Team Members: Alan Nguyen Issa Frayeh Sarkis Arabyan Mentor

1

Team Members:Alan NguyenIssa Frayeh

Sarkis Arabyan

Mentor:Professor Keyue Smedley

2

Table of Contents

0. Cover Page ---------------------------------------------------------------------- 1

1. Introduction --------------------------------------------------------------------- 3

2. Applications ---------------------------------------------------------------------- 3

3. Specifications --------------------------------------------------------------------- 4

4. Theory ----------------------------------------------------------------------------- 6

5. Design Calculations --------------------------------------------------------------13

6. PCB Layout ----------------------------------------------------------------------- 16

7. Test Simulation Results and Analysis-----------------------------------------18

8. Conclusions ------------------------------------------------------------------------30

9. Suggestions to Improve Project------------------------------------------------31

10. Constraints ----------------------------------------------------------------------- 31

3

Section 1: Introduction

The purpose of this project is to design, fabricate and test a circuit board that converts a low dc voltage to high dc voltage using pulse, known as a boost converter. The boost converter uses a capacitor, inductor, and power MOSFET to maintain a steady current and higher voltage output. The MOSFET will be connected to a Pulse Width Modulator in order to control the switching of the Boost Converter. The output voltage to input voltage ratio is a function of the duty cycle of the switching circuit. The dc to dc converter utilizes pulse width modulation (PWM) and feedback control to produce a regulated output voltage greater than the input. Since power is conserved, the resulting output current is lower than the input current, such that the product of the input voltage and current is equivalent to the product of the output voltage and current. The particular circuit designed in this project is expected to have an efficiency of 90-95%. The project requires the design of a PCB layout using a given circuit schematic and bill of materials (BOM). After completion of a PCB design, the layout is sent out for fabrication. Once received, the components are soldered onto the fabricated circuit board and the entire circuit is analyzed for functionality.

Section 2: Applications

In battery powered systems, placing batteries in series is required in order to achieve higher voltages. In many cases, however, this may be problematic due to space limitations. Boost converters serve as an alternative to stacking batteries by providing increases voltage while requiring little space. Boost converters are also used in hybrid electric vehicles and lighting systems. For instance, the motor of a hybrid electric vehicle, such as a Toyota Prius, requires 500V to operate. Without the use of boost converters, such a motor would require 417 batteries stacked in series. It is because of boost converters that they only used 138 batteries to boost 202V to 500V. As in this example, step up transformers increase the voltage and reduce the cells used, hence decreasing area. In smaller devices, such as white LED lights, boost converters allow the used of one alkaline battery of 1.5V to support a much higher needed 3.3V of output. The unique feature of our boost converter is its 90-95% efficiency, as normal amplifiers generally have a much lower efficiency.

4

Section 3: Specifications

Specification of the Boost ConverterInput Voltage: 20V-40VOutput Voltage: 70VRated Output Power: 20 WattsOutput voltage Ripple: <0.2%Switching Frequency: 200kHz

Conduction Mode:CCM for Vin=20V, 40VDCM for Vin=30V

Major Components

Power StageMOSFET: IRF510Diode: MUR415Inductor: 105uHOutput Capacitor: 100uF electrolytic cap.

PWM control and Driving CircuitPWM controller: SG3524MOSFET driver: IR4427

The inductor and the output capacitor values meet the necessary ripple values. The MOSFET IRF510 blocks the output voltage of 70V during the off switching period. The Diode MUR415 is a power rectifier.

5

BOM – Bill of Materials

6

Section 4: Theory

Before jumping into describing the boost converter, some theory needs to be explained about what is PWM and how is it applied. PWM works by switching the power ON and OFF rapidly to produce a square wave signal altering between full on and zero. By adjusting the duty cycle of the signal, the average power can be varied. Depending on the switching state of the MOSFET, ON or OFF, the equivalent circuit diagrams are shown in figures 3.3 and 3.4 respectively. Starting with the ON circuit (fig 3.5) we have

VL = Vg and ic =-iR= where VL is voltage across inductor, ic and iR are currents through the

capacitor and inductor respectively and Vo is the output voltage across the load R.As for the off circuit (fig 3.6):

VL = Vg – Vo, iL- ic= .

