Experiment 4 Introduction to Photovoltaic Systems and Power Electronics

ECEN 4517

Team Members:

Ali Abu AlSaud Hassan AlAhmed

Tuesday’s Lab - Bench 2

Date Performed: April 4, 2017 Instructor: Professor Khurram Afridi

Table of Contents 1. Objectives 3

2. Analog Pulse-Width Modulator 3 2.1. Controller Circuit Construction 3 2.2. Controller Circuit Demonstration 4

3. Open-Loop Power Stage 12 3.1. Inductors Construction 12 3.2. Two-stage Boost Converter Construction 15 3.3. Boost Converter Testing 16

4. Feedback Controller 20 4.1. Transfer Function Measurement 20 4.2. Transfer Function Simulation 22 4.3. Compensator Circuitry Design 24 4.4. Feedback loop testing 27 4.4.1 Closed-Loop Voltage Regulation: Load Test 27 4.4.2 Closed-Loop Voltage Regulation: Variations in Input Voltage 29 4.4.3 Soft Start 31

5 Conclusion 32

1. Objectives Design, construct, test, and demonstrate a step-up dc-dc converter that converts the

low-voltage dc produced by the lead-acid battery to high-voltage dc as needed by the dc-ac inverter.

Design, construct, test, and demonstrate a closed-loop analog feedback loop that regulates the dc output of the boost converter.

This report is organized in the same order as Experiment 4’s procedure document.

2. Analog Pulse-Width Modulator In this section, a circuit around the UC3525A chip will be designed and implemented in

order to achieve an output PWM signal from the UC3525A chip, such that the gate drive signal applied to the gates of the power MOSFETs in order to have a 100 KHz switching frequency. The circuit includes a potentiometer that allows adjusting the duty cycle. Note that the maximum duty cycle is limited to a point that is less than 100% (it is limited to 92%). Also, the circuit includes bypass capacitors on all power supplies. In addition, the following points were taken into consideration and were designed in the circuit:

Between the positive supply and ground terminals of the analog controller circuit board, there is an electrolytic capacitor of 10µF.

For the UC3525A chip, a ceramic capacitor of 0.1µF is connected between each power supply pin and the ground pin as close as possible to the chip.

2.1. Controller Circuit Construction In order to change the duty cycle, a potentiometer and a voltage divider were used

between V_REF and NI Input pins. To calculate the switching frequency, the following relationship were used:

f = 1C (0.7 R + 3 R )t t d

By choosing:

= 1nFC t = 12k Rt Ω = 0 (this is a not required discharging resistor)Rd Ω

The following is obtained:

kHzf = 110 (0.7 (12 10 )+ 3 (0))−9

*3 ≈ 1

The following controller circuit was implemented in order to achieve 85 kHz switching

frequency:

Figure 1: PWM Controller circuitry

2.2. Controller Circuit Demonstration After constructing the controller circuit, the circuit was tested. The following results were

collected when the duty cycle is 60%. The following are the voltage waveforms in all the pins of the UC3525A chip:

Figure 2: Pin 1

Figure 3: Pin 2

Figure 4: Pin 3

Figure 5: Pin 4

Figure 6: Pin 5

Figure 7: Pin 6

Figure 8: Pin 7

Figure 9: Pin 8

Figure 10: Pin 9

Figure 11: Pin 10

Figure 12: Pin 11

Figure 13: Pin 12

Figure 14: Pin 13

Figure 15: Pin 14

Figure 16: Pin 15

Figure 17: Pin 16

3. Open-Loop Power Stage In this section, a two-stage boost converter will be designed, implemented, and tested. To

do that, two inductors need to be constructed, as well as building the boost. The two-stage boost converter must convert 12V to 200V. To do that, and by having a common duty cycle for both boosts, the duty cycle is calculated as follows: The relationship between the output voltage of the boost converter and the output voltage is given by:

Using the previous relationship twice, the duty cycle is calculated. The calculation is

shown below:

In addition, a switching frequency of 85 kHz is used from the controller circuit.

3.1. Inductors Construction Before implementing the boost converter, two inductors, one for each stage, were

designed and constructed. For the first inductor, L1, the PQ 26-25 core was used. Some of the core’s parameters are

provided in the following table

Parameter Value

Ac 1.18 cm2

WA 0.503 cm2

MLT 5.62 cm

Bmax 0.33 T

Ku 0.55

In addition to the core’s parameters, the following table has some important parameters in order to design the inductor

Parameter Value

Frequency 85 kHz

Duty Cycle 755%

Input Voltage 12V

Inductor ripple ( ΔiL ) 10%

Inductor current( iL ) 7 A

The inductor value was calculated by using the following equation:

For the first inductor, L2, the PQ 32-20 core was used. Some of the core’s parameters are provided in the following table

Parameter Value

Ac 1.70 cm2

WA 0.471 cm2

MLT 6.71 cm

Bmax 0.33 T

Ku 0.55

In addition to the core’s parameters, the following table has some important parameters in order to design the inductor

Parameter Value

Frequency 85 kHz

Duty Cycle 755%

Input Voltage 50V

Inductor ripple ( ΔiL ) 10%

Inductor current( iL ) 1.715A

The inductor value was calculated by using the following equation:

After having the values for both inductors, the inductors were designed. To do that, the

following equations were used to design both inductors:

For the first inductor, the following parameters were calculated using the equations

above:

Parameter Value

lg 0.12 mm

N 4.7

Awire 0.055 cm2

AWG 12

For the second inductor, the following parameters were calculated using the same

equations above:

Parameter Value

lg 0.13 mm

N 19.7

Awire 0.013 cm2

AWG 16

After some iterations, both inductors values were successfully reached, and the values

were checked on the RLC meter.

