Design of Boost Converter (DC-DC Step Up Converter)

A boost converter (shown in figure 1) is simply is a particular type of power converter with an output DC voltage greater than the input DC voltage. This circuit is used to ‘step-up’ a source voltage to a higher, regulated voltage, allowing one power supply to provide different driving voltages.

Figure 1

The key principle that drives a boost converter is the tendency of an inductor to resist change in the current.

Boost Converter operates in two in two modes, continuous and discontinuous mode. In discontinuous mode, the amount of energy required by the load is too small. Therefore, the current through the inductor falls to zero during that period. 

In continuous mode, the current through the inductor never falls to zero.

• When the switch is open (shown in figure 2) the stored energy is transferred through the inductor to the voltage output in a controlled manner through the flyback diode D, the capacitor C and the load R

Figure 2

Eq. 1

• When the switch is closed (figure 3), current flows through the inductor in clockwise direction and the inductor stores the energy magnetically.

Figure 3

Eq. 2

The inductor current has to be the same at the start and end of the commutation cycle. This means the overall change in the current is zero.

Eq. 3

Using the voltage and current ripple, the value of Inductance and Capacitor values are calculated.

Design of Buck Converter

The basic topology of DC-DC buck, or step down converter is shown in Figure 1. Buck Converter operates in two modes, continuous and discontinuous mode. In discontinuous mode, the amount of energy required by the load is too small. Therefore, the current through the inductor falls to zero during that period. The inductor is completely discharged at the end of the commutation cycle.

Figure 1

A buck converter operates in continuous conduction mode, if the current in the conductor never falls to zero during commutation. The operating principle is described below:

• When the switch is closed (i.e. ON state) as shown in figure 2, the voltage across inductor is V(L) = Vi - Vo. As the diode is reverse biased by the voltage source V, no current flows through it.

Figure 2 - Buck Converter when switch is closed (ON State)

Eq. 1

• When the switch is opened (i.e. OFF state) as shown in figure 3, the diode is forward biased and the voltage across the inductor V(L) = -V.

Figure 3 - Buck converter when switch is open (OFF State)

Eq. 2

Here D is the Duty cycle and T is time period of the buck operation.

Increase in current when the switch is turned ON, must be equal to decrease in current when the switch is turned OFF, as the net change in flux in the inductor must be zero.

From Eq. 1:

From Eq. 2:

Equating the above equations:

Calculation of ripples:

From the equation of current ripple the maximum and minimum value of the inductor can be calculated.

Assuming 100% efficiency, in an ideal converter Pin = Pout. This gives the relation between input and output current as Iout = Iin/D . The value of D is always less than 1 therefore, the value of Iout will be greater than the value of Iin. While the buck converter steps down the voltage it steps up the current to transfer power with minimum loss.

Switched Mode Converters used in Solar Systems

DC-DC converters are electronic devices that are used whenever we want to change DC electrical power efficiently from one voltage level to another. Figure 1 is a very basic switched mode converter. The switched mode converter or switching regulator is a simple switch (and hence ideally no resistance or very low resistance). This switch goes on and off at a fixed frequency.

Figure 1

The Duty Cycle for the switch is determined by the Eq. 1.

Eq. 1

The time that the switch remains closed during each switch cycle is varied to maintain a constant output voltage. The switching regulator is much more efficient than the linear regulator achieving efficiencies as high as 80% to 95% in some circuits. In contrast, the linear regulator usually exhibits only 50% to 60% efficiency. With higher efficiency smaller heat sinks will be required because lesser heat is dissipated.

There is also another advantage of Switching Regulators and that is the energy stored by inductor & capacitor can be transformed to output voltages that can be greater than input (boost), negative (inverter), or can be transferred through a transformer to provide electrical isolation with respect to the input. 

DC-DC converters can be divided into two broad categories:

a. Non-isolated dc/dc converters

b. Isolated dc/dc converters

Non-Isolated DC/DC Converters

The non-isolated converter usually employs an inductor, and there is no dc voltage isolation between the input and the output. The vast majority of applications do not require dc isolation between input and output voltages. The non-isolated dc-dc converter has a dc path between its input and output. 

