Claimed OU circuit of Rosemary Ainslie

[Rosemary Ainslie]

i wonder what tack his argument will take when he actually uses the current and differential probes...

Hi Wilby.  I also wonder.  I think this is a perfect example of ad hominem.  There are always limits to measurements.  Tektronix will not argue this.  Therefore Tektronix, by some curious twist in a logical extension of this argument thereby concede that it's ability to measure under test conditions has its limitations.  Therefore tektronix further admits that on this application too it has limitations.  Which therefore and through another reckless reach into mad logic then becomes an admission that these test parameters now WHOLLY AND ENTIRELY exceed the capability of the TDS.  Golly Poynty.  When it comes to casting the odd aspersion - there is none to equal you.  LOL

Anyway - I have NOTHING that I need to substantiate.  I know the capabilities and the constraints of the DSO and its probes.  I have no desire to convince you that you're wrong.  Nor will I enter into an argument here. I would - however point out that you have stated this - and I quote

"The limits of this measurement technique have been EXCEEDED for this application, and again Tektronix would not argue with this". 

Either you know something that is not included in the manuals - and are not telling us.  Or you are relying on aspersions to discredit the instrument which we're using.  If the former - you really need to tell us.  If the latter you really need to retract this statement.  The only thing that you now cannot do - is leave this statement hanging.  You may not, under any circumstances bring Tektronix's good name nor the excellence of their equipment to question - under any circumstances whatsoever.  I am left with the mouth agape at your extraordinary arrogance and presumption.  Talk about 'fighting dirty'.  How desperate are you?

But justify this quoted statement with reference to an acknowledged inability to measure the tested and required voltage at the tested and given frequency if you can.  Else retract.  Those are now the only options open to you.

[poynt99]

Poynt,

You seem to be hung up on the differential probes. The reality of it is this, using those probes is step in the wrong direction. The differential probes available for purchase have various voltage and bandwidth restrictions that call into question whether or not we may miss some of the actual energy signatures important to our calculations.

Utilizing true differential probes is not only going in the right direction, but is critical for obtaining usable results in this application.

The Tektronix P5205 is a 100MHz, 1300V (differential), 1000V (common-mode) active high voltage differential oscilloscope probe with TEKPROBE interface, so it will work perfectly with the TDS3054C.

The rise-time of Glen's Drain voltage spike is approximately 50ns. This represents a spectrum bandwidth of about 7MHz. It is generally accepted, and touted in many papers and articles (including Tektronix), that to adequately capture the signal bandwidth and its rise-time, the measurement instrument (oscilloscope or probe) must have a bandwidth and rise-time from 3 to 5 times that of the signal being measured. With a 7:1 ratio of instrument+probe to signal bandwidth, the measurement error is less than 1%. A 7:1 ratio requires only a 49MHz capable instrument. With the 100MHz bandwidth of the P5205 probe, this error will probably fall to the 0.2% region.

Therefore, it is quite clear that loss of data will not be an issue using one of these probes for this application.

At the moment we are not in a position to procure them either. Now, realistically, do you honestly propose that the data provided by Channel 2 and Channel 4 in our tests is anything but accurate?

These probes can be rented. Ideally, Tektronix would loan you one.

Channel 2 (VDrain) and Channel 4 (Vbat)? Do you mean Channel 1 (Vshunt) and Channel 4 (Vbat)?

Battery voltage and battery current (low-side shunt) are the two parameters in question, as they produce the average POS when all the samples are processed in Excel. This is the pivotal measurement that makes or breaks the associated claims and thesis, therefore it is critical that it be measured absolutely correctly.

Referring again to the posted table of results of average delivered battery power from Glen's test #5, it is quite evident that the measurements used to compute those values are erroneous and therefore unusable.

Not only is the shunt voltage wave form in great question due to the shunt inductance and probe ground lead inductance (and ground point), but the battery voltage wave form on Channel 4 is also in question. Previous pictures of Glen's setup show the battery voltage being measured at some point quite distant from the actual battery + terminal. It has been shown conclusively, that this will produce a falsely-elevated representation of the actual battery voltage due to the inductance of the wire between the actual battery + terminal and the scope probe tip, not to mention the probe ground lead location issues as well.

