In March Dr Hamish Laird presented the High Performance Digital Control professional education seminar at this year’s Applied Power Electronics Conference in Long Beach California. Attendance at the seminar was high, with more than four hundred people coming to hear Dr. Laird speak. The large number of attendees asking questions at the end of the presentation showed how much demand there is for this type of knowledge and training.
The response to the presentation was very positive with a number of people asking if Dr. Laird could write a book containing information from the seminar.
Hamish Laird’s Reaction
Dr. Hamish Laird says, “As the power electronics design industry moves to digital control there is a great need to ensure that the quality of the digital control is high enough to ensure safe and reliable power supply operation. We at ELMG Digital Power hope to contribute to the constant improvement in the quality of the control”.
The ELMG Digital Power Chief Technology Officer also announced that “…as a result of the really large interest we are making the slides from the presentation available to download via our website. And we are working up a plan to present the three hour seminar content as a multipart webinar”.
Digital Modulators Explained
The first part of the APEC seminar covered how digital modulators such as Variable Period Oscillators (VPO) and Pulse Width Modulators PWM) differ from analogue modulators while the second part covered the other issues and know how to do with compensators, analogue to digital converters and anti-aliasing filters.
Dr. Laird again – “From the number of questions we received after the presentation it looks like digital control is well on the way to becoming established in the smaller switchmode power space. We look forward to helping achieve high quality digital control.”
Over the last two years ELMG Digital Power CTO, Dr. Hamish Laird, has helped supervise (the now Dr.) Rabia Nazir in the pursuit of her Doctoral studies.
Hamish Laird says
“The research that Rabia has completed in the area of fractional delays in recursive filters for current control in grid tied inverters gives great control tools in the implementation of control for GTIs in grids where the AC system frequency is varying. It is always great to help with PhD research as I learn so much so thanks to Rabia for letting me help.”
Congratulations to Dr. Rabia Nazir on her successful oral defense of here work. Dr Laird again
“It was fantastic to attend Rabia’s defense. I am so proud of and pleased with the work she did in analysing, simulating and building power converter hardware to show her findings. It was a great learning experience for me.”
Recently (now Dr.) Rabia Nazir presented a paper at a conference in Sicily on the use of Taylor Series expansion based fractional delay filters for recursive control of grid inverter currents.
What is the latest news from the Linkedin Digital Power Electronics Control group?
The group now has over 600 members. To join the group please click here.
In the group we have lively discussions with people contributing their knowledge and experience. To all who have contributed and made the group so much fun – a really big thank you.
In the group we have had discussions that cover
Digital PWM modulators
ADC (analogue to digital converters)
Choosing the number of bits in a digitally controller,
Implementing digital filters,
Processor choices and coding.
How many bits are required in digital control of power electronics
Recently the how many bits are required was brought up and addressed in this way by John S.
There are two aspects to selecting the number of bits: measurement resolution and control variable resolution. For measurement resolution, the ADC resolution is the deciding factor, and this can be increased by oversampling.
In a current control application I was recently working on, I required 14 bits of resolution to meet a target requirement of +-1% of ADC quantitization error at 10% current. This resolution was relatively simple to achieve by oversampling the 12 bit ADC in the uC.
The resolution of the control variable, e.g. uC timer resolution, is just as critical in fast systems. If the resolution is too coarse, jitter in the control variable will appear at the output. With slower system dynamics, e.g. in voltage controlled systems, jitter in the control variable causes less jitter in the output variable because of the smoothing effect of the output capacitor. Ideally the resolution of the control variable should match that of the measured variable.
Thanks also to all the other contributors.
Other discussions cover measuring stability of power converters, what is the effect of PWM timer quantisation and what is the best converter for isolated bi-directional DC to DC.
Look out for the group identified with the z-transform image like that shown above.
Recent Group Discussions
Recent discussions have included one on the effects of precision limits for PWM timers
Another thread covers how to get started in Digital Control of Power electronics.
There are some well known and expert members in the group who share experience and insight.
The group is private to prevent the spam that has been increasing on Linkedin so please ask to join.
Where to begin with digital control
The discussion on where to begin with digital control is an ongoing and popular thread. The thread has 49 replies from group members offering advice on what is important in digital control of power electronics
Another discussion covering a very specific issue is on handling slip-strike problems. Slip strike is what happens where the system output gets “stuck” and then suddenly moves. This typically happens in mechanical or hydraulic systems where the friction coefficient falls as movement begins. A relatively simple solution is to use a high derivative gain to reduce the effect. Other solutions include linearising the system to allow higher controller gains and higher bandwidth.