Most of these dc-dc converter ICs use either an internal or external synchronous rectifier. Their only magnetic component is usually an output inductor and thus less susceptible to generating electromagnetic interference. For the same power and voltage levels, it usually has lower cost and fewer components while requiring less pc-board area than an isolated dc-dc converter. 

Non-Isolated DC/DC Converters are further subdivided into following types:

a. Buck Converter

b. Boost Converter

c. Buck-Boost Converter

d. Cuk Converter

Isolated DC/DC Converters

For safety considerations, there must be isolation between an electronic system’s ac input and dc output. Isolation requirements cover all systems operating from the ac power line, which can include an isolated front-end ac-dc power supply followed by an isolated “brick” dc-dc converter, followed by a non-isolated point-of-load converter. An isolated converter employs a transformer to provide dc isolation between the input and output voltage which eliminates the dc path between the two. 

Isolated dc-dc converters use a switching transformer whose secondary is either diode-or synchronous-rectified to produce a dc output voltage using an inductor-capacitor output filter. This configuration has the advantage of producing multiple output voltages by adding secondary transformer windings. For higher input voltages (48V) transformer isolated converters are more viable.

Isolated DC-DC converters work in different configurations as shown below:

a. Flyback Converters

b. Cuk Converters

c. Push Pull Converters

d. Half Bridge Converters

e. Full Bridge Converters

Pulse Width Modulation

Pulse-width modulation (PWM) or pulse-duration modulation (PDM) is a modulation technique that conforms the width of the pulse, formally the pulse duration, based on modulator signal information. Although this modulation technique can be used to encode information for transmission, its main use is to allow the control of the power supplied to electrical devices, especially to inertial loads such as motors. 

The simplest way to generate a PWM signal is the intersective method, which requires only a sawtooth or a triangle waveform (easily generated using a simple oscillator) and a comparator. When the value of the reference signal (the red sine wave in figure 2) is more than the modulation waveform (blue), the PWM signal (magenta) is in the high state, otherwise it is in the low state.

Figure 2

Maximum Power Point Tracking

Introduction

Solar panels have a nonlinear voltage-current characteristic, with a distinct maximum power point (MPP), which depends on the environmental factors, such as temperature and irradiation. In order to continuously harvest maximum power at any point of time from the solar panels, MPPT algorithms need to be employed. The calculations result in an output that delivers maximum current at the required voltage at any point in time. During low light level situations it will compensate for the low light level and find the new point at which the solar cell delivers its maximum power output.

MPPT Algorithms

Over the past decades many methods to find the MPP have been developed and published. These techniques differ in many aspects such as required sensors, complexity, cost, range of effectiveness, convergence speed, correct tracking when irradiation and/or temperature change, hardware needed for the implementation or popularity among others. A complete review of 19 different MPPT algorithms can be found

Among the above discussed methods, three methods have been studied and analysed in detail.

Classification of MPPT algorithms

Constant Voltage Tracking

This is comparatively an easy and inefficient method to find the maximum power point of any solar photovoltaic module. It is assumed that the maximum power point of solar PV module lies at about 0.75 times the open circuit voltage (Voc). So by measuring the open circuit voltage of the PV module, a reference voltage can be generated and feed forward voltage scheme can be implemented to bring solar PV module voltage to a point of maximum power. The drawbacks of this method are:

• The maximum power point of a solar PV module does not always lies at 0.75*Voc. Hence the tracking efficiency is low.

• The open circuit of the solar PV module varies with the temperature. Hence, open circuit voltage is to be measured continuously for temperature variations.

The Perturb & Observe (P&O) Algorithm

The Perturb and Observe method is a widely used approach to MPPT. As the name suggests, this method works by perturbing the system by increasing or decreasing the PV module operating voltage and observing its impact on the output power supplied by the module. As shown by the flow chart in Figure 1, PV system controller change PV module output with a small step in each control cycle. The step size is generally fixed and it can be increased or decreased. Both PV module output voltage and output current can be the control object, so this process is called "perturbation". Then, by comparing PV array output power of the cycles before and after the perturbation, this method determines the maximum power point.

