Iacdrive|Automation Solution

Impedance analyzer

A graphical impedance analyzer with good phase resolution is a must. Some brands have all the bells and whistles, but not the phase resolution necessary to accurately measure high Q (100+) components over the instrument’s full frequency range (which should extend at least into the low megahertz). Of course the Agilent 4294A fills the performance bill, but with a $40k+ purchase bill, it also empties the budget (like similar high end new models from Wayne Kerr). Used models from Wayne Kerr work very well, and can be had for under $10K but they are very heavy and clunky with very ugly (but still useable) displays.

Perhaps the best value may be the Hioki IM3570, which works extremely well with superior phase resolution, has a very nice color touch screen display (with all the expected engineering graphing formats), is compact and lightweight, and costs around $10k new. Its only downside is that its fan is annoyingly loud and does not reduce its noise output during instrument idle.

But where should an impedance analyzer rank on the power electronics design engineer’s basic equipment list (and why)?

Beyond the basic lower cost necessities such as DMMs, bench power supplies, test leads, soldering stations, etcetera, I would rank a good impedance analyzer second only to a good oscilloscope. The impedance analyzer allows one to see all of a component’s secondary impedance characteristics and to directly compare similar components. Often overlooked is the information such an instrument can provide by examining component assemblies in situ in a circuit board assembly. Sometimes this can be very revealing of hidden, but influential layout parasitics.

Equally importantly, an impedance analyzer allows accurate SPICE models to be quickly formulated so that simulation can be used as a meaningful design tool. Transformer magnetizing and leakage inductances can be measured as well as inter-winding capacitance and frequency dependent resistive losses. From these measurements and with proper technique, a model can be formulated that nearly exactly matches the real part. Not only does this allow power circuits and control loops to be initially designed entirely by simulation (under the judicious eye of experience, of course), but it even allows one to effectively simulate the low frequency end of a design’s EMI performance.

FETs in ZVS bridge

Had run into a very serious field failure issue a decade ago due to IXYS FETs used in a phase-shifted ZVS bridge topology. Eventually, the problem was tracked to failure of the FETs’ body diode when the unit operated at higher ambient temperature.

When FETs were first introduced for use in hard switching applications, it was quickly discovered that under high di/dt commutating conditions, the parasitic bipolar transistor that forms the body diode can turn on resulting in catastrophic failure (shorting) of the FET. I had run into this issue in the mid ’80s and if memory serves me correctly, IR was a leader in making their FET body diodes much more robust and capable of hard commutation. Having had this experience with FET commutation failures and after exhausting other lines of investigation which showed no problem with the operation of the ZVS bridge, I built a tester which could establish an adjustable current through the body diode of the FET under test followed by hard commutation of the body diode.

Room temperature testing of the suspect FET showed the body diode recovery characteristic similar to that of what turned out to be a more robust IR FET. Some difference was seen in the diode recovery as the IXYS FET was a bit slower and did show higher recovered charge. However, was unable to induce a failure in either the IXYS or IR FET even when commutating high values of forward diode current up to 20A when testing at room temperature.

The testing was then repeated in a heated condition. This proved to be very informative. The IXYS FETs were found to fail repeatedly with a case temperature around 80C and forward diode current prior to commutation as low as 5A. In contrast, the IR devices were operated to 125C case temp with forward diode currents of 10A without failure.

This confirmed a high temperature operating problem of the IXYS FETs associated with the body diode. Changing to the more robust IR devices solved the field failure issue.
Beware when a FET datasheet does not provide body diode di/dt limits at elevated ambient.

A more complete explanation of the FET body diode failure mechanism in ZVS applications can be found in application note APT9804 published by Advanced Power Technology.

I believe FETs can be reliably used in ZVS applications if the devices are carefully selected and shown to have robust body diode commutation characteristics.

Paralleling IGBT modules

I’m not sure why the IGBTs would share the current since they’re paralleled, unless external circuitry (series inductance, resistance, gate resistors) forces them to do so?