The diagram below shows how VL with time depending on switching state of the MOSFET.

Inductor Voltage v.s time (Fig 4.1)

7

During steady state conditions the amount of energy stored in each component must be the same at the beginning and at the end of the cycle. Thus, the current has to be the same too.

Using the balance concept to calculate :

and

Vg Ton + (Vg – Vo) Toff = 0VgdT + (Vg - Vo) d’ T=0 where d is the duty ratio (fraction of time a device is operated) And d’=1-d 0<d<1

So we have gain = =

The diagram below how the gain varies with duty cycle. Looking at the diagram we can see that as the duty ratio increase the gain increases as expected.

Gain v.s duty ratio fig 4.2.1

To find the inductor’s current ripple:

(voltage across inductor)

∆il= where L is the inductance and ∆il is the

inductor’s current ripple.

Using =

We have ∆il =

8

Inductor current ripple v.s time fig (4.2.2)

As for the capacitor current ripple:

We know that current across a capacitor ic=

Using some geometry and looking at fig’s 3.2.3 and 3.2.4 we get that the current ripple is equal to

2∆Vc= where C is the capacitance.

Capacitor current v.s time (fig 4.2.3) Capacitor voltage v.s time (fig 4.2.4)

9

The diagram below (fig 3.3) shows the circuit components of the Boost Converter. For one to understand how the Converter works exactly, one has to understand the basic function of each component and why it is used. The converter is made up of 6 basic components inductor, capacitor, switching MOSFET, the Diode, the MOSFET driver and a PWM (Pulse Width Modulation) controller.

Boost Converter (fig 4.3) The Boost converter has two functional parts: the power stage and control stage. The power stage stores and transfers power from input to output during each switching cycle. The control stage provides a pulsed driver signal which switches the MOSFET on and off.

A) Power StageComponents:-Power MOSFET IRF510-105uH inductor-Power rectifier MUR415-100uF electrolytic output capacitor

Boost Converter power stage circuit (fig 4.4)

10

The power stage is shown in fig 3.4. The operation of the circuit depends on controlling the switching state of the MOSFET. When the MOSFET, the control element, switch is ON (fig 3.5), the current in the inductor ramps up and energy is stored in the inductor. No current flows through the Diode, and the load, isolated by the diode, is supplied by the charge stored in the capacitor. When the control element is off (fig 3.6), on the other hand, the energy stored in the inductor is added to the input voltage and is released into the load. To minimize sudden changes current (voltage spikes) while switching the state of the MOSFET, the diode is there to provide a discharge path for the inductor thus allowing it to discharge slowly. Therefore, current still moves in the same direction supplying load as well as restoring charge to the capacitor.

A) MOSFET ON, diode OFF (fig 4.5) b) MOSFET OFF, diode on (fig 4.6)

B) Control StageComponents:-Regulating PWM UG3524-high-speed power MOSFET driver IR4427

The purpose of the control stage is to generate a periodic frequency pulsed signal, which controls the MOSFET and diode switching ON and OFF. The oscillator controls the frequency of the SG2524 and is programmed by RT and CT. The desired frequency is defined as f= 1.3/ ( RTCT). For the given resistance RT=5.6kΩ and capacitance CT=1nF, f=232 kHz.

Figure 3.7 provides a detailed view of the circuitry. Starting at the top of circuit, the series connection of capacitor Cr and the resistor Rr prevent sudden rise in voltage by saturating the voltage across the MOSFET and is called a Snubber. To further enhance the function of the MOSFET, the combination of Rdr with the parasitic gate capacitor of the MOSFET reduce noise by creating a low pass filter. This is established by allowing DC signals to pass through while high frequency signals are short to ground. To the right of the Snubber, there is voltage divider that reduces output voltage to about 5V at the node between RF1 and RF2. This is done to prevent drawing high voltage to the chip IC1 (the PWM).

The PWM controller’s main purpose is to provide a fixed frequency output signal by adjusting the Rpoti which controls the duty cycle of the converter. A square wave signal that will drive the MOSFET is generated in the PWM controller. This is achieved by inputting a saw-tooth

11

signal, generated by the oscillator, and voltage reference Vref (potentiometer voltage) in to the comperator as seen in fig 3.9. If Vref > Vsawtooth then voltage Vdr is outputted. Otherwise output voltage is zero as seen in fig 3.8.

