Chapter 5: Power Electronics: Devices and Circuits potx

Power electronics is all about using electronic devices and circuits with storage ement to control the level of voltage and current, either in the form of AC or DC.. Owing to the intrins

Power Electronics: Devices and Circuits

5.1 Introduction

Power electronics is an enabling technology for all electrical and electronic apparatusrequiring electric power to drive Over the past twenty years, the power electronicsindustry has grown tremendously Its growth is a result from increasing demand of re-liable, efficient, compact and cost effective power supplies for telecommunication, com-puter, and motor drive industries as well as for medical equipments and military use.This growth is facilitated by the significant improvement in semiconductor technology

in which smaller packaging and higher power handling devices have been marketed Inresponse to the advancement in semiconductor and magnetics technology, power elec-tronics researchers and engineers have strived to thoroughly employ these technologiesthrough new circuit design and topologies, optimized control and packaging techniques,

in order to meet the industry demands

Power electronics is all about using electronic devices and circuits with storage ement to control the level of voltage and current, either in the form of AC or DC Powerelectronics circuits are switching converters with periodic switching actions to processthe electrical energy to meet the design specification Apart from semiconductors, in-ductor and transformer are the critical magnetic components in the power switchingconverter Their functions such as storage element, power splitting, and safety isolation

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5.2 Electrical Energy Conversion by Switching

The characteristics of power conversion by power electronics converters are summarized

4 Power Electronics technologies are based on switching on and off the power source

by power semiconductors The electrical energy conversion process can be cisely controlled in a manner far much better than electromechanical devices

pre-5 Power Electronics applications include power supplies for computers, cation equipment, machine drives, lighting, automobile and many applications

communi-6 Electrical energy conversion can be classified into the following four categories :

AC to AC, AC to DC, DC to DC, and DC to AC

Figure 5.1: Four categories of electrical energy conversion.

Figure 5.2: Diode: (a) Symbol, (b) I-V characteristic, (c) idealized characteristics.Sources: Mohan 1995 [2]

Figure 5.3: Diode switching characteristics Sources: Mohan 1995 [2].

5.3 Power Semiconductor Devices as Switches

5.3.1 Diodes

A diode performs as a switch It is driven by the voltage applied across its two terminals:anode and cathode Fig 5.2(a) shows the symbol of a diode A is the anode, the positiveterminal K is the cathode, the negative terminal When a diode is forward biased, vd

is positive (i.e potential at A is higher than K), the arrow shows the direction of thediode current iD When the diode conducts, a small forward voltage drop denoted as

VF is established and the magnitude is usually around 1V When the diode is reversebiased, it is blocked and the diode current becomes slightly negative This is due to thecontribution of reverse saturation current For example, 1N4004 has a reverse current

of 50µA And this reverse current is of temperature-dependent; when temperature ishigher the reverse current is increased and vice versa The reverse voltage applied on

a diode has a limit Beyond the limit the diode will breakdown and becomes a shortcircuit This limit is usually called the peak inverse (or reverse) voltage

Another interesting fact of semiconductor is that it can handle repetitive pulsecurrent which has a magnitude much higher than the continuous diode current Forexample, 1N4004 has a maximum forward current at 1A but its allowable repetitivepulse current is at 10A This property is of particular interest to power electronicscircuit because of its switching nature

Owing to the intrinsic resistance, inductance and capacitance of a diode, it riences voltage overshoot (especially in power diode) and reverse recovery transition.When the diode is forward biased, large amount of excess carriers are driven across thejunction and the depletion region is reduced This behaves like charging a capacitorplus the ohmic resistance and inductance that cause the voltage overshoot When thecharging action is finished the forward current IF becomes steady and the effect ofdi/dt on inductance becomes zero, and thus the drop after the overshoot When thediode is reversed biased, it will turn off and the current decreases If it was an idealdiode, current would have dropped to zero and remained zero afterwards In practice,

expe-IF becomes negative for a while before it settles to zero This period is known asreverse recovery period The reverse recovery period is due to charge storage in thediode when it is forward biased When the diode turns off the charge storage has to be

Figure 5.4: N-Channel MOSFET: (a) Symbol, (b) I-V characteristic, (c) idealizedcharacteristics Sources: Mohan 1995 [2].

removed before the junction can become reverse biased again

The effects of reverse recovery of diode are not only increasing the power pation of diode itself but also increaseing the losses of other devices connected Forpower electronics circuits switching at high frequency in the range of hundred kHz tofew MHz, fast recovery time diodes are preferred

dissi-There are at least three different types of diode:

• Line frequency diode or general purpose diode - on-state or forward voltage oftheses diodes is made as low as possible but higher trr, which is acceptable forline frequency applications (50 Hz or 60 Hz)

• Fast recovery diode - small reverse recovery time, trrless than a few microseconds,for high frequency switching circuits

• Schottky diode - these diodes have low forward voltage drop(typically 0.3V) andthe diode dissipation is reduced However Schottky diodes are limited in theirvoltage blocking capabilities, typically less than 250V

The MOSFET has three terminals: Gate (G), Drain (D) and Source (S) It is a controlled device which needs a voltage across gate-to-source (VGS) be greater than athreshold voltage Vthto drive the transistor on When the transistor is on, the drain-to-source becomes a channel for electric current to pass through at both directions This

voltage-channel has an internal resistance called on-state resistance RON which is and temperature-dependent In general, the higher the voltage and temperature, thehigher the on-state resistance RON is at the range from a few milli-ohms to a fewohms Besides, due to the formation structure of MOSFET, there is a body diodeacross the source-to-drain terminals.

