Duty cycle challenge

1. Why power electronics circuits are efficient? In practice, which types of losses occur in power electronics circuits?    

Power Electronics is the study of switching electronic circuits in order to control the flow of electrical energy. Power Electronics is the technology behind switching power supplies, power converters, power inverters, motor drives, and motor soft starters. Some of the power electronics are Diodes, Power Bipolar Junction Transistors, MOSFETs, Thyristors, Silicon Controlled Rectifier (SCR), Insulated Gate Bipolar Transistors (IGBT). Some of the power converter circuits are Rectifiers, Inverter, Chopper, cycloconverter. 

The power electronics circuit are mainly used in switching 

In the ideal switching regulator, the current is zero when the switch is open and the power loss is zero, thus VIN is being chopped. When the switch is closed, the voltage across it is zero and the power loss is also zero. An ideal switch implies zero losses, thus offering
100% efficiency.

Power loss comes mainly in the form of the tow factors listed below.

1. Switching losses - The switching losses are loss which occurs during the On & Off of the switches. So losses in power converter come from the core and core losses. Gate-drive losses are also switching losses because they are required to turn the FETs on and off. For the feedback circuit, the voltage divider, error amplifier, and comparator bias currents contribute to power loss. 

2. Conduction losses - This type of loss occurs during the freewheeling of the circuit.  The total power dissipation during conduction is computed by multiplying the on-state voltage and the On-state current (Ic). In PWM application the conduction loss must be multiplied by the duty factor to obtain the average power dissipation. Conduction loss is the on-state loss or steady-state loss. The average power dissipated by the power converter circuit is given by,

Iout – load current of the boost converter.

Fsw – Switching frequency.

Vin – input voltage.

Iripple - Ripple current. 

Vout – output voltage.

Qgs - Gate charge.

2. MATLAB model for duty cycle control signal for a simple power converter circuit.

The Average-Value DC-DC Converter block represents a controlled average-value DC-DC converter. The diagram shows the equivalent circuit for the block. The converter contains a controlled current source and a controlled voltage source. Use the duty cycle port to convert the electrical energy between the connected components on either side of the converter.

where

d - duty cycle.

Vi - Input voltage.

Vo - Output voltage.

Results

3. Comparison of square wave inverter output voltage with sine modulated one. 

The device which converts DC into AC is called Inverter. The inverter is used to convert DC to variable AC. This variation can be in the magnitude of voltage, number of phases, frequency or phase difference.

According to the output characteristic of an inverter, there can be three different types of inverters.

• Square Wave Inverter
• Sine Wave Inverter
• Modified Sine Wave Inverter

1) Square wave inverter

The output waveform of the voltage for this inverter is a square wave. This type of inverter is least used among all other types of inverter because all appliances are designed for sine wave supply. If we supply square wave to sine wave-based appliance, it may get damaged or losses are very high. The cost of this inverter is low but the application is rare. It can be used in simple tools with a universal motor.

2) Sine wave

The output waveform of the voltage is a sine wave and it gives us very similar output to the utility supply. This is the major advantage of this inverter because all the appliances we are using, are designed for the sine wave. So, this is the perfect output and gives a guarantee that the equipment will work properly. This type of inverters is more expensive but widely used in residential and commercial applications.

3) Modified sine wave

The construction of this type of inverter is complex than simple square wave inverter but easier compared to the pure sine wave inverter. The output of this inverter is neither pure sine wave nor the square wave. The output of such an inverter is some of the two square waves. The output waveform is not exactly sine wave but it resembles the shape of a sine wave.

Variable frequency drive

It is a type of adjustable-speed drive used in electro-mechanical drive systems to control AC motor speed and torque by varying motor input frequency and voltage. VFDs are used in applications ranging from small appliances to large compressors. About 25% of the world\'s electrical energy is consumed by electric motors in industrial applications. Systems using VFDs can be more efficient than those using throttling control of fluid flow, such as in systems with pumps or fans. Over the last four decades, power electronics technology has reduced VFD cost and size and has improved performance through advances in semiconductor switching devices, drive topologies, simulation and control techniques, and control hardware and software.

Vector Control

Vector control called field-oriented control (FOC) is a variable frequency drive (VFD) control method in which the stator currents of a three-phase AC electric motor are identified as two orthogonal components that can be visualized with a vector. One component defines the magnetic flux of the motor, the other the torque. The control system of the drive calculates the corresponding current component references from the flux and torque references given by the drive\'s speed control. The pulse-width modulation of the variable-frequency drive defines the transistor switching according to the stator voltage references that are the output of the PI current controllers.

FOC is used to control the AC synchronous and induction motors. It was originally developed for high-performance motor applications that are required to operate smoothly over the full speed range, generate full torque at zero speed, and have high dynamic performance including fast acceleration and deceleration. However, it is becoming increasingly attractive for lower performance applications as well due to FOC\'s motor size, cost and power consumption reduction superiority.

Why VFD can\'t be used for EV application?

The flux and current are coupled so current variation affects the flux value. Using the VFD control method is scalar so we can\'t decouple the current & flux. Stator drop compensation must be considered. Precision control of EV applications can\'t be achieved. The VFD control method has low efficient so the overall efficiency of the vehicle reduces.