Waveform shows how Vref controls the duty cycle in Vdr (fig 4.8)

To improve stability of the PWM, the Rcomp and Ccomp provide a zero to cancel the pole introduced from chip. As for the Rsoft and Csoft, they ensure that the chip shuts down safely. Since the square wave pulse current is insufficient to drive the MOSFET, the MOSFET Driver amplifies the current and drives it to the MOSFET.

12

Detailed circuitry of the boost converter in Closed-Loop (fig 4.7)

13

PWM Controller (fig 4.9)Operating Principle:

CCM (Continuous Mode conduction): During CCM mode, current flowing through the inductor (IL) never falls to zero (inductor does not fully discharge) as seen in below in fig 3.10.

Waveforms of current and voltage in CCM mode (4.10)

DCM Mode ( Discontinuous Conduction Mode):

Occurs when current through the inductor falls to zero, as in fig 3.11; thus the inductor completely discharges. This occurs when energy required by load is transferred in less time than the period T.

14

Waveforms of current and voltage in DCM Mode (fig 4.11)

Section 5: Design Calculations

=

Max Duty Ratio:

= 70 -70d = 2070d = 50dmax = = 0.714

Avg. Duty Ratio:

= 70 -70d = 3070d = 40davg = = 0.571

Min Duty Ratio:

= 70 -70d = 4070d = 30dmin = = 0.429

P = VI :

Iout = = = 0.286 ARL = = = 245 Ω IL = = = 0.5 A

15

If IL ≥ ∆iL then in CCM

∆iL = <

Using ∆iL = IL = 0.5004A and Vg = 40V to get largest inductance for worst case

L = = = 85.7 µH

C > = = 3.6 µF - To eliminate ripple, we use a much larger capacitor

Voltage Stress = V0 + ∆v0 = 70V + [(70V)(0.002)] = 70.14V

Current Stress = IL + ∆iL = 1A + 1A = 2A

K = if K < dd’2, then in DCM Mode If K ≥ dd’2, then in CCM Mode

By increasing R we decrease K hence we get into DCM mode.

Dd’2 = (0.571)(1-0.571)2 = 0.105

For K < 0.105 DCM ModeFor K ≥ 0.105 CCM Mode

Inductor Design

IL = ≈ 1 A = ∆iL =

16

For V0 = 70V and Vg = 20V, dmax = = 0.714 which is the worst case condition

IL = = 1AIL-Pk = IL + ∆iL = 2A [ IL = ∆iL ]

Since L = 85.7 µH, we chose L = 100 µH in order to stay in CCM mode.

Design Procedures

1. Pick the core with area productAp ≥ where B = 0.3T, K = 0.5, J = 5A/mm2

I = 0.8A, L = 109.4µH

Ap ≥ = 0.0533cm4

Ap = 0.187cm4 for OP-42213Since 0.0533 < 0.187, MAGNETICS OP-42213 meets the operation requirements

2. Determine the number of turns N byN = = = 10.4

N = 11 turns without air gap

3. Determine wire cross section = = 0.002cm2

Aw = πr2 [ For AWG22 wire dwire = 0.0701cm ]Aw = π(cm)2 = 0.003859 cm2

Since < Aw , multiple wires are not needed.

4. Skin effect depth @ 200kHz

r < σ = = = 0.0527 cm

r = 0.03535cmσ = 0.0527cm

5. Find Air GapLg = 0.2mm

17

N = = = 15.8

Use N = 16 with air gap

6. Copper Losses

PCu = 1.6 mW

7. Core LossesPCore = afsc·Bd = (0.0434)(200kHz)1.63(0.3T)2.62 = 267.2uW

Pcore = 267.2uW

Section 6: PCB Layout

We used “Express SCH” and “Express PCB” for drawing the PCB Layout. By first drawing the circuit in express SCH and linking it with Express PCB, it was easier for us to location the connections in Express PCB. Below is out final results from express PCB. Some of the problems that we encountered was that this was our first time designing a PCB layout. We had to learn the software by ourselves. Also since we were new we did not know much about line thickness, through hole sizes, and bend angles. Everything turned out to be fine in our final product except the four sockets came out to be small so the sockets would not fit into the holes. Since the board cost us $86.86 to be fabricated, money and time was a major issue in getting the board made again. Therefore we used the board provided by Professor Smedley.

