Iacdrive|Automation Solution

How to suppress chaotic operation in a DCM flyback at low load

I would like to share these tips with everybody.
A current mode controlled flyback converter always becomes unstable at low load due to the unavoidable leading edge current spike. It is not normally dangerous but as a design engineer I don’t like to look at it and listen to it.

Here are three useful and not patented tips.

First tip:
• Insert a low pass filter, say 1kohm + 100pF between current sense resistor and CS input in your control IC.
• Split the 1kohm in two resistors R1 to the fet and R2 to the control IC. R1 << R2.
• Insert 0,5 – 1pF between drain and the junction R1/R2. This can be made as a layer-to-layer capacitor in the PCB. It does not have to be a specific value.
• Adjust R1 until the spike in the junction in R1/R2 is cancelled.
You will see that the current spike is always proportional to the negative drain voltage step at turn-on. Once adjusted, the cancellation always follows the voltage step, and you some times achieve miracles with it. Cost = one resistor.

Second tip:
Having the low pass filter from first tip, add a small fraction of the gate driver output voltage to the current sense input, say 0,1V by inserting a large resistor from ‘Drive Out’ to ‘CS input’. The added low pass filtered step voltage will more or less conceal the current spike. You should reduce your current sense resistor accordingly. Cost = one resistor.

Third tip:
In a low power flyback, you some times just need an RC network or just an extra capacitor from drain to a DC point, either to reduce overshoot or to reduce noise. Connect the RC network or the capacitor to source, not to ground or Vcc. If you connect it to ground or Vcc, you will measure the added discharge current peak in the current sense resistor. Cost = nothing – just knowledge.

All tips can be used individually or combined => Less need for pre-load resistors on your output.

Right Half Plane Pole

Very few know about the Right Half Plane Pole (not a RHP-Zero) at high duty cycle in a DCM buck with current mode control. Maybe because it is not really a problem.
It is said that this instability starts above 2/3 duty cycle – I think that must be with a resistive load. If loaded with a pure current source, it starts above 50% duty cycle.

Here is a little down-to-earth explanation:
If you run a buck converter at high duty cycle but DCM, it probably works fine and is completely stable. Then imagine you suddenly open the feedback loop, leaving the peak current constant and unchanged. The duty cycle will then rush either back to 50% or to 100% if possible. You now have a system with a negative output resistance – if Voltage goes up, the output current will increase.

You can see it by drawing some triangles on a piece of paper: A steady state DCM current triangle with an up-slope longer than the down-slope and a fixed peak value. Now, if you imagine that the output voltage rises, you can draw a new triangle with the same peak current. The up-slope will be longer, the down-slope will be shorter but the sum of times will be longer than in the steady state case. The new triangle therefore has a larger area than the steady state triangle, which means a higher average output current. So higher output voltage generates higher output current if peak current is constant. Loaded with a current source, it is clear that this is an unstable system, like a flipflop, and it starts becoming unstable above 50% duty cycle.

However, when you close the feedback loop, the system is (conditionally) stable and the loop gain is normally so high at the RHP Pole frequency that it requires a huge gain reduction to make it unstable.

It’s like when you drive on your bike. A bike has two wheels and therefore can tilt to either side – it is a system with a low frequency RHPP like a flipflop. If you stand still, it will certainly tilt to the left or to the right because you have no way to adjust your balance back. But if you drive, you have a system with feedback where you can immediately correct imbalance by turning the handlebars. As we know, this system is stable unless you have drunk a lot of beers.

Experience: Flyback

My first SMPS design was a multiple output flyback. This was in 1976, when there were no PWM controllers. So I used a 556 (1/2 osc -30 kHz, and 1/2 PWM generator) plus used a 3904 NPN where the VBE was the reference and also provided gain for the error amp function. I hap-hazardly wound the windings on a 25 mm torroid. It ranglike a tank circuit. I quickly abandoned the transformer and after a year, and many hours on the bench, I had a production-grad SMPS.
Since it went into a private aircraft weather reader system, I needed an exterier SMPS which was a buck converter. I used an LM105 linear regulator with positive feedback to make it oscillate (one of nationals ap notes). It worked, but I soon learned that the electrolytic capacitors lost all of their capacitance at -25 deg C. It later worked with military-grade capacitors.

