Introduction To Power Inverter

Published Date: 02 Nov 2017

Chapter 1

A power inverter is a device that converts DC (Direct Current) power into AC (Alternating Current) power. The converted AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits.

An inverter converts the DC electricity from sources such as batteries, solar panels, or fuel cells to AC electricity. The electricity can be at any required voltage; in particular it can operate AC equipment designed for mains operation, or rectified to produce DC at any desired voltage.

Types of Inverter

Power inverters produce one of three different types of wave output:

Square Wave

Modified Square Wave (Modified Sine Wave)

Pure Sine Wave (True Sine Wave)

The three different wave signals represent three different qualities of power output and consequently, three different price categories. Square wave inverters result in uneven power delivery that is not efficient for running most devices. They could be harmful to some electronic equipment, especially equipment with transformers or motors. The square wave output has a high harmonic content which can lead such equipment components to overheat. Square Wave units were the pioneers of inverter development.

The most common, general-use inverters available are of the Modified Sine Wave variety, usually available at more moderate pricing compared to pure sine wave models. Modified Square Wave (or "Modified Sine Wave" and "Quasi Sine Wave") output inverters are designed to have somewhat better characteristics than Square Wave units, while still being relatively inexpensive. Although designed emulate a Pure Sine Wave output, Modified Square Wave inverters do not offer the same perfect electrical output. As such, a negative by-product of Modified output units is electrical noise, which can prevent these inverters from properly powering certain loads. For example, many TVs and stereos use power supplies incapable of eliminating common mode noise. As a result, powering such equipment with a Modified Square Wave may cause a "grain" or small amount of "snow" on the video picture, or "hum" in the sound system. Likewise, most appliances with timing devices, light dimmers, battery chargers, and variable speed devices may not work well, or indeed, may not work at all.

Pure or True Sine Wave inverters provide electrical power similar to the utility power available from the outlets in home or office, which is highly reliable and does not produce electrical noise interference associated with the other types of inverters. With its "perfect" sine wave output, the power produced by the inverter fully assures that the sensitive loads will be correctly powered, with no interference. Some appliances which are likely to require Pure Sine Wave include computers, digital clocks, battery chargers, light dimmers, variable speed motors, and audio/visual equipment. If your application is an important video presentation at work, surveillance video, a telecommunications application, any calibrated measuring equipment, or any other sensitive load, Pure Sine Wave inverter must be used.

Power inverters are also usually described as having either a high or low switching frequency. Switching frequency refers to the rate at which the input DC voltage is oscillated to create an AC output. Low frequency inverters oscillate a DC voltage at 50 Hz. Then they step that voltage up to the desired amplitude using a bulky and a heavy transformer. High frequency inverters, on the other hand, use a small and lightweight transformer. A high frequency inverter will produce many harmonics near the range of the switching frequency. However, most of the harmonics are relatively higher in order than the 50 Hz fundamental frequency. These harmonics can be isolated using a small low-pass filter. In turn, isolation of harmonics will result in less buzzing in audio equipment and less interference in other electronic equipment such as radios and televisions.

Problems associated with typical inverters

When it comes to mobility, a unit that’s the size of a laptop doesn’t seem awfully large. Considering the current trend in electronics, a laptop seems gigantic as compared to some of the microscopic devices that are being massed produced. Therefore the recent trend in electronics is miniaturization. Size and bulk determines mobility. And for a unit as useful as a power inverter, smallness should be one of the top priorities in designing this unit. In order to create a more compact unit, it requires the use of as many devices of negligible size as possible. These devices, or integrated circuits, must also be able to accomplish as many features as possible within their small stature. The increase in demand for mobile AC power sources has led to an increase in market supply. However, these inverters that use the "square wave" technology tend to produce a lot of heat due to power loss. Their efficiency is also less than proficient. The price of an inverter like this is considerably less than one with a pure sine wave output, but it is also reflected in their operational efficiency. The design that we will implement will solve the problem associated with "square wave" inverters by using a microprocessor to obtain a more efficient and smooth means of switching the inverter’s transistors This will reflect, in the overall design, a greater efficiency, less power loss to heat and cost reduction of overall product.

