Q4

Question 1

Q4. slide 1 main ideas

Answer 1

• Reason for using PFC
• Benefits of using PFC: PF and THD

Question 2

why do we need PFC?

Answer 2

• In a single-phase full bridge rectifier i.e. input I is highly distorted
• diodes of bridge conduct only during small interval of time in each cycle when input V is higher than ouptut V, resulting in non sinusoidal waveforms and large peak I.
• large distortion in I causes high THD and low PF.

Question 3

benefits of PFC

Answer 3

• allows to achieve PF close to 1. Most of power drawn from grid is used effectively and minimum I is required to transfer certain amount of power, transmission losses are reduced.
• THD is reduced, allowing to meet standards of harmonic distortion at points of common coupling

Question 4

Q4. slide 2 main ideas

Answer 4

• structure of common single-phase PFC: full-bridge rectifier+boost converter.
• derivation of equations for voltage gain
• Justification for using boost converter: infinity voltage gain, continuous input I.
• Considerations of output voltage>input voltage

Question 5

describe structure of common-single phase PFC

Answer 5

• consists of a full-bridge rectifiver, boost converter, and the controller.

Question 6

justify need of infinity voltage gain for PFC

Answer 6

• assuming an ideal PFC, input I should be proportional to ac input voltage.
• The input voltage is sinusoidal and rectified voltage is absolute value of this sinusoidal voltage
• a constant dc output voltage is desired at the output of the converter, voltage gain M is equal to output voltage divided by rectified voltage.
• rectified voltage varies between 0 and Vm, voltage gain M must vary between infinity and Vo/Vm

Question 7

reasons for using boost converter as pfc

Answer 7

• since rectified voltage varies between 0 and Vm, voltage gain of converter must vary between infinity and Vo/Vm
• boost converter ideally can produce a voltage gain between 1 and infinite
• input I of boost converter is continuous, there is a path during on-state and off-state of switch, it helps to shape input I to nearly a sinusoidal waveform, achieving very high PF.

Question 8

main control targets of PFC

Answer 8

• inductor I
• output voltage

Question 9

Q4. slide 3 main ideas

Answer 9

• inductor I is main control target of PFC since it is equal to absolute value of ac input I
• general idea of control on inductor I
• characteristics of CCM operation for PFC
• advantages of CCM operation

Question 10

general idea of control on inductor I

Answer 10

• when inductor I is below the sinusoidal reference I, switch is closed so that inductor I increases
• when inductor I is above reference I, switch is opened so that inductor I decreases
• so in general at each switching cycle average inductor L is equal to the reference I, method known as average-current mode control

Question 11

characteristics of average-current-mode control

Answer 11

• requires higher bandwidth, much higher than grid frequency so that inductor I follows reference with minimum error
• boost converter must operate in CCM, inductance is large and I ripple is relatively small

Question 12

advantages of average-current-mode control

Answer 12

• input current has almost sinusoidal waveform with small ripple
• very high PF and low THD can be achieved
• peak I smaller compared to other techniques

Question 13

Q4. slide 4 main ideas

Answer 13

• output voltage is the second control target since usually a DC-DC converter is connected to PFC and it is neede to keep output voltage within a specified range
• voltage control-loop to determine peak of inductor I
• Output C
• Bandwidth of voltage loop control

Question 14

explain why voltage-control loop must set the input of peak inductor I

Answer 14

• if inductor I is insufficient for given load, output voltage decreases and error increases
• therefore, voltage controller increases the magnitude of the reference I signal to increase output voltage

Question 15

why output C of PFC must be large

Answer 15

• since output I consists of dc component and ac component at twice the grid frequency
• C must be large to attenuate the ripple caused by 2nd harmonic of output I

Question 16

why voltage loop must have low bandwidth

Answer 16

• to attenuate component at the 2nd harmonic frequency of output I and avoid third-harmonic distortion that can affect I control loop

Question 17

Q3. slide 5 describe PFC schematic with average current control

Answer 17

• input I is measured by sensing voltage across a shunt R and usually amplified with a OA
• output of OA compared to reference signal generated by multiplier block to produce error signal
• using the error signal the current controller produces the control signal that is compared to a triangular waveform to drive the switch

Question 18

Q4. slide 6

Answer 18

• Current controller
• Voltage controller
• Switching frequency
• Operation mode
• Output voltage level

Question 19

characteristics of CrCM of PFC

Answer 19

• Constant on-time, variable off-time
• MOSFET is switched on when inductor I reaches zero
• Peak inductor current depends on applied input voltage
• Average inductor I equal to 1/2 of peak
• Small inductor value and size
• Control is simpler and low-cost ICs are available
• Higher peak I and additional EMI filtering

Question 20

Characteristics of CrCM flyback as PFC

Answer 20

• Behave as a R when operated in DCM or CrCM
• Constant on-time, variable off-time
• average input I equal to 1/2 of peak I in each switching cycle
• Improves PF
• achieves voltage regulation
• provides electrical isolation
• reduces number of components
• Higher peak I
• Higher filter requirements

Question 21

Characteristics of bridgeless PFC

Answer 21

• reduce number of conducting elements and therefore conduction losses
• reduces number of elements
• input I is continuous, allowing to achieve very high PF
• complex voltage sensing
• complex current sensing
• No electrical isolation