Our board was built on 2 layers. Since we used Express PCB for PCB design, we just had them fabricate our board for us. They had a minimum order of 2 boards per order. In order to have had a better board which had the silk layer and solder protection, they asked for $270. It is economically not suitable for this project to spend that much on just the PCB layout fabrication. Since we were not able to draw some of the connections on the top layer without intersecting other lines, we used via to drill a hole and use the ground plane for those connections. Those two connections are in green in the PCB layout picture. The following schematic and layout are what we have constructed.

18

Boost Converter Circuit Diagram – Closed-Loop (fig 6.1)

19

Boost Converter PCB Layout (6.2)

Section 7: Simulation Results and Analysis

IC1 – SG3524 (fig 7.1) IC2-IR4427 (fig 7.2)

IC1-SG3524 Vsaw(p-p)=3.25V (pin7) Vcc=11.98V (pin15) Vref=5.105V (pin 16)

20

Saw-tooth of SG3524 measured at pin 7 (fig. 7.3)

IC2-IR4427Vcc=11.98V (pin 6)fpwm=194.9kHz (pin 7)

Square-wave of IR4427 measured at pin 7 at 50% Duty ratio (fig 7.4)

a) Open-Loop Test

21

Boost Converter Circuit Diagram – Open-Loop (fig. 7.5)

i) Vg=20V

For the open-loop test, by manually adjusting the Rpoti potentiometer, we were able to obtain output voltage of 70V. For this part we used Vg=20V. The voltage supply had a current limit of 1A, which protected the circuit from overloading.

Pin=VI=(20V)(1A)=20WVg=20.04VVout=69.5VD=0.71Vds=70.1V

Doing KVL analysis we find that VL=Vin-Vout=20V-70V = -50V and Vds=Vg-VL= (20V)-(-50V)=70V. Hence Vds=Vout when MOSFET is off and the diode is on.

22

Vds waveform of open-loop test for Vg=20V (fig 7.6)

Inductor Current for Vg=20V

is the dc part of the inductor current. The inductor current IL for Vg=20V is shown

below in fig 7.7. For D=0.714, R=245Ω, and Vg=20V IL=1A. The inductor current ripple is

2ΔIL=156mA. Hence ΔIL=78mA, which corresponds to current ripple.

Inductor Current Waveform of open-loop test for Vg=20V (fig 7.7)

Output Voltage Ripple for Vg=20V

From the given specifications, Δvout<0.2% or (70V*0.002=140mV). From our plot we measured that Δvc=96mV, which meets the given requirement. Our measured output voltage ripple,

. Below (fig 7.8) is the output voltage ripple waveform for Vg=20V. The

output voltage ripple is kept small as a result of co1//co2. The two capacitors add up to 100.68uF. co1 is used to construct a low pass filter at the output, while co2 is used to maintain the output voltage at 70V.

23

Output voltage ripple waveform for open-loop test at Vg=20V (fig 7.8)

ii) Vg=40V

Pin=VI=(40V)(1A)=40WVg=40.13VVout=69.5VVds=69.2VD=0.42

Vds waveform for open-loop test at Vg=40V (fig 7.9)



below in fig 7.6. For D=0.714, R=245Ω, and Vg=20V IL=0.5A.The inductor current ripple is


24

Inductor Current Waveform of open loop test for Vg=40V (fig 7.10)


From the given specifications, Δvout<0.2% or 140mV. From our plot we measured that Δvc=110mV, which meets the given requirement. Our measured output voltage ripple,

. Below (fig 7.11) is the output voltage ripple waveform for Vg=40V.