Regarding the Current Probe, yes I do agree that having one would be a comfort toward validating the voltage drops we are seeing across the CSR*, but with a truly non-inductive resistor of high tolerance (say 1%) we may actually be more accurate than the current probes available. The reason for this, is that the current probes are driven by inductance and this circuit has a very unconventional inductance signature due to the aperiodicity and multi-harmonic nature of the resonant action. Using an inductive Current Probe outside of a Faraday Cage would always raise the question as to whether it was introducing its own induced values into the readings.

I have considered placing the load resistor in a cage, but we would then have to ensure that the cage itself is not interacting with the resonant nature of the resistor or absorbing the energy we would like to have converted to heat. Thus we end up with a cage several feet in diameter to be happy and that is probably outside the scope of this endeavor at this time. A small cage over the circuit may be a good test however, if the load resistor is at a moderate distance. All things considered, the 'bakers rack' behind the resistor in Glen's tests no doubt becomes involved on some level with the magnetic field traversing it. At some point during the month Glen will probably relocate the test to ensure it is consistent and environment independent.

There is a chance that using a non-inductive shunt may produce accurate results if implemented and measured correctly, but the limitations (and inherent challenges) of the single-ended probe with its associated inductive ground lead are still at issue here. That leads us back to the requirement for a true differential probe at the very least.

The recommended current probe is the Tektronix TCP202: DC-50MHz, 0.1 Ohm insertion impedance at 5MHz. The bandwidth is more than adequate, as detailed above for the P5205 differential probe. These two probes have a matched propagation delay within +/- 2ns of each other.

The latter part of the quote is speculative and outside the scope of the main issue at hand, and therefore requires no comment at this time.

The existing data collected is highly accurate and usable for presentation once we have the precise inductance for both the load resistor and the CSR.

I disagree as I have already pointed out above, and I have not seen anything that would substantiate the notion that the data is accurate.

It is unfortunate that the manufacturer of the CSR did not include any inductance specifications for use pro tempore. If you have any resistors of that exact model, we could use your measurements pro tempore - but eventually, we will need to accurately measure (calibrate) the specific devices used.

Manufacturers don't offer an inductance specification for inductively wire-wound resistors, because it is assumed that they are not being used in high frequency applications, or at least those that do not require a constant impedance.

I mentioned to Glen already that I am fairly confident his resistor inductance measurement will come out very close to the theoretical value of 20.4uH as per my own measurements and comparisons. If you wish I will measure Glen's resistor and shunt with my meter. That offer was already pre-rejected by Rose, but it still stands if Glen is so inclined.

Regarding the inductive reactance - if we know specifically the true inductance of the parts, we can then accurately extrapolate the expected current in a specific instance in time. This would also give us the ability to accurately determine the phase shift of that current relative to the voltage during charging and discharging of the magnetic field (inductors). With those two pieces of information, the instantaneous power could then be accurately calculated.

I am uncertain how you would go about doing what you propose, and moreover, the quality of any data massaging performed will be limited by the quality of the raw data a priori.



The power dissipated in the load resistor is easily obtained and is accurate. This is 50% of the battle already completed. All that remains to prove or disprove the associated claims and thesis is to obtain a reasonably-accurate measurement of the supplied power. A method has been offered that is easy to build and is reliable. The question remains as to why it is not being implemented? The last 50% is easily attainable and within reach--it's just a matter of moving forward with it so the ultimate question can finally be put to bed.

.99

[Rosemary Ainslie]

Poynt - get used to this.  Under no circumstances will I burden the team nor Tektronix with the demand for differential probes as this is entirely UNNECESSARY.  No differential probe is required. 

You can talk about this to your heart's content - but you will be talking to yourself.  When you show us your name and your credentials as a measurement expert we may revise this.  Until then your application here will be ignored.

You need to address the point made in my previous post.  Let me copy it for your convenience.