1.4 Applications

DC POWER SOURCE UTILIZATION

Inverter designed to provide 115 VAC from the 12 VDC source provided in an automobile. The unit provides up to 1.2 Amps of alternating current, or just enough to power two sixty watt light bulbs.

An inverter converts the DC electricity from sources such as batteries, solar panels, or fuel cells to AC electricity. The electricity can be at any required voltage; in particular it can operate AC equipment designed for mains operation, or rectified to produce DC at any desired voltage.

Grid tie inverters can feed energy back into the distribution network because they produce alternating current with the same wave shape and frequency as supplied by the distribution system. They can also switch off automatically in the event of a blackout.

Micro-inverters convert direct current from individual solar panels into alternating current for the electric grid.

UNINTERRUPTIBLE POWER SUPPLIES

An uninterruptible power supply is a device which supplies the stored electrical power to the load in case of raw power cut-off or blackout. One type of UPS uses batteries to store power and an inverter to supply AC power from the batteries when main power is not available. When main power is restored, a rectifier is used to supply DC power to recharge the batteries.

It is widely used at domestic and commercial level in countries facing Power outages.

INDUCTION HEATING

Inverters convert low frequency main AC power to a higher frequency for use in induction heating. To do this, AC power is first rectified to provide DC power. The inverter then changes the DC power to high frequency AC power.

HVDC POWER TRANSMISSION

With HVDC power transmission, AC power is rectified and high voltage DC power is transmitted to another location. At the receiving location, an inverter in a static inverter plant converts the power back to AC.

VARIABLE-FREQUENCY DRIVES

A variable-frequency drive controls the operating speed of an AC motor by controlling the frequency and voltage of the power supplied to the motor. An inverter provides the controlled power. In most cases, the variable-frequency drive includes a rectifier so that DC power for the inverter can be provided from main AC power. Since an inverter is the key component, variable frequency drives are sometimes called inverter drives or just inverters.

Chapter 2

Introduction to Transformer-less technology

2.1 Transformer

A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core, and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF) or "voltage" in the secondary winding. This effect is called mutual induction.

If a load is connected to the secondary, an electric current will flow in the secondary winding and electrical energy will be transferred from the primary circuit through the transformer to the load. In an ideal transformer, the induced voltage in the secondary winding (VS) is in proportion to the primary voltage (VP), and is given by the ratio of the number of turns in the secondary (NS) to the number of turns in the primary (NP) as follows:\frac{V_{S}}{V_{P}} = \frac{N_{S}}{N_{P}}

By appropriate selection of the ratio of turns, a transformer thus allows an alternating current (AC) voltage to be "stepped up" by making NS greater than NP, or "stepped down" by making NS less than NP.

Transformers range in size from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge units weighing hundreds of tons used to interconnect portions of power grids. All operate with the same basic principles, although the range of designs is wide.

2.2 Basic Principle

The transformer is based on two principles: firstly, that an electric current can produce a magnetic field (electromagnetism) and secondly that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction). Changing the current in the primary coil changes the magnetic flux that is developed. The changing magnetic flux induces a voltage in the secondary coil.

An ideal transformer is shown in the adjacent figure. Current passing through the primary coil creates a magnetic field. The primary and secondary coils are wrapped around a core of very high magnetic permeability, such as iron, so that most of the magnetic flux passes through both the primary and secondary coils.

Transformer universal EMF equation

If the flux in the core is purely sinusoidal, the relationship for either winding between its rms voltage Erms of the winding , and the supply frequency f, number of turns N, core cross-sectional area a and peak magnetic flux density B is given by the universal EMF equation:

E_{rms} = {\frac {2 \pi f N a B_{peak}} {\sqrt{2}}} \! \approx 4.44 f N a B

From above equation, the EMF of a transformer at a given flux density increases with frequency. Also, the cross section area of the transformer inversely relates to the frequency. So by operating at higher frequencies, transformers can be physically more compact because a given core is able to transfer more power without reaching saturation and fewer turns are needed to achieve the same impedance. However, properties such as core loss and conductor skin effect also increase with frequency.