Output voltage ripple waveform for open-loop test at Vg=40V (fig 7.11)

Open-Loop System Efficiency:

Power (W) Vg (V) Ig (A) VR (V) RL (Ω) V0 (V) I0 (A) Pin (W) Pout (W) η5 20 0.278 0.288 980 70 0.071 5.480 5 0.912

10 20 0.553 0.572 490 70.05 0.143 10.743 10.014 0.93215 20 0.814 0.842 326.67 70.1 0.215 15.594 15.043 0.96520 20 1.080 1.118 245 70.03 0.286 20.393 20.017 0.982

25

Power (W) Vg (V) Ig (A) VR (V) RL (Ω) V0 (V) I0 (A) Pin (W) Pout (W) η5 40 0.147 0.152 980 70 0.071 5.858 5 0.854

10 40 0.273 0.283 490 70.08 0.143 10.843 10.023 0.92415 40 0.395 0.409 326.67 70.01 0.214 15.639 15.004 0.95920 40 0.527 0.545 245 70 0.286 20.793 20 0.962

iii) Vg=30V - Discontinuous-Conduction Mode

26

By increasing the load, we can obtain DCM mode. As soon as IL<ΔiL, the converter starts operating in DCM mode. For Vg=30, IL=0.667A. The corresponding load resistance which makes the circuit operate in DCM mode is Rload=410Ω.

CCM and DCM boundary waveform of an open loop at Vg=30V, which occurred at 410Ω load resistance (fig 7.12)

b)Closed-Loop Test

i. Vg=20V

For the closed-loop test, the PWM controller automatically adjusts the duty ratio in order to maintain the output voltage at 70V. Hence the closed-loop configuration acts as a negative feedback as a response to disturbances at the input. For this part we used Vg=20V. The voltage supply had a current limit of 1A, which protected the circuit from overloading.

Pin=VI=(20V)(1A)=20WVg=20.05VVout=69.5VVcc=11.98VD=0.70Vds=70.1V

IC1- SG3524Vinv=-5.74V (pin 1)

27

Vnon-inv=2.512V (pin 2)

Vds waveform for closed-loop test at Vg=20V (fig 7.13)



below in fig 7.6. For D=0.714, R=245Ω, and Vg=20V IL=1A. The inductor current ripple is


28

Inductor Current Waveform of closed-loop test for Vg=20V (fig 7.14)


From the given specifications, Δvc<0.2% or 140mV. From our plot we measured that Δvc=22.65mV, which meets the given requirement. Our measured output voltage ripple,


29

Output voltage ripple waveform for closed-loop test at Vg=20V (fig 7.15)

ii. Vg=40V

Pin=VI=(20V)(1A)=20WVg=20.05VVout=69.5VVcc=11.98VD=0.70Vds=70.1V

IC1- SG3524Vinv= -1.321V (pin 1)Vnon-inv=2.49V (pin 2)

30

Vds waveform for closed-loop test at Vg=40V 9fig. 7.16)



below in fig 7.6. For D=0.714, R=245Ω, and Vg=20V IL=0.5A.The inductor current ripple is

2ΔIL=37.5mA. Hence ΔIL=18.75mA, which corresponds to current ripple.

31

Inductor Current Waveform of closed-loop test for Vg=40V (fig 7.17)Output Voltage Ripple for Vg=40V

From the given specifications, Δvout<0.2% or 140mV. From our plot we measured that Δvc=57mV, which meets the given requirement. Our measured output voltage ripple,


Output voltage ripple waveform for closed-loop test at Vg=40V (fig 7.18)

Closed-Loop System Efficiency:

Power (W) Vg (V) Ig (A) VR (V) RL (Ω) V0 (V) I0 (A) Pin (W) Pout (W) η

5 20 0.27 0.279 980 70 0.071 5.325 5 0.93910 20 0.535 0.554 490 70 0.143 10.404 10 0.96115 20 0.8 0.828 326.67 70.01 0.214 15.338 15.004 0.97820 20 1.1 1.139 245 70 0.286 20.748 20 0.964

32

Power (W) Vg (V) Ig (A) VR (V) RL (Ω) V0 (V) I0 (A) Pin (W) Pout (W) η

5 40 0.139 0.144 980 70.02 0.071 5.540 5.003 0.90310 40 0.274 0.284 490 70.01 0.143 10.882 10.003 0.91915 40 0.401 0.415 326.67 70 0.214 15.874 15.000 0.94520 40 0.533 0.552 245 70 0.286 21.026 20 0.951

33

iii. Vg=30V - Discontinuous-Conduction Mode

By increasing the load, we can obtain DCM mode. As soon as IL<ΔiL, the converter starts operating in DCM mode. For Vg=30, IL=0.667A. The corresponding load resistance which makes the circuit operate in DCM mode is Rload=520Ω.

