.  DUT input current exceeds 4.5 A for 80 ms

To be honest, at the time of the visit, I wasn’t sure if our new product could do this. The product is so new and so feature-rich that I am not yet familiar with all of its capabilities. But when I returned to my office, I set it up and found it was very easy to do! Here is the solution:

I used the advanced signal routing and logical trigger expressions built into our N7952A APS to setup both requirements. I could have sent SCPI commands to setup the same trigger configuration, but our free Power Assistant Software (N7906A) made this even easier. Figure 1 shows the software with the configuration.

If, after creating the configuration, I want all of the SCPI commands that correspond to it for a program, I could use the software feature “SCPI to clipboard” that creates them from the configuration. See Figure 2.

Take a look at this feature in action. Figure 3 shows a scope trace of the current waveform. As you can see, currents that are less than 4.5 A do not trip the protection. And currents above 4.5 A for less than 80 ms (and below 6 A) also do not trip the protection. But as soon as the current exceeds 4.5 A for 80 ms (and remains below 6 A), the protection tripped – the output shut off causing the current to go to zero amps.

This is just one example of how versatile the N6900/N7900 APS power supplies are. For more information about how these advanced power systems can help you in your power application, please use this link: www.agilent.com/find/aps. To explore this advanced signal routing and logical trigger expressions feature even more, take a look at a post from one of my collegues: http://gpete-neil.blogspot.com/2013/10/protecting-your-dut-during-test-with.html

What is a power supply’s over current protect (OCP) and how does it work?

One feature we include in our Agilent system DC power supplies for providing additional safeguard for overload-sensitive DUTs is over current protect, or OCP. While some may think this is something separate and independent of current limiting, OCP actually works in concert with current limiting.

Current limiting protects overload-sensitive DUTs by limiting the maximum current that can be drawn by the DUT to a safe level. There are actually a variety of current limit schemes, depending on the level of protection required to safeguard the DUT during overload. Often the current limit is relatively constant, but sometimes it is not, depending on what is best suited for the particular DUT. Additional insights on current limits are provided in an earlier posting, entitled “Types of current limits for over-current protection on DC power supplies“.

By limiting the current to a set level may DUTs are adequately protect from too much current and potential damage. When in current limit, if the overload goes away the power supply automatically goes back to constant voltage (CV) operation. However, current limit may not be quite enough for some DUTs that are very sensitive to overloads. This is where OCP works together with the current limit to provide an additional level of protection. With OCP turned on, when the DC power supply enters into current limit OCP takes over after a specified time delay and shuts down the output of the DC power supply. The delay time is programmable. This prevents OCP from shutting down the DC power supply from short current spikes and other acceptably short overloads that are not considered harmful. Like over voltage protect or OVP, after tripping the output needs to be disabled and an Output Protect Clear needs to be exercised in order to reset the power supply so that its output can be re-enabled.  Unlike OVP, OCP can be turned on and off and its default is usually off. In comparison, OVP is usually always enabled and cannot be turned off. A typical OCP event is illustrated in Figure 1.

Figure 1: OCP operation

When powering DUTs, either on the bench or in a production test system, it is always imperative that adequate safeguards are taken to protect both the DUT as well as the test equipment from inadvertent damage. Over current protect or OCP is yet another of many features incorporated in system DC power supplies you can take advantage of to protect overload-sensitive DUTs from damage during test!

Protecting your DUT using a power supply’s remote inhibit and fault indicator features

Paramount in most any good electronic test system is the need to adequately protect the device under test (DUT), as well as the test equipment, from inadvertent damage due to possible faults with the yet-untested DUT, accidental misconnections, misapplication of power, and a large number of other unanticipated events that can occur. It is no surprise that a lot of these unanticipated events by nature are related to the powering of the DUT. For this reason good system DC power supplies incorporate a number of features designed to protect both the DUT, as well as the power supply, in the event of an unanticipated fault occurring.  Two related protection features incorporated into our DC system power supplies are the remote inhibit and the discrete fault indicator (RI/DFI). These features provide real-time protection enabling immediate shutting down the power supply, as well as enabling the power supply to take immediate action, on the event of detecting the occurrence of an unanticipated event or fault.