I would be pretty leery of paralleling these modules. As far as the PN diodes go, reverse recovery currents in PN diodes (especially if they are hard switched to a reverse voltage) are usually not limited by their internal semiconductor operation until they reach “soft recovery” (the point where the reverse current decays). They are usually limited by external circuitry (resistance, inductance, IGBT gate resistance). A perfect example: the traditional diode reverse recovery measurement test externally limits the reversing current to a linear falling ramp by using a series inductance. If you could reverse the voltage across the diode in a nanosecond, you would see an enormous reverse current spike.

Even though diode dopings are pretty well controlled these days, carrier lifetimes are not necessarily. Since one diode might “turn off” (go into a soft reverse current decreasing ramp, where the diode actually DOES limit its own current) before the other, you may end up with all the current going through one diode for a least a little while (the motor will look like an inductor, for all intents and purposes, during the diode turn-off). Probably better to control the max diode current externally for each driver.

Paralleling IGBT modules where the IGBT but not the diode has a PTC is commonly done at higher powers. I personally have never done more than 3 x 600A modules in parallel but if you look at things like high power wind then things get very “interesting”. It is all a matter of analysis, good thermal coupling, symmetrical layout and current de-rating. Once you get too many modules in parallel then the de-rating gets out of hand without some kind of passive or active element to ensure current sharing. Then you know it is time to switch to a higher current module or a higher voltage lower current for the same power. The relative proportion of switching losses vs conduction losses also has a big part to play.

Power converter trend

The trend toward lower losses in power converters is not apparent in all of the applications of power converters. It is also not apparent that the power converter solution and its losses for a given market will be the same when it comes to losses. In terms of the market shift that you mention, Prof. the answer is probably that each market is becoming split into a lower efficiency and higher efficiency solution.

From my limited view the reason for this is the effort and time required to do the low loss development. The early developers of low loss converters are now ahead and those that were slower may never catch them. This gap is in a number of converter markets widening, with both higher loss and lower loss offerings continuing to be used and sold. This split is not apparent with different levels of development or geographically.

Some markets already have very efficient solutions, other markets not so efficient and others had high power loss solutions. The customers accepted these solutions. The path to lower loss converters is for some markets not yet clear and in some markets the requirement may never actually become real.

It does seem that there is a real case to make for any power converter market splitting in two as the opportunities presented by lowering the power loss are taken.

All low loss converters present significant challenges and are all somewhat esoteric.

For me power supply EMI control consists of designing filtering for differential and common mode conducted emissions. The differential mode filtering attenuates the primary side differential lower frequency switching current fundamental & harmonic frequencies. The common mode filtering provides a low impedance return path for high frequency noise currents resulting from high dV/dt transitions during switching transitions present on the power semiconductors (switching mosfet drain, rectifier cathods). These noise currents ring at high frequencies as they oscillate in the uncontrolled parasitic inductance and capacitance associated with their return to source path. Shortening and damping this return path allows the high frequency noise currents to return locally instead of via the measurement copper bench and conducted emi current or voltage (LISN) probe as well as providing a more damped ringing frequency. Shorting this return path has the added benefit of decreasing radiated emissions. In addition proper layout of the power train so as to minimize the loop area associated with both the primary and secondary side switching currents minimizes the associated radiated emissions.

When I mentioned the criticism of resonant mode converter as related to the challenges of emi filitering I was referring to the additional differential mode filtering required. For example if you take a square wave primary side current waveform and analyze the differential frequency content the fundamental magnitude with be lower and there will be higher frequency components as compared to a purely resonant approach at the same power level. It is normally the lower frequency content that has to be filtered differentially.

Given these differences the additional emi filtering volume/cost of the resonant approach may pose a disadvantage.

Conditional stability

Conditional stability, I like to think about it this way:

The ultimate test of stability is knowing whether the poles of the closed loop system are in the LHP. If so, it is stable.

We get at the poles of the system by looking at the characteristic equation, 1+T(s). Unfortunately, we don’t have the math available (except in classroom exercises) we have an empirical system that may or may not be reduced to a mathematical model. For power supplies, even if they can be reduced to a model, it is approximate and just about always has significant deviations from the hardware. That is why measurements persist in this industry.

Nyquist came up with a criterion for making sure that the poles are in the LHP by drawing his diagram. When you plot the vector diagram of T(s) is must not encircle the -1 point.