2.3 Introduction to transformer-less inverter

As discussed above, the cross-section area of transformer is inversely related to its operating frequency. Therefore, the area and thus the size of the transformer can be reduced by operating it at high frequency. The transformers that operate at high frequencies are made of ferrite core.

The transformer used in this project is used to step up the voltage from the Push Pull converter to provide an approximate voltage of 340 VAC with an

1000W Low Frequency Bulky transformer

1000W High Frequency ferrite transformer

approximate frequency of 100 kHz. Choosing such a high switching frequency has allowed a decrease in the size of the transformer needed

2.4 Ferrite Core

A ferrite core is a structure on which the windings of electric transformers and other wound components are formed. It is used for its properties of high magnetic permeability coupled with low electrical conductivity (which helps prevent eddy currents).

There are two broad applications for ferrite cores which differ in size and frequency of operation: signal transformers are of small size and higher frequencies, power transformers are of large size and lower frequencies. Cores can also be classified by shape: there are toroidal cores, shell cores, cylindrical cores, and so on.

The ferrite cores used for power transformers are working in the range of low frequencies (1 to 50 kHz usually) and are quite big in size, can be toroidal, or shell or C shape and are useful in all kinds of switching electronic devices (especially power supplies from 1 watt to 100 watts maximum because usually powerful applications are out of range of ferritic single core and required grain oriented laminations cores).

The ferrite cores used for signals have a range of applications from 1 kHz to many MHz, perhaps as much as 300 MHz, and have found their main application in electronics.

Ferrite is a class of ceramic material with useful electromagnetic properties and an interesting history. Ferrite is rigid and brittle. Like other ceramics, ferrite can chip and break if handled roughly. Luckily it is not as fragile as porcelain and often such chips and cracks will be merely cosmetic. Ferrite varies from silver gray to black in color. The electromagnetic properties of ferrite materials can be affected by operating conditions such as temperature, pressure, field strength, frequency and time.

The problem in the ferrite transformer is that it require a very complex circuitry for its operation, but on the other hand pure sinusoidal can only be possible with the ferrite transformer.

Chapter 3

Transformer Design Considerations

3.1 Obstacles in Ferrite core transformer

The operating frequency of power transformer increased from 25 cycles per second to line frequency of 50/60 Hz in the first of 20th century, it kept increasing in the last two decades to tens of KHz to few MHz or even more. The structure and winding configurations have been changed to overcome the problems generated by eddy currents. The laminated core materials and soft new magnetic materials have been discovered and used to minimize the eddy current losses in the magnetic cores.

High frequency magnetic materials, such as ferrites, have improved to suit the requirement of high frequency operation. Still, transformers and inductors have many obstacles for high frequency applications. Parasitic elements, such as eddy currents, leakage inductance, skin and proximity effects, make it very difficult to design high frequency transformer.

3.2 Flux Saturation

Faraday’s Law states that the flux through winding is equal to the integral volt-seconds per turn. This requires that the voltage across any winding of any magnetic device must average zero over a period of time. The smallest dc voltage component in an applied ac waveform will slowly but inevitably "walk" the flux into saturation. In a low frequency mains transformer, the resistance of the primary winding is usually sufficient to control this problem. As a small dc voltage component pushes the flux slowly toward saturation, the magnetizing current becomes asymmetrical. The increasing dc component of the magnetizing current causes an IR drop in the winding which eventually cancels the dc voltage component of the drive waveform, hopefully well short of saturation. In a high frequency switch mode power supply, a push-pull driver will theoretically apply equal and opposite volt-seconds to the windings during alternate switching periods, thus "resetting" the core (bringing the flux and the magnetizing current back to its starting point). But there are usually small volt second asymmetries in the driving waveform due to inequalities in MOSFET RDS(ON) or switching speeds. The resulting small dc component will cause the flux to "walk" The high frequency transformer, with relatively few primary turns, has extremely low dc resistance, and the IR drop from the dc magnetizing current component is usually not sufficient to cancel the volt-second asymmetry until the core reaches saturation. Flux walking is not a problem with the forward converter. When the switch turns off, the transformer magnetizing current causes the voltage to backswing, usually into a clamp.

