Sunday, June 30, 2013

What is Command Processing Time?

Hello everybody,

We have a new intern here at Agilent Power & Energy HQ named Patrick.  Gary, Patrick, and I have been having a philosophical debate on what the term command processing time means.  This is a very important number for many of our customers since it tells them what kind of throughput they can get out of our test equipment.  A fast command processing time allows you to reduce your test times and therefore increase your throughput.  The question that we have been debating is:  what is command processing time and how can we measure it?  We have been discussing three scenarios.   Let’s go through them.

The first option is the amount of time that it takes the processor to take one command off the bus so that it can get to the next command.  This tells you how quickly you can send commands to the instrument.  The only issue with this is that some instruments have a buffer so it is not actually “processing” the command, just bringing it into the buffer and letting you send the next command.  Obviously this is useful but it really does not address the throughput question.  This is pretty easy to test by sending a command in a loop and timing it.  You record the time before the command is sent and the time after the loop and then divide by the number of loops you executed.  This would yield a pretty good approximation of the time.

The second option is the amount of time from when the instrument receives a command until it starts performing the action.  I believe that this is what we list in our manuals for the Command Processing Time Supplemental Characteristic.  This does address the throughput issue.  This is also easy for us at Agilent to measure.  We have a breakout for GPIB that allows us to monitor the attention line.  The test that we did was send a VOLT 5 to the instrument.  We looked at the GPIB attention line.  The time from when the attention line toggles until the power supply starts slewing the voltage up would be our command processing time (measured with an always awesome Agilent Oscilloscope).  This is what I consider to be the command processing time.

The third option includes what I spoke about in the last paragraph but also includes the slewing of the voltage.  The processing time would be the time that it takes to take the command and complete all the actions associated with it (for example settling at 5 volts after being sent a VOLT 5 command).  I do not think that this is a bad option but we have a Supplemental Characteristic for voltage rise time that addresses the slewing of the voltage.   The test method would be the same as above using an oscilloscope but watching for where the voltage settles at five volts.

What do you, our readers and customers think the correct interpretation of command processing time is?  Also, please stay tuned for a future installment where we try to figure out what the quickest interface is: LAN, USB, or GPIB.  

Tuesday, June 25, 2013

Current limit setting affects voltage response time

The current limit setting in a power supply is primarily used to protect the device under test (DUT) from excessive current. You should set your current limit setting higher than the maximum amount of current you expect your DUT to draw, but low enough so that if your DUT fails as a short or low impedance, it does not draw an amount of current that can damage wires, connectors, or the DUT itself due to excessive current. The power supply will limit the current at the current limit setting and reduce the voltage accordingly. If you want, you can turn on over-current protection (OCP) and then the power supply output will turn off if the output transitions into constant current (CC) mode. For previous posts on this topic, click here and here.

Current limit plays an important role in protecting your DUT. But you should also know that the current limit setting can affect the voltage response time, specifically the up-programming speed. Voltage up-programming speed is the time it takes the output voltage to go from a lower voltage to a higher voltage. For example, the up-programming output response time for an Agilent N5768A power supply (rated for 80 V, 19 A, 1520 W) is specified to be no more than 150 ms with a full load (settling band is 1% of the rated output voltage). This spec assumes the current limit is set high enough to not limit the current. The output capacitor of this power supply will draw current as the voltage on the cap rises (Ic = C * dVc/dt). The output current and the cap current flow through the current monitoring resistor which is where the current is measured and compared to the current limit setting. See Figure 1. Therefore, the output cap current adds to the output current and can cause the power supply to momentarily go into CC mode as the output cap charges. If this happens, the output voltage will rise more slowly than if the power supply stayed in constant voltage (CV) mode the entire time the output voltage was rising and charging the output cap.

So, the current limit setting can slow down the voltage response time if set too low causing the power supply to momentarily go into CC mode as the output voltage is rising and the output cap is charging. This effect is shown in Figure 2 for various current limit settings on the N5768A power supply. As you can see, the lower the current limit setting (Iset), the longer it takes for the voltage to reach its final value.

If fast up-programming response time is important to you in your power supply application, make sure you set your current limit high enough to provide current to your DUT and to charge the power supply’s output capacitor without going into CC mode. Once the output voltage reaches its final value, you can always lower the current limit again to properly protect your DUT.

Thursday, June 20, 2013

How can I measure output impedance of a DC power supply?

In my last posting “DC power supply output impedance characteristics”, I explained what the output impedance characteristics of a DC power supply were like for both its constant voltage (CV) and constant current (CC) modes of operation. I also shared an example of what power supply output impedance is useful for. But how does one go about measuring the output impedance of a DC power supply over frequency, if and when needed?

There are a number of different approaches that can be taken, but these days perhaps the most practical is to use a good network analyzer that will operate at low frequencies, ranging from 10 Hz up to 1 MHz, or greater, depending on your needs. Even when using a network analyzer as your starting point there are still quite a few different variations that can be taken.