Bode realized that the Nyquist diagram was not good for high gain since it plotted a linear scale of the magnitude, so he came up with his Bode plot which is what everyone uses. The Bode criteria only says that the phase must be above -180 degrees when it crosses over 0 dB. There is nothing that says it can’t do that before 0 dB.

If you draw the Nyquist diagram of a conditionally stable system, you’ll see it doesn’t surround the -1 point.

If you like, I can put some figures together. Or maybe a video would be a good topic.

All this is great of course, but it’s still puzzling to think of how a sine wave can chase itself around the loop, get amplified and inverted, phase shifted another 180 degrees, and not be unstable!

Having said all this about Nyquist, it is not something I plot in the lab. I just use it as an educational tool. In the lab, in courses, or consulting for clients, the Bode plot of gain and phase is what we use.

1:1 ratio transformer

A 1:1 ratio transformer is primarily used to isolate the primary from the secondary. In small scale electronics it isolates the noise / interference collected from the primary from being transmitted to the secondary. In critical care facilities it can be used as an isolation transformer to isolate the primary grounding of the supply from the critical grounding system of the load (secondary). In large scale applications it is used as a 3-phase delta / delta transformer equipment to isolate the grounding of the source system (primary) from the ungrounded system of the load (secondary).

In a delta – delta system, the equipment grounding is achieved by installing grounding electrodes of grounding resistance not more 25 ohms (maximum or less) as required by the National electrical code. From the grounding electrodes, grounding conductors are distributed with the feeder circuit raceways and branch circuit raceways up to the equipment where the equipment enclosures and non-current carrying parts are grounded (bonded). This scheme is predominant on installations where most of the loads are motors like industrial plants, or on shipboard installations where the systems are mostly delta-delta (ungrounded). In ships, the hull becomes the grounding electrode. Electrical installations like these have ground fault monitoring sensors to determine if there are accidental line to ground connections to the grounding system.

Self Excited Induction Generator (SEIG)

The output voltage and frequency of a self excited induction generator (SEIG) are totally dependent on the system to which it is attached.

The fact that it is self-excited means that there is no field control and therefore no voltage control, instead the residual magnetism in the rotor is used in conjunction with carefully chosen capacitors at its terminal to form a resonant condition that mutually assists the buildup of voltage limited by the saturation characteristics of the stator. Once this balance point is reached any normal load will cause the terminal voltage to drop.

The frequency is totally reliant upon the speed of the rotor, so unless there is a fixed speed or governor controlled prime mover the load will see a frequency that changes with the prime mover and drops off as the load increases.

The above characteristics are what make SEIGs less than desirable for isolated/standalone operation IF steady well regulated AC power is required. On the other hand if the output is going to be rectified into DC then it can be used. Many of these undesirable “features” go away if the generator is attached to the grid which supplies steady voltage and frequency signals.

The way around all the disadvantages is to use a doubly fed induction generator (DFIG). In addition to the stator connection to the load, the wound rotor is provided with a varying AC field whose frequency is tightly controlled through smart electronics so that a relatively fixed controllable output voltage and frequency can be achieved despite the varying speed of the prime mover and the load, however the costs for the wound rotor induction motor plus the sophisticated control/power electronics are much higher than other forms of variable speed/voltage generation.

Differences of Grounding, Bonding and Ground Fault Protection?

Grounding (or Earthing) – intentionally connecting something to the ground. This is typically done to assist in dissipating static charge and lightning energy since the earth is a poor conductor of electricity unless you get a high voltage and high current.

Bonding is the intentional interconnection of conductive items in order to tie them to the same potential plane — and this is where folks get the confusion to grounding/earthing. The intent of the bonding is to ensure that if a power circuit faults to the enclosure or device, there will be a low-impedance path back to the source so that the upstream overcurrent device(s) will operate quickly and clear the fault before either a person is seriously injured/killed or a fire originates.