I know the capabilities and the constraints of the DSO and its probes.  I have no desire to convince you that you're wrong.  Nor will I enter into an argument here. I would - however point out that you have stated this - and I quote

"The limits of this measurement technique have been EXCEEDED for this application, and again Tektronix would not argue with this".

Either you know something that is not included in the manuals - and are not telling us.  Or you are relying on aspersions to discredit the instrument which we're using.  If the former - you really need to tell us.  If the latter you really need to retract this statement.  The only thing that you now cannot do - is leave this statement hanging.  You may not, under any circumstances bring Tektronix's good name nor the excellence of their equipment to question - under any circumstances whatsoever.  I am left with the mouth agape at your extraordinary arrogance and presumption.  Talk about 'fighting dirty'.  How desperate are you?

But justify this quoted statement with reference to an acknowledged inability to measure the tested and required voltage at the tested and given frequency if you can.  Else retract.  Those are now the only options open to you.

[poynt99]

Harvey, Rose, Glen, et al:

I strongly urge you to do some serious reading up on oscilloscope probes, their uses, selection, limitations, aberrations, etc. Clearly you all would benefit greatly by doing so.

I would suggest the following very relevant article by our friends at Tektronix is an excellent place to begin. I've highlighted some important relevant points:


Dynamic Measurements Help Power Supply Designers Spot Problems Early

The supply and utilization of system power is a critical factor in the design of every type of electronic product. Personal computers, telecommunications and broadcast equipment, and military equipment are all pressuring the humble power supply to provide ever more current in smaller and smaller packages. Due to their compactness and lower cost, high-frequency switching supplies (switchers) have become the preferred solution for most applications. Although more complex than traditional linear supplies, switchers are without peer in delivering current all out of proportion to their size.

The behavior of any power supply must be well understood before it is designed into a product or released to the market. After all, it will be the foundation of an end product's efficiency, safety, and reliability. The prudent designer will characterize a power supply's behavior thoroughly during the design stage. Components in the supply, particularly active components, may encounter momentary conditions that far exceed their average operating levels. The engineer must be aware of these peaks and account for them when choosing components like power MOSFETs (metal oxide semiconductor field effect transistors) and IGBTs (insulated gate bipolar transistors).

A broad repertoire of accepted power supply measurement procedures exists, but several basic methodologies are essential in power supply design evaluation. Historically these approaches have relied on static current and voltage measurements taken with digital multimeters. However, meaningful data about dynamic performance is needed to detect hidden problems in switching components. Consequently, the oscilloscope has become a cornerstone of switching power supply evaluation. With switching frequencies and edge speeds on the increase, a full-featured wideband instrument is needed to capture subtle signal details.

This article will discuss some of the tools and techniques used to observe a power MOSFET's in-circuit behavior in a power supply, emphasizing the tools and preparations needed to ensure accurate measurements. The article will show how these readings can detect instantaneous power peaks that elude static measurement techniques.

The Unit Under Test

Figure 1 shows a simplified circuit for the input side of a switcher. The MOSFET power transistor configuration is typical to switching power supplies and most power conversion products-PWM motor drives, electronic ballasts, and many others. The MOSFET is floating-it has no reference to either the input AC ground or the output ground terminal. Simplistic ground-referenced measurements with a conventional oscilloscope setup are not possible here because connecting the scope probe's ground lead to any of the transistor's terminals would short-circuit that point.

Figure 1. The circuit under test, showing the converter MOSFET and the test points for the drain-to-source voltage measurement , Vds.

Defining the Measurement and the Tools

The power measurement across the MOSFET is made up of two constituent waveforms: voltage and current. The product of these two variables at any instant in time is the instantaneous power. The graph of these products over time is the power waveform. The shape, amplitude, phase, and timing of the power waveform combine to tell a story about the real-world stresses on the MOSFET.

A lab-quality DSO, the Tektronix TDS 510A, has been chosen as the measurement platform for two reasons: 1.) Its ability to display not only voltage and current waveforms, but also to compute and display power waveforms with direct readout in watts; 2.) Its fully integrated interface to precision differential voltage and current probes. Moreover, the scope's 500 MS/s sample rate can capture very fast switching transients faithfully.

