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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: Hysteretic controller

We can see that the hysteretic controller is a special case of other control techniques. For example, “sliding mode control” usually uses two state variables to determine one switching variable (switch ON or OFF). So the hysteretic controller is a special case of “1-dimensional” sliding mode. In general, there are many techniques under the name of “geometric control” that can be used to prove the stability of a general N-state system under a given switching rule. So I believe that you can apply some of these techniques to prove the stability of the hysteretic controller, although I have not tried to do this myself. The book “elements of power electronics” by Krein discusses that in chapter 17.

But I can talk more about one technique that I have used and in my opinion is the most general and elegant technique for non-linear systems. It is based on Lyapunov stability theory. You can use this technique to determine a switching rule to a general circuit with an arbitrary number of switches and state variables. It can be applied to the simple case of the hysteretic controller (i.e. 1 state variable, 1 switching variable) to verify if the system is stable and what are the conditions for stability. I have done this and verified that it is possible to prove the stability of hysteretic controllers, imposing very weak constraints (and, of course, no linearization needed). In a nutshell, to prove the system stable, you have to find a Lyapunov function for it.

What can expand is to go beyond a simple window comparator for hysteretic control.

#1) control bands, or switching limits can be variable and also part of a loop, especially if one wants to guarantee a nearly fixed frequency.

#2) using a latch or double latch after the comparator(s), one can define (remember) the state and define operations such as incorporating fixed Ton or Toff periods for additional time control… this permits the “voltage boost” scenario you previously said could not be done. This also prevents common “chaos” operation and noise susceptibility that others experience with simpler circuits.

#3) additional logic can assure multiphase topologies locked to a system clock and compete very well with typical POL buck regulators for high-end processors that require high di/dt response.

Time or state domain control systems such as this, can have great advantages over typical topologies. There really is no faster control method that provides a quicker load response without complete predictive processing, yet that can also be applied to hysteretic control.

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: Constant on-time control

There are three different more or less widely used types of constant on-time control. The first one is where the off-time is varied with an error signal. A loop with this type of control has a control-to-output voltage frequency response (or Bode plot if you prefer) similar to that of the constant-frequency voltage-mode control. The second one is where the off-time is terminated with a comparator that monitors the inductor current, and when that current goes below a level set by the error signal, the switch is turned on. This control (also called constant on-time valley-current control) has a control-to-output voltage frequency response similar to the constant-frequency valley-current control. The main difference is that its inner current-control loop does not suffer from the subharmonic instability of the constant-frequency version, so it does not require a stabilizing ramp and the control-to-output voltage response does not show the half-frequency peaking. The third version is where the off-time is terminated when the output voltage (or a fraction of it) goes below the reference voltage. This control belongs to the family of ripple-based controls and it cannot be characterized with the usual averaging-based control-to-output frequency response, for the reason that the gain is affected by the output ripple voltage itself.

As for the hysteretic control, the current-mode version is a close relative of the constant on-time valley-current-control. The version that uses the output ripple voltage instead of the inductor current ripple for turning on and off the switch (also called “hysteretic regulator”) is a close relative of the constant on-time ripple-based control.

Although the ripple-based control loops cannot be characterized with the usual Bode plots, the converters can still be unstable, but not in the meaning of the traditional control-loop instability that power-supply engineers are used to. Furthermore the hysteretic regulator is essentially unconditionally stable. The instabilities with ripple-based control are called “fast-scale” because the frequency of the instability is closely related to the switching frequency (either subharmonic, similar to the inner-loop instability of some of the current-mode controller, or chaotic in nature).

The paper I wrote a couple of years ago (“Ripple-Based Control of Switching Regulators—An Overview”) is a good introduction to ripple-based control and discusses some of the stability issues. There are also quite a few papers with detailed analyses on the stability of converters with feedback loops where the ripple content of the feedback signal is significant.

