New Firmware Available for the N6700B, N6701A, and N6702A Modular Power System Frames.

Hello everybody!

I recently posted a new firmware file for the N6700B, N6701A, and N6702A mainframes to the Agilent website.  You can access the new firmware at: http://www.agilent.com/find/N6700firmware.  The latest firmware revision is D.01.09.  There are two new measurement features that I wanted to highlight.  These two new features are External Datalogging and Power Measurements. 

External Datalogging (ELOG for short) is a feature that we have had in the N6705 DC Power Analyzer for some time now that we have just added to the modular power mainframes.  This feature allows you to take averaged measurements at a specified time interval for however long you want (be careful though, you can fill your hard drive).  The time interval can be anywhere from 102.4 us to 60 s depending on the number of parameters being logged.  You can take those measurements and store them in whatever format you want (I usually store everything in a CSV file).  You can only access ELOG using a SCPI program (you cannot ELOG from the front panel).  I plan on writing more about ELOG in the future but here is a quick peek of what the SCPI commands to set up and execute an ELOG look like:

SENS:ELOG:FUNC:CURR ON,(@2)                     !Turn current logging on

SENS:ELOG:FUNC:CURR:MINM ON,(@2)        !Turn current min/max logging on

SENS:ELOG:FUNC:VOLT ON,(@2)                     !Turn voltage logging on

SENS:ELOG:FUNC:VOLT:MINM ON,(@2)        !Turn voltage min/max logging on

SENS:ELOG:PER 0.0007,(@2)                            !Sets an integration time of 700 us

TRIG:ELOG:SOUR BUS,(@2)                              !The ELOG will start when there is a bus trigger

INIT:ELOG (@2)                                                   !Tell the unit to wait for a trigger

*TRG                                                                      !Trigger

This is an example of how you would read back the logged data:

while current time<the time you want to log for

FETC:ELOG? 4096 ,(@2)                                    !Read back a maximum of 4096 ELOG records

store into a file

loop

This command kills the ELOG:

ABOR:ELOG (@2)                                              !Return the unit to its normal state

Stay tuned to this blog for more information.

The other feature that we added to a few of our modules was the ability to measure power.  We can now measure power on the N676xA, N6781A, N6782A, and N6784A modules.  Why is it only on these few modules you ask?  That is because these modules have two measurement digitizers that allow it to measure both voltage and current at the same time.  Since power = voltage * current, you need to have a simultaneous voltage and current measurement to get an accurate power measurement. 

That is all I have for today.  If you have any questions, please just let us know.

Why Does My Power Supply Overshoot at Current Limit? Insights on Mode Crossover

One often encountered issue with power supply use is expecting that the current limit will clamp the current to no greater than the set value, only to discover the current initially overshoots when the DUT demands current in excess of the set limit. In some cases the short surge of excess current may be enough to damage a sensitive DUT. Those experienced with power supplies will recognize this as a dynamic characteristic of mode crossover.

What is mode crossover? Mode crossover is the transition point between Constant Voltage (CV) and Constant Current (CC) modes. The dynamic response characteristic of mode crossover is an aspect that separates real-world from ideal-world power supplies. To start it will be helpful to review a previous posting on “How Does a Power Supply regulate its Output Voltage and Current?” Here it is shown there are two control loops in most power supplies, one for regulating the voltage and one for regulating the current. Only one is in control at any given time while the other is “open loop”. The error amplifier that is open loop is up against it stops. When load conditions change such that the power supply transitions through mode crossover the open loop error amplifier needs to recover and gain control of the output. In the more common case of the power supply operating as a voltage source there can be a current overshoot during the brief moment when the load increases beyond the power supply’s current limit setting. Conversely, for a current source, there can be a voltage overshoot during the brief moment when the load decreases, causing the output voltage to rise to the voltage limit setting.

The magnitude of the overshoot depends on many factors relating to both the power supply and the DUT. Supplementary circuitry usually surrounds the error amplifiers to clamp them from being driven into saturation or cutoff so that they can more quickly recover when needed. Amplifiers are carefully selected for their recovery characteristics. Careful design is required to assure a stable transition between modes during crossover while at the same time minimizing the delay and overshoot.  The magnitude of the overshoot also depends on how quickly and to what extent the DUT transitions between loading conditions.

Figure 1 shows the mode crossover current overshoot of a 50 volt, 3 amp general purpose power supply, set for 10 volts and 1 amp output.  The loading DUT is an electronic load set to transition from no load to 10 amps with a slew of 0.8 amps per microsecond. This loading represented a worst case for all practical purposes. When the load transitions to full (i.e. overload) it takes about 6 milliseconds for the current limit control loop to fully take over and bring the current down. During this mode crossover period the current overshoot plateaus at 5 amps, which is the gross current limit capacity of the power supply. Basically this is the point where the power supply runs out of drive.