True Differential Voltage Measurements Make a Difference

The solution of choice for measuring the MOSFET voltage waveforms is a differential measurement. The voltage excursion is measured between two points (for example, the voltage between source and drain, Vds in Figure 1), neither of which need be at ground potential. Depending on the range of the power supply, these voltage waveforms may be riding on top of a voltage ranging from tens of volts to hundreds of volts. There are several ways to accomplish this measurement, in ascending order of preference:

    * Elevate the scope's chassis ground. This extremely unsafe method endangers the operator, instrument, and unit under test. Moreover, it yields very imprecise measurements. This approach doesn't merit further discussion.

    * Use two conventional scope probes (with their ground leads connected only to each other) and the built-in channel summing capability of an oscilloscope. This is known as a quasi-differential measurement. Unfortunately, the passive scope probes in combination with the scope's amplifiers lack the CMRR (common mode rejection ratio) to block the common mode voltage adequately. This setup cannot capture the measurement with good accuracy.

    * Use a commercially-available probe isolator to isolate the scope's chassis ground. Thus the probe's "ground" lead is no longer at ground potential and can be connected directly to a test point. Probe isolators are an effective solution but are very costly, on the order of 2- to 5 times the cost of good differential probes.

    * Use a battery-operated scope with individually isolated inputs, for example the Tektronix THS 720 TekScopeTM. When used with carefully chosen probes, this method delivers good results, especially in field service applications.

    * Use a true differential probe on a wideband oscilloscope. This is the most appropriate method for critical measurements like those used to predict power supply component reliability and performance.

A true differential voltage probe (the Tektronix P5205) was chosen for this measurement because of its high CMRR (common mode rejection ratio), low circuit loading (only 7 pF input capacitance), and 100 MHz bandwidth.

Picking Up the Current Waveform

Of course, making the voltage measurement is only half the job. Acquiring current waveforms is a discipline all its own, with specialized tools and techniques. The common digital multimeter, though suitable for static current readings, lacks the ability to display the waveform properties in an AC environment. Here again the oscilloscope is the best tool for examining amplitude, timing, and phase characteristics.

The Tektronix TCP202 current probe was chosen for this application. This is a "non-invasive" probe; that is, it doesn't require breaking into the circuit to connect the probe. Its clip-on pickup acquires the signal by induction. Like the P5205 probe, the TCP202 relies on the scope's TekProbeTM interface to provide automatic ranging, scaling, and readout of the measurement in engineering units.
Preparing for the Measurement

A little time spent setting up the scope/probe system in advance can help ensure stable, repeatable power measurements. Both the current and the voltage probes are affected:

   1. There is a simple "nulling" procedure that should precede any instantaneous power measurement. Both the P5205 and the TCP202, and other probes of their type, have built-in DC offset trimmers. With the unit under test turned off and the scope and probes fully warmed up, set the oscilloscope to measure the mean of both the voltage and current waveforms. Use the sensitivity settings that will be used in the actual measurement. With no signal present, adjust the trimmers to null the mean level for each waveform to OV, or as close as possible. This step ensures that the "quiescent" voltages and currents in the measurement system are not added to the levels at the test points.

   2. It is essential to use current and voltage probes with well-matched delay characteristics. Otherwise the power measurement-actually the product of instantaneous voltage and current readings-might be corrupted by delays that, for example, shift the current waveform relative to the voltage waveform. Such a shift would in turn displace the peaks in the power waveform, possibly leading to an incorrect assessment of the transistor's behavior.

      The P5205 differential voltage probe and the TCP202 current probe are inherently matched to within ±2ns, close enough for most applications. In addition, some scopes itself provide an adjustment for further delay equalization (deskew) between the probes, if needed.

   3. In spite of the differential voltage probe's high (80 dB) CMRR, it's wise to verify the probe's performance in the actual measurement environment. To do so, simply connect both leads to the same test point, for example the drain of the MOSFET. Both probe tips see the same signal-a "common mode" signal. Ideally, the differential probe should reject the whole signal and display a flat trace on the scope screen. In reality a small amount of the signal is passed through, and the resulting trace reveals common mode error. While this simple test isn't definitive, it will expose gross CMRR problems that might affect the measurement outcome.