The reverse voltage causes the magnetizing current to decrease back to zero, from once it started. The reverse volt-seconds will exactly equal the volt- seconds when the switch was ON. Thus the forward converter automatically resets itself (assuming sufficient reset time is allowed, by limiting the maximum duty cycle). The flux walking problem is a serious concern with any push-pull topology (bridge, half-bridge or push-pull CT), when using voltage mode control.

Remedy for flux saturation problem:

As discussed above that unlike the core transformers, ferrite transformer easily saturates due to the very high switching frequency.

Saturation is the state reached when an increase in applied external magnetizing field H cannot increase the magnetization of the material further, so the total magnetic field B levels off.

The dc current can saturate the inductor quickly if an air gap is not introduced into the core's magnetic path. The powdered iron materials have an inherent air gap that is distributed throughout the core, which gives them a soft saturation curve. Ferrite material must have an air gap physically inserted or ground between the mating surfaces of the core halves.

Following figure shows the top quadrant of a typical B/H loop for a ferrite core with a small air gap, a ferrite core with a large air gap, and an iron powder core without an air gap.

From figure, the ferrite core without an air gap saturates more quickly as compared to the core which contains an air gap. The air gap reduces the slope of the B/H loop, reducing permeability and inductance, and hence increasing the magnetizing current in the primary. Remember that magnetizing current flows in the primary — even if the secondary is open circuit.

The circular area seen below in ferrite transformer is rubbed to create the air gap to avoid saturation.

3.3 Voltage Turn Ratio Calculation

In this section, only general calculation is being used to find out the turn ratios of the transformer. The detailed calculation procedure is given in the appendix section of the report.

Frequency = f =100kHz

Area=a = 1.5cm*1.9cm=1.78cm2

B=5000 Gauss

So, Practically: N1=1 T

Max pulse width of TL494=0.97%

Therefore, N2’=0.97*230/24= 9 3T

Because of voltage drop, we select N2= 2*N2’

So, N2=18.6 T

Practically N2=19T

So,

N1=1T

N2=19T

Chapter 4

DC-DC Converter

4.1 Introduction

Dc-DC power converters are employed in a variety of applications, including power supplies for personal computers, office equipment, spacecraft power systems, laptop computers, and telecommunications equipment, as well as dc motor drives. The input to a dc-dc converter is an unregulated dc voltage. The converter produces a regulated output voltage, having a magnitude (and possibly polarity) that differs from unregulated one.

High efficiency is invariably required, since cooling of inefficient power converters is difficult and expensive. The ideal dc-dc converter exhibits 100% efficiency; in practice, efficiencies of 70% to 95% are typically obtained. This is achieved using switched-mode, or chopper, circuits whose elements dissipate negligible power. Pulse-width modulation (PWM) allows control and regulation of the total output voltage. This approach is also employed in applications involving alternating current, including high-efficiency dc-ac power converters (inverters and power amplifiers), ac-ac power converters, and some ac-dc power converters (low-harmonic rectifiers).

4.2 Converter circuit topologies

A large number of dc-dc converter circuits are known that can increase or decrease the magnitude of the dc voltage and/or invert its polarity. Selection of the converter circuit depends upon the required power rating. There are two basic types of DC-DC converters topologies.