Choosing the control processor
The discussion on choosing the processor for digital control is also still popular. This covers which is the best micro controller or processor for digital control of power electronics. This is an ongoing and recurring theme as more people use digital control for their power electronics. Other subjects in the thread include
analogue to digital converter choice
settling time and
how to design a digital loop compensator
Numeric precision and the effect of limited PWM timer bit limits are also mentioned.
Join the community
We look forward to seeing you there.
Come and join the community at the Digital Power Electronics Control group on LinkedIn.
Google are always shaking things up. And they are shaking the digital power electronics and low power loss converter space with this competition.
For those of you who don’t know, the Little Box Challenge is a competition where you can win $1 Million USD for creating a 2kVA inverter with a power density of 50W per cubic inch. For those not familiar with this unit I calculate this to be 3.4 kW per liter. Other specifications are around efficiency, AC voltage and current THD and noise, and the DC side current ripple. The inverter is a single phase device.
This is an interesting challenge. It is also put together and presented in an interesting way. The marketing website has pictures of hybrid cars, laptops and washing machines. The inverters in these are three phase inverters. The fan in your laptop is driven by a three phase inverter as is the motor in your hard drive. Plugin hybrid car AC to DC converters for battery charging are single phase but they are probably not inverters and in a strange juxtaposition, there is a picture of a hairdryer. Then I realised I had misunderstood. The inverter is to power the appliances and charge the electric vehicle.
When you read on it becomes clear that the the little box challenge technical problem is single phase and more to do with solar inverters or probably standby inverters for your household single phase loads. Sorry to those at Google for misunderstanding the picture.
The minimum efficiency is do-able now with silicon and/or SiC. I have an in-production 2.88 kW isolated digitally controlled AC to DC converter that I had downstairs on my desk now. It is more efficient by some distance than the minimum efficiency required in the contest. It has SiC diodes in the PFC stage. Aside from that it is all silicon. And it is from six years ago. Just measuring it now and it is 10 inches by 1 and 5/8 inches by 4 and 3/4 inches for a total volume of 77.1875 cubic inches. This gives 37 W /cubic inch so it is in the ballpark. All of you who do this stuff know that to get to 50W per cubic inch will take some effort. Having said that looking in my power supply there is some air space.
The Littlebox requires a a DC side ripple current that is quite low. I haven’t worked out the 120Hz filtering volume needed to do this with capacitors but am assuming the volume will be significant. By adding the DC side ripple requirement the challenge becomes really quite different and is more about energy storage replacement or the equivalent of valley filling. The solution to that might be some sort of integrated reasonable switching frequency active filter on the DC side that only operates at certain conditions so the 20% current ripple can be met. Perhaps the active filter could gyrate the storage capacitor. Or maybe some sort of pulse number increase like the old HVDC ripple current re-injection schemes. I might dig a few of those papers out to see what use they’d be.
The AC side specification doesn’t seem too harsh. A good inverter feedforward lineariser for PWM precision, dead-time and minimum pulse width with a good repetitive or resonant notch filter based current controller will meet this THD and noise requirement. It’d be good to know the measuring instruments noise measurement approach. The magnetising current of the transformer is supplied by the inverter so this will mean that getting both the current THD and the voltage THD below a certain limit may not be actually possible. This will depend on the transformer magnetising characterisitcs.
It is possible that you will be able to meet the power density with the minimum efficiency as the cooling might be OK. That said the description of the testing doesn’t cover exhaust air temperatures as would a UL standard. There may be room for that kind of really high temperature exhaust air that UL are always so troubled by. It might be that the contest rules need to be tightened on this.
For those of you considering this from outside the US make sure you get some US fuses as the UL and IEC definition of the current is significantly different. Maybe a part number specification for the fuse for the testing would be useful. US fuses will be hard to buy outside the US in any case.
It’d also be good to get some idea of what the transformer in the test is so any EMC filter design can be optimised for that transformer.
The solution to the DC ripple current was solved by Tesla (Nikola – not the car company) back when he was doing his thing. It is three phase AC systems where balanced three phase currents give constant power flow and so constant DC side current. If only we had three phase appliances.
Unfortunately most of the people who legitimately have a good shot at meeting this challenge will not be allowed to as they are already doing this type of stuff for a job and so will not be able to meet the disclosure requirements in the terms and conditions.
Other issues might be getting your gear into the US.