A Power Module

Reading Time: 17 minutes

Special Thanks to

  1. Prof Varsha Shah, EE Dept, SVNIT
  2. Mr Anand Aggarwal’s 6.002 MIT OCW Course 

Above is a team of thousands of different power modules, better known as the control room of a power plant.🦾

Are you ready to know one of them???

INTRODUCTION

Control and measurement of system parameters is a crucial facet for reliable and safe operation of any electrical systems, particularly for real-time system. By real-time system we mean the system whose parameters like current, voltage, impedance, power, etc. changes with time, thus to maintain the parameters under threshold limits, we first require to monitor them i.e. take measurements in real-time.

For most of the electrical circuits, the voltages and currents are two parameters of greatest interest, as they are solely responsible for safe operation. When unchecked one leads to electrical breakdown and another a thermal breakdown.

Consider the following cases:

  1. A battery backup system, constant monitoring of battery terminal voltage is necessary to stop the battery from getting over-discharged. Also, current drawn has to be monitored to check that the battery doesn’t overheats and catch flames.
  2. In the power system, bus voltages and currents in line have to be maintained very precisely, which again requires first taking measurements.
  3. For metering of electrical energy consumed by a consumer, we need voltage, current and power factor measurement.
  4. Majority of control systems in industrial system employ a negative feedback technique which essentially requires sampling/monitoring of a particular output parameter, which is itself a form of measurement.

PREREQUISITES

KVL and KCL, 😅😅 rest leave on us!!!

THE PROJECT IN SHORT:

Under this project, we set out to build a dynamic power module for measurement of current and voltage in DC circuits and current, voltage, power factor and frequency in AC systems in real-time to constantly monitor them and trigger necessary safeguards.

It’s pretty obvious that real-time operations are best executed with the help of microcontrollers. Microcontrollers are equipped with a group of pins called ADCs which basically read analog voltage level and convert them to n-bit digital data. Problem is that these microcontrollers can hardly survive above 5 V pressure.

So, if we wish to measure higher AC/DC values then we are required to take proper samples of voltage and current, then do proper conditioning, and finally process the data to compute the parameters.

ARDUINO UNO and ADC

With first boards appearing in 2005, Italy based Arduino.cc is open-source software and hardware company which gives a range of affordable microcontroller. With a broad computational power range, they are easy to use platform for purposeful use in industry, education, art etc.

A Power Module

Atmel based Arduino UNO introduced in year 2010 with 32KB of flash memory is best suited to serve the purpose for this module.

Now we are interested in ADC function, for that Arduino UNO has following specifications:

A Power Module

The meaning of these specifications is:

“UNO contains 6, 10-bit channels for analog to digital conversion. It maps analog input voltage at these pins from 0-5 V into integer values between 0- 1023, yielding a resolution of 5V/1024 or 4.8828 mV/unit. It takes 100 usec to read one input, so max reading speed is 10000 times a second.”

Syntax assigned is analogRead(pinname). It reads pin “pinname” and returns 10-bit int accordingly.

Sample code:

A Power Module

ADC DISCRPTION AVIALABLE AT ARDUINO.CC

https://www.arduino.cc/reference/en/language/functions/analog-io/analogread/

VOLTAGE MEASUREMENT

A basic voltage divider with appropriate resistor values can be used to scale down higher voltages V to microcontroller compatible levels, Vs.

A Power Module

A Power Module

Suppose readings are to be made in 0-50 V range. We need to scale down this 0-50 V range to 0-5 V.

A Power Module

Now current has to maintained as minimum as possible, to reduce errors. Let current be 0.5 mA. R1 should be of 100K range (5/0.5m). So,

A Power Module

As already stated, UNO has 10-bit resolution so voltage from 0 to 5V would be mapped into integers value from 0 to 1023, which is 5/1023 =4.88 mV per unit, which is fairly good accuracy.

A Power Module

The digital data from ADC can be easily used to manipulated to get the actual voltage, as follows:

A Power Module

GETTING LARGER RANGE

For a range of 500 V:

A Power Module

A Power Module

For current to be 0.5 mA, R1 should be of 1M range (500/0.5m). Thus,

A Power Module

Which is not a standard value so let R2 be 10K.

