POWER INVERTER DESIGNINTRODUCTION:Power inverters are electrical devices designed to convert direct current (DC) to alternating current (AC). DC is the power that is produced by batteries and solar panels while AC is the standard power needed to run electrical equipment. A power inverter does the opposite of a rectifier and is used in places and situations where AC power is not available. The method in which the low voltage DC power is inverted, is completed in two steps. The first being the conversion of the low voltage DC power to a high voltage DC source, and the second step being the conversion of the high DC source to an AC waveform using pulse width modulation. Another method to complete the desired outcome would be to first convert the low voltage DC power to AC, and then use a transformer to boost the voltage to 120 volts.TYPESPower inverters produce one of three types of power wave signals. They are basically classified as Square Wave : This is one of the simplest waveforms an inverter design can produce and is useful for some applications. Square Wave signals produced power that was not reliable or consistent. Square Wave units 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 and, like the horse and buggy, are no longer relevant for modern use. Modified Sine Wave Inverter : A modified sine wave inverter can adequately power some household appliances and power tools. It is cheaper, but may present certain compromises with some loads such as computers, microwave ovens, laser printers, clocks and cordless tool chargers. Virtually all low cost inverters are "modified sine wave". A modified sine wave is easier and cheaper to produce than a sine wave inverter. They usually lack many features such as auto-start or any type of tweaking ability. The devices are usually about 70% efficient, so expect some significant power losses if you are using a modified sine wave inverter in your system.

Pure Sine Wave Inverter : Pure sine wave inverters are used to operate sensitive electronic devices that require high quality waveform with little harmonic distortion. In addition, they have high surge capacity which means they are able to exceed their rated wattage for a limited time. This enables power motors to start easily which can draw up to seven times their rated wattage during startup. Virtually any electronic device will operate with the output from a pure sine wave inverter. These devices are able to run more sensitive devices that a modified sine wave may cause damage to such as: laser printers, laptop computers, power tools, digital clocks and medical equipment. This form of AC power also reduces audible noise in devices such as fluorescent lights and runs inductive loads, like motors, faster and quieter due to the low harmonic distortion.

Inverter Design Formulas The internal voltage reference is 1.245 voltsis the peak inductor current,is the maximum output load current

tonand toff> 10 ms andton+ toff< 50 ms

Timing capacitor Minimum output filter capacitor Converter efficiency Input currentInverter Design Example (AN711)Vin= 12 voltsVout= -15 voltsIout (max)= 500 maVripple< 1%Step 1 Calculate the peak current

Step 2 Calculate the current sense resistorStep 3 Calculate theton/toffratio

Step 4 Calculate the values fortonandtoffSinceandLetthenStep 5 Calculate the timing capacitorCT

Step 6 Determine the inductor size

Step 7 Determine the minimum storage capacitor size

Step 8 Determine the feedback resistor sizes

Step 9 Determine the switching transistor bias resistorsSelectwhere( current sense threshold voltage)(of the external transistor)

INVERTER PARAMETERS

Sinusoidal Pulse Width Modulation In sinusoidal PWM the distortion factor and lower order harmonics are reduced. The frequency of reference signal determines inverter output frequency and its peak amplitude controls the modulation index M and and RMS output voltage. Number of pulses per half cycle depends on carrier frequency. The rms output voltage can be varied from 0 to Vs volts by varying the modulation index M from 0 to 1. A DC voltage source can be made to look like an AC signal across a load by altering the duty cycle of the PWM signal.PWM signal varies can be generated through simple analog components, a digital microcontroller, or specific PWM integrated circuits.We chose the microcontroller over the analog system for several reasons. First, it would be simpler to adapt. With an analog system, it would be difficult to make changes for the desired output. The second advantage of using a microcontroller is that it can allow for easy feedback to control the power flowing through the load.

BLOCK DIAGRAMThe block diagram shown below shows the various parts of the project what will be addressed. The control block is simply the microcontroller. It generates both the PWM and square wave signals needed in controlling the MOSFET drivers. The signals from the drivers are then used to drive the four N-channel MOSFETs in the H-bridge configuration. The output signal form the h-bridge is then sent through an low-pass LC filter so that the final output is a pure sine wave.

For H Bridge it is beneficial to use N-channel MOSFETs as the high side switches as well as the low side switches because they have a lower ON resistance and therefore less power loss. The MOSFET driver operates from a signal input given from the microcontroller and takes its power from the battery voltage supply that the system uses. In order to achieve the extra voltage necessary to switch on the device, a MOSFET driver is used with a bootstrap capacitor.

Figure 1a on & off time for PWM Figure 2 H Bridge Schematic

The difficulty in this system however is that in the high side MOSFETs, the gate voltage (12V) needs to be at least 10V higher than the drain to source voltage (170V). To meet this requirement, a bootstrap capacitor is needed to maintain the voltage difference approximately 10V above the drain to source voltage.

The formula for the minimum bootstrap capacitor values can be obtained from:

For MicrocontrollerPulse width modulation (PWM) is a technique of controlling the amount of power delivered to an electronic load using an on-off digital signal. The fraction of the period for which the signal is on is known as the duty cycle. The average DC value of the signal can be varied by varying the duty cycle. PIC16F628A has a built-in hardware, called Capture/Compare/PWM (CCP) module, to generate a PWM signal. The Capture and Compare features integrate closely with the 16-bit TMR1 and the PWM feature uses the third timer, the 8-bit TMR2. The CCP module has two 8-bit registers, called CCPR1L and CCPR1H. In PWM mode, the RB3/CCP1 pin can output a 10-bit resolution periodic digital waveform with programmable period and duty cycle. To operate in PWM mode, the CCP1 pin must be configured for output.

Code for PWM

For the Output filter The final component necessary to output a pure sine wave signal is an output filter. For our circuit we need a basic LC lowpass filter.

The Transformer