Author: ABBdriveX

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AQ: 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.

AQ: What causes VFD driven motor bearing current?

There are several things involved, all with varying degrees of impact.

Large machines are – generally speaking – made of pieces (segments) because the circle for the stator and/or rotor core is too large to manufacture from a single sheet. This leads to some breaks in the magnetic flux path symmetry, both in the radial (right angles to the shaft) and axial (parallel to the shaft) directions.

For the most part, the windings of large machines are formed and installed by hand. This too can lead to symmetry issues, as the current paths are not identical which in turn will create some differences in the magnetic field flux.

Output waveforms from power electronics are only approximations of true sinusoids. The presence of additional harmonics distorts the sinusoidal nature and results in changes that are not symmetric in the magnetic field strength … which in turn means a non-symmetric flux distribution.

Two other items contribute to potentially damaging bearing currents as well. One of these is the Common Mode Voltage which is present (to some degree) in all drives. Essentially this is a signal that is present at both the drive input and output … I tend to think of it as an offset. It’s not something that traditional grounding addresses, and can create an elevated potential in the shaft which then discharges through the bearing path.

A second item is not related to the presence (or absence) of drives at all; it is related to the mechanical arrangement of the process drive train. For example, a shaft that has a sliding seal (like the felt curtain on a dryer section), or one that turns a blade against a gas or liquid (like a compressor) can generate a static charge at the point of contact. If there is no means of isolating this charge to the portion of the shaft where the sliding is occurring, it can pass through to the motor shaft and thence through the motor bearings.

Lastly – the frequency of the variable frequency drive harmonics in the output waveform is significantly higher than line frequency. This requires specific accommodations for grounding as traditional methods are insufficient due to the attenuation caused by the relatively high resistance ground path.

AQ: Determine coefficient of grounding

Determination of required grounding impedance is based on determination of coefficient of grounding which represents ratio of maximum phase voltage at phases which aren’t exposed by fault and line voltage of power network:

kuz=(1/(sqrt(3)))*max{|e(-j*2*π/3)+(1-z)/(2+z)|; |e(+j*2*π/3)+(1-z)/(2+z)|}
z=Z0e/Zde

where are:

kuz-coefficient of grounding,
z-ratio of equivalent zero sequence impedance viewed from angle of place of fault and equivalent direct sequence impedance viewed from angle of place of fault,
Z0e-equivalent zero sequence impedance viewed from angle of place of fault,
Zde-equivalent direct sequence impedance viewed from angle of place of fault.

So, after this explanation, you can get next conclusions:
if kuz=1 then power network is ungrounded because Z0e→∞, which is a consequence of existing more (auto) transformers with ungrounded neutral point than (auto) transformers with grounded neutral point (when kuz=1 then there aren’t (auto) transformers with grounded neutral point),
if kuz≤0,8 then power network is grounded because Z0e=Zde, which is a consequence of of existing more (auto) transformers with grounded neutral point than (auto) transformers with ungrounded neutral point.

Fault current in grounded power networks is higher than fault current in ungrounded power networks. By other side, in case of ungrounded power networks we have overvoltages at phases which aren’t exposed by fault, so insulation of this conductors could be seriously damaged or in best case it could become older in shorter time than it is provided by design what is the main reason for grounding of power networks.
Coefficient of grounding is very important in aspect of selecting of insulation of lighting arresters and breaking power of breakers, because of two next reasons:
1. in grounded power networks insulation level is lower than insulation level in ungrounded power networks,
2. in grounded power networks value of short circuit current is higher than value of short circuit current in ungrounded power networks.

AQ: flyback & boost applications

For flyback & boost applications, powder cores such as Kool-mu, Xmu, etc… are usually best performing and lowest cost. Even these may need to be gapped and if CCM operation is required, a “stepped-gap” is preferred to allow a large load compliance. Center stepped gaps reduce the fringe flux greatly as there is never a complete gap, only localized saturation. This permits the inductor’s value to “swing” more and accommodate the required operation.
With only the center leg with a gap, the outer copper band can be applied without significant loss.

To explore further, dissimilar core materials can be used in parallel, ferrite & powdered types, such that different materials provide function at different operating points within the same construction. Some decades ago, we had some high power projects that utilized fixed magnets within a ferrite’s gap to provide a flux bias offset for a forward topology.

Abe Pressman wasn’t big on exploring magnetic losses, however he operated at lower frequencies than are typical today. MPPs are great with large DC bias, but suffer high loss if AC swing is large and fast. Toroids also have the least efficient winding window, however, they are best to mitigate emi.

AQ: Heavily discontinuous mode flyback design

With a heavily discontinuous mode flyback design, the transformer’s ac portion of current can be larger than the dc portion. When a high perm material is used for the transformer core, the required gap can be quite large in order to reach the low composite permeability required while the core size will likely be driven by winding and core loss considerations rather than just simply avoiding saturation. Normally the gap is put in the center leg only (with E type topology cores) in order to minimize the generation of stray fields. However, in designs such as yours (high ac with a high perm core) the needed core gap can lead to a relatively large fringing zone through which foil or solid wire may not pass without incurring excessive, unacceptable loss. Possible solutions are to use Litz wire windings or inert spacers (e.g., tape) around the center leg in order to keep the windings far enough away from the gap (the rule of thumb is 3 to 5 gap lengths, which can eat up a lot of the window area).