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: Simulation interpretation in automation industry

Related to “automation industry”, there are generally 3 different interpretations of what simulations is:
1) Mechanical Simulations – Via various solid modeling tools and cad programs; tooling, moving mechanisms, end-effectors… are designed with 3D visualizations, connecting the modules to prevent interference, check mass before actual machining…
2) Electronics Simulations – This type of simulations are either related to the manufacturers of “specific instrumentations” used in automation industry (ultrasonic welders, laser marking systems,…) or the designers of circuit boards.
3) Electrical & Controls Simulations.
A) Electrical Schematics, from main AC disconnect switch, down to 24VDC low amps for I/O interface.
Simulation tools allow easy determinations of system’s required amperage, fuse sizes, wire gauges, accordance with standards (CE, UL, cUL, TUV…)…
B) Logic Simulations, HMI interface, I/O exchange, motion controls…
a) If you want to have any kind of meaningful simulations, get in the habit of “modular ladder logic” circuit design. This means, don’t design your ladder like one continuous huge program that runs the whole thing; simulating this type of programs is almost impossible in every case. Break down the logic to sub-systems or maybe even down to stand alone mechanisms (pick & place, motor starter…), simulating and troubleshooting this scenario is fairly easy.
b) When possible, beside automated run mode of the machine or system, build “manual mode logic” for it as well. Then via physical push-buttons or HMI, you should have “step forward” & “step back” for every “physical movement or action”.

Simulating the integrity of the “ladder logic program” and all the components and interfaces will be a breeze if things are done meticulously upfront.

AQ: Active power losses in electrical motor

Equivalent active power losses during electrical motor’s testing in no-load conditions contain next losses:
1. active power losses in the copper of stator’s winding which are in direct relation with square of no-load current value: Pcus=3*Rs*I0s*I0s,

2. active power losses in ferromagnetic core which are in direct relation with frequency and degree of magnetic induction (which depends of voltage):
a) active power losses caused by eddy currents: Pec=kec*f*(B)x
b) active power losses caused by hysteresis: Ph=(kh*d*d*f*f*B*B)/ρ

3. mechanical power losses which are in direct relation with square of angular speed value: Pmech=Kmech*ωmech*ωmech,

Comment:
First, as you can see, active power losses in ferromagnetic core of electrical motor depend of voltage value and frequency, so by increasing voltage value you will get higher active power losses in ferromagnetic core of electrical motor.

Second, you can’t compare two electrical motors with different rated voltage and different rated power because active power losses in the ferromagnetic core, as I have already said above, depend of voltage value and frequency while active power losses in the copper of stator’s windings depend of square of no-load current value which is different for electrical motors with different rated power.

Third, when you want to compare active power losses in no-load conditions of two electrical motors with same rated voltage and rated power, you need to check design of both electrical motors because it is possible that one of them has different kind of winding, because, maybe in the past, one of them was damaged, so its windings had to be changed, what could be the reason for different electrical design and that has a consequence different no-load current value.

AQ: PPE (Personal Protective Equipment)

When I think of using PPE as a controls engineer, I think
about electrical shock and arc-flash safety in working with electrical devices.

The PPE (Personal Protective Equipment) requirements to work on live electrical
equipment is making doing commissioning, startup, and tuning of electrical
control systems awkward and cumbersome. We are at a stage where the use of PPE
is now required but practice has not caught up with the requirements. While
many are resisting this change, it seems inevitable that we will need to wear
proper PPE equipment when working on any control panel with exposed voltages of
50 volts or more.

With many electrical panels not labeled for shock and arc-flash hazard levels,
the default PPE requires a full (Category 2+) suit in most cases, which is very
awkward indeed. What can we do to allow us to work on live equipment in a safe
manner that meets the now not so new requirements for shock and arc-flash
safety?

Increasingly the thinking is to design our systems for shock and arc-flash
safety. Typically low voltage (less than 50 volts), 120VAC, and 480 VAC power
were often placed in the same control enclosure. While this is cost effective,
it is now problematic when wanting to do work on even the low voltage area of
the panel. The rules do not appear to allow distinguishing areas of a panel as
safe, while another is unsafe. The entire panel is either one or the other. One
could attempt to argue this point, but wouldn’t it be better to just design our
systems so that we are clearly on the side of compliance?