3.2. Two-stage Boost Converter Construction Before constructing the two-stage boost converter, two power MOSFETs and two

Schottky diodes were mounted on the heat-sinks. Insulators were used to insulate the MOSFETs and the diodes cases from the heat-sink. A thin layer of thermal paste was used also on both sides of the insulator to ensure good thermal conduction. After that, the two-stage boost converter was implemented. The following diagram shows the two-stage boost converter circuitry.

For the power stage of the converter, #18 AWG wire was used to make the

interconnections in the power stage. In addition, the distances between the input voltage, transistor, and the diode as well as the distance between the PWM chip and the MOSFETs were minimized to reduce the parasitic inductance. In addition, a large bypass capacitors was used in the input of the power stage for both stages of the converter. Finally, the power stage was implemented. The bypass capacitors used in the boost converter are capacitors that have a large voltage rating; since the voltage of the boost will go up to 200V.

3.3. Boost Converter Testing After constructing the two-stage boost converter, the boost converter was tested. Initially,

the boost was tested with a small input voltage and a large load; to ensure that the voltage and the current don’t go high. When the input voltage is 5V and the duty cycle of the controller is 34%, the following waveforms were recorded:

Figure 18: Drain to Source waveform of Q1

Figure 19: Drain to Source waveform of Q2

Figure 20: Waveform of the output voltage

After testing and ensuring that the boost is working with low voltages, the boost was

tested with 12V and with a load that makes the input current 7A to set the input power to 85W (the PV panel rated power). When doing that, the following waveforms were collected:

Figure 21: Output of the boost at 85W

Figure 22: Drain to Source signals of both Q1 and Q2

Figure 20: PWM signal for reaching 200V

Observation: At high output voltage, 200V, the waveforms look clean. Even though

there are some spikes on the output of the boost converter, the spikes have small amplitude, which will not affect the output of the boost converter. In addition, it is observed that when connecting the PWM signal to the boost, the fall time decreases slightly. However, after some adjustments, the PWM signal looks a lot cleaner (see figure 20).

From the waveforms above, at an input of 85W, the output voltage was 195V, and the output current was 370mA. The output power can be found as:

In addition, the efficiency of the two-stage boost converter can be calculated as:

4. Feedback Controller In this section, a feedback controller will be designed, constructed, and tested. The

purpose of the feedback controller is to regulates the dc output voltage of the two-stage boost converter to a constant value regardless of the load or the input voltage variations.

4.1. Transfer Function Measurement In order to measure the transfer function, the converter needs to operate open-loop, as

shown in section 3. A network analyzer will be used to measure the transfer function of the converter. Before connecting the converter to the network analyzer, the output load and the duty cycle were adjusted to make the output voltage 170V and the input power equals 85W. The duty cycle that does that is 73%. After that, the boost converter was connected to the network analyzer as shown below.

The following is an oscilloscope screenshot of pin 5 of the UC3525 chip:

When connecting the boost converter to the network analyzer as shown above, the following transfer functions were recorded:

Figure 21: Frequency Amplitude of the Boost

Figure 22: Phase of the Boost

Observation: The frequency amplitude of the boost looks as expected. However, the

phase of the boost looks weird. In addition, from the plot above, the following data was collected:

Parameter Value

DC Gain 36.4dB

Corner Frequency 500 Hz

Q-Factor 39.6dB

Zeros 3.17 kHz

From the measured transfer function and the data collected above, the transfer function

can be written as:

4.2. Transfer Function Simulation After finding the transfer function experimentally, the transfer function was found using

LtSpice. The following circuit was built in LtSpice in order to do the simulation.

Figure 23: LtSpice boost circuit

The circuit was tested with frequencies that start with 150 Hz and stop with 15 kHz.

When doing that, the following plot was recorded:

Figure 24: Results from the simulation

Observation: From the plot above, the following data was collected:

Parameter Value

DC Gain 64dB

Corner Frequency 342 Hz

Q-Factor 81.5dB

Zeros 4 kHz

From the simulated transfer function and the data collected above, the transfer function

can be written as:

Comparison between measured and simulated results: In general, the simulated and

measured values are very similar to each other with the observation that the measured values are slightly smaller than the simulated ones. Specifically, the simulated DC gain is almost twice as the measured DC gain. In addition, the measured corner frequency is similar to the simulated one (a difference of about 100 Hz). The simulated Q-factor is a lot bigger than the measured one (the simulated Q-factor is almost twice the measured Q-factor). Finally, the zeros are very close to each other with a difference that is less than 1 kHz.