New scale is:

A Power ModuleA Power Module

A Power Module

Putting the value of Vs:

A Power Module

DC CIRCUITS MEASUREMENTS

What we just did was for DC voltage measurement in 0-500V range.

AC CIRCUITS MEASUREMENTS

For AC measurement we have to make few modifications. As voltage range is only +ve (0 -5 V) so we need to either shift whole waveform above zero, or flip the negative cycle or simply chop it, and then take readings. Using suitable algorithms, AC values (RMS/ PEAK) could be found.

  1. READING RMS: Since UNO takes 100 usec for every reading. For a typical frequency of 50 Hz, a half-cycle consisting of 10 msec, so UNO can make 10m/100u or 100 readings. For these 100 readings, the formula to compute RMS can be applied as follows.

A Power Module

A Power Module

Sampling a half-sino, Image courtesy: Internet

  1. READING PEAK:
    1. Using sampling: Max value from the 100 readings from can be found out using some sorting algorithm and RMS can be simply computed from it.
    2. Using time-delay: Since peak occurs at t/4, so using a timer function to generate a time delay of 5 msec after zero crossing and then taking the reading would directly give the peak value.

Any of these three techniques can be used for AC measurement.

IMP: It might possibly be case that the waveform may not be crossing the zero when the ADC starts taking measurement case would result in wrong results or the ADC never captures the maxima case 2a will be faulty , to deal with this is to take a larger number of samples like 2000-3000 to reduce the probability of error occurring.

Sample code for case2a:

A Power Module

The above code can also be used to measure DC voltages.

The final circuit becomes:

A Power Module

Since the sample voltage can be directly fed to microcontroller so no conditioning is required. Let’s see is that the case for current measurement too???

CURRENT MEASUREMENTS

The underlying idea to measure current is to obtain proportional voltage samples for any given load current, read voltage value at ADC port and process the ADC output bit to get the current value.

A proportional voltage can be obtained simply by forcing the current through pure resistor. If the value of this resistor is extremely small compared to the load resistance the equivalent load impedance would hardly change thus load current remain the same and in turn a small but proportional voltage drop is obtained across the external resistance.

A Power Module

Assuming the load current range from 0-2 A. Keeping the external series resistance R, as small as 0.1Ω, the sample voltages will be in range of 0- 0.2 V i.e. (0 -200 mV).

Accuracy would be significantly compromised for small load current if this range of sample voltage is used at ADC.

GETTING LARGER RANGE

Here the sampled voltage requires a proper conditioning.

So, all we need to do is to boost up the sample voltage from 0-200 mV to 0-5 V range.

How would you do that???

Well this is a typical day-job in analog engineering. Technically this is called the signal amplification. Giving a signal a required gain to push the level to a higher value.

The Operational Amplifier

Let us just step back from the current project and take a dive to depths which is certainly not required as far as the project is concerned, but for the sake of spirit of learning more and better, in the name of love of subject. 🍹🍹

What are the Operational Amplifiers? 

This class of devices singly forms the backbone of the modern analog industry. Just as the gates in digital electronics, the induction motor in power systems, the IC engine in mechanical systems, the library functions in the computer engineering field, these operational amplifiers are the basal workhorses of the modern analog systems. These little beasts are characterized by a versatile application, which includes amplifier, voltage source, current source, filters, actuator driver, comparator, etc.

The very first need for amplifier circuits typically appeared in long telephone lines to obtain proper signal conditioning at the receiving ends. The problem of the available amplifier in those days were their highly undependable gain due to the inherent nature of the active components used, vacuum tubes in 1930s and transistors after 1947. The gain varied enormously for small changes in working temperature and supply voltage. External condition like season, weather, humidity all of them made the gain of amplifier almost uncontrollable.

Harold Black, an electrical engineer at the bell laboratories in 1927 came up with a revolutionary technique that has now became so ubiquitous in all electronic circuit for control applications, it is called the negative feedback concept.

THE BIG IDEA: Use an amplifier made of undependable active elements to get a very large gain, typically infinite, and then use dependable passive element to provide negative feedback to it to obtain any reduced desired gain or transfer function.