Ground Fault Protection is multi-purpose, and I will stay in the Low Voltage (<600 volts) arena. One version, that ends up being seen in most locations where there is low voltage (220 or 120 volts to ground) utilization, is a typically 5-7 mA device that’s looking to ensure that current flow out the hot line comes back on the neutral/grounded conductor; this is to again protect personnel from being electrocuted when in a compromised lower resistance condition. Another version is the Equipment Ground Fault Protection, and this is used for resistive heat tracing or items like irrigation equipment; the trip levels here are around 30 mA and are more for prevention of fires. The final version of Ground Fault Protection is on larger commercial/industrial power systems operating with over 150 volts to ground/neutral (so 380Y/220, 480Y/277 are a couple typical examples) and — at least in the US and Canada — where the incoming main circuit interrupting device is at least 1000 amps (though it’s not a bad idea at lower, it’s just not mandated); here it’s used to ensure that a downstream fault is cleared to avoid fire conditions or the event of ‘Burn Down’ since there’s sufficient residual voltage present that the arc can be kept going and does not just self-extinguish.

In the Medium and High Voltage areas, the Ground Fault Protection is really just protective relaying that’s monitoring the phase currents and operating for an imbalance over a certain level that’s normally up to the system designer to determine.

Hazardous area classification

Hazardous area classification has three basic components:
Class (1,2) : Type of combustible material (Gas or Dust)
Div (I, II) : Probability of combustible material being present
Gas Group (A,B,C,D): most combustible to least combustible (amount of energy required to ignite the gas)

Hazardous Area Protection Techniques: There are many, but most commonly used for Instrumentation are listed below:
1) Instrinsic Safety : Limits the amount of energy going to the field instrument (by use of Instrinsic Safety Barrier in the safe area). Live maintenance is possible. Limited for low energy instruments.
2) Explosion proof: Special enclosure of field instrument that contains the explosion (if it occurs). Used for relatively high energy instruments; Instrument should be powered off before opening the enclosure.
3) Pressurized or Purged: Isolates the instrument from combustible gas by pressurizing the enclosure with an inert gas.

Then there are encapsulation, increased safety, oil immersion, sand filling etc.

Weather protection: Every field instrument needs protection from dust and water.
IP-xy as per IEC 60529, where
x- protection against solids
y- protection against liquids
Usually IP-65 protection is specified for field instruments i onshore applications (which is equivalent of NEMA 4X); IP-66 for offshore application and IP-67 for submersible service.

SCADA & HMI

SCADA will have a set of KPI’s that are used by the PLCs/PACs/RTUs as standards to compare to the readings coming from the intelligent devices they are connected to such as flowmeters, sensors, pressure guages, etc.

HMI is a graphical representation of your process system that is provided both the KPI data and receives the readings from the various devices through the PLC/PAC/RTUs. For example you may be using a PLC that has 24 i/o blocks that are connected to various intelligent devices that covers part of your water treatment plant. The HMI software provides the operator with a graphical view of the treatment plant that you customize so that your virtual devices and actual devices are synchronized with the correct i/o blocks in your PLC. So, when an alarm is triggered, instead of the operator receiving a message that the 15th i/o block on PLC 7 failed, you could see that the pressure guage in a boiler reached maximum safety level, triggering a shutdown and awaiting operator approval for restart.

Here is some more info I got from my colleague who is the expert in the HMI market, this is a summary from the scope of his last market study which is about a year old.

HMI software’s complexity ranges from a simple PLC/PAC operating interface but as plant systems have evolved, HMI functionality and importance has as well. HMI is an integral component of a Collaborative Production Management (CPM) system; simply you can define that as the integration of Enterprise, Operations, and Automation software into a single system. Collaborative Production Systems (CPS) require a common HMI software solution that can visualize the data and information required at this converged point of operations and production management. HMI software is the bridge between your Automation Systems and Operations Management systems.

An HMI software package typically performs functions such as process visualization and animation, data acquisition and management, process monitoring and alarming, management reporting, and database serving to other enterprise applications. In many cases, HMI software package can also perform control functions such as basic regulatory control, batch control, supervisory control, and statistical process control.

“Ergonometrics,” where increased ergonomics help increase KPI and metric results, requires deploying the latest HMI software packages. These offer the best resolution to support 3D solutions and visualization based on technologies such as Microsoft Silverlight. Integrating real-time live video into HMI software tools provide another excellent opportunity to maximize operator effectiveness. Live video provides a “fourth dimension” for intelligent visualization and control solutions. Finally, the need for open and secure access to data across the entire enterprise drives the creation of a single environment where these applications can coexist and share information. This environment requires the latest HMI software capable of providing visualization and intelligence solutions for automation, energy management, and production management systems.