1- One switch power converter

2- Two switch Power Converter

Once switch is used for low power demand and while the two switch power converter is used for high power rating. According to our requirements, we need two switch power converter. The most common topologies used for one switch power converter are:

And the most common topologies for two switch power converter are following:

Push Pull

Half Bridge

Full Bridge

4.3 Push Pull Characteristics

A push–pull converter uses a transformer to change the voltage of a DC power supply. The transformer's ratio is arbitrary but fixed. In many circuit implementations the duty cycle of the switching action can be varied to affect a range of voltage ratios. The primary advantages of push–pull converters are their simplicity and ability to scale up to high power throughput, earning them a place in industrial DC power applications.

The Push-Pull topology is basically a Forward converter with two primaries. The primary switches alternately power their respective windings. T1 and T2 switches are alternately turned-on during a time ton. The secondary circuit operates at twice the switching frequency. A dead time td between the ends of conduction of one switch and the turn-on time of the other one is required in order to avoid simultaneous conduction of the two switches.

Shown herer is oscilloscope waveforms for the Drain voltages of the two primary switches and the output inductor current. When a given primary is active the Drain voltage is zero and the alternate switches Drain is 2X the input voltage. This is due to the transformer voltage bringing "reflected" from the active primary to in-active primary. When neither switch is active then both Drain voltages are at the input voltage.

4.4 Transistors for PUSH-PULL Converter

The output power of our inverter circuit is 500 Watts. So it has to handle heavy currents. In push-pull converter, power transistors can be used but Power MOSFETs are most appropriate due to their high current switching capability and their inherently low ON resistance. A POMOS is used to pull up the NMOS power transistor gate and an NMOS is used to pull down the PMOS gate.

The operation of the circuit means that both transistors are actually pushing, and the pulling is done by a low pass filter (coil) in general, and by a center tap of the transformer in the converter application. But because the transistors push in an alternating fashion, the device is called a push–pull converter.

4.5 Switching Constraints

If both transistors are in their on state, a short circuit results. On the other hand if both transistors are in their off state, high voltage peaks appear due to back EMF.

If the driver for the transistors is powerful and fast enough, the back EMF has no time to charge the capacity of the windings and of the body-diode of the MOSFETs to high voltages.

To avoid this, PWM (pulse width modulation) technique is used. PWM can be done using micro-controller. In our circuit, we have used TL494 IC for this purpose. The datasheet of the IC is available in the appendix.

The primary reason for selecting this IC is that it contains two PWM channels. 1 for each MOSFET.

Chapter 5

Pulse Modulation Schemes

5.1 PULSE-AMPLITUDE MODULATION

In PAM the successive sample values of the analog signal s(t) are used to affect the amplitudes of a corresponding sequence of pulses of constant duration occurring at the sampling rate. No quantization of the samples normally occurs. In principle the pulses may occupy the entire time between samples, but in most practical systems the pulse duration, known as the duty cycle, is limited to a fraction of the sampling interval. Such a restriction creates the possibility of interleaving during one sample interval one or more pulses derived from other PAM systems in a process known as time-division multiplexing (TDM).

(a)Analog signal, s(t). (b) Pulse-amplitude modulation. (c) Pulse-width modulation. (d) Pulse position modulation

5.2 PULSE-WIDTH MODULATION

In PWM the pulses representing successive sample values of s(t) have constant amplitudes but vary in time duration in direct proportion to the sample value. The pulse duration can be changed relative to fixed leading or trailing time edges or a fixed pulse center. To allow for time-division multiplexing, the maximum pulse duration may be limited to a fraction of the time between samples.

5.3 PULSE-POSITION MODULATION

PPM encodes the sample values of s(t) by varying the position of a pulse of constant duration relative to its nominal time of occurrence. As in PAM and PWM, the duration of the pulses is typically a fraction of the sampling interval. In addition, the maximum time excursion of the pulses may be limited.

5.4 WHY PULSE WIDTH MODULATION?

Pulse-width modulation (PWM) of a signal or power source involves the modulation of its duty cycle, to either convey information over a communications channel or control the amount of power sent to a load.