Figure 1: Constant voltage to constant current mode crossover for 10 V, 1A power supply settings

In Figure 2 the power supply current limit was reduced to 0.1 amps and the mode crossover was again captured. This had an interesting impact on the current overshoot. While the peak current still hit a plateau of 5 amps, the duration of the overshoot was considerably reduced to about 0.5 milliseconds.  The reason for this is there was a much larger difference driving the error amplifier’s input, causing it to transition more quickly. The peak level remained unchanged as it is determined by the power supply’s gross current limit capacity, which is fixed.

Figure 2: Constant voltage to constant current mode crossover for 10 V, 0.1A power supply settings

The extent of an overshoot during mode crossover depends on the power supply as well as the DUT. A power supply optimized for voltage sourcing usually has very little voltage overshoot at mode crossover, but then can have significant current overshoot, as we see here. Conversely, a power supply optimized for current sourcing usually has very little current overshoot at mode crossover, but then can have significant voltage overshoot. Higher performance power supplies may provide faster and better mode crossover performance, but this usually comes at greater expense. Some useful things to do include:

·         Be aware that overshoot during mode crossover is a reality that exists in most all power supplies

·         Try not to oversize the power supply. Be aware that the peak level of voltage or current during mode crossover may be governed more by the maximum voltage and current ratings of the power supply and less by the settings. Using an oversized power supply with its limit set to 5% of its capacity will likely yield a much larger overshoot than a smaller one with it limit set to 50% of its capacity.

·         Understand the nature of your DUT, behavior or fault modes that may cause it to draw an overload, and how sensitive it is to an overload

·         If your DUT is sensitive to an overload, include evaluating the response characteristics of mode crossover as part of your evaluation, using realistic conditions that reflect the characteristics of your DUT.

Recognizing that there is dynamic response characteristics associated with mode crossover of “real-world” power supplies, and they need to be considered, may save a lot of surprise and frustration later on!

How Does a Power Supply regulate It’s Output Voltage and Current?

We have talked about Constant Voltage (CV) and Constant Current (CC) power supply operation in many various ways and applications here on the “Watt’s Up?” blog in the past. Indeed, CV and CC are fundamental operating modes of most all power supplies. But what exactly takes place inside the power supply that endows it with the ability to regulate either its output voltage or current, depending on the load? If you ever wondered about this, wonder no longer!

Most all power supplies regulate either their output voltage or output current at a constant level, depending on the load resistance relative to the power supply’s output voltage and current settings. This can be summarized as follows:

·         If R load > (V out / I out) then power supply is in CV mode

·         If R load < (V out / I out) then power supply is in CC mode

To accomplish this most all power supplies have separate voltage and current feedback control loops to limit either the output voltage or current, depending on the load. To illustrate this Figure 1 shows a circuit diagram of a basic 5 volt, 1 amp output series regulated power supply operating in CV mode.

Figure 1: Basic DC Power Supply Circuit, Constant Voltage (CV) Operation

The CV and CC control loops/amplifiers each have a reference input value. In this case the reference values are both 1 volt. In order to regulate output voltage the CV error amplifier compares its 1 volt reference against a resistor divider that divides the output voltage down by a factor of 5, limiting the output voltage to 5 volts. Likewise the CC error amplifier compares its 1 volt reference against a 1 ohm current shunt resistor located in the output current path, limiting the output current to 1 amp. For Figure 1 the load resistance is 10 ohms. Because this load resistance is greater than (V out / I out) = 5 ohms, the power supply is operating in CV mode. The CV error amplifier takes control of the series pass transistor by drawing away excess base current from the series pass transistor, though the diode “OR” network. The CV amplifier is operating in closed loop, maintaining its error voltage at zero volts. In comparison, because the actual output current is only 0.5 amps the CC amplifier tries to turn the current on harder but cannot because the CV amplifier has control of the output. The CC amplifier is operating open loop. Its output goes up to its positive limit while it has -0.5 volts of error voltage. The output I-V diagram for this Constant Voltage operation is shown in Figure 2.

Figure 2: Power Supply I-V Diagram, CV Operation

Now say we increase the load by lowering the output load resistance from 10 ohms down to 3 ohms. Figure 3 shows the circuit diagram of our basic 5 volt, 1 amp output series regulated power supply revised for operating in CC mode with a 3 ohm load resistor.

