pwm inverter

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Design a 200 Watt, 150 Vrms PWM Bipolar Inverter

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Abstract-This paper is concentrate on design procedure which including design

requirement, calculation respect to design, circuit building and simulation result. The

simulation in this assiment by using MULTISIM software and it is base on components

that availale in market.

I. INTRODUCTION

Power inverters are circuits used to convert Direct Current (DC) to Alternating current (AC).

The input of the inverter may come from a DC source or from rectified AC input. There are

two main categories for switch mode voltage source inverters: square wave and pulse width

modulated (PWM). Square wave inverters are the simplest to implement. The simplicity of

the square wave inverter comes along with the disadvantage of harmonics close to the

fundamental frequency. PWM inverters function by comparing a sinusoidal control signal at

the desired output frequency with a triangular carrier signal at switching the frequency. The

harmonics of PWM inverters are located at multiples of the carrier signal frequency which is

typically in the kHz range. This simply means the output waveform of PWM appears more

sinusoidal than a square wave inverter. Also, higher frequency harmonics are easier to filter

than harmonics near the fundamental frequency.

The requirement of this asignment is to design full-bridge inverter that can produce an ac

output voltage of 150 Vrms with frequency of 50 Hz. The output power is 200 Watt. The

modulation index (mi) is 1.0 and the frequency of the carrier is 5 kHz. The PWM modulation

type is the bipolar based PWM. This paper highlight:

The design of bipolar PWM

The THD voltage before filtering

The low-pass filter design

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II. METHODOLOGY

To make the flow ease and simple, we discribe this design methodology by refering to the

block diagram in Figure 1

Figure 1: PWM inverter block diagram

Two main block in designing the PWM inverter are the PWM generator and the basic

topology of the square wave inverter. The main differ is the PWM inverter use PWM wave as

a switching signal instead of using square wave. The PWM generator is discribed in the dash

box in the Figure 1 above while the PE inverter use the same topology of square wave inverter

as Figure 2.

Figure 2: Inverter topology

Sine wave generator

Triangle wave generator

comparator amplifier Inverter / buffer

PE Inverter

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Sine wave and triangle wave is feed to comparator for comparison to produce PWM signal.

This signal then amplified by emplifier. Since the T1, T2 apply the same signal and T3, T4

switching signal inverse from T1, T1, thus the digital inverter/buffer is used to perform the

operation. This inverter/buffer output then feed to the T1, T2 and T3, T4 inverter.

III CALCULATION AND DESIGN

The requirement of this design as follow:

i) ac output voltage of 150 Vrms with frequency of 50 Hz.

ii) output power is 200 Watt

iii) modulation index (mi) is 1.0

iv) frequency of the carrier, fc is 5 kHz.

i) output voltage, Vo = 150 Vrms

Vds drop is ≈ 30V due to characteristic of the mosfet (refer mosfet datasheet), thus

Vdc need to increased to suit the Vo = 150Vrms

150푉푟푚푠 = √2

= 212.12 푉dc

Therefore,

212.12 + 30 = 242.12 푉dc

242.12 Vdc need to be feed to Vdd (see figure 3) and resulting the PWM inverter

output as Figure 15. The modulation frequency, fm is 50Hz applied.

ii) The output power, Po = 200W

Po =

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Since Vo = 212.12, thus

200 =212.12푟

푟 = 225 훺

iii) modulation index (mi) is 1.0

Vmodulation, Vm = 10 Vpic and

Vcarrier Vc = 10 Vpic

mi =

mi = 1

All the calculated value then applied to the circuit below by using MultiSim software. The

mosfet type 2SK2978 was selected which has the characteristic as in mosfet datasheet. (see

mosfet datasheet attached)

Figure 3: PWM inverter (full circuit)

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V1 = Vin when mi = 1

Calculation for THDV

1

)sin()(n

nwtVntVout

T

wtdnwttVVnk0

)()sin()(2

Performing the integration

)(212 kkCosnkCosnkCosnVdcVnk

mf = 푃 = , ,

= = 100

When mf >10, or so, the harmonics can be normalised as shown in the Figure 4. It is desirable

to have mf as large as possible and the 100 mf is quite good. This will push the harmonic at

higher frequencies on the spectrum. Thus filtering requirement is reduced.

Figure 4: Frequency spectrum for bipolar PWM with mi = 1

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Each courier frequency spectrum. The harmonic amplitudes are a function of mi=1

THDv= − 1

=150

0.6 푥 212.12 − 1

%62THDv

Although the voltage THD improvement is not significant, but the current THD will improve

greatly because the load normally has some current filtering effect.

By additional of filter; the largest harmonic (see Figure 4) can be eliminated. A simple

method reducing inverter output harmonic uses the LC low-pass passive filters. Second order

filter is represented by LC circuit, where the inductor is shunt with the Equations below

express LC filter transfer function as derived from voltage-divider rule (Anca, 2009)

Use of LC-filter lowers cost and losses of the inverter system. Equation below is for filter

inductance. The maximum ripple current was chosen to be 5%-20% (typical value of

maximum ripple current is 20% of rated current (Wang, 2003)).

Filter capacitance is determinable by the reactive power absorbed in the filter capacitor;

Equation below defines it. being the reactive power factor, its value was selected to be less

than 5%(Wang, 2003).