I had small hills of dead MOSFETs and the directly attached controllers. When the first power MOSFETs emerged in 1979, I blew-up so many that I almost wrote them off. They had some real issues with D-S voltage overstress. They have come a long way since.

As far as very wide range flyback converter, please dig-up AN1327 on the ONSEMI website. This describes a control strategy (fixed off-time, variable on-time) and the transformer design.
The processor to that was a 3W flyback that drove 3 floating gate drive circuits and had an input range of 85 VAC to 576 VAC. It was for a 3 phase industrial motor drive. The toughest area was the transformer. To meet the isolation requirements of the UL, and IEC, it would have required a very large core, and bobbin plus a lot of tape. The PCB had the dimensions of 50 mm x 50 mm and 9 mm thick A magnetics designer named Jeff Brown from Cramerco.com is now my magnetics God. He designed me a custom core and bobbin that was 10 mm high on basically an EF15 sized core. The 3 piece bobbin met all of the spacing requirements without tape. The customer was expecting a 2 – 3 tier product offering for the different voltage ranges, but instead could offer only one. They were thrilled.

Can be done, watch your breakdown voltages, spacings and RMS currents. I found that around 17 -20 watts is about the practical limit for an EF40 core before the transformer RMS currents get too high.

Experience: Design

I tell customers that at least 50% of the design effort is the layout and routing by someone who knows what they are doing. Layer stackup is very critical for multiple layer designs. Yes, a solid design is required. But the perfect design goes down in flames with a bad layout. Rudy Severns said it best in one of his early books that you have to “think RF” when doing a layout. I have followed this philosophy for years with great success. Problems with a layout person who wants to run the auto route or doesn’t understand analog layout? No problem, you, as the design engineer, do not have to release the design until it is to your satisfaction.

I have had Schottky diodes fail because the PIV was exceeded due to circuit inductance causing just enough of a very high frequency ring (very hard to see on a scope) to exceed the PIV. Know your circuit Z’s, keep your traces short and fat.

Fixed a number of problems associated with capacitor RMS ratings on AC to DC front ends. Along with this is the peak inrush current for a bridge rectifier at turn on and, in some cases, during steady state. Unit can be turned on at the 90 deg phase angle into a capacitive load. This must be analyzed with assumptions for input resistance and/or a current inrush circuit must be added.

A satellite power supply had 70 deg phase margin on the bench, resistive load, but oscillated on the real load. Measured the loop using the AP200 on the load and the phase margin was zero. Test the power supply on the real load before going to production and then a random sampling during the life of the product.

I used MathCAD for designs until the average models came out for SMPS. Yes, the equations are nice to see and work with but they are just models none the less. I would rather have PsPice to the math while I pay attention to the models used and the overall design effort. Creating large closed form equations is wrought with pitfalls, trapdoors, and landmines. Plus, hundreds of pages of MathCAD, which I have done, is hard to sell to the customer during a design review (most attendees drift off after page 1). The PsPice schematics are more easily sold and then modified as needed with better understanding all around.

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.

Grain Storage system

A Grain Storage system usually consists of the following elements

1. A means of measuring Grain coming in and out- Usually a truck scale or a bulk weighing system. In addition some applications require measuring grain between transfers to different bins and a bulk weighing system is usually used for that.

2. A means of transferring between different operations or storage location- Conveyors, screws, buckets, pneumatic, wheel house.

3. Dust Collection

4. And Equipment for the operations that will be performed, drying, cleaning, screening, grading, sampling, roasting, steaming, packaging etc.

