Gallium Nitride: Is it the End of the Road for Silicon in Power Conversion?

Hi folks,

This article is regarding upcoming GaN or wide band gap technology. I am also attaching the links to brief application notes and short video tutorials. Go through it if you find this article interesting. 

For over three decades, power management efficiency and cost showed steady improvement as innovations in power MOSFET structures, technology, and circuit topologies paced the growing need for electrical power in our daily lives. In the new millennium, however, the rate of improvement slowed as the silicon power MOSFET asymptotically approached its theoretical bounds.

Power MOSFETs first started appearing in 1976 as alternatives to bipolar transistors. These majority carrier devices were faster, more rugged, and had higher current gain than their minority-carrier counterparts. As a result, switching power conversion became a commercial reality. AC-DC switching power supplies for early desktop computers were among the earliest volume consumers of power MOSFETs, followed by variable speed motor drives, fluorescent lights, DC-DC converters, and thousands of other applications that populate our daily lives.

 HEMT (High Electron Mobility Transistor) gallium nitride (GaN) transistors first started appearing in about 2004 with depletion-mode RF transistors made by Eudyna Corporation in Japan. Using GaN on silicon carbide (SiC) substrates, Eudyna successfully brought into production transistors designed for the RF market. The HEMT structure was based on the phenomenon first described in 1975 by T. Mimura et al and in 1994 by M. A. Khan et al, which demonstrated unusually high electron
mobility described as a two-dimensional electron gas (2DEG) near the interface between an AlGaN and GaN heterostructure interface. Adapting this phenomenon to gallium nitride grown on silicon carbide, Eudyna was able to produce benchmark power gain in the multi-gigahertz frequency range. In 2005, Nitronex Corporation introduced the first depletion mode RF HEMT transistor made with GaN grown on silicon wafers using their SIGANTIC® technology.
 
As shown in Figure, SiC and GaN both have a superior relationship between on-resistance and breakdown voltage due to their higher critical electric field strength. This allows devices to be smaller and the electrical terminals closer together for a given breakdown voltage requirement. GaN has an extra advantage compared with SiC as a result of the enhanced mobility of electrons in the 2DEG. This translates into a GaN device with a smaller size for a given on-resistance and breakdown
voltage.

 Enhancement mode (eGaN®) transistors have characteristics very similar to the power MOSFET, but with improved high speed switching, lower on-resistance, and a smaller size than their silicon predecessors. These new capabilities, married with a step forward in high-density packaging, enable power conversion designers to reduce power losses, reduce system size, improve efficiency, and ultimately, reduce system costs. These are the early years of a great new technology.

EPC Application Note on GaN fundamentals:
http://epc-co.com/epc/Portals/0/epc/documents/product-training/Appnote_GaNfundamentals.pdf

EPC Video tutorials on GaN:
https://www.youtube.com/watch?v=WxZs5zM1nyA&list=PL0Nwh_j9InYyXYUff-ixuRAjpa63Z-m7j

Inverter Basics : Building Blocks of Modern Power Electronics, as we know it!

Hi folks,

The purpose of inverters is to convert AC power into DC power. They are used in several applications including Uninterruptible Power Supplies, Control of Electrical Machines, Active Power filtering and Grid integration of renewable. Before we start with how inverter functions, we will have a look at fundamentals of AC and DC power. On screen, you can see six waveform. Now try to identify yourself, which one is AC and which one is DC. This might be very easy for waveform A and B, as they are familiar. How about other four? Are they AC or DC? (Please forgive me for my drawing skills and I expect you to think before you scroll down for the answer.)

The key concept here is any periodic waveform with average value over a cycle equal to zero is an AC waveform. Any waveform with non-zero average value over a cycle, may be positive or negative, is a DC waveform. Now you can easily identify that waveform C and D are AC waveform whereas waveform E and F are DC waveform.

Now that we have a clear concept of AC and DC waveform, Let us see how inverter works. We have a DC source and we need to generate an AC waveform. To do this, we are supposed to generate a waveform that has zero average value over a cycle. The simplest way to do this is to have a mechanism, which can connect DC source directly to the AC load for some time, reverse connect the DC source directly to the AC load for some time and should be able to do this repeatedly. We all know that the frequency of AC supply available at our home is 50 Hz. So this means, we need to change the connection between DC and AC side for every 10 milliseconds which is not possible manually or by using mechanical switches. So here, we take help from semiconductor devices which can turn on and off a million times per second.