The remote inhibit is a digital input control while the discrete fault indicator is a digital output control signal, incorporated into the digital I/O port on our system DC power supplies. An example of a digital I/O port is illustrated in Figure 1. When the digital I/O port is configured for fault/inhibit (also called RI/DFI) pins 1 and 2 are the open collector and emitter of an isolated transistor, to serve as a digital output control, and pin3 and 4 are the digital input and common for the inhibit control input. The remote inhibit and the fault indicator can be used independently as well as in combination, for protecting the DUT.

Figure 1: Multi-function digital I/O port on Agilent 6600A series system DC power supplies

As the name implies, the remote inhibit is a digital control input, when activated, immediately disables the DC power supply’s output. One way this is commonly used is to connect an emergency shutdown switch that can be conveniently activated in the event of a problem. This may be a large pushbutton, or it may be a switch incorporated into a fixture safety cover. This arrangement is shown in Figure 2.

Figure 2: Remote inhibit using external switch

The fault indicator (i.e. FLT, FI, or DFI) digital output signal originates from the system DC power supply’s status system. The status system is a configurable logic system within the power supply having a number of registers that keep track of its status for operational, questionable, and standard events. Many of these events can be logically OR’ed together as needed to provide a fault output signal when particular, typically unanticipated, events occurs with the power supply. Items tracked by questionable status group register, like over voltage and over current, for example, are commonly selected and used for generating a fault output signal. An overview of the power supply status register system was discussed by a colleague in a previous posting. If you are interested in learning more; click here.

The fault indicator output can in turn be used to control an external activity for protecting the DUT, such as opening a disconnect relay to isolate the DUT, as one example, as depicted in Figure 3.

Figure 3: Fault output controlling an external disconnect relay

For DUTs that require multiple bias voltage inputs it is usually desirable that if a fault is detected on one bias input, that the other bias inputs are immediately shut down in conjunction with the one detecting a fault. The fault outputs and remote inhibit inputs on several DC power supplies can be used in combination by chaining them together, as depicted in Figure 4, to accomplish this task, to safeguard the DUT.

Figure 4: Chaining fault indicators and remote inhibits on multiple DC power supplies

The remote inhibit and fault indicator digital control signals on system DC power supplies provide a number of ways to disable power and take other actions for safeguarding the DUT. Their action is immediate, not requiring communication to, and intervention from, the test system controller. At the same time the system DC power supply generates status signals and can issue a service request (SRQ) to the test system controller so that it is notified of a problem condition and take appropriate correction action as well. The remote inhibit and fault indicator digital control signals are just two of many features found in many good system DC power supplies to assure the DUT is always adequately protected during test!

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

Protect your DUT with power supply features including a watchdog timer

The two biggest threats of damage to your device under test (DUT) from a power supply perspective are excessive voltage and excessive current. There are various protection features built into quality power supplies that will protect your DUT from exposure to these destructive forces. There are also some other not-so-common features that can prove to be invaluable in certain applications.

Soft limits
The first line of defense against too much voltage or current can be using soft limits (when available). These are maximum values for voltage and current you can set that later prevent someone from setting output voltage or current values that exceed your soft limit settings. If someone attempts to set a higher value (either from the front panel or over the programming interface), the power supply will ignore the request and generate an error. While this feature is useful to prevent accidentally setting voltages or currents that are too high, it cannot protect the DUT if the voltage or current actually exceeds a value due to another reason. Over-voltage protection and over-current protection must be used for these cases.

Over-voltage protection
Over-voltage protection (OVP) is a feature that uses an OVP setting (separate from the output voltage setting). If the actual output voltage reaches or exceeds the OVP setting, the power supply shuts down its output, protecting the DUT from excessive voltage. The figure below shows a power supply output voltage heading toward 20 V with an OVP setting of 15 V. The output shuts down when the voltage reaches 15 V.

