Sunday, March 31, 2013

Remote sensing can affect load regulation performance

Back in September of 2011, I posted about what load effect was (also known as load regulation) and how it affected testing (see 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.

For example, the Agilent 6642A power supply (20 V, 10 A, 200 W) has a voltage load regulation specification of 2 mV. This means that for any load current change between 0 A and 10 A, the output voltage will change by no more than 2 mV. The 6642A also has a remote sensing capability spec (really, a “supplemental characteristic”). It says that each load lead is allowed to drop up to half the rated output voltage. The rated output voltage for the 6642A is 20 V, so half is 10 V meaning when remote sensing, you can drop up to 10 V on each load lead. Also included in the 6642A remote sensing capability spec is a statement about load regulation. It says that for each 1 volt change in the + output lead, you must add 3 mV to the load regulation spec. For example, if you were remote sensing and you had 0.1 ohms of resistance in your + output load lead (this could be due to the total resistance of the wire, connectors, and any relays you may have in series with the + output terminal) and you were running 10 A through the 0.1 ohms, you would have a voltage drop of 10 A x 0.1 ohms = 1 V on the + output lead. This would add 3 mV to the load regulation spec of 2 mV for a total of 5 mV.

There are other ways in which this effect can be shown in specifications. For example, when remote sensing, the Agilent 667xA Series of power supplies expresses the load regulation degradation as a formula that includes the voltage drop in the load leads, the resistance in the sense leads, and the voltage rating of the power supply. Output voltage regulation is affected by these parameters because the sense leads are part of the power supply’s feedback circuit, and these formulas describe that effect:

One more example of a way in which this effect can be shown in specifications is illustrated by the Agilent N6752A. Its load effect specification is 2 mV and goes on to say “Applies for any output load change, with a maximum load-lead drop of 1 V/lead”. So the effect of load-lead drop is already included in the load effect spec. Then, the remote sense capability section simply says that the outputs can maintain specifications with up to a 1 V drop per load lead.

When you are choosing a power supply, if you want the output voltage to be very well regulated at your load, be sure to consider all of 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.

Watt's Up with Datalogging and Digitizing?

All of our power supplies offer the ability to take an average measurement using either the front panel or the MEAS SCPI commands.  Some of our newer power supplies have some more advanced measurement capabilities.   The two capabilities that we are going to look at today are digitized measurements and datalogging.   Let’s take a short look at each one and then talk about when to use each one.

The digitizer has been in our products for a while now.  With the digitizer, you define three parameters and the measurement uses these parameters to return an array of measurements back to you.  The three parameters are: the number of points, the time interval, and the points offset.  The number of points is pretty simple.  It is the number of measurements that you want to take as well as the size of the array that you are going to read back.  The time interval is the pace of the measurements.  This is also the time between the points in the array.  The points offset is a way that you vary the starting point of the array.  This offset can be negative to return measured points before the trigger or positive to delay the start of the measurement.  The most points that we can measure and the fastest time interval is with our N678xA SMU modules.  These modules have a time interval of 5.12 us and a total number of points of 512 Kpoints (keep in mind that 1 Kpoint is 1,024 points).  This yields a total time of 5.12 us x 512 x 1,024 which yields a result of 2.68 seconds.  So the longest measurement that you can make is 2.68 seconds.  The largest time interval that we can measure is 40,000 seconds.  Setting this with the highest number of points would yield 40,000 s x 512 x 1,024 yields a total acquisition of 20,971,520,000 seconds.  That is 666.83 years! 

The other advanced measurement capability that we are going to talk about is our datalogger.  With the datalogger, you set a total acquisition time and an integration time.  The integration time is the amount of time that the power supply will average measurements.  The measurement system is still running at its maximum digitizing rate but it is averaging those measurements and returning that averaged measurement.  The digitizer on the N6705B DC Power Analyzer also will return the maximum measured value and the minimum measured value of each integration period.  The quickest integration time on the N6705B is 20.48 us.  The only limitation in the amount of data that you can log with the internal datalogger is the file size (the maximum file size is somewhere near 2 gB).  If you want to datalog huge files, you can use the external datalog feature (I wrote another blog post about this) or use our 14585A software where the only limitation is the free space on your hard drive.  The catch on the external datalogger is that that the quickest integration time is 102 us.

So when do you use one over the other?  It is pretty simple.  When you want to make a long term measurement (days, weeks, etc.) at a fast rate you should use the datalogger.  You would use this when you are looking to measure something like long term battery drain.  If you are looking for a more short term, faster measurement you would use the digitizer.  You would use the digitizer to measure something like inrush current. 

These are a few of the great features available in our power supplies.  Please let us know if you have any questions on these features or any of the features of our power supplies.          

Wednesday, March 20, 2013

Open sense lead detection, additional protection for remote voltage sensing

A higher level of voltage accuracy is usually always needed for powering electronic devices under test (DUTs). Many devices provide guaranteed specifications for operating at minimum, nominal, and maximum voltages, so the voltage needs to accurate as to not require unacceptable amounts of guard banding of the voltage settings.

One very significant factor that affects the accuracy of the voltage at the DUT is the voltage drop in the wiring between the output terminals of the power supply and the actual DUT fixture, due to wiring’s inherent resistance, as shown in Figure 1.

 A standard feature of most all system DC power supplies is remote voltage sensing. Instead of the voltage being regulated at the output terminals of the DC power supply’s output terminal, it is instead sensed and regulated at the DUT itself, compensating for the voltage drop in the wiring. Additional details of this are documented in an earlier posting: “Use remote sense to regulate voltage at your load”

While remote voltage sensing addresses the problem of voltage drop in wiring affecting the voltage accuracy at the DUT, it then raises the concern of what happens if one of the sense lines becomes disconnected. Will the DC power supply voltage climb up to it maximum potential causing my DUT to be damaged?  Although this is a very legitimate concern, often the voltage is usually kept within a reasonable range of the setting by a feature referred to as “open sense lead protection”. A deeper dive on the issue of open sense lines and open sense lead protection are discussed at our posting: “What happens if remote sense leads open?”

Even with open sense lead protection and the voltage being kept within a reasonable range of the setting, this can be a concern for some customers who are relying on a high level of DC voltage accuracy at the DUT for test and calibration purposes. One categorical example of this is battery powered devices, where ADC circuits that need to precisely monitor the battery input voltage have to be accurately calibrated. If the voltage from the DC power supply has significant error, the DUT will be miss-calibrated.

One issue with open sense lead protection is it is a passive protection mechanism. It is simply a back up that takes over when a sense line is open. There is no way of knowing the sense lead is open. No error flag is set or fault condition tripped. The voltage being read back is the same as that is being regulated by the voltage sensing error amplifier, which is the same as the set voltage, so all looks fine from a read-back perspective. This is where open sense lead detection takes over. Open sense lead detection is a system that actively checks to see if the sense lines are doing their job. If not it lets the test system know there is a fault.

Open sense detection is not a common feature for most system DC power supplies. As one example we do employ it in our 663xx series Mobile Communications DC Sources as these are used for powering, testing and calibrating battery powered wireless devices. In the case of an open sense line condition it generates a fault condition and it keeps the output of the DC source powered down. It also provides status information on which of the sense lines are open as well.

Tuesday, March 12, 2013

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!