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Equation below defines the resonance frequency of the LC filter circuit (Khaled, 2007):

Since the MultiSim cannot provide the current waveform, thus the value of LC cant be

calculated properly and the THDi correction cannot be obtained.

IV SIMULATION RESULT AND DISCUSSION

Figure 5: PWM generator

The PWM signal is produced by comparing sine wave and triangle wave (5 Hz and 5 kHz as

required in this design). This can be done by using LM311H comparator (refer Figure 6).

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Figure 6: Comparator input with natural sampling (m=1, fc = 5kHz, fm = 50Hz)

Figure 7: Virtual Comparator output

Since the Comparator LM311H output change between 12V and 30V (refer Figure 8 and

LM311H datasheet), we need to change the Vmin from 12V to 0V, thus the higher value then

12V need as a comparison value.

Figure 8: Comparator LM311H output

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20V has been selected for this design feed to the 741 op-amp VS+ input (refer Figure 5) and

the ap-amp output generated as Figure 9 (also refer 741 op-amp datasheet).

Figure 9: 741 Op-amp output

Since the switching freq for Q1, Q3 is the same and Q2, Q4 is inverse from Q1, Q3, thus we

use the inverter/buffer ic 4041BP to perform and the resulting output (switching signal) as

Figure 10 and Figure 11.

Figure 10: Digital 4041BP inverter/buffer output – switching signal Q1 and Q2

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Figure 11: Digital 4041BP inverter/buffer output – switching signal Q3 and Q4

This PWM signal then feed to the inverter at point IO1 and IO2 as Figure 12.

Figure 12: PWM inverter

The PWM inverter output is produced as Figure 13, Figure 14 and Figure 15; each taken from

different point (XSC6, XSC 7 and XSC8).

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Figure 13: output point XSC6

Figure 14: output point XSC7

Inverter output Vo = Vab

Vds drop is ≈ 30V due to characteristic of the mosfet (refer mosfet datasheet), thus Vdc need to increased to suit the Vo = 150Vrms

242.12 Vdc need to be feed to Vdd as calculated in DESIGN AND CALCULATION section

previously (see Figure 3) and resulting the PWM inverter output in Figure 15 as required in

this design.

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Figure 15: PWM inverter output point XSC8

Since the MultiSim software cannot generate the current waveform, thus the current

waveform was obtained by theoretically and the output should be as Figure below (blue line).

Figure 16: PWM inverter voltage and current output

V CONCLUSION

After flow through all the process, its can be conclude that designing the PWM converter was

not easy as square wave converter. Instead of square wave switching signal, we need to

generate PWM signal. PWM switching signal can be genarate by comparing sinewave and

trianglewave via comparator. In this design, natural sampling was used. One major limitation

with natural sampling PWM is the difficulty of its implementation in a digital modulation

system, because the intersection between the reference waveform and the triangular waveform

is defined by a transcendental equation and is complex to calculate. An analogue circuit

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possesses the advantages of a low cost with a fast dynamic response, but suffers from a

complex circuitry to generate complex PWM, limited function ability and difficulty to

perform in circuit modifications (Mekhlief , 1999). To overcome this limitation the modern

popular alternative is to implement the modulation system using a regular sampling PWM

strategy. This technique was introduced to provide a more flexible way of designing the

system. The system offers simple circuitry, software control and flexibility in adaptation to

various applications. The two most common regular sampling techniques are regular

symmetrical and asymmetrical sampling (Ledwich, 1991).

Simulation with an actual components parameter usually will cause waveform deviation

comparing to the ideal parameter. Thus the modification from the original topology is

necessary. The operation and characteristic of the ideal and the actual components parameter

need to understand. Waveform deviation that we obtained from LM311H comparator output

from the actual signal that we want push us to feed the signal via 741 op-amp to obtained the

right signal. Since the switching signal for Q1, Q2 and Q3, Q4 inverse each other, it is

necassary to feed this output signal of 741 op-amp through the digital inverter/buffer 4041BP.

In this simulation we took the sinewave and the squarewave direct from the signal generator

provided by MultiSim but for the real design, the both signal should be obtained by a circuit

design or generated by special components that available in market such as crystal oscillator

to generate sinewane and 555 timer circuit with some additional circuit to generate

trianglewave which not discuss in this paper.

On the fullwave inverter side is just the same as original squarewave inverter topology. The

chalenging on this side is to find the write mosfet that available is MultiSim software and

finally the n-channel mosfet by Hitachi 2SK2978 was selected. Since we use the MultiSim

software text book edition, even its provide the actual component parameter but not all of the

component available is the sistem. For cormesial circuit design, it is necessary to buy the

expensive solfware (e.g. professional edition) that provide more actual components parameter.

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REFERENCE

[1] Daniel W. Hart, Introduction to Power Electronics, Prentice Hall International Inc,

2003.

[2] Robert W. Erickson, “DC-DC Power Converters”, Department of Electrical and

Computer Engineering, University of Colorado

[3] Rashid, M.H., Power Electronics, Circuits, Devices and Applications, Pearson/Prentice

Hall, 2004

[4] Mohan, Undeland & Robbins, Power Electronics – Converters, Applications and

Design, 2nd edition, John Wiley & Sons, 2003.

[5] Datasheet download at www.ic-on-line.net

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ATTACHMENT

pwm inverter

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