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


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,,


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


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


Some examples showing the effects of K (k ) andSome examples showing the effects of Kdist (kd) and Kdisp (kφ) on PF and THD


Passive Power Factor Corrector

Series‐tuned LC harmonic fil PFfilter PF corrector

Low‐pass inductive filter

Inductive input PF corrector


Inductive‐input PF corrector

Shunt and series active filters


PFCs are usually cascaded with loadPFCs are usually cascaded with load



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.( )


(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].


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


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.


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


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


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.


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:


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


Study pros and cons of CCM, DCM and CRM PFCs from the reference book.

Overview of PFC circuit topologies


Principle of CCM


(A) the input current waveform of the PFC circuit and (B) the line current waveform.

Principle of DCM


(A) the input current waveform of the PFC circuit and (B) the line current waveform.

Principle of CRM

Department of EEE, KUET 24(A) the input current waveform of the PFC circuit and (B) the line current waveform.

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.


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.


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


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.


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


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


Bl k di f PFC t ithBlock diagram of PFC converter with CCM shaping technique


CCM operation of boost PFC


Boost corrector using average current control.

k i 2 d lBuck corrector using I2 average current mode control.


Comparison of CCM shaping techniques


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


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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EE 6904 Ad dAd vanced Power El iEl ectronics

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