Chapter 6

Pulse Width Modulation (PWM)

6.1 Introduction

Pulse Width Modulation: A modulation technique that uses a digital circuit to create a variable analog signal. PWM is a simple concept: open and close a switch at uniform, repeatable intervals. Analog circuits that vary the voltage tend to drift, and it costs more to produce ones that do not than it does to make digital PWM circuits. In addition, control of almost everything today is already in the digital realm.

For example, PWM is widely used to control the speed of a DC motor and the brightness of a bulb, in which case the PWM circuit is used to open/close a power line. If the line were opened for 1ms and closed for 1ms, and this were continuously repeated, the target would receive an average of 50% of the voltage and run at half speed or half brightness. If the line were opened for 1ms and closed for 3ms, the target would receive an average of 25%.

Today, PWM technique has been used in wide applications, such voltage control, current control, motor control, power control, UPS, inverter.

6.2 How it Works

In a nutshell, PWM is a way of digitally encoding analog signal levels. Through the use of high-resolution counters, the duty cycle of a square wave is modulated to encode a specific analog signal level. The PWM signal is still digital because, at any given instant of time, the full DC supply is either fully on or fully off. The voltage or current source is supplied to the analog load by means of a repeating series of on and off pulses. The on-time is the time during which the DC supply is applied to the load, and the off-time is the period during which that supplies is switched off. Given a sufficient bandwidth, any analog value can be encoded with PWM.

Following figure shows three different PWM signals. 1st Figure shows a PWM output at a 10% duty cycle. That is, the signal is on for 10% of the period and off the other 90%. 2nd & 3rd figures show PWM outputs at 50% and 90% duty cycles, respectively.

These three PWM outputs encode three different analog signal values, at 10%, 50%, and 90% of the full strength. If, for example, the supply is 9V and the duty cycle is 10%, a 0.9V analog signal results.

6.3 ADVANTAGES OF PWM

Using pulse width modulation has several advantages over analog control.

I. The entire control circuit can be digital, eliminating the need for digital-to-analog converters.

II. Using digital control lines will reduce the susceptibility of your circuit to interference.

III. Finally, motors may be able to operate at lower speeds if you control them with PWM. When you use an analog current to control a motor, it will not produce significant torque at low speeds.

IV. The output voltage control can be obtained without any additional components.

V. With this method, lower order harmonics can be eliminated or minimized Along with its output voltage control.

VI. As higher order harmonics can be filtered easily the higher order harmonics can be minimized.

6.4 Application of PWM in Power Electronics

PWM can be used to reduce the total amount of power delivered to a load without losses normally incurred when a power source is limited by resistive means. This is because the average power delivered is proportional to the modulation duty cycle. With a sufficiently high modulation rate, passive electronic filters can be used to smooth the pulse train and recover an average analog waveform.

High frequency PWM power control systems are easily realizable with semiconductor switches. The discrete on/off states of the modulation are used to control the state of the switch(es) which correspondingly control the voltage across or current through the load. The major advantage of this system is the switches are either off and not conducting any current, or on and have (ideally) no voltage drop across them. The product of the current and the voltage at any given time defines the power dissipated by the switch, thus (ideally) no power is dissipated by the switch. Realistically, semiconductor switches such as MOSFETs or bipolar junction transistors (BJTs) are non-ideal switches, but high efficiency controllers can still be built.

Nevertheless, during the transitions between on and off states, considerable power is dissipated in the switches, but the change of state between fully on and fully off is quite rapid relative to typical on or off times, so the average power dissipation is quite low compared with the power being delivered,

6.5 PWM Controllers

Many microcontrollers include on-chip PWM controllers. For example, ATMega32 includes four, each of which has a selectable on-time and period. The duty cycle is the ratio of the on-time to the period; the modulating frequency is the inverse of the period. To start PWM operation, the data sheet suggests the software should:

Set the period in the on-chip timer/counter that provides the modulating square wave

Set the on-time in the PWM control register

Set the direction of the PWM output, which is one of the general-purpose I/O pins

Start the timer

Enable the PWM controller

Although specific PWM controllers do vary in their programmatic details, the basic idea is generally the same.