      If problems do arise (for example, when using a lesser-quality differential probe) the common mode error can be subtracted mathematically by the scope. While triggering on the current waveform, capture the common mode error waveform as previously explained and save it in the scope's reference memory. Then subtract this fixed quantity from each measurement using the oscilloscope's built-in math function.

The Moment of Truth

After all the preparations, the measurements themselves are relatively simple. The object is to examine the nature of the switching transitions in the 40 kHz converter circuit depicted in Figure 1. For the differential voltage reading, the probe tips are connected to MOSFET source and drain terminals. The resulting Vds waveform is shown in Figure 2 (upper trace).

Figure 2. The TDS 510A display, showing voltage, current, an power waveforms, in addition to numerical readouts.

For the current measurement, the clamp-on probe must acquire a signal from a conductor passing through its inductive pickup core. If it isn't physically possible to clamp around the conductor of interest (in this case the lead coming from the MOSFET's drain), then it will be necessary to add a loop of wire in series with the signal as a test point. In fact, this technique can be used to increase the sensitivity of the current probe if necessary. Instead of just one loop of wire, use several turns-the sensitivity of the probe will be multiplied by the number of turns. Figure 3 illustrates the technique.

Figure 3. A non-invasive current probe (the Tektronix TCP 202) attached to a multi-turn wire loop in series with the signal path. This technique increases the sensitivity of the current measurement.

The current waveform from the MOSFET measurement is shown in Figure 2 (middle trace). This particular reading did not require the increased sensitivity technique mentioned above, and therefore gives a correctly scaled current waveform and readout.

At this point we can begin to see the direction this measurement is heading. The traces are almost complements of one another: on the voltage trace, voltage is at its maximum when no current flows, and at its minimum when current is at its peak. However, a brief transient in the current waveform disturbs an otherwise smooth switching transition. This transient occurs during the time when there is still approximately 60 V Vds voltage across the MOSFET.

The bottom trace in Figure 2 is the power measurement, automatically computed by the TDS 510A oscilloscope. It reveals just what the voltage and current traces promise: a single strong peak that coincides with the current transient. This is the reading that summarizes the circuit behavior. Assume the MOSFET was chosen for an average power capacity of, say, 20 watts. Conventional DMM current and voltage readings would indicate that the transistor was operating well within safe limits. But can this MOSFET withstand a 30W peak in every switching cycle? Just as importantly, is this peak raising the average power dissipation of the circuit to unacceptable levels? Why is the trailing current transition so much cleaner than the leading edge? These questions can point to solutions for the problem, which may range from changing the switching characteristics to simply using a larger MOSFET. Conversely, the true power, as calculated by using the scopes' Mean function to determine the mean value of the instantaneous power readings, may indicate that the transistor operating within safe limits. Either way, the designer can make informed decisions about the components in the circuit.

Looking at the power measurement, it's easy to see why all the preparation before the measurement was important. For example, small DC offsets in the probing tools, when compounded by scale factors and multiplication, can lead to large numerical errors. Likewise, a delay difference between the current and voltage probes would change the relative positions of the two respective waveforms. As a result, the peak in the power waveform would be dislocated, or might disappear altogether! This could lead to a design that wouldn't be tested until it was in the marketplace-a situation that nobody likes to risk.

An accurate power measurement system using a high performance oscilloscope, a true differential probe, and a precision current probe is the best toolset for characterizing the active components in a switching power supply. The scope-based measurement methodology helps designers evaluate the variables that produce cost-effective, yet reliable and market-worthy power supply designs.

http://www2.tek.com/cmswpt/tidetails.lotr?ct=TI&cs=afs&ci=14789&lc=EN

.99

[Rosemary Ainslie]

Poynt - I have just spoken to Tektronix and have been assured that the probes being used are more than adequate for the tests.  Now.  Yet again I need you to retract your statement or substantiate it.  I do not want a slew of information about differential probes.  I want to know, from you, and specifically - why are the probes under use are inadequate for the purposes to which they're applied.  You really, really need to answer this.