Not all two-quadrant power supplies are the same when operating near or at zero volts!

Occasionally when working with customers on power supply applications that require sourcing and sinking current which can be addressed with the proper choice of a two-quadrant power supply, I am told “we need a four-quadrant power supply to do this!” I ask why and it is explained to me that they want to sink current down near or at zero volts and it requires 4-quadrant operation to work. The reasoning why is the case is illustrated in Figure 1.

 Figure 1: Power supply sinking current while regulating near or at zero volts at the DUT

As can be seen in the diagram, in practical applications when regulating a voltage at the DUT when sinking current, the voltage at the power supply’s output terminals will be lower than the voltage at the DUT, due to voltage drops in the wiring and connections. Often this means the power supply’s output voltage at its terminals will be negative in order to regulate the voltage at the DUT near or at zero volts.

Hence a four-quadrant power supply is required, right? Well, not necessarily. It all depends on the choice of the two-quadrant power supply as they’re not all the same! Some two-quadrant power supplies will regulate right down to zero volts even when sinking current, while others will not. This can be ascertained from reviewing their output characteristics.

Our N6781A, N6782A, N6785A and N6786A are examples of some of our two-quadrant power supplies that will regulate down to zero volts even when sinking current.  This is reflected in the graph of their output characteristics, shown in Figure 2.

Figure 2: Keysight N6781A, N6782A, N6785A and N6786A 2-quadrant output characteristics

What can be seen in Figure 2 is that these two-quadrant power supplies can source and sink their full output current rating, even along the horizontal zero volt axis of their V-I output characteristic plots. The reason why they are able to do this is because internally they do incorporate a negative voltage power rail that allows them to regulate at zero volts even when sinking current. While you cannot program a negative output voltage on them, making them two-quadrants instead of four, they are actually able to drive their output terminals negative by a small amount, if necessary. This will allow them to compensate for remote sense voltage drop in the wiring, in order to maintain zero volts at the DUT while sinking current. This also makes for a more complicated and more expensive design.

Our N6900A and N7900A series advanced power sources (APS) also have two-quadrant outputs. Their output characteristic is shown in Figure 3.

Figure 3: Keysight N6900A and N7900A series 2-quadrant output characteristics

Here, in comparison, a certain amount of minimum positive voltage is required when sinking current. It can be seen this minimum positive voltage is proportional to the amount of sink current as indicated by the sloping line that starts a small maximum voltage when at maximum sink current and tapers to zero volts at zero sink current.  Basically these series of 2-quadrant power supplies are not able to regulate down to zero volts when sinking current. The reason why is because they do not have an internal negative power voltage rail that is needed for regulating at zero volts when sinking current.

So when needing to source and sink current and power near or at zero volts do not immediately assume a 4-quadrant power supply is required. Depending on the design of a 2-quadrant power supply, it may meet the requirements, as not all 2-quadrant power supplies are the same! One way to tell is to look at its output characteristics.

New performance options for the N6900A Advance Power System gives greater versatility for your test needs

Our N6900 and N7900 series Advanced Power System (APS) DC power supplies are some of our most sophisticated products, setting new levels of performance and capabilities on many fronts. They come in 1kW and 2kW power levels as shown in Figure 1 and can be grouped together to provide much greater power levels as needed.

Figure 1: N6900 and N7900 Advanced Power System 1kW and 2kW models

Most noteworthy is that these can be turned into full two-quadrant DC sources by connecting up the optional 1kW N7909A Power Dissipator (2 needed for 2kW units) providing 100% power sinking capability. This makes APS an excellent solution for battery, battery management and many alternative energy applications, where both sourcing and sinking power are needed.

• The N6900 series DC power supplies are designed for ATE applications where high test throughput and high performance is critical.
• The N7900 series dynamic DC power supplies are designed for ATE applications where high speed dynamic sourcing and measurement is needed, in additions to high performance.

A lot more about these products is covered in another post on our General Purpose Electronic Test Equipment (GEPTE) blog when they were first announced. This is a great resource for learning more about APS and can be accessed from the following link: “New Advanced Power System: Designed to Overcome Your Toughest Test Challenges”

If you are a regular visitor to the “Watt’s Up?” blog no doubt you have seen we have shared a lot about how to do things with the N6900 series and N7900 series APS to address a number of difficult test challenges. A lot of times it would have otherwise required additional equipment or custom hardware to accomplish these tasks. While many of these examples are suitable for the N6900 and N7900, a good number of times examples make use of the additional capabilities only available in the N7900 series.