Figure 3: Basic DC Power Supply Circuit, Constant Current (CC) Operation

Because the load resistor is lower than (V out / I out) = 5 ohms, the power supply switches to CC mode. The CC error amplifier takes control when the voltage drop on the current shunt resistor increases to match the 1 volt reference value, corresponding to 1 amp output, drawing excess base current from the series pass transistor though the diode “OR” network. The CC amplifier is now operating closed loop, regulating the output current to maintain its input error voltage at zero. In comparison, because the actual output voltage is now only 3 volts the CV amplifier tries to increase the output voltage but cannot because the CC amplifier has control of the output. The CV amplifier is operating open loop. Its output now goes up to its positive limit while it has -0.4 volts of error voltage. The output I-V diagram for this Constant Current operation is shown in Figure 4.

Figure 4: Power Supply I-V Diagram, CC Operation

As we have seen most all power supplies have separate current and voltage control loops to regulate their outputs in either a Constant Voltage (CV) or in a Constant Current (CC) mode. One or the other takes control, depending on that the load resistance is in relation to what the power supply’s output voltage and current settings are. In this way both the load and power supply are protected by limiting the voltage and current that is delivered by the power supply to the load. By understanding this theory behind a power supply’s CV and CC operation it is also easier to understand the underlying reason for why various power supply characteristics are the way they are, as well as see how other power supply capabilities can be created by building on top of this foundation. Stay tuned!

Validating Battery Capacity for End-use Conditions

In my previous posting “Some Basics on Battery Ratings and Their Validation” I discussed the importance of making certain you are getting the most out of your battery as a key task for optimizing the battery run-time of a mobile battery powered device. You do not want to just rely on what is specified for the battery but you really need to validate it. Indeed, when I did, I found a battery’s capacity to be 12% lower than its rated value. That is a lot of unexpected loss of run-time to try to make up for! On further testing and investigation I indeed confirmed it was the battery and not something I did with inadequate charging or discharging.

Once you get a handle on the battery’s stated ratings, based on recommended charging and discharging conditions, you should then validate the capacity you are able to get under the loading conditions your device subjects the battery to. Most modern mobile battery powered devices draw high peak pulsed, low average current from the battery. Batteries subject to pulsed loading deliver less capacity in comparison to being subjected to the comparable loading that is only DC.  The amount of impact depends on the battery’s design and its ability to handle high peak pulsed loading. Furthermore two different batteries with the same ratings can deliver substantially different results in the end-use application. The bottom line is you need to validate the battery under end-use conditions to assess how much impact it has on the battery’s performance.

Creating end-use operating conditions for devices of course depends on the type of device. In some cases it may be fairly simple but in many cases it can be rather complex. A smart mobile phone, for example, requires a set up that can emulate the wireless network it normally operates in and then place it in a representative active operating state under which to run down the battery. The battery’s run down voltage and current in turn needs to be logged until the battery reaches its proper discharge termination point, in order to assess the amount of capacity it delivers under end-use conditions. An example of such a set up is shown in Figure 1.

Figure 1: End-use battery run down test set up for a mobile phone

As you are trying to assess battery capacity under end-use conditions you will likely want to run trials several times and for different batteries, you will want to control conditions as closely as possible so that you can confidently compare results knowing they were done under comparable test conditions. You also need to be careful about (not) relying on the mobile device’s internal battery management system for end-of-life discharge termination as it is a possible source of error. A technique I resorted to was to record a representative portion of the end-use pulsed current drain drawn by the mobile phone which I then “played back” continuously through our N6781A SMU, acting as an electronic load, to discharge the battery. The N6781A had the required fidelity, accuracy, and “playback” hooks to faithfully reproduce loading of the actual device.  Further details on this record and playback approach are documented in a technical overview “Simplify Validating a Battery’s Capacity and Energy for End-Use Loading Conditions”. The results of my validating the battery’s capacity under end-use conditions are shown in Figure 2.

Figure 2: End-use battery capacity validation results

In this case the battery delivered 3% less capacity under end-use pulsed loading in comparison to the results when validated using comparable DC-only loading. Here the battery appears well suited for its end-use application. Many times however, the impact can be much greater. As always, make certain to take appropriate safety precautions when working with batteries and cells.

Test of Time power supply contest winners announced

Earlier this week, the winners of Agilent’s Test of Time power supply contest were announced. Here is a link to the press release:

http://www.agilent.com/about/newsroom/presrel/2012/25jun-em12080.html

I found the Test of Time contest to be quite interesting and I was honored to be one of the judges for the contest. The contest invited engineers who were using vintage Agilent or Hewlett-Packard (is there a “vintage” Agilent supply?) or even the older Harrison Labs power supplies (HP power supplies started as Harrison Labs power supplies) to describe their application, writing about how the instrument has been used over the years and how they are using it today. We received quite a few entries and we had extensive discussions to choose what we considered the best entry based on the contest rules. Go to this link for the home page of the contest and select the Gallery tab to see all of the entries:
http://powercontest.tm.agilent.com/

Richard Factor, of Little Ferry, NJ (I did not know he was from NJ until after I submitted my choices as a judge) won one of the prizes: an N6705B DC Power Analyzer with three modules installed. Quite a nice prize, and well deserved based on Richard’s entry!