This remarkable concept is the underlining principle of all the practical operational amplifiers used today.

Now to understand op-amp, as known popularly, there are two standpoints. One is this:

A Power ModuleImage courtesy: Internet

And the other is this:

A Power Module

And we have no doubt that you would like to understand it through the second stand point.

Now recall the first part of the basic idea i.e. building infinite gain op-amp, more or less this comes under the domain of pure analog electronics but given the versatility of operational amplifier the second part of basic idea, the design of negative feedback circuit using passive elements (resistors, capacitors, inductors) comes under the realm of the electrical engineering. Also, it largely deals with core electrical circuit theories like KVL, KCL, Thevenin’s, etc.

The first standpoint leads to accomplishment of first part of the basic idea, and the second stand-point leads to the realization of second part of the idea.

Now you would wonder how can we execute second part without knowing the first part, and here comes a great powerful tool to do this for you, it is the hack of all the complicated system around us.

“THE ABSTRACTIONS”

Without a solid-thorough understanding of how that horrendously intricate mesh of transistor and resistances work to produce infinite gain, we can still build a perfect negative feedback circuit to obtain exact desired transfer function (gain), using the abstractions.

This concept is so crucial and pervasive in building all modern perplexing systems the microcontrollers, computers, airplanes, particle accelerators, etc.

Consider this more striking example: the “printf” library function which we take for so granted, is an abstraction of the all the icky logics that goes into it to print a given string on some terminal or a display device. Try building your own function to print a string, you would be shocked at the complexity behind this little command.

The point is that we cannot keep on dwelling on basal stuffs if we wish to build something magnificent, if we do-we will never end up building an app, a website, a power converter, and so forth.

So, use of abstraction is a proven tool to reduce complexity, we can use this tool to derive some common results and build or understand large systems layer by layer.

What you see below is the abstraction of the mesh of transistor shown earlier.

A Power Module

A Power Module

WHAT DOES OP-AMP DO FOR US?

Well, they just simply produce output proportional to the difference in the voltage between the two input terminals. The proportionality constant is very high, order of 10^5, called the system gain, note Ed is in μV and o/p in V.

Mathematically,

A Power Module

The output characteristics for the device is as follows:

A Power Module

  • The magnitude of output voltage depends on the difference between in the input terminal voltage in active region and it saturates once output hits the supply voltage magnitude.
  • The polarity of output is same as the polarity of V+ wrt V-, thus V+ is called non-inverting terminal.

THE REAL OP-AMP

  • The variety of op-amps available are many LM324, LM339, LM258, etc. Most popular is IC 741. In our project we will use LM358, as it is single supply dual op-amp, so it will reduce complexity a bit.

A Power Module

A Power Module

      Image courtesy: ON Semiconductors

THE RULES OF OPERATION

  1. The difference in input voltage is very small (typically in μV) so the two input terminals can be assumed to be virtually shorted, i.e. at same voltage.
  2. The input impedance is very high, so both the input currents are zero.
  3. The gain is infinity.

This is all one need to know about op-amp, using these three rules op-amps can be used very easily to get required gain.

Let’s check it out.

ROUTINE EXAMPLES

Inverting configuration:

A Power Module

Just calmly apply the rules one by one.

  1. Input terminals at same voltage, so voltage at 2 is voltage at 1, i.e. zero.
  2. No current through input terminals. Apply KCL at terminal 2:

A Power Module

A Power Module

The characteristics become:

A Power Module

Non-inverting configuration:

A Power Module

Again, apply the same rules:

  1. Input terminals at same voltage, so voltage at 2 is voltage at 1, i.e. V1.
  2. No current through input terminals. Apply KCL at terminal 2:

A Power Module

A Power Module

The output characteristics become:

A Power Module

So, are you now able to appreciate the beauty of these curves we just obtained??? We began with an op amp with typically infinite gain (10^5), showing very creepy dependence on the temperature and external factors and here is a calm stable op-amp with desired finite gain by just using simple passive resistances.

New gain of system become (Rf/Rb) and (1+ Rf/Rb), which remains fairly constant for a wide temp range.

HOW THE NEGATIVE FEEDBACK WORKS?