4.3. Compensator Circuitry Design After measuring the transfer function using the network analyzer, the transfer function

will be used to design and construct the compensator. The compensator design used in this document is the PD design. The PD design looks like the following:

To do that, several parameters need to be calculated. To begin with, the DC gain needs to be converted from decibels to gain. To do that, the following equation is used:

Using the previous equation, the DC gain is equal to 66. Also, fz and fp have to be

calculated. Both frequencies can be calculated using the following equations:

Using the previous equations, some resistors and capacitors values can be calculated in

order to design the compensator. The following equations show how to calculate the values of the resistors and the capacitors:

Using these equations and picking R1 to be 100 ohm, the following table shows all the parameters and their corresponding values:

Parameter Value

GC0 66

fZ 176.77 Hz

fP 1414.21 Hz

R1 100Ω

R2 6.6 kΩ

C1 9 uF

C2 0.17 nF

Putting everything together, the compensator circuit looks like the following:

The potentiometer is used as a voltage divider in order to change the duty cycle of the boost converter. For the sensing part, HV, a voltage divider was used to pull the 170V down to approximately 5V.

4.4. Feedback loop testing After constructing the compensator, the compensator was tested. After changing the duty

cycle and the input voltage slowly, the following plot was recorded:

Figure 25: Compensation duty cycle and output

Observation: From the plot above, the output voltage is around 170V, which is the voltage that the compensator designed to reach. In addition, the switching frequency went up to 300 kHz. Even though the switching frequency went very high, the frequency is acceptable since it is not too high since it shouldn’t affect the efficiency. In other words, if the switching frequency was greater than 500 kHz, it needed to be adjusted since it will make the efficiency lower significantly.

4.4.1 Closed-Loop Voltage Regulation: Load Test After ensuring that the boost works with the compensator circuitry, the boost converter

was tested with different loads that gives high power. The following plots show some readings with different loads. Note that the input and output power are less than 85W; and that’s because there were no available power resistors in the lab that can supply 85W.

Figure 26: Boost output reading

Figure 27: Boost output reading

In addition, the following table shows the input and output voltages and currents, as well

as the input and output power and the efficiency of the boost converter:

Vg Ig Pg Vo Io Po Efficiency

12V 6.29A 75.48W 179V 0.35A 58W 76.84%

12V 4.3A 51.6W 178W 0.25A 41.6W 80.62%

Observation: When using the compensator circuitry, the efficiency of the boost went

down a little bit. The reason behind that is that the loss of the opamp is added. In addition, the compensator was a little bit far away from the boost converter, which decreased the efficiency a little bit. However, the output voltage was a lot more constant than using the boost converter with open-loop conditions.

The closed-loop voltage regulation can be calculated using the following equation:

Using the two readings above, the closed-loop regulation is equal to 0.56%. The

closed-loop voltage regulation is very small because the compensator gain at DC and low frequencies is very large.

4.4.2 Closed-Loop Voltage Regulation: Variations in Input Voltage After testing the boost converter and the compensator circuit with an input of 12V, the

system was tested with different input voltages between 11V and 14V. When the input voltage is 14V, the system was tested with two different loads. The following plots show some readings with different loads when the input voltage is 14V. Note that the input and output power are less than 85W; and that’s because there were no available power resistors in the lab that can supply 85W.

Figure 28: Boost output reading

Figure 29: Boost output reading

In addition, the following table shows the input and output voltages and currents, as well

as the input and output power and the efficiency of the boost converter:

Vg Ig Pg Vo Io Po Efficiency

14.05V 3.86A 54.233W 171V 0.25A 43W 79.29%

14.05V 4.02A 56.48W 172W 0.26A 43W 76.13%

Observation: The duty cycle that gives 170V from the 14V is 90%, which is very similar

to the duty cycle that gives 170V when the input is 12V. Also, when the input voltage increases, the efficiency of the boost converter decreases slightly. In addition, when changing the input voltage, the output voltage almost stays the same.

The closed-loop voltage regulation can be calculated using the following equation:

Using the two readings above, the closed-loop regulation is equal to 0.58%. The

closed-loop voltage regulation is very small because the compensator gain at DC and low frequencies is very large.

4.4.3 Soft Start In order to prevent a large inrush current during the startup transient that can overload the

input power source or damage the converter power components, a soft start circuitry is used. The soft start circuitry consists of a capacitor that is connected between the soft start pin, which is pin 8, of the UC3525 chip and ground. The capacitor value used is 10µF. The following figure shows the entire PWM circuit including the soft start circuit.

Figure 30: Soft Start Circuit

The following capture shows the start-up output voltage of the PWM signal:

Figure 31: Start-up output voltage of the PWM signal

When using the 10µF capacitor, the duty cycle reaches the nominal value after a little less

than 1 second.

5 Conclusion To sum up, a PWM control circuit was designed and implemented successfully. In

addition, a two-stage boost converter that is capable of boosting 12V to 200V was constructed

successfully. The boost converts the power with an efficiency of almost 85%. After that, a compensator circuit was designed and implemented successfully. When using the compensator circuit, the efficiency went a little bit down, and that is because the compensator was a little bit far away from the boost converter.