It is mainly for these reasons that placing half the gap in an E type core’s outer legs might be worth the trouble of dealing with the magnetic potential between the core halves (and you have seen first hand what trouble an ill designed shield band can be).

To avoid eddy current losses, the shield band should be spaced well away from the outer leg gap, probably 5 gap lengths or more. Also to be a really effective magnetic shield, it should be 3 to 5 gap lengths thick.

Bear in mind that with a high frequency, high ac current inductor design proximity effects in the winding may become very significant. This is why many of these type of inductors have single layer windings or winding wound with Litz wire (foil is the worst winding type here). One advantage of an equally gapped E type core design is that the proximity effect on the windings is significantly less because there are two gaps in series (a quasi distributed gapped core design). Not only layer-to-layer, but turn-to-turn proximity effects can sometimes be problematic in an ac inductor (or flyback) design. Just as with the gap, these are reduced by adding appropriate spacing, for example making the winding coil loose or winding it bifilar with a non-conductive filament.

AQ: Power supply prototype failures

I remember my very first power supply. They threw me in the deep end in 1981 building a multi-output 1 kW power supply. I was fresh from college, thought i knew everything, and consumed publications voraciously to learn more. Exciting times.

But nothing prepared me for the hardware trials and tribulations. We built things and they blew up. Literally. We would consume FETs and controllers at an alarming rate. The rep from Unitrode would come and visit and roll his eyes when we told him we needed another dozen controllers since yesterday.

The reasons for failure were all over the map . EMI, heat, layout issues, design issues, bad components (we had some notorious early GE parts – they exited the market shortly afterwards.)
Some of the issues took a few days to fix, some of them took weeks. We had two years to get the product ready, which was faster than the computer guys were doing their part, so it was OK.

90% of the failure issues weren’t talked about in any paper, and to this day, most of them still aren’t.

So, fast forward to today, 32 years later. I still like to build hardware – you can’t teach what you don’t practise regularly, so I keep at it.

With all the benefit of 3 decades of knowledge I STILL blow things up. Everything progresses along fine, then i touch a sensitive circuit node, or miss some critical design point and off it goes. I’m faster now at finding the mistakes but I still find there are new ones to be made. And when it blows up with 400 V applied, it’s a mess and a few hours to rebuild. Or you have to start over sometimes, if the PCB traces are vaporized.

So my first prototype, while on a PC board, always includes the controller in a socket because I know I will need that. Magnetics too, when possible, I know I’ll revise them time and again to tweak performance. PC boards will be a minimum of two passes, probably three.

AQ: Experience: Power Supply

My first big one: I had just joined a large corporation’s central R and D in Mumbai (my first job) and I was dying to prove to them that they were really very wise (for hiring me). I set up my first AC-DC power supply for the first few weeks. Then one afternoon I powered it up. After a few minutes as I stared intently at it, there was a thunderous explosion…I was almost knocked over backwards in my chair. When I came to my senses I discovered that the can of the large high-voltage bulk cap had just exploded (those days 1000uF/400V caps were real big)…the bare metal can had taken off like a projectile and hit me thump on the chest through my shirt (yet it was very red at that spot even till hours later). A shower of cellulose and some drippy stuff was all over my hair and face. Plus a small crowd of gawking engineers when I came to. Plus a terribly bruised ego in case you didn’t notice. Now this is not just a picturesque story. There is a reason why they now have safety vents in Aluminum Caps (on the underside too), and why they ask you never never to even accidentally apply reverse polarity, especially to a high-voltage Al cap. Keep in mind that an Al Elko is certainly damaged by reverse voltage or overvoltage, but the failure mechanism is simply excessive heat generation in both cases. Philips components, in older datasheets, used to actually specify that their Al Elkos could tolerate an overvoltage of 40% for maybe a second I think, with no long-term damage. And people often wonder why I only use 63V Al Elkos as the bulk cap in PoE applications (for the PD). They suggest 100V, and warn me about surges and so on. But I still think 63V is OK here, besides being cheap, and I tend to shun overdesign. In fact I think even ceramic caps can typically handle at least 40% overvoltage by design and test — and almost forever with no long term effects. Maybe wrong here though. Double check that please.