Though equations obtained by reasonable mathematical approximations shows us independence of overall gain, but intuition is still lacking….

So, how does the results we just obtained is manifested actually????

So, lets simply heat an op-amp working in a non-inverting configuration, and see what happens.

As heating begins the gain begins to rise, and so does the output voltage. Corresponding to it there will be rise in voltage at terminal 2. Which would result in lowering the difference between the two input terminals, and consequently the output drops. This drop in output leads to drop in voltage at terminal 2 which results in increased differential voltage resulting in increased output. These oscillations die out soon and result is stable output displaying temperature independent gain. This is in general how a negative feedback principle works.

IMPORTANT

Ensure that the power is never off when the inputs and output are connected in the op-amp.

……………………………………….

So, it’s time to get back from where we left, the need to boost up the sample voltage from 0-200 mV to 0-5 V range.

Pretty cakewalk now, isn’t???

You would say it’s a lockdown. Where should I get the op-amp?

Cool, there are whole lot of gadgets and sensors where you can find it.

Say, for example, we extracted an op-amp from an IR sensor.

A Power Module

Due to unavailability of any solder iron, we just simply cut out the resistors, IR LEDs, POTs.

A Power ModuleUsing careful examination of the IC we traced out the whole IC circuit diagram.

A Power Module

And simply bought out the required terminals of op-amp by normal connectors.

A Power Module

To boost the sample voltage from 0-200 mV to 0-5 V range, we need a gain of (5/200m = 25), since inverting is not desired hence using the non-inverting configuration.

A Power Modulewhere,

A Power ModuleRequired gain is 25.

A Power Module

Let the resistances be: A Power Module

Finally, we used pair of resistors in combination to get the required ohms and made the op-amp circuit with a gain of 24.6 as follows:

DC CIRCUITS MEASUREMENTS

What we finally got is a 0-5 V scale that would end up giving us 0-1023 integer, we have to get all the way back to current, lets gooooo!!!!!🚀🚀🚀🚀🚀

Analog Voltage at the ADC:

A Power Module

Voltage input to the non-inverting amplifier:

A Power ModuleCurrent through the external resistance:

A Power ModuleThe overall conversion factor becomes:

A Power ModulePutting our design values:

A Power Module

AC CIRCUITS MEASUREMENTS

Contribute

FREQUENCY AND POWER FACTOR MEASUREMENT

Underdev

DISPLAY LCD

All about LCD interfacing with the Arduino can be very easily understood by referring this short 1 min read at Arduino.cc.

https://www.arduino.cc/en/Tutorial/LiquidCrystalDisplay

We tried the same, followed every step very accurately but unfortunately, results didn’t show up except this blank screen.

A Power Module

Help us to troubleshoot the problem by coming up with possible errors.

We build the icky circuit thrice from zero, and then checked and rechecked every connection, but failed.

MORAL:

  1. Life isn’t fair always, sometimes no matter how hard we try, no matter how dedicated our purpose is, we are destined to fail. We have realised this truth, and we hope to develop temperament to mindfully accept such failures in life.
  2. Connecting the circuit three times hadn’t yielded us the result, but surely planted in us the seed of perseverance to go through that nasty process. Surely, we raised our patience wall a little higher.

And it is these lessons and quality we wish to learn and develop by involving in these projects, not just simply putting things up.

THE FINAL CIRCUIT

Well do you think it’s done???

No, it’s not.

Since we are using same apparatus for the measurement of AC and DC. We hadn’t done anything to identify them. It can be done via program codes or by hardware.

By providing high or low manually on a digital pin we can indicate the microcontroller about it, say high for AC measurements and low for DC measurements.