Measuring the output impedance requires injecting a disturbance at the particular frequency the network analyzer is measuring at. This signal is furnished by the network analyzer but virtually always needs some amount of transformation to be useful. Measuring the output impedance of a voltage source favors driving a current signal disturbance into the output. Conversely, measuring the output impedance of a current source favors driving a voltage signal disturbance into the output. The two set up examples later on here use two different methods for injecting the disturbance.

The reference input “R” of the network analyzer is then used to measure the current while the second input “A” or “T” is used to measure the voltage on the output of the power supply being characterized. Thus the relative gain being measured by the network analyzer is the impedance, based on:
zout = vout/iout = (A or T)/R
The output voltage and current signals need to be compatible with the measurement inputs on the network analyzer. This means a voltage divider probe may be needed for the voltage measurement, depending on the voltage level, and a resistor or current probe will be needed to convert the current into an appropriate voltage signal. A key consideration here is appropriate scaling constants need to be factored in, based on the gain or attenuation of the voltage and current probes being used, so that the impedance reading is correct.

Figure 1: DC power supply output impedance measurement with the Agilent E5061B

One example set up using the Agilent E5061B network analyzer is shown in Figure 1, taken from page 15 of an Agilent E5061B application note on testing DC-DC converters, referenced below. Here the disturbance is injected in through an isolation transformer coupled across the power supply output through a DC blocking capacitor and a 1 ohm resistor. The 1 ohm resistor is doing double duty in that it is changing the voltage disturbance into a current disturbance and it is also providing a means for the “R” input to measure the current. The “T” input then directly measures the DC/DC converter’s (or power supply’s) output voltage.

A second, somewhat more elaborate, variation of this arrangement, based on using a 4395A network analyzer (now discontinued) has been posted by a colleague here on our Agilent Power Supply forum: “Output Impedance Measurement on Agilent Power Supplies”. In this set up the disturbance signal from the network analyzer is instead fed into the analog input of an Agilent N3306A electronic load. The N3306A in turn creates the current disturbance on the output of the DC power supply under test as well as provide any desired DC loading on the power supply’s output. The N3306A can be used to further boost the level of disturbance if needed. Finally, an N278xB active current probe and matching N2779A probe amplifier are used to easily measure the current signal.

Hopefully this will get you on your way if the need for making power supply output impedance ever arises!

Reference: “Evaluating DC-DC Converters and PDN with the E5061B LF-RF Network Analyzer” Application Note, publication number 5990-5902EN (click here to access)

Monday, June 10, 2013

DC power supply output impedance characteristics

In a previous posting; “How Does a Power Supply regulate It’s Output Voltage and Current?” I showed how feedback loops are used to control a DC power supply’s output voltage and current.  Feedback is phenomenally helpful in providing a DC power supply with near-ideal performance. It is the reason why load regulation is measured in 100ths of a percent. A major reason for this is it bestows the power supply, if a voltage source, with near zero impedance, or as a current source, with high output impedance. How does it do this?

The impedance of a typical DC power supply’s output stage (like the conceptual one illustrated in the above referenced posting) is usually on the order of an ohm to a couple of ohms. This is the open-loop output impedance; i.e. the output impedance before any feedback is applied around the output.   If no feedback were applied we would not have anywhere near the load regulation we actually get. However, when the control amplifier provides negative feedback to correct for changes in output when a load is applied, the performance is transformed by the ratio of 1 + T, where T is loop gain of the feedback system. As an example, the output impedance of the DC power supply operating in constant voltage becomes:

Zout (closed loop) = Zout (open loop) / (1+T)

The loop gain T is approximately the gain of the operational amplifier times the attenuation of the voltage divider network. In practical feedback control systems the gain of the amplifier is quite large at and near DC, possibly as high as 90 dB of gain. This reduces the power supply’s DC and low frequency output to just milliohms or less, providing near ideal load regulation performance. Another factor in practical feedback control systems is the loop gain is rolled off in a controlled manner with increasing frequency in order to maintain stability. Thus at higher frequency the output impedance of a DC power supply operating as a voltage source increases towards its open loop impedance value as the loop gain decreases. This is illustrated in the output impedance plots in Figure 1, for the Agilent 6643A DC power supply.

Figure 1: Agilent 6643A 35V, 6A system DC power supply output impedance

As can be seen in Figure 1, for constant voltage operation, the 6643A DC power supply is just about 1 milliohm at 100 Hz, and exhibits an inductive output characteristic with increasing frequency as the loop gain decreases.

As also can be seen in Figure 1, feedback control works in a similar fashion for constant current operation. While a voltage source ideally has zero output impedance, a current source ideally has infinite impedance.  For constant current operation the 6643A DC power supply exhibits 10 ohms impedance at 100 Hz and rolls off in a capacitive fashion as frequency increases. However, for the 6643A, it is not so much the constant current control loop gain dropping off with frequency but the output filter capacitance dominating the output impedance. While the 6643A can be used as an excellent, well-regulated current source (see posting: “Can a standard DC power supply be used as current source?”) it is first and foremost optimized for being a voltage source. Some output capacitance serves towards that end.

An example of one use for the output impedance plots of a DC power supply is to estimate what the amount of load-induced AC ripple might be, based on the frequency and amplitude of the current being drawn by the load, when powered by power supply operating in constant voltage.