Another historic explosion I heard about after I had left an old power supply company. I deny any credit for this though. My old tech, I heard, in my absence, was trying to document the stresses in the 800W power supply which I had built and left behind. The front-end was a PFC with four or five paralleled PFC FETs. I had carefully put in ballasting resistors in the source and gates of each Fet separately, also diligently symmetrical PCB traces from lower node of each sense resistor to ground (two sided PCB, no ground plane). This was done to ensure no parasitic resonances and good dynamic current sharing too. There was a method to my madness it turns out. All that the tech did was, when asked to document the current in the PFC Fets, placed a small loop of wire in series with the source of one of these paralleled Fets. That started a spectacular fireworks display which I heard lasted over 30 seconds (what no fuse???), with each part of the power supply going up in flames almost sequentially in domino effect, with a small crowd staring in silence along with the completely startled but unscathed tech (lucky guy). After that he certainly never forgot this key lesson: never attempt to measure FET current by putting a current probe in its source— put it on the drain side. It was that simple. The same unit never exploded after that, just to complete the story.

AQ: How to get confidence while powering ON an SMPS prototype?

I never just put power to a first prototype and see what happens. Smoke and loud sounds are the most likely result and then you just know that something was not perfect. So how would you test the next prototype sample?

A good idea is to put supply voltage to your control circuit from an external supply first – often something like 12V. Check oscillator waveform, frequency, gate pulses etc. If possible, use another external power supply to put a voltage to your output. Increasing this voltage slowly, you should see the gate pulses go from max. to min. duty cycle when passing the desired output voltage. If this does not happen, check your feedback path, still without turning main power on.

If everything looks as expected, remove the external supply from the output but keep the control circuit powered from an external source. Then SLOWLY turn up the main input voltage while using your oscilloscope to monitor the voltage waveforms in the power circuit and a DC voltmeter to monitor output voltage etc. Keep an eye on the ampere-meter on the main power source. If something suspicious occurs, stop increasing input further and investigate what’s happening while the circuit is still alive.

With a low load you should normally expect the output voltage to hit the desired value soon, at least in a flyback converter. Check that this happens. Then check what happens with a variable load – preferably electronic.

If you did not calculate your feedback loop, very likely you will see self oscillation (normally not destructive). If you don’t, use the step load function in your electronic load to check stability. If you see a clear ringing after a load step, you still have some work to do in your loop. But feedback and stability is another huge area which Mr. Ridley has taught us a lot about.

And yes – the world needs powerful POWER ENGINEERS desperately!

AQ: Caution is the key to success in power converters

I work across the scale of power electronics in voltages and currents. From switchers of 1W for powering ICs to 3kW telco power supplies up to multi-megawatt power converters for reactive power control in AC transmission networks and into power converters for high voltage transmission.

There is a difference in how you can work on these different scale converters. This difference is down to how much the prototype you are destroying costs, how long it takes to rebuild it and how easily it will kill you. When you spend more than 2 million on the prototype parts then you do not ever blow it up. If the high voltage on your converter is 15kV or more then there is no way to probe it with an oscilloscope directly and no possibility to be anywhere near that voltage without being hurt. So the level of care at these bigger power levels is higher and the consequence of a mistake is so high that the process needs to be much more detailed and controlled mostly for safety’s sake. We find that our big power converter processes really help when working on smaller converters. The processes include sign offs for safety, designed and prescribed safety and earthing systems for each converter, no scope probes put on and off live parts and working in pairs at all times with agreed planned actions. Pair working is one thing that may save you in the event of an electric shock. These processes seem very slow and cumbersome to engineers who work on low voltage (<1000V) but they are very useful even at low voltage.

Having said all that, experienced cautious engineers prevent converter blow ups. Add just a little bit of process and success can go up significantly. I think that an analysis of Dr Ridley’s failure list will point to actions that will improve success.

As my boss at one of those really large converter companies used to say “Stamp out converter fires”.

AQ: Power supply prototypes is the best way to learn it

I have been designing power supplies for over 15 years now. We do mostly off line custom designs ranging from 50 to 500W. Often used in demanding environments such as offshore and shipping.
I think we are the lucky ones who got the chance to learn designing power supplies using the simple topologies like a flyback or a forward converter. If we wanted to make something fancy we used a push-pull or a half bridge.

Nowadays, straight out of school you get to work on a resonant converter, working with variable frequency control. Frequencies are driven up above 250kHz to make it fit in a matchbox, still delivering 100W or more. PCB layouts get almost impossible to make if you also have to think about costs and manufacturability.
Now the digital controllers are coming into fashion. These software designers know very little about power electronics and think they can solve every problem with a few lines of code.

But I still think the best way to learn is to start at the basics and do some through testing on the prototypes you make. In my department we have a standard test program to check if the prototype functions according to the specifications (Design Verification Tests), but also if all parts are used within their specifications (Engineering Verification Tests). These tests are done at the limits of input voltage range and output power. And be aware that the limit of the output power is not just maximum load, but also overload, short circuit and zero load! Start-up and stability are tested at low temperature and high temperature.

With today’s controllers the datasheets seem to get ever more limited in information, and the support you get from the FAE’s is often very disappointing. Sometime ago I even had one in the lab who sat next to me for half a day to solve a mysterious blow up of a high side driver. At the end of the day he thanked me, saying he had learned a lot!
Not the result I was hoping for.