THE FINAL CODES

#include <LiquidCrystal.h>
LiquidCrystal lcd(12, 11, 5, 4, 3, 2);
#define DCV_multiplier 0.499
#define DCC_multiplier 1.9531*10^-3
int read_voltage = A3;
int read_current = A4;
int select_pin = 7;
int voltage_adc_value = 0;
int current_adc_value = 0;
int voltage_peak_value = 0;
float dc_voltage= 0;
float ac_voltagerms= 0;
float dc_current = 0;
unsigned long sample_count = 0;
void setup()
{
  pinMode(A3, INPUT);
  pinMode (A4, INPUT);
  pinMode (7, INPUT);
  lcd.begin(16, 2);
  lcd.setCursor(0, 0);
  lcd.print("POWER MODULE");
  delay(3000);
  lcd.clear();
}
void loop()
{
  //Voltage Measurement//
  for(sample_count = 0; sample_count < 2000; sample_count ++)
  {
      voltage_adc_value = analogRead(read_voltage);
      if(voltage_peak_value < adc_value)
      voltage_peak_value = adc_value;
      else;
  }
      dc_voltage = voltage_peak_value * DCV_MULTIPLIER;
      ac_voltagerms = dc_voltage / 1.414;
  //Current measurement//
      current_adc_value = analogRead(read_current);
      dc_current = current_adc_value * DCC_MULTIPLIER;
  if ( select_pin==0 )
  {
      lcd.clear();
      lcd.setCursor(0, 0);
      lcd.print("DC SYSTEM");
      lcd.setCursor(0, 1);
      lcd.print(dc_voltage);
      lcd.print("V");
      lcd.print (" ");
      lcd.print (dc_current);
      lcd.print ("A");
 else
      lcd.print("AC SYSTEM");
      lcd.setCursor(0, 1);
      lcd.print(ac_voltagerms);
      lcd.print("V");
      delay(500);
  }
}

A Power Module

LOCKDOWN SPECIAL

We could arrange a low resistance of required power handling capacity, so current measurements cannot be made. Moreover, the resistors required for the voltage multiplier are also not available with us. So, we used op-amp to get the required gain, slight code modifications and obtained the results using the serial monitor. Battery voltage is 12.25 V power module shows 12.33 V, 0.65% error.

A Power ModuleA Power Module

CONCLUSION

These power modules can be custom build for battery monitoring for systems like drones, etc. by removing AC measuring components and using small uC like Arduino NANO. They could be used for real-time monitoring of some load. They could be used to trigger some protective measures like triggering a relay, blowing a buzzer, etc. when any parameter beyond a limit.

The development or building of measurement systems is less about dwelling on rigorous electrical concepts rather more appropriately it could be categorized as a form of art, which requires small intuition of some very basic electrical phenomenon and rest is all about creativity to obtain the desired result by arranging the already available elementary elements.

If we could imagine force exerted on a wire carrying current in a magnetic field, if we can imagine emf induced in a changing magnetic field, if we can imagine magnetic field due to a coil carrying current, we are ready to go, about learning, understanding, modifying and building interesting measurement systems.

🍹🍹🍹

Keep reading, keep learning

AANTARAK DIVISION,

TEAM CEV!

References

  1. Special thanks to Prof Varsha Shah
  2. MIT OCW 6.002 Circuits and Electronics
  3. Mr Anand Aggarwal’s Fundamental of Analog and Digital Electronics
  4. Arduino.cc

 

Inverter Circuits: The Basics

Reading Time: 9 minutes

INTRODUCTION

The Renewable energy is showing a great ramp up in these early decades of 21st century era. The trends and prediction show a promising future for solar, wind, tidal etc. The solar energy harnessed in the form of solar photovoltaics and heat are boosting ever since the early 2000s, and in the year 2019 it shot to gigantic 25% growth rate and increasing.

Inverter Circuits: The Basics

 

Note: A better-edited version of this blog is available in original MS Word format, link below:

https://drive.google.com/open?id=1a63QhjDmNMEEZI1iW9Ik_POmSW0UGre7

Follow for an eminent IEA report “World Energy Outlook 2019” to know all about our planet’s current energy scenario:

https://www.iea.org/reports/world-energy-outlook-2019/renewables#introduction

Inverter Circuits: The Basics

The problem with renewables is not of power scale but of irregular availability. For example, in a single hour, the amount of power from the sun that strikes the Earth is more than the entire human race consumes in a year, a hurricane every second releases the energy equivalent of 10 atomic bombs but last only a few minutes or hours.

Inverter Circuits: The Basics

However, it an obvious fact that we couldn’t tap tiny fraction of these mighty energy resources due to present technology constraints, but nothing could be so soothing, to hear of energy availability at such minimal cost to our environment when climate scientist predicts of human race extinction due to fossil-fuel induced climate crisis.