Chapter 7

DC – AC Conversion

The DC which was stepped up using DC - DC Converter, is now to be converted in AC at a frequency of 50Hz. There are many different power circuit topologies and control strategies used in inverter designs. Different design approaches address various issues that may be more or less important depending on the way that the inverter is intended to be used.

The issue of waveform quality can be addressed in many ways. Capacitors and inductors can be used to filter the waveform. If the design includes a transformer, filtering can be applied to the primary or the secondary side of the transformer or to both sides. Low-pass filters are applied to allow the fundamental component of the waveform to pass to the output while limiting the passage of the harmonic components. If the inverter is designed to provide power at a fixed frequency, a resonant filter can be used. For an adjustable frequency inverter, the filter must be tuned to a frequency that is above the maximum fundamental frequency.

Since most loads contain inductance, feedback rectifiers or anti-parallel diodes are often connected across each semiconductor switch to provide a path for the peak inductive load current when the switch is turned off. The anti-parallel diodes are somewhat similar to the freewheeling diodes used in AC/DC converter circuits.

The most appropriate topology suitable for our design is H-Bridge.

7.1 H – Bridge

The term "H-bridge" is derived from the typical graphical representation of such a circuit. An H-bridge is built with four switches (solid-state or mechanical). When the switches S1 and S4 (according to the following figure) are closed (and S2 and S3 are open) a positive voltage will be applied across the load. By opening S1 and S4 switches and closing S2 and S3 switches, this voltage is reversed, allowing reverse operation of the load.

Using the nomenclature above, the switches S1 and S2 should never be closed at the same time, as this would cause a short circuit on the input voltage source. The same applies to the switches S3 and S4. This condition is known as shoot-through.

7.2 Construction

A solid-state H-bridge is typically constructed using reverse polarity devices (i.e., PNP BJTs or P-channel MOSFETs connected to the high voltage bus and NPN BJTs or N-channel MOSFETs connected to the low voltage bus).

The most efficient MOSFET designs use N-channel MOSFETs on both the high side and low side because they typically have a third of the ON resistance of P-channel MOSFETs. This requires a more complex design since the gates of the high side MOSFETs must be driven positive with respect to the DC supply rail. However, many integrated circuit MOSFET drivers include a charge pump within the device to achieve this.

Alternatively, a switch-mode DC-DC converter can be used to provide isolated ('floating') supplies to the gate drive circuitry. A multiple-output flyback converter is well-suited to this application.

Another method for driving MOSFET-bridges is the use of a specialized transformer known as a GDT (Gate Drive Transformer), which gives the isolated outputs for driving the upper FETs gates. The transformer core is usually a ferrite core.

We will use the last method in our design.

Chapter 8

H-Bridge MOSFET Drive Control

8.1 Introduction

High-power MOSFETs present a significant load to their associated gate drive circuit, efficient operation requires the proper drive. One requirement for the gate driver in high frequency applications is to minimize the effect of parasitic circuit elements by placing the high-current driver physically close to the load. Also, newer power supply control ICs that target high operating frequencies may not incorporate onboard gate drivers at all. Their PWM outputs are only intended to drive the high impedance input to an external gate driver. In addition, the control device may be under thermal stress due to power dissipation, and an external driver can help by moving the heat from the controller to an external package.

Gate driving circuitry provides an interface between low-power controllers and MOSFETs. One application is as a high-power buffer stage between the PWM output of the control device and gates of the primary power switching MOSFET.

Certain gate driving ICs are available in market but they are quite expensive. Our intention is to keep the cost of the inverter affordable so that it can survive in market.

A very economical technique is explained below. Its only drawback is its complexity.

