EE 6904 Ad dAd vanced Power El iEl ectronics
Transcript of EE 6904 Ad dAd vanced Power El iEl ectronics
EE 6904 Ad d P El iEE 6904: Advanced Power Electronics
Lecture 06
Power Factor Correction (PFC)DCM and CCM operations of buck andDCM and CCM operations of buck and
boost PFCs
Dr. Md. HabibullahAssociate Professor, EEE, KUET
Power Factor Correction (PFC)(Power conditioning)
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Some examples of nonlinear loads are rectifier pc cell phone VSD printer
Nonlinear loads draw harmonic current from the power supply; this is
Some examples of nonlinear loads are rectifier, pc, cell phone, VSD, printer, TVs, telecom systems that use SMPS etc.
Nonlinear loads draw harmonic current from the power supply; this is bad for power system.
PFC is needed to reshape the input current as the supply voltage
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PFC is needed to reshape the input current as the supply voltage.
Effect of PFC circuit integration in SMPSSMPS switch mode power supplySMPS‐switch mode power supply
Pulsating sinusoidal
Input characteristics of a typical
Pulsating input current
sinusoidal input current
Input characteristics of a typical switched‐mode power supply without PFC
Input characteristics of a power supply with near‐perfect PFC
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Definition of PF and THDPF gives a measure of how effective the real power utilization in the system is. It also represents a measure of distortion of the line voltage and the line current and phase shift between them.
For sinusoidal voltage and current,
For non‐sinusoidal voltage and current, Look at the wave shapes, current is very much distorted and no longer sinusoidaldistorted and no longer sinusoidal
For a symmetrical case, both IDC and VDC are zero
Hence,,
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Since the supply voltage is sinusoidal
for power electronics system
Depends on load type
p y
Hence improvement of PF in a power electronics system will improve the currentquality in terms of harmonics and phase displacement with respect to supplyvoltagevoltage
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Total Harmonic Distortion (THD)Another important parameter which measures the percentage ofAnother important parameter which measures the percentage of current distortion is known as the current total harmonic distortion (THDi), which is defined as follows:
Typical waveform of poor PF rectifier system
This shows how PF and THD are interrelated
THD 55‐65%PF=0.65
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Some examples showing the effects of K (k ) andSome examples showing the effects of Kdist (kd) and Kdisp (kφ) on PF and THD
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Passive Power Factor Corrector
Series‐tuned LC harmonic fil PFfilter PF corrector
Low‐pass inductive filter
Inductive input PF corrector
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Inductive‐input PF corrector
Shunt and series active filters
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PFCs are usually cascaded with loadPFCs are usually cascaded with load
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Active Power Factor Corrector
Buck corrector
Comparing with the other type of high‐frequency PFC circuits, the buck corrector offersinrush‐current limiting, overload or short‐circuit protection, and overvoltage protection forinrush current limiting, overload or short circuit protection, and overvoltage protection forthe converter due to the existence of the power switch in front of the line. Anotheradvantage is that the output voltage is lower than the peak of the line voltage, which isusually the case normally desired.y y
(a) When the output voltage is higher than the line voltage, the converter draws no current from
Drawbacks:
the line, resulting in significant line current distortion near the zero‐across of the line voltage.(b) The input current is discontinuous, leading to high differential mode EMI.(c) The current stress on the power switch is high.( )
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(d) The power switch needs a floating drive.
Some Other Active Power Factor Correctors
Boost corrector Buck‐Boost corrector
Cuk corrector SEPIC corrector
Also see the zeta and flyback correctors from the reference book [1].
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PFC principle
Where, is constant value that is presenting the average of the instantaneous input power pin(t).
It is important to clarify that only this part (Pin) of power is contributing for the production of the effective energy that can be used by the load; therefore, it is often called the real
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power.
The instantaneous input power would be equal to the average output power, i.e.
Simplifying the above expression, we can get the instantaneous output voltage as follows
It can be seen that the output voltage contains a constant average voltage and a double line frequency sinusoidal ripple voltage with p‐p amplitude Viniin/2voutωlc.
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in in out l
Energy balance in PF corrector
Id l t t f d /d tIdeal output of a dc/dc converter
It can be seen clearly that the converter has to transfer a peak power equals to twice the average output power, whereas this power has to be stored and then restored to the converter
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output side.