The supply and demand cannot be achieved using renewables without proper engineering. Energy storage provides the only feasible solution to link the two. Storing when excess availability and delivering when demand exceeds generation.

There is massive science community working in the area of energy storage technologies: mega batteries, large pumped hydro storage, thermal storage, chemical storage, etc.

MIT Lecture on future of Energy Storage: https://www.youtube.com/watch?v=E76q-9q7ZDg

Now the other face of story is that the modern power system is in transitional period, now DC power is again gaining ground on consumer side like the highly efficient BLDC motors, semiconductor applications like large database centers, home appliances, and jillion low power devices operating on DC. But still AC is major form of consumption and unfortunately major storage technologies (most importantly batteries) don’t store AC power directly.

And here comes the topic of the blog: The Inverter Circuits.

Inverter Circuits are systems designed for efficient conversion of DC electricity into AC electricity, and comes in wide range of power ratings from small solar charging LED lighting system to medium house emergency system to massive receiving stations of HVDC transmission lines using different conversion principles.

Under this project, team Aantarak aimed to look upon inverter technologies and tried to develop an insightful understanding of the operational principles of inverter circuits, with a vision to further improve the existing technology.

In this first blog intuitive explanation of inverter principle and mathematical analysis of system performance, typically available topologies, major circuit engineering challenges and scope of improvement are explored.

BASIC IDEA OF DC TO AC CONVERSION

Suppose we have V Volts DC battery source and we want to power an AC load whose input is specified as f Hz.

How would you do that, think for a second.

To get an AC supply from a DC source, all we have to do is to interchange the supply terminal across the load at the required rate. So, for 1/2f  seconds we are giving +V, then zero and then -V for next 1/2f  seconds.

Inverter Circuits: The Basics

Well that is a square wave.

Inverter Circuits: The Basics

If you recall the mathematical treatment of waves shapes in previous blogs, the Fourier series expansion of this wave give the component waveforms.

Let square wave be represented as:

Inverter Circuits: The Basics

Fourier series expansion of the waveform is:

Inverter Circuits: The Basics

Graphically plotting (in the time domain) the components of a square wave:

Inverter Circuits: The Basics

Plotting the components in the frequency domain:

Inverter Circuits: The Basics

But for now, we can say we have obtained a voltage waveform which has a sinusoidal component of f Hz in it along with its multiples.

We will look at implication of multiple frequency components later.

If we try to realize this physical interchanging of the terminals using some electrical/electronic device then we are half done for building an inverter circuit. 🙌🙌🙌

So, let us now use this concept to layout circuit basic idea using some imaginary god-switch which is capable of doing what we want here exactly.

Assume an electrical circuit that works in following manner:

Inverter Circuits: The Basics

Between load terminals: LT1 and A & D and between LT2 and C & B we have a single pole double throw switch whose operational characteristics could satisfy following operations as described below.

So, for the first half time-period, load should be supplied by +ve voltage, for that:

  1. A & LT1 and B and LT2 must be shorted.
  2. D & LT1 and C & LT2 must not conduct and should withstand voltage appearing across them.

Inverter Circuits: The Basics

Then for a very small-time instant voltage should across load must be zero.

For that:

  1. A & LT1 and C and LT2 must be shorted or,
  2. D & LT1 and B and LT2 must be shorted.

Inverter Circuits: The Basics

For another half time-period, load should be supplied by -ve voltage, for that:

  1. D & LT1 and C and LT2 must be shorted
  2. A & LT1 and B & LT2 must not conduct and should withstand voltage appearing across them.

Inverter Circuits: The Basics

Now the vague principle of interchanging supply terminals to obtain an AC voltage waveform across a load using a constant voltage battery has become much clearer. Before we go on laying out exact switches required at the load terminals, we have to give a thought on the load current waveform too.

Real-world AC load

When the load is purely resistive- things are simple, the load current will follow the voltage linearly and the direction of current would be from A to B I positive cycle, zero current at t=t/2, and reverse direction in next half cycle.

When you search for switches with such characteristics, then the 21st century Doremon pocket’s hobbyist electronics store (Easy Electronics for SVNITians😅😅😅) will give you a BJT.