If the power output is increased at a particular cycle, then according to this method, the system controller will change the step in the same direction as the previous cycle and checks for further increase in power of PV module. While if the output power observed is decreased, then the system controller change the step in direction opposite to the previous cycle. In this way, the actual operating point of PV module can move closer to the maximum power point, and finally in steady state, oscillates around the maximum power point in a very small area. This causes a power loss which depends on the step width of a single perturbation. If the step width is large, the MPPT algorithm will be responding quickly to sudden changes in operating conditions with the trade-off of increased losses under stable or slowly changing conditions. If the step width is very small the losses under stable or slowly changing conditions will be reduced, but the system will be only able to respond very slowly to rapid changes in temperature or insolation. The value for the ideal step width is system dependent and needs to be determined experimentally.

Figure 1

The Incremental Conductance Algorithm

The disadvantage of perturb and observe method to track the peak power under fast varying atmospheric condition is overcome by IC method. The IC can determine that the MPPT has reached the MPP and stop perturbing the operating point. If this condition is not met, the direction in which the MPPT operating point must be perturbed can be calculated using the relationship between dl/dV and –I/V (Eq. 1). This relationship is derived from the fact that dP/dV is negative when the MPPT is to the right of the MPP and positive when it is to the left of the MPP. This algorithm has advantages over P&O in that it can determine when the MPPT has reached the MPP, where P&O oscillates around the MPP. Also, incremental conductance can track rapidly increasing and decreasing irradiance conditions with higher accuracy than perturb and observe. One disadvantage of this algorithm is the increased complexity when compared to P&O.

Eq. 1

The flowchart depicts the working of Incremental Conductance method.

Characteristics of a Photovoltaic Cell (PART - 2)

I-V Characteristics of a Photovoltaic Cell

From Figure 1 (Click Here), the current generated in the solar cell by the current source (IL) is proportional to the amount of light falling on it. When there is no load connected to the output Vo, almost all of the generated current flows through diode D. The resistors Rs and Rsh represent small losses due to the connections and leakage respectively. There is very little change in Voc for most instances of load current. However, if a load is connected to the output then the load draws current away from the diode D. As the load current increases more and more, current is diverted away from the diode D.

So, as the output load varies, so does the output current while the output voltage Voc remains largely constant. That is until so much current is being drawn by the load that diode D becomes insufficiently biased and the voltage across it diminishes with increasing load. This results in I‐V characteristics as shown in Figure 2.

Figure 1: I-V Characteristics of PV Cell for various insolation levels

Characteristics of a Photovoltaic Cell (PART 1)

Equivalent Circuit of a Photovoltaic Cell

Mathematical Model of PV System

To understand the electronic behaviour of a solar cell, it is useful to create a model which is electrically equivalent, and is based on discrete electrical components whose behaviour is well known. An ideal solar cell may be modelled by a current source in parallel with a diode; in practice no solar cell is ideal, so a shunt resistance and a series resistance component are added to the model. The resulting equivalent circuit of a solar cell is shown in figure 1 below:

Figure 1

Characteristic equation

From the equivalent circuit it is evident that the current produced by the solar cell is equal to that produced by the current source, minus that which flows through the diode, minus that which flows through the shunt resistor:

The current through these elements is governed by the voltage across them:

Substituting these into the first equation produces the characteristic equation of a solar cell, which relates solar cell parameters to the output current and voltage:

Open-circuit voltage and short-circuit current

When the cell is operated at open circuit, I = 0 and the voltage across the output terminals is defined as the open-circuit voltage. Assuming the shunt resistance is high enough to neglect the final term of the characteristic equation, the open-circuit voltage Voc is:

Similarly, when the cell is operated at short circuit, V = 0 and the current I through the terminals is defined as the short-circuit current. It can be shown that for a high-quality solar cell (low Rs and Io, and high Rsh) the short circuit current Isc is :

NOTE: It should be noted that it is not possible to extract any power from the device when operating at either open circuit or short circuit conditions.

Introduction

Solar power is a renewable source of energy, which has become increasingly popular in modern times. A shift away from conventionally-fuelled systems towards greater use of renewable energy fuel sources is needed to manage its increasing usage and associated adverse environmental and human health impacts. Solar powered systems suppress polluting gas emissions within urban areas. These systems utilize electrical power generation from solar energy, reducing the demand from power generating stations using non-renewable fuel sources.

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