Driver And Output Stage Of A 3.5Kva Inverter

ABSTRACT

A driver stage prepares a small electrical signal for further amplification or processing. It is used to boost the signal strength to drive the transformer of an inverter

In inverter circuit,  the received oscillating frequency is suitably amplified to high current levels using Mosfets.

Though the boosted response is an AC, but is still at the battery supply voltage level and therefore cannot be used to operate electrical appliances which work at higher voltage AC potentials. The amplified voltage is therefore finally applied to the output transformer secondary winding.

CHAPTER ONE

1.0                                                      INTRODUCTION

An inverter converts DC power (also known as direct current), to standard AC power (alternating current). Inverters are used to operate electrical equipment from the power produced by a car or boat battery or renewable energy sources, like solar panels or wind turbines. DC power is what batteries store, while AC power is what most electrical appliances need to run so an inverter is necessary to convert the power into a usable form. For example, when a cell phone is plugged into a car cigarette lighter to recharge, it supplies DC power; this must be converted to the required AC power by a power inverter to charge the phone.

There are different blocks in an inverter that makes up an inverter which include pulse width controller, driver stage, output circuit, feedback circuit, and battery. However, this work is focusing on the driver stage of the inverter.

In inverter, a driver is an electrical circuit used to control another circuit or component, such as a high-power transistor (MOSFET). MOSFETs are a type of transistor with a low on-resistance and a high current-handling capacity. The main benefit of the power MOSFET is that the base current for BJT is large compared to almost zero for MOSFET gate current. Since the MOSFET is a depletion channel device, voltage, not current, is necessary to create a conduction path from drain to source. The gate does not contribute to either drain or source current. Turn on gate current is essentially zero with the only power dissipated at the gate coming during switching. Losses in MOSFETs are largely attributed to on-resistance. The calculations show a direct correlation to drain source on-resistance and the device blocking voltage rating, BV_dss.

Switching times range from tens of nanoseconds to a few hundred microseconds, depending on the device. MOSFET drain source resistances increase as more current flows through the device. As frequencies increase the losses increase as well, making BJTs more attractive. Power MOSFETs can be paralleled in order to increase switching current and therefore overall switching power. Nominal voltages for MOSFET switching devices range from a few volts to a little over 1000 V, with currents up to about 100 A or so. Newer devices may have higher operational characteristics. MOSFET devices are not bi-directional, nor are they reverse voltage blocking.

They are usually used to regulate current flowing through a circuit or to control other factors such as power that flow to the transformer of an inverter, some devices in the circuit. The term is often used, for example, for a specialized integrated circuit that controls high-power switches in switched-mode power converters.

Drivers stage convert the logic level PWM signals to a bipolar signal that can be used to drive the gate of power semiconductors. They also isolate the power section from the control section.

1.2                                             OBJECTIVE OF THE PROJECT

The main objective of this work is to design and construct a driver stage of an inverter that will be used to power 3.5kva inverter. This inverter drive stage may be called upon to supply power range 3.5kwatts in an inverter to a load (transformer).

1.3                                                 SCOPE OF THE PROJECT

Driver stage of inverter functions as switch that is used in order to allow power flow in the On state and to stop power flow when it is in the Off state. Driver stage of inverter works by applying voltage to a semiconductor component, therefore changing its properties to block or create an electrical path.

1.4                                         APPLICATIONS OF THE PROJECT

Apart for using this device in inverter, this device is used in high power applications such as:

• Appliance motor drives
• Electric vehicle motor drives
• Power factor correction converters
• Uninterruptible power supplies
• Solar inverters
• High frequency welders
• Inductive heating cookers

1.5                                              PURPOSE OF THE PROJECT

The purpose of this project were to produce a driver stage of a DC AC inverter that would accept a frequency of 50Hz,220volts RMS with 3500 watt output which would be cheap to manufacture, and fairly efficient in the method in  which it produces it.

1.6                                 LIMITATION/PROBLEM OF THE PROJECT

1. The gate of the driver, that is MOSFET, must be protected with resistors.
2. During load condition MOSFET emit heat. If too much power is dissipated, this junction gets too hot and the transistor will be destroyed, a typical maximum temperature is between 100°C and 150°C, although some devices can withstand higher maximum junction temperatures. The maximum power output available from a power transistor is closely linked to temperature, and above 25°C falls in a linear manner to zero power output as the maximum permissible temperature is reached. heat-sink is designed to remove heat from a transistor and dissipate it into the surrounding air as efficiently as possible. Heat-sinks take many different forms, such as finned aluminium or copper sheets or blocks, often painted or anodised matt black to help dissipate heat more quickly. Good physical contact between the transistor and heat-sink is essential, and a heat transmitting grease (heat-sink compound) is smeared on the contact area before clamping the transistor to the heat-sink. Where it is necessary to maintain electrical insulation between transistor and heat-sink a mica layer is used between the heat-sink and transistor. Mica has excellent insulation and very good heat conducting properties.