With proper base supply, we can obtain the desired characteristics.

Inverter Circuits: The Basics

Inverter Circuits: The Basics

But when it comes to real-world, rarely you would find an AC load of unity power factor or we can say purely resistive load in another way.

For non-resistive loads, the current through it and the voltage across the terminals are not linear. For inductive load the current will lag and for capacitive load the current would lead.

Consider a case when the voltage across the load somehow obtained is sinusoidal, which will be obtained in advanced inverter circuits.

The load being inductive, will show the following characteristics.

Inverter Circuits: The Basics

As indicated in the graph at the onset of every -ve half-cycle due to lagging nature, load current won’t reverse its direction similarly for +ve cycle. The above circuitry would reverse the voltage across terminal but it would not be able to sustain the required load current of opposite polarity. This could be easily achieved if we connect a diode to conduct current in short time period at the starting of +ve or-ve voltage waveform.

So, the new circuit now becomes:

Inverter Circuits: The Basics

Let us try to analyze the circuit operating over one complete cycle.

Inverter Circuits: The Basics

+VE VOLTAGE WAVEFORM:

From t = 0 to t = T2, transistors T1 and T4 should be ON and transistors T3 and T4 must be in OFF state.

  • t = 0 to t = T1: Current is negative so diode D1 and D4 will conduct.
  • t = T1 to t = T2: Current becomes positive hence transistor T1 and T4 conducts.

VE VOLTAGE WAVEFORM:

From t = T2 to t = T4, transistor T2 and T3 should be ON and transistors T1 and T2 must be in OFF state.

  • t = T2 to t = T3: Current is positive so diode D3 and D2 will conduct.
  • t = T3 to t = T4: Current becomes negative hence transistor T2 and T3 conducts.

In the following table we have required states of transistor (T1, T2, T3 and T4) to obtain the AC waveform across the load and which diodes are conducting and not-conducting current during the particular time-period.

Inverter Circuits: The Basics

We can replace the transistor and diode combination by another power electronics device, called power MOSFET. The greatest advantage of using this device is its capability for conduction of current in both directions. Additionally, it gives more power handling and reliable operation. The device is known by name of IRF540.

Inverter Circuits: The Basics

Internal Schematic Diagram:

Inverter Circuits: The Basics

NOTE: The diode shown in the symbol is parasitic. To know more follow the notes or the datasheet of IRF540.  

Inverter Circuits: The Basics

A generalized statement can be made that all the four switches are capable of conducting current in both directions during ON state.

Now let us have a glance on most important parameters of an IRF540 taken for datasheet:

Inverter Circuits: The Basics

The drain source breakdown voltage is 100V, while most of AC load operate on 230 V RMS voltage i.e. peak value of 325 V. So, this H- bridge configuration is modified for obtaining relatively higher AC peak voltage for low DC battery voltages. This is done by use of a three winding step-up transformer.

Inverter Circuits: The Basics

In this circuit when M1 is OFF and M2 is ON, the battery injects current to the primary winding (marked 3+) and the voltage appear across the load is +ve. When M1 is ON and M2 is OFF the battery supply the primary winding (marked 2+) and the voltage induces on secondary is -ve (opposite polarity than the first cycle).

Since the MOSFETs can carry bidirectional current through the source and the drain terminal thus this circuit can ve used to power non-linear loads also.

Gate Triggering Circuit

The MOSFETs M1 and M2 should be turned ON and OFF alternatively for 10 ms each for obtaining an AC output voltage of 50 Hz frequency. Numerous IC can be used to design the triggering circuits like IC4047, IC4049, etc.

CONCLUSION

The output voltage wave obtained using the circuit drawn above is a square wave. The Harmonic content would be little more than required for optimal performance.

Using a minor modification to this circuit we can get a modified sine wave.

Inverter Circuits: The Basics

It resembles more closely to pure sine wave hence improved harmonic performance. In the next blog will see how the harmonic performance is computed, measured and improved in inverter circuits, some practical circuits, and much more.

NOTE: Other advanced technique used is PWM (Pulse Width Modulation) technique.

Keep reading, keep learning!

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