Richard used an old HP 6186B in an application that basically turned his Toyota Prius into a backup generator for his house during a power failure. Now that’s what I call a unique application! You can read more about it at these links:

http://priups.com/agilent%20contest/hp-6186b-entry.htm

http://www.priups.com/ (you must select the correct answer….I’m sure you’ll figure it out….)

Simon Jensen of Husum, Germany, also won an N6705B for his entry. Simon’s entry was chosen by readers of the stories who voted for their favorite. Simon used an Agilent 6632B power supply as an inexpensive load to sink current from a switching power supply he built. As Simon correctly points out, the nameplate on this power supply does not reveal all of its capabilities. It says 0 to 5 A, but it can also sink a programmable, regulated current like an electronic load. So the nameplate should say -5 A to +5 A, as pointed out by Simon!

Having worked for HP/Agilent on power products for more than 32 years (since 1980), it was not too surprising for me to see so many interesting applications for our products. I was also delighted, and not too surprised, to see how many of our older power supplies are still out there, providing power, decades after they were introduced! Now that’s what I call “vintage voltage”!!

Using Power Supply Status Registers in your Program – Not So Scary

Hello everyone!  Today I am going to talk about how to use the Operation Status and Questionable Status Registers on Agilent’s power supplies.  All of Agilent’s SCPI based power supplies use these registers.  Figure 1 is a pictorial representation of the status system of the Agilent N6700 Modular Power System:

 

Figure 1 N6700 Status Model

 

Looks pretty daunting, doesn’t it?  After reading this blog post, it won’t be so scary anymore.   There are many great uses for these registers in your programs.

The Questionable Status Group lets you know if your power supply is in an abnormal operating state.  Sometimes your power supply will be in protect mode when these states are encountered.  You typically want to query this to make sure that your power supply does not transition to one of these abnormal states.  Here is a list of all the members of the Questionable Status Group:

Figure 2 N6700 Questionable Status Group

The Operation Status Group provides the normal operating modes of the power supply.  You typically want to query this register to either make sure that the power supply is either in the correct operating state or that it has completed some task (such as initiating a trigger or performing a measurement).  Here are the different members of the Operation Status Group:

 Figure 3 Operation Status Group

There are multiple ways to program using the registers.  The main way that I query the resisters is using the Condition Register.  The Condition Register will give you the real time status of the register.  Reading this register does not clear it.   A good example of using this is when you are initiating a triggered measurement:

 

INIT:ACQ (@1)                                    //This initiates the Acquire trigger system

Do

STAT:OPER:COND? (@1)       //Check the status register

               status = read

Loop Until (status And 8) = 8

*TRG  

Looking at Fig 3, 8 is the WTG_MEAS bit.  Once this is true, the unit is ready to be triggered.  This lets you make sure that you are not triggering the unit before the initiation is done.  Note that all of the statuses are referred to by the instrument by their by the bit weight (listed in the tables as the decimal value).  When you are looking for multiple statues, you need to add the bit weights.

 

The other main method of Querying the registers is using the Event register.  The Event Register is different than the Condition Register in that it keeps track of transitions in the statuses.  It does not matter when the status change happened, the Event Register will catch it and keep it until it is read back.   You do need to tell the power supply which statuses you are concerned with using the Negative Transition (STAT:OPER:NTR) and the Positive Transition (STAT:OPER:PTR) commands.  The Event Register is a latching register that will clear after it is read.  An example of using this register is when you want to make sure that there were no momentary transitions into an unwanted status.  In the following example, you want to make sure that your power supply does not go into the Unregulated mode (UNR) before you make a current measurement:

STAT:QUES:PTR 1024,(@1)            // This is the UNR bit

STAT:QUES:NTR 0, (@1)                // We are only interested in a positive transition

Body of  your program

STAT:QUES:EVEN?                        //Query the register

Status = readback

If status And 1024 =1024 then

 exit                                     //If the unit went UNR, exit

Else

                MEAS:CURR? (@1)      //measure current

                Curr=readback

End if

 

In this case, UNR is bit weight 1024.  When the unit transitions to the unregulated state (the bit transitions from 0 to 1), this bit gets set. 

As you can tell by Figure 1, there is a bunch more that you can do using status, including Service Requests but I will save those for a possible future blog post.

Please feel free to post any questions or comments here.