Q4 Flashcards
1
Q
Q4. slide 1 main ideas
A
- Reason for using PFC
- Benefits of using PFC: PF and THD
2
Q
why do we need PFC?
A
- 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.
3
Q
benefits of PFC
A
- 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
4
Q
Q4. slide 2 main ideas
A
- 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
5
Q
describe structure of common-single phase PFC
A
- consists of a full-bridge rectifiver, boost converter, and the controller.
6
Q
justify need of infinity voltage gain for PFC
A
- 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
7
Q
reasons for using boost converter as pfc
A
- 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.
8
Q
main control targets of PFC
A
- inductor I
- output voltage
9
Q
Q4. slide 3 main ideas
A
- 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
10
Q
general idea of control on inductor I
A
- 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
11
Q
characteristics of average-current-mode control
A
- 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
12
Q
advantages of average-current-mode control
A
- 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
13
Q
Q4. slide 4 main ideas
A
- 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
14
Q
explain why voltage-control loop must set the input of peak inductor I
A
- 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
15
Q
why output C of PFC must be large
A
- 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
16
Q
why voltage loop must have low bandwidth
A
- to attenuate component at the 2nd harmonic frequency of output I and avoid third-harmonic distortion that can affect I control loop
17
Q
Q3. slide 5 describe PFC schematic with average current control
A
- 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
18
Q
Q4. slide 6
A
- Current controller
- Voltage controller
- Switching frequency
- Operation mode
- Output voltage level
19
Q
characteristics of CrCM of PFC
A
- 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
20
Q
Characteristics of CrCM flyback as PFC
A
- 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
21
Q
Characteristics of bridgeless PFC
A
- 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
