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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.

AQ: 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.

AQ: Remote diagnostic

Remote diagnostic is a must now a days. All CNC machines must be able to undergo remote access to undergo diagnostic and it must be two way. The problem mostly with remote diagnostic is it has to be two way and you have to have a qualified technician or an operator who is well verse with machine operations and its features, always on your machine he must be trained on how to be able to recover from lost of communication and the most important is to be able to engage E-stop when needed. The remote operator is a trained technician as well and knows a procedures and protocols that will help prevent accidents that can harm both man and machine. Mostly remote access is good for updates and upgrades, training and assistance needed. We offer the first year as free to make sure we can get the customer up an about during the learning curve on how to familiarize with control functions. We also need a land line or cell phone to be able to have a voice interchange. We use Webex for remote and another pc laptop or desktop as a dedicated bridge with controls that run with older versions of Windows such as windows XP. The dedicated PC is primarily secured as level four security compliance and must be turned off when remote diagnostic is needed. You can add assign a dedicated that is level four compliant as part of the control you will have two computers one on standby for remote diagnostic primiraly use for remote diagnostic, another for CNC function.

In regards to data collection new CNC’s are monitoring activities such as error messages that are categorized in different areas. This can be with the communication between PLC’s, CNC and station cards, lost of communication or timing problems errors common with the system, CNC errors due to plc warnings and prompts, operator prompts to name a few. Mostly this is error messages have a day and time stamp so it can easily be cyphered if the condition of errors are intermittent or consistent. We can all set up the option of recording what nc programs are run and how long it took to complete a job. It can also be set to count the number of hours the tools is used. Since this is a text format you design a spread sheet that can put them in named cells. The extent of data is a chosen through the logging option and in our case is stored in the Logging directory. It helps with monitoring intermitent problems and monitor if this is a NC program error, System error, human error, machine problem etc. It is a must now a days for ease of data gathering for management and troubleshooting.

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: Switching frequency selection

Switching frequency selection is actually a tradeoff, and follows the below guidelines:

  1. Lower frequency (Eg 30kHz) means bulkier magnetics and capacitors; Higher frequency (Eg 1Mhz)) means smaller parts, hence more compact PSU.
  2. Stay away from exact 150kHz as this is the low end of any EMI compliance; So, if your frequency happens to be exactly 150kHz, then your PSU will be a strong emitter; For many commercial low cost PSUs, 100 KHz has been used for many years, which is why many inductors and capacitors are specified at 100kHz.
  3. Higher frequency >/= 1MHz converters provide for better transient response. Obviously, the control IC should be capable of supporting. There are plenty of resonant converters available.
  4. Higher frequency results in higher switching losses; To control that, you will need faster switching FETs, Diodes, capacitors, magnetics and control ICs.
  5. Higher frequency MAY result in more broadband noise; its not always true, since noise can be controlled by good PCB layout and good magnetics designs.

Board power DC/DC converters are commonly built using 1MHz switchers.
Chassis power Telecom/Server PSUs seem to stay with 100-300KHz range.

Manufacturers are able to achieve exceptional density by virtue of High frequency resonant topologies, but they have to achieve high efficiencies too; Else, they will generate so much heat that they cannot meet UL/IEC safety requirements.
In some cases, they will leave the thermal problem to the user.  Usually, the first few paragraphs of any reference design discusses the tradeoffs.

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.