Here are my thoughts to improve electrical shock and arc flash safety by
designing this safety into electrical control panels.

1. Keep the power components separate from the signal level components so that
maintenance and other engineers can work on the equipment without such hazards
being present. That’s the principle. What are some ideas for putting this into
practice?

2. Run as much as possible on 24VDC as possible. This would include the PLC’s
and most other panel devices. A separate panel would then house only these shock
and arc-flash safe electrical components.

3. Power Supplies could be placed in a separate enclosure or included in the
main (low voltage) panel but grouped together and protected separately so that
there are no exposed conductors or terminals that can be reached with even a
tool when the control panel door is opened.

4. Motor Controls running at anything over 50 volts should be contained in a
separate enclosure. Try remoting the motor controls away from the power devices
where possible. This includes putting the HIM (keypad) modules for a VFD
(Variable Frequency Drive) for example on the outside of the control panel, so
the panel does not have to be opened. Also, using the traditional MCC (Motor
Control Centers) enclosures is looking increasing attractive to minimize the
need for PPE equipment.

For example “finger safe” design does not meet the requirements for arc-flash
safety. Also making voltage measurements to check for power is considered one
of, if not the most hazardous activity as far as arc-flash goes.

AQ: Voltage transmission & distribution

If you look back over history you will find how things started out from the early engineers and scientists looking at materials and developing systems that would meet their transmission goals. I recall when drives (essentially ac/dc/ac converters) had an upper limit around 200 to 230 volts). In Edison and Tesla days there was a huge struggle to pick DC or AC and AC prevailed mainly because it was economical to make AC machines. Systems were built based on available materials and put in operation. Some worked great some failed. When they failed they were analyzed and better systems built. Higher and higher voltages lowered copper content and therefore cost as insulators improved. Eventually commitees formed and reviewed what worked and developed standards. Then by logical induction it was determined what advances could be made in a cost effective and reliable manner. A lot of “use this” practice crept in. By this I mean for example, I worked at a company and one customer bought 3,000 transformers over the course of ten years, They had a specific size enclosure they wanted.

Due to high volume purchase the cost of the enclosure was low. Other small jobs came thru and this low cost enclosure was used on them to expedite delivery and keep cost minimum. Guess what, that enclosure is now a standard enclosure there because it was used on hundreds of designs over ten years. Is it the most economical box, probably not in the pure engineering sense but changing something that works is seldom a good idea. Today, they are raising voltage levels to new high values. I read of a project in Germany to run HVDC linesover huge distance. They are working to overcome a problem they foresee. How do you break the circuit with HVDC economically. If you ever put DC thru a small contactor maybe 600VDC you find quickly that the arc opening the contactor melts the contacts. Now, what do you do at 800kVDC or 1.2MVDC. What will the cost of the control circuit be to control this voltage level. (Edison and Tesla all over again)And there you have it, my only push for the subject of history to be taught.

AQ: Motor design

When I was doing my PhD in motor design of reluctance machines with flux assistance (switched reluctance machines and flux switching machines with magnets and/or permanently energised coils) my supervisor was doing research on the field of sensorless control (it wasn’t the area of my research but it got me thinking about it all). At the time I had thought (only in my head as a PhD student daydream) that I would have to initially force a phase (or phases) to deliberately set the rotor into a known position due to the phase firing then start a normal phase firing sequences to start and operate the motor for a normal load without the need for any form position detection (all this was assuming I had the motor running from stationary to full speed at normal expected load with use of a position sensor to start with so I could link phase firing, rotor position and timings all together to create a “map” which I could then try to use to re-program a firing sequence with no position detection at all but only if I could force the rotor to “park” itself in the same position every time before starting the machine properly – the “map” having the information to assume that the motor changes speed correctly as it changes the firing sequences as it accelerates to full speed). But any problem such as unusual load condition or fault condition (e.g. short circuit or open circuit in a phase winding) would render useless such an attempt at control with no form of position detection at all. The induction machine being sensorless and on the grid being measured.