A system I have just completed was 24 containers + 3 buildings for storage with 76 conveyors, 3 drop-off and 3 loading points. Connection to ERP system and weigh scales to weigh trucks and send them to the correct bay. Local HMI on each bay ensured correct lorry goes to correct bay. Main conveyor runs are automatically selected. Manual option to run all conveyors to move grain around.

System used Ethernet infrastructure with hubs mounted strategically around tank farm. Also implemented soft starter with Ethernet connectivity, thus allowing easy monitoring of current consumption + for maintenance.

The future-proof design will allow customer to install level and humidity measurement in the future using the Analogue IO connected on Ethernet.

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”.

How to design an Panel required for PLC / MCC / Drive

1. The regular industrial standard size panel available with most of the panel fabricator’s.
2. Type of protection (used to say as IP).
3. Spacing depends upon the Power handled by the conductors inside Panel and the ventilation system.
4. Cable Entry / Bus bar entry may depend on the application and site condition. it may be at rear/bottom or at the top.
5. When comes to Drive, if the site condition is too hot then an industrial ac is required usually attached at the side of the panel.
6. Drive to drive required spacing (Check the manual of the drive}, since the power switching activity take place inside the drive.
7. Provide required space for the transformers and AC-choke since they create magnetic flux in ac circuits.
8. Don’t mix the Control cable, Power cable, Signal cable and Communication cable together in the cable tray… Otherwise you will be wired…
9. Keep the control on mcb/mccb/mpcb in handy location. So that its easier for operator to control it frequently and not disturbing other circuits..
10. Plc will be acquiring the top position in the panel since there is nothing to do with it once installed. Just we will be monitoring the status.
11. Don’t place the Plc nearby to the incoming or outgoing heavy power terminals..
12. Mcc panel are easy thing to do, but do the exact calculation for the ACB selection in the incomer side. Since each feeder will be designed with tolerance level.

There being a lot more than 12 guidelines to follow. What
about back-up power for the PLC? What about internal heat flow considerations
(not just does it need an AC or not)? How much space between terminal blocks
and wireway? What about separate AC and instrument grounds? What about wireway
fill? What about wire labels? What about TSP shields? What about surge
protection? In my experience, there are plenty of people that can design a
panel but if they haven’t gone to the field with it then they haven’t been able
to learn from their design mistakes.

The best thing you can do is start your design but you
really need to be guided by an experienced designer.

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!

Avoid voltage drop influence

My cable size and transformer size should give me maximum 3% on the worst 6% to 10%. If it is the single only equipment on the system then maybe you can tolerate 15%. If not, dip factor may affect sensitive equipment and lighting.

This is very annoying for office staff each time a machine starts lights are dimming. It does not matter what standard you quote I cannot accept 10%- 15% make precise calculation and add a 10% tolerance to avoid.

In most cases, this problem comes from cable under sizing so we have to settle with a Standard giving 15% Max.

Just recently I had to order a transformer and cable change for a project which was grossly undersized.
I have had to redesign the electrical portion of a conveyor and crushing system to bring the system design into compliance with applicable safety codes. The site was outdoor at a mine in Arizona where ambient temperatures reach 120F. The electrical calculation and design software did not include any derating of conductor sizes for cable spacing and density within cable trays, number of conductors per raceway, ambient temperature versus cable temperature rating, etc. Few of the cables had been increased in size to compensate for voltage drop between the power source and the respective motor or transformer loads.

Feeder cables to remote power distribution centers were too small, as voltage drop had not been incorporated in the initial design. The voltage drop should not be greater than 3%, as there will be other factors of alternating loads, system voltage, etc. that may result in an overall drop of 5%.

The electrical system had to be re-designed with larger cables, transformer, MCCS, etc, as none of the design software factors in the required deratings specified in the National Electric Code NFPA70 nor the Canadian Electric Code, which references the NEC.