CCM and DCM boundary waveform of an open loop at Vg=30V, which occurred at 520Ω load resistance (fig 7.19)

Section 8: Conclusion

While the project procedure is quite straightforward, construction and simulation of the circuit design proved to be more challenging than expected. Many difficulties were encountered during the fabrication process, mainly due to inexperience creating PCB layouts using unfamiliar software. A poor layout design resulted in a fabricated circuit board that was inconsistent with the original circuit schematic and, thus, was unusable. Consequently, circuit simulations could not be performed until the faulty circuit board was replaced with a functional one.

Once the circuit board was replaced, it was necessary to solder each component onto the board according to the design schematic. Among these components was an inductor that was created according to a theoretically calculated number of turns. During circuit construction, however, issues arose regarding inaccurate lead spacing, making it difficult to solder certain components to the board. The integrity of the component connections was also compromised by poor soldering tools. In particular, the soldering irons provided did not produce enough heat to

34

completely melt the solder and the solder used was of poor quality. Ultimately, the components were soldered onto the circuit board and the connections were checked for shorts. Following the assembly of the completed circuit, simulations were conducted to verify its functionality by measuring output voltages and currents, making sure that they meet project specifications as well as stay consistent with theoretical calculations. A recurring problem experienced during simulation was adjusting the potentiometer and input voltage to achieve 20-40V. Frequently, doing so resulted in the power supply “overloading” and limiting the voltage to 4-5V. The circuit also stopped working at one point during the test procedure, failing to produce the PWM or saw tooth signal. After debugging and checking for short circuits, the problem was diagnosed to be a burnt IC, which was promptly replaced. Few problems were experienced thereafter. In general, the simulation results stayed consistent with theoretical values, but few discrepancies existed. These discrepancies may have been caused by various factors. Excessive noise, for example, may have been introduced to the test environment, causing incorrect measurements from the oscilloscope. Also, some of the components used are sensitive to static electricity, which they may have been exposed to throughout the project. This may have damaged the ICs, and compromised their functionality.

Section 9: Suggestions to Improve Project

The project assignment introduced many problems and difficulties that may have been avoided with a few changes. Firstly, there was very little guidance in designing the PCB layout, which eventually resulted in an incorrect design sent to fabrication. The software used, Express PCB, was recommended to use for the PCB layout, but very little instruction was given on how to use the software. Also, the equipment used in the lab to test the circuit board was unreliable and inconsistent, which eventually had to be replaced. Having fully functional test equipment would provide more accurate test results and reduce the complications experienced during testing. Lastly, the project abstract specified the design of a boost converter, but due to the complications caused by the boost converter’s instability, it may be better to create a buck-converter.

Section 10: Constraints

Economic: The DC/DC boost converter designed in this project is based on an external inductor, which is relatively expensive compared to inductorless DC/DC converters. As a trade off for high efficiency, the inductor-based converter sacrifices cost, specifically due to the number of components, board size, and design time. Design time for an inductor-based converter is much greater than an inductorless converter since there are various aspects regarding the converter’s design that must be considered including: switching methods, feedback control schemes, and operating modes. Cost is also dependent on the external components chosen. The inductor, output capacitor, and switching diodes/FETs must be chosen in such a way as to reduce cost, while maintaining efficiency and low ripple.

35

Manufacturability: The inductor-based boost converter also suffers from manufacturability constraints, particularly in terms of size and number of components. As discussed, the inductor-based DC/DC converter utilizes many external components and requires a more complex design. Also, the current design of the inductor-based boost converter will be compromised by future technology improvements.

Team Members: Alan Nguyen Issa Frayeh Sarkis Arabyan Mentor

Documents

Transcript of Team Members: Alan Nguyen Issa Frayeh Sarkis Arabyan Mentor