Some power supplies have an SCR (silicon-controlled rectifier) across their output that gets turned on when the OVP trips essentially shorting the output as quickly as possible. Again, the idea here is to protect the DUT from excessive voltage by limiting the voltage magnitude and exposure time as much as possible. The SCR circuit is sometimes called a “crowbar” circuit since it acts like taking a large piece of metal, such as a crowbar, and placing it across the power supply output terminals.

Over-current protection
Over-current protection (OCP) is a feature that uses the constant current (CC) setting. If the actual output current reaches or exceeds the constant current setting causing the power supply to go into CC mode, the power supply shuts down its output, protecting the DUT from excessive current. The figure below shows a power supply output current heading toward 3 A with a CC setting of 1 A and OCP turned on. The power supply takes just a few hundred microseconds to register the over-current condition and then shut down the output. The CC and OCP circuits are not perfect, so you can see the current exceed the CC setting of 1 A, but it does so for only a brief time.

The OCP feature can be turned on or off and works in conjunction with the CC setting. The CC setting prevents the output current from exceeding the setting, but it does not shut down the output if the CC value is reached. If OCP is turned off and CC occurs, the power supply will continue producing current at the CC value basically forever. This could damage some DUTs as the undesired current flows continuously through the DUT. If OCP is turned on and CC occurs, the power supply will shut down its output, eliminating the current flowing to the DUT.

Note that there are times when briefly entering CC mode is expected and an OCP shutdown would be a problem. For example, if the load on the power supply has a large input capacitor, and the output voltage is set to go from zero to the programmed value, the cap will draw a large inrush current that could temporarily cause the power supply to go into CC mode while charging the cap. This short time in CC mode may be expected and considered acceptable, so there is another feature associated with the OCP setting that is a delay time. Upon a programmed voltage change (such as from zero to the programmed value as mentioned above), the OCP circuit will temporarily ignore the CC status just for the delay time, therefore avoiding nuisance OCP tripping.

Remote inhibit
Remote inhibit (or remote shutdown) is a feature that allows an external signal, such as a switch opening or closing, to shutdown the output of the power supply. This can be used for protection in a variety of ways. For example, you might wire this input to an emergency shutdown switch in your test system that an operator would use if a dangerous condition was observed such as smoke coming from your DUT. Or, the remote inhibit could be used to protect the test system operator by being connected to a micro switch on a safety cover for the DUT. If dangerous voltages are present on the DUT when operating, the micro switch could disable DUT power when the cover is open.

Watchdog timer
The watchdog timer is a unique feature on some Agilent power supplies, such as the N6700 series. This feature looks for any interface bus activity (LAN, GPIB, or USB) and if no bus activity is detected by the power supply for a time that you set, the power supply output shuts down. This feature was inspired by one of our customers testing new chip designs. The engineer was running long-term reliability testing including heating and cooling of the chips. These tests would run for weeks or even months. A computer program was used to control the N6700 power supplies that were responsible for heating and cooling the chips. If the program hung up, it was possible to burn up the chips. So the engineer expressed an interest in having the power supply shut down its own outputs if no commands were received by the power supply for a length of time indicating that the program has stopped working properly. The watchdog timer allows you to set delay times from 1 to 3600 seconds.

Other protection features that protect the power supply itself
There are some protection features that indirectly protect your DUT by protecting the power supply itself, such as over-temperature (OT) protection. If the power supply detects an internal temperature that exceeds a predetermined limit, it will shut down its output. The temperature may rise due to an unusually high ambient temperature, or perhaps due to a blocked or incapacitated cooling fan. Shutting down the output in response to high temperature will prevent other power supply components from failing that could lead to a more catastrophic condition.

One other way in which a power supply protects itself is with an internal reverse protection diode across its output terminals. As part of the internal design, there is often a polarized electrolytic capacitor across the output terminals of a power supply. If a reverse voltage from an external power source was applied across the output terminals, the cap (or other internal circuitry) could easily be damaged. The design includes a diode across the output terminals with its cathode connected to the positive terminal and its anode connected to the negative terminal. The diode will conduct if a reverse voltage from an external source is applied across the output terminals, thereby preventing the reverse voltage from rising above a diode drop and damaging other internal components.