Now below we can see a simplest circuit that can satisfy our requirements. This is a half bridge inverter. When we turn on switch S1 and turn off S2, the voltage difference between point A and Point B is Vdc/2. Then we turn off S1 and turn on S2 and voltage difference between point A and Point B is -Vdc/2. If we do this process repeatedly, we will generate AC power from DC power. The important drawback of this circuit is: even though we have total DC voltage of Vdc, we can apply only half of it at a time across AC load. Can you think of a circuit that can apply the complete DC voltage across the AC load and also able to change the polarity of it?

This circuit is called full bridge inverter or single phase inverter. More popularly, it is known as H bridge structure. When we turn on switch S1 and S4, the voltage difference between point A and Point B is Vdc. Then we turn off S1 and turn on S2 and voltage difference between point A and Point B is -Vdc. So now we have a circuit that can give us complete utilization of available DC voltage. This circuit operation also has a drawback. Many applications, especially control of electrical machines, require variable AC voltage. In this circuit the only way to vary the output AC voltage is to change the value of Vdc. To change the value of Vdc, additional DC-DC converter will be required that will lead to additional circuitry, losses and extra cost.

Can we operate the same structure in such a way that we can vary the output AC voltage while keeping the DC voltage constant? This can be done using various modulation strategies. Let us discuss this in our upcoming articles. 

Thank you for your interest. Please let me know about the topics you want to read, I will try to post accordingly. Please leave comments if you have any doubt or suggestions.

Quasi Z and Extended Quasi Z DC-DC converter

Hi folks.

I have told you that I have been working on DC-DC converters. Here is link to my paper published in IJEE.

Its about quasi Z source and its applications in photovoltaic (PV) systems. It covers basic topologies of converters used, its working and operational states, its suitability towards PV applications.

 http://www.irphouse.com/ijee/ijeev8n1_02.pdf 

Please make sure that you do not copy any part of paper and use it as your own. Violating copyright is a serious offense by law.

In case you have any queries or suggestions, please feel free to comment or you can mail me at dnachiketa1010@gmail.com

Z- source based DC-DC converter

Hi folks.

Lets start or MISO systems' discussion with a trending technology - Z source impedance network. This was proposed in 2002 and up till now, it is utilized in almost every possible power conversion circuit. 

We start our discussion with the simplest application i.e. DC-DC converter. Z source based DC-DC converter can reduce in-rush and harmonic current, provide larger range of output dc voltage and improve reliability. It can operate in voltage-fed and current-fed when the place of the source and load is exchanged each other (bidirectional operation), and it can be perform buck-boost function in these two conditions.

The figure shows voltage fed Z source based DC-DC converter.

It has two operating states named shoot through and non shoot through states. 
The Z-network of the Z-source dc-dc converter is  symmetrical, that is, the inductors L1 and L2 and capacitors C1 and C2 have the same inductance (L) and capacitance (C), respectively.

so we have, 

In non shoot through state , the switch S1 (or diode forward biased, you can say) is turned on and S2 turned off. The dc source charges the z-network capacitors, while the inductors discharge and transfer energy to the load. 

                                

The interval of the converter operating in this state is (1-D)T, where D is the duty ratio of switch S2, and T is the switching cycle then one has,

In the shoot through state (remember this name, its very important),  the switch S2 is turned on and S1 turned off (diode reverse biased by capacitor voltage to protect source from short circuiting- very important function of diode) . The z-network capacitors discharge, while the inductors charge and store energy to release and transfer to the load. 

The interval of the converter operating in this state is DT, one has,

Where Vi is the value of the dc voltage source. 

The average voltage of the inductors over one switching period (T) in steady state should be zero. (ref = volt-sec balance theory - inductor and capacitor basics we talked about) 

so we have, 

So, this is how it works. It has large number of applications. A friend of mine used it in MPPT (maximum power point tracking) of solar system. I used quasi Z and Extended quasi Z source for the same application. We both are working on Z source inverter as our project and its not as easy as it seems..! 

In the upcoming session we will talk about MISO systems that I have proposed through my papers. Please feel free to leave a comment or you can mail me your opinions on nachiketa1010@gmail.com. You can also mail me topics of your interest or any other useful information that you want to share. 