8.2 Isolated Ground Approach

We are using IRF840 which is an N-channel MOSFET, as the main power switch because of its lower price, higher speed and lower on-resistance. Using N-channel devices as a high side switch necessitates a gate drive circuit which is referenced to the source of the MOSFET. The driver must tolerate the violent voltage swings occurring during the switching transitions and drive the gate of the MOSFET above the positive supply rail of the power supply. In most cases, the gate drive voltage must be above the highest DC potential available in the circuit. All these difficulties make the high side driver design a challenging task.

In the easiest high side applications the MOSFET can be driven directly by the PWM controller or by a ground referenced driver. Two conditions must be met for this application:

VGS < VGS, MAX

VIN < VDRV - VGS, Miller

A typical application schematic is illustrated in following figure with an optional PnP turn-off circuit.

(The selection of this circuit is based on a research paper by Laszlo Balogh in 2001. Further details about the research paper are mentioned in reference section.)

As the H-Bridge is to handle high current, so it should be isolated from the other PWM circuit. For isolation purpose, we have used opto-coupler PIC817.

The circuit for MOSFET drive control is available in the appendix.

Chapter 9

Micro-controller Based Wave Generation Scheme

9.1 Overview of 89C2051 Micro-controller

The AT89C2051 is a low-voltage, high-performance CMOS 8-bit microcomputer with 2K bytes of Flash programmable and erasable read-only memory (PEROM). By combining a versatile 8-bit CPU with Flash on a monolithic chip, the Atmel AT89C2051 is a powerful microcomputer which provides a highly-flexible and cost-effective solution to many embedded control applications. The AT89C2051 provides the following standard features: 2K bytes of Flash, 128 bytes of RAM, 15 I/O lines, two 16-bit timer/counters, a five vector two-level interrupt architecture, a full duplex serial port, a precision analog comparator, on-chip oscillator and clock circuitry. In addition, the AT89C2051 is designed with static logic for operation down to zero frequency and supports two software selectable power saving modes. The Idle Mode stops the CPU while allowing the RAM, timer/counters, serial port and interrupt system to continue functioning. The power-down mode saves the RAM contents but freezes the oscillator disabling all other chip functions until the next hardware reset.

9.2 Pin Configuration & Block Diagram

The IC package/footprint is 20 Lead PDIP/SOIC.

Block Diagram of 89C2051 Micro-controller

9.3 Sinusoidal PWM Generation using Micro-controller

Sinusoidal PWM is generated using AT89C2051. This method is very fast and reliable. To generate sinusoidal PWM, the pulse width is to be varied continuously. There are certain other methods to generate sinusoidal PWM using several ICs. In those methods, a triangular wave is generated and is compared with a constant reference voltage. But there are certain problems associated with those methods.

Therefore, micro-controller is much preferred. The micro-controller having sufficient memory is preferred because the values of pulse width are stored in its memory. The program fetches the value from memory and changes the pulse width continuously to achieve the sinusoidal PWM. If the memory of available controller is not enough, than we can use an external EEPROM for storage purpose. Also, the operating frequency of the controller should be in MHz ranging from 8 to 20 MHz.

The PWM generated is then used to control the H-Bridge via the control circuit as explained in previous chapters.

9.4 Filter Design

The output obtained across the H-Bridge is not a pure sine wave. It contains odd harmonics which are not suitable for the load. The harmonics are generated due to fast switching of the MOSFETs. A low pass filter is required to finish these harmonics. A low-pass filter is a filter that passes low-frequency signals but attenuates (reduces the amplitude of) signals with frequencies higher than the cutoff frequency.

One simple electrical circuit that will serve as a low-pass filter consists of a resistor in series with a load, and a capacitor in parallel with the load. The capacitor exhibits reactance, and blocks low-frequency signals, causing them to go through the load instead. At higher frequencies the reactance drops, and the capacitor effectively functions as a short circuit. The combination of resistance and capacitance gives the time constant of the filter Ï„ = RC. The break frequency, also called the turnover frequency or cutoff frequency (in hertz), is determined by the time constant:

f_\mathrm{c} = {1 \over 2 \pi \tau } = {1 \over 2 \pi R C}