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WORKSHOP 3 POWER ELECTRONICS ELEN90075

1. Report You must show the completed tasks to your workshop demonstrator to get the full in workshop mark (40% of the total mark of each workshop) The report should have a brief executive summary detailing what has been observed in the workshop. The remaining part of the report should consist of the outcome of the tasks below (plots, measurements, etc.). (60% of the total mark of each workshop) 2. Closed Loop Control of the Buck Converter In this workshop we design the closed-loop controller for the Buck converter that powers a Raspberry Pi. To achieve this we investigate each element of the feedback loop of Figure 1. 2.1. Reference Voltage. “That’s some catch, that Catch-22,” he observed. “It’s the best there is,” Doc Daneeka agreed. In this section we investigate the following conundrum: Who gave us the first “clean” reference voltage? Answer: Not Prometheus. Power input

Switching converter

Load +

vg

H (s )

v

G

− iload Transistor Gate Driver

Compensator d

PWM

Gc (s)

vc

- v ve

+

vref

Figure 1. The buck converter with feedback. 1

Sensor

− +

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(1) Use an LT1004-2.5 zener diode along with appropriately chosen resistor and capacitor as described in Figure 2 to generate a reference signal. (The VC C voltage is the voltage that powers all the op-amps, gate-driver, and the PWM chip.) VCC Rref vref Cref

LT1004-2.5

Figure 2. A reference voltage (2) Why does this circuit work? What is the value for the reference voltage that is obtained via this circuit? (3) Explain why this circuit cannot be used as a voltage source. 2.2. Output Sensor. Now we focus on realising the sensing circuitry used to measure the output of the converter. (1) Given the value of the reference signal obtained in the previous section and the output voltage requirement from the previous workshop, design a sensing circuit that measures the output v . (2) What is the transfer function, H(s), of the sensing circuit? 2.3. Compensator Design. In this section we realise the compensator circuit and describe the desired transient properties of the circuit. (1) What is the expected Q of the transfer function of the system for different output loads? (2) Using the averaged circuits analysis plot the magnitude and phase asymptotes of the uncompensated (Gc (s) = 1) loop gain for the typical bare-board active current consumption scenario. (3) It is desired to design the a compensator that keeps the cross over frequency constant while maintaining a phase margin of at least 52○ . Design a PI compensator that accomplishes this. Sketch the magnitude and phase asymptotes of the resulting loop gain, and its label important features. Note that the low frequency gain should be large enough, e.g. 20 dB. (4) For the circuit depicted in Figure 3, it is known that Vo =

Z2 (s) (V1 − V2 ). Z1 (s)

Choose appropriate resistances and capacitances to construct appropriate impedances such that the lead compensator calculated above can be realised as the circuit of Figure 3.

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(5) Replace the compensator block Gc (s) with this circuit, as well as the PWM and H(s) with circuits constructed earlier in this workshop along with the circuit constructed in the prevous workshop and plot the output voltage of the circuit and the control input. (6) Vary the value of Cref from ∼ 10 nF to ∼ 10 µF (logarithmically) and observe the output. What role does Cref play? (7) What happens if the input voltage source is corrupted by a sinusoidal disturbance with frequency 10 Hz and amplitude of 1 V? Is the output voltage ripple specification satisfied? Z1 (s)

Z2 (s)

V2

− + +

Vo −

Z1 (s) V1 Z2 (s)

Figure 3. An op-amp subtractor circuit....