In certain test situations the N6900 series APS would be a great solution and lower cost than the N7900 series, if only it also had a certain additional capability. To this end Keysight has recently announced four new performance options for the N6900 series APS to address a specific test need you may have, as follows:

1. Accuracy Package (option 301): Adds a second seamless measurement range for current
2. Measurement Enhancements (Option 302): Adds external data logging and voltage and current digitizers with programmable sample rates
3. Source and Speed Enhancements (Option 303): Adds constant dwell arbitrary waveforms and output list capability, and faster up and down programming speed
4. Disconnect and Polarity-Reversal Relays (Option 760 and 761): Provides galvanic isolation and allows output voltage to be switched between positive and negative values

 Additional details about the N6900 series APS and the four new performance options are available from the recent press release, available at the following link: “Keysight Technologies adds Versatile Performance Options to Industry’s Fastest Power Supplies”

With these new options you now have a spectrum of choices in the Advanced Power System product family to better address any test challenges you may be faced with!

Why does the response time of OCP vary on the power supply I am using and what can I do about it? Part 2

In the first part of this posting (click here to review) I highlighted what kind of response time is important for effective over current protection of typical DUTs and what the actual response characteristic is for a typical over current protect (OCP) system in a test system DC power supply. For reference I am including the example of OCP response time from the first part again, shown 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.  As can be seen, for reasons previously explained, the response time clearly depends on the amount of overdrive beyond the current limit setting.

As the time to cause over current damage depends on the amount of current in excess of what the DUT can tolerate, with greater current causing damage more quickly, the slower response at lower overloads is generally not an issue.  If however you are still looking how you might further improve on OCP response speed for more effective protection, there are some things that you can do.

The first thing that can be done is to avoid using a power supply that has a full output current rating that is far greater than what the DUT actually draws. In this way the overdrive from an overload will be a greater percentage of the full output current rating. This will normally cause the current limit circuit to respond more quickly.

A second thing that can be done is to evaluate different models of power supplies to determine how quickly their various current limit circuits and OCP systems respond in based on your desired needs for protecting your DUT. For various reasons different models of power supplies will have different response times. As previously discussed in my first part, the slow response at low levels of overdrive is determined by the response of the current limit circuit.

One more alternative that can provide exceptionally fast response time is to have an OCP system that operates independently of a current limit circuit, much like how an over voltage protect (OVP) system works. Here the output level is simply compared against the protect level and, once exceeded, the power supply output is shut down to provide near-instantaneous protection. The problem here is this is not available on virtually any DC power supplies and would normally require building custom hardware that senses the fault condition and locally disconnects the output of the power supply from the DUT. However, one instance where it is possible to provide this kind of near-instantaneous over current protection is through the programmable signal routing system (i.e. programmable trigger system) in the Keysight N6900A and N7900A Advanced Power System (APS) DC power supplies. Configuring this triggering is illustrated in Figure 2.

Figure 2: Configuring a fast-acting OCP for the N6900A/N7900A Advanced Power System

In Figure 2 the N7909A software utility was used to graphically configure and download a fast-acting OCP level trigger into an N7951A Advanced Power System. Although this trigger is software defined it runs locally within the N7951A’s firmware at hardware speeds. The N7909A SW utility also generates the SCPI command set which can be incorporated into a test program.

Figure 3: Example custom-configured OCP system response time vs. overdrive level

Figure 3 captures the performance of this custom-configured OCP system running within the N7951A. As the OCP threshold and overdrive levels are the same this can be directly compared to the performance shown in Figure 1, using the conventional, current limit based OCP within the N7951A. A 5 millisecond OCP delay was included, as before. However, unlike before, there is now virtually no extra delay due to a current limit control circuit as the custom-configured OCP system is totally independent of it. Also, unlike before, it can now be seen the same fast response is achieved regardless of having just a small amount or a large amount of overdrive.

Because OCP systems rely on being initiated from the current limit control circuit, the OCP response time also includes the current limit response time. For most all over current protection needs this is usually plenty adequate.  If a faster-responding OCP is called for minimizing the size of the power supply and evaluating the performance of the OCP is beneficial. However, an OCP that operates independently of the current limit will ultimately be far faster responding, such as that which can be achieved either with custom hardware or making use of a programmable signal routing and triggering system like that found in the Keysight N6900A and N7900A Advanced Power Systems.

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.