MISO systems

Hi folks,

I know its been a long time. lets resume our discussion from MISO (multiple input single output) systems that I am working on. (actually I have been working a lot of arenas lately - MISO systems, 9 switch inverter, Z source derivatives etc n published 3 ieee conference papers too).

The thing about DC-DC converter is they become cost inefficient n bulky when it comes to interfacing more number of sources. Say boost converter, for 10 sources, we need 10 inductors, 10 diodes, 10 capacitors n 10 switches. so there is need of a topology that reduces number of inductors and capacitors with giving us independent control over each source.

Have a look.

The MISO system I was working on fuses boost converter and conventional Z source to reduce passive elements and improve gain of the system. It also reduces ripples in source current and voltage stress on each source. It provides independent control for each source. It can also work for voltage sources having different magnitudes. This is a feature that very few MISO systems can achieve.
we will talk about it.

I would like to have your views on MISO. In case you have any doubts, feel free to comment.

interview question explained-8.1

Hi folks.
Here we are going to continue our discussion about differences in metering CT and protective CT.

Let us talk about the magnetising curve first.

 This is typical magnetisation characteristics of a ferromagnetic material. Imagine it as leg of the person sitting on chair. The first point is similar to his ankle and second point is similar to his knee. The important point to note here is the region between these 2 points is linear. So we get output proportional to input (true reflection). Below ankle point and above knee point is not possible (due to magnetic inertia and due to saturation respectively, if you want to know).

For a metering CT, we want linear reflection of input from 20% to 120% of the load current. So this region should fall between the 2 markers we have set. Also when fault occurs, The currents values rise to very large extent (even 20 times the rated value). Under these conditions, CT should saturate and protect the measuring devices connected across it. So measuring CT is operated just below the knee point. The core material used is not of superior quality as it is supposed to get saturated at 120% of load.

For protective CT, we want linear reflection of the fault current. If we do not get linear reflection of normal current, not an issue. So 1.5 to almost 20 (or even higher) times of the rated secondary current should fall in the linear region. So protective CT is operated near ankle point. The core material used is of superior quality as we do not want saturation even the 50 times the rated secondary current of CT.

Thank you for your time. Please feel free to leave comments about your views. You can mail me your doubts at dnachiketa1010@gmail.com, I will be very glad to post about them.

interview question explained-8

Hi folks.
Here we are going to talk about a very important practical aspect of power system engineering.

Q. Why secondary of the Current transformer is never kept open or should not be opened?

Let us first see what is current transformer. It has same working principle (self and mutual induction) as that of voltage transformer. I want to measure the AC current flowing through a conductor. For that purpose, I put a circular magnetic core around it. I wrap a wire on that core. Now the rate of change of flux in the core in responsible for the induced EMF in the wrapped wire. That flux will be produced by the straight conductor carrying current. So the current flowing through the wrapped wire is giving me the reflection of the current flowing through the straight conductor. This is nothing but current transformer.

To have the transformer action, the flux produced by primary (straight conductor) should be opposed by the flux produced by the secondary (wrapped conductor). If we open the secondary of the Voltage transformer (traditional transformer, you know from 12th), the primary current is very small (no load current). So the flux produced in the core is very small and will not cause any undesirable effects.

For a current transformer, The current in primary is not depending on the current in secondary. It is depending on the load connected across it. So when secondary is opened, The opposing flux produced by secondary is vanished. So the only flux present in core is flux due to primary and it is very high. This large flux induces extremely high voltages across open secondary. This voltages may rise to several KV that is enough to damage any equipment, or the insulation or in worst cases to kill the operator.

You can also understand this as CT always tries to maintain constant secondary current. When there is some resistance across its secondary, (typical relay coil resistance is 0.5 Ohm) Small voltage is enough to make flow rated current. When the secondary is opened, To make flow the same current through high air resistance, the CT develops high voltages across it.

So there is spacial care taken provide a shorting switches across the secondary of the transformer. Many relay constructions have a spring operated shorting switch that keeps secondary of CT shorted when the body of relay is removed form its plastic casing. Please keep this thing in mind. Just for information, The voltage developed by a protective CT should be greater then the voltage developed by metering CT. We will soon talk about Difference between these two as it is interesting to see how hardware construction changes to satisfy peculiar purpose.

Thank you for your time and please feel free to leave comments about your views. You can also mail me at dnachiketa1010@gmail.com