The operation principle of a PFC circuit is to process the input power in a certain way that it stores the excessive input energy when pin(t) is greater than Po and releases the stored energy when p (t) is less than Po
We can estimate the instantaneous excessive power as
energy when pin(t) is less than Po.
Hence, the instantaneous excessive input energy, wex(t) is given by
The PFC circuit has to ensure this both functionality of storing the excessive energy andThe PFC circuit has to ensure this both functionality of storing the excessive energy andrealizing the stored excessive energy by integrating the basic dynamic components (inductorand capacitor) in the PFC circuit.
The energy stored in the inductor (1/2LiL 2 (t)) cannot completely match the change of theexcessive energy. Therefore, to maintain the output power constant, another energy storagecomponent (usually the output capacitor) is needed
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component (usually the output capacitor) is needed.
System configurations of PFC power supply
(A) two stage scheme (B) one stage(A) two‐stage scheme, (B) one‐stage scheme, and (C) parallel scheme.
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See the reference book [1] for advantages and disadvantages of each PFC configuration
The relation between P1 and P2 based on the calculation of the areas corresponding to each power is:
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Overview of PFC control techniques
The primary objective of PFC is to force the inputcurrent to trace the shape of line voltageStudy pros and cons of CCM DCM and CRM PFCs from
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Study pros and cons of CCM, DCM and CRM PFCs from the reference book.
Overview of PFC circuit topologies
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Principle of CCM
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(A) the input current waveform of the PFC circuit and (B) the line current waveform.
Principle of DCM
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(A) the input current waveform of the PFC circuit and (B) the line current waveform.
Principle of CRM
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DCM operation PFC circuits
To simplify the analysis of different topologies of converter working in DCM forhi i th i bj ti f t h i th f ll i ti
DCM operation PFC circuits
achieving the main objective of current shaping, the following assumptions weremade:
1. The converter is lossless.
2. The duty ratio of the converter is kept constant throughout the entire line cycle.
3. The switching frequency is much higher than the line frequency.
4 The leakage inductances of transformers are negligible4. The leakage inductances of transformers are negligible.
5. Despite of slight changes in DC bus voltage, it is considered to be constant during
each line cycle.
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DCM operation of buck PFC
Input I‐V characteristic of basic buck converter operating in DCM: (A) buck converter, (B) input current, and (C) input I‐V characteristic.
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The instantaneous output current of the rectifier is presented as follows:
The average output current of the rectifier in one switching cycle is given by
The average line current or the average input current of the rectifier during one switching cycle is given as follows:
the basic buck converter cannot fulfill the requirement of a good candidate that can be used
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to achieve the DCM input PFC.
DCM operation of boost PFC
Input I‐V characteristic of boost converter operating in DCM: (A) boost converter, (B) input current, and (C) input I‐V characteristic.
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The instantaneous input current of the rectifier is presented as follows:
The average instantaneous input current of the boost converter in one switching cycle is given as follows:
Using volt sec balance of theUsing volt‐sec balance of the boost inductor L
The average line current or the average input current of the rectifier during one switching cycle is given as follows:cycle is given as follows:
boost converter can operate properly only when the output voltage is higher than its input
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voltage. When low‐voltage output is needed, a step‐down DC‐DC converter must becascaded.
Comparison of basic converter topologies operating for DCM input h itechnique
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Bl k di f PFC t ithBlock diagram of PFC converter with CCM shaping technique
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CCM operation of boost PFC
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Boost corrector using average current control.
k i 2 d lBuck corrector using I2 average current mode control.
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Comparison of CCM shaping techniques
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References
1. Power Electronics Handbook, Fourth Edition‐‐‐‐‐‐‐‐‐‐‐ by M. H. Rashid
2. Lecture series from Prof. Mehdi Ferdowsi: https://www.youtube.com/watch?v=5YtIpfc9YNM
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Assignment: Explain the principles of DCM operation of buck boost PFCAssignment: Explain the principles of DCM operation of buck‐boost PFCDeadline: 28/06/20 (before lecture 07)
Lecture 07Applications of DC/DC converters in SMPS, UPS, welding and lighting
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