voltage-The MOSFET has intrinsic capacitances across all its terminals Of particularconcern is the capacitances across G-S and G-D These capacitances will cause delay inthe turning on or off the MOSFET This leads to switching losses In order to minimizethe losses, the gate driver circuit (i.e to provide VGS) has to be of high current andfast switching response We will discuss that in more detail in the last Section of thischapter

The gate-to-source voltage needs to stay above the threshold voltage to maintainthe transistor on One important point to stress is that, for a practical MOSFET, VGS

and drain current ID are inter-related For example, in Fig 5.5, the drain current IDonly reaches 6.5A maximum when VGS is at 4.5V ID increases when VGS increases.When the VGSdecreases to zero, the transistor is off and ID decreases to zero CurrentMOSFET can sustain a reverse voltage up to 1kV

5.4 Basic Power Converter Topologies

The buck converter with MOSFET is shown in Fig 5.6 The buck converter performsvoltage step-down function That is, Vo is less than VS By the switching actions ofMOSFET, the buck converter can be described by two basic operation stages as shown

in Fig 5.7 The switching waveforms of the buck converter is shown in Fig 5.8 Inthis operation mode, the inductor current does not reach zero This mode is calledcontinuous (inductor) conduction mode (CCM)

Stage 1

Prior to this stage, the switch Q1 is turned off But there is current flowing in theinductor L When Q1 is turned on at the beginning of this stage (t = 0), the voltage

Figure 5.5: N-Channel MOSFET IRF540N I-V characteristic Sources: InternationalRectifier.

? Q1

.

.

∆V C

.

.

? 6

∆I L

Figure 5.8: Key switching waveforms of a buck converter

applied across the inductor is

This stage begins when switch Q1 is turned off The drain-to-source terminal becomes

an open circuit Input current is ceased to flow However, the current in the inductorcannot change abruptly Without a path to continue IL, the energy stored in theinductor will be released suddenly that appears as a destructive voltage spike on theMOSFET and eventually would burn it out Fortunately, a free wheeling diode Df w ispresented in the buck converter IL is diverted to flow through this diode Neglectingthe forward voltage drop of diode VDf w, the voltage applied across L has reversedpolarity and its magnitude is described as

One important property of magnetic component is the change of inductor current

is proportional to the change of magnetic flux of the inductor or transformer core If

which is the voltage conversion ratio of buck converter at operating at CCM Since D

is always less than 1, it implies that Va is always less than Vs

Critical Inductance

From Fig 5.8, we know that Ia = (I2−I1)/2 In order to maintain inductor current

at CCM, we must ensure the following inequality

where fs is the switching frequency

Input and Output Ripples

The average charging or discharing current of the output capacitor C is equal to thetrangular area IC(t) covered Take the capacitor charging part as an example It can

∆IL

8 dt =

T ∆IL

Substitute (5.3) and (5.9) into (5.14), the capacitor ripple voltage is expressed as

As the input current of buck converter is pulsating, it may affect the equipment

or power sources connected to the input of the buck converter A common practice is

to insert an LC filter between the source VS and the switch Q1 The filter also reducesthe magnitude of electromagnetic interference (EMI)

Example: The buck converter shown in Fig 5.6 has an input voltage of 12V.The switching frequency is 50kHz The load requires an average voltage of 5V with amaximum ripple voltage of 20mV The maximum ripple current of the output inductor

is 0.2A Determine: (a) the duty cycle, (b) the output inductance, (c) the outputcapacitance, and (d) the output inductance if the switching frequency is increased to100kHz

Solution: (a) From (5.9),

(c) From (5.15)

C = VSD(1 − D)8f2

We can see that by doubling the switching frequency, the inductance is reduced by half

Discontinuous Condution Mode (DCM)

In discontinuous conduction mode (DCM), there is an additional stage after Stage 2 inwhich the inductor current has already reached zero, as shown in Figs 5.9 and 5.10.The output voltage is sustained by the output capacitor C

Since the average voltage across the inductor is zero, we can write the following:

in-be stored and transferred from inductive element such as inductor and transformer It

is in this circuit modeled as Io Vd is the power source, vT and iT are the voltage andcurrent through the transistor respectively

Figure 5.11: Cause of switching losses in transistor Sources: Mohan 1995 [2].

Switching Loss

When the switch control signal is at On state, after certain delay td(on) the transistor

is closed The current of the transistor increases while the voltage across it decreases.The duration takes tc(on) = tri + tf v for iT to reach Io and vT to reach Von Von isnon-zero in practical transistor as it has internal resistance

From 5.11(c), it shows the energy losses to switching on and off of the transistor(shaded area), which can be approximately written as

Wc(on)= 1

2VdIotc(on) (5.22)

Wc(of f )= 1

2VdIotc(of f ) (5.23)And the power dissipation of the transistor due to switching losses can be written as

Ps = 1

2VdIo(tc(on)+ tc(of f ))fs (5.24)

It can be seen that the switching loss increases with the switching frequency Thisbecomes the trade-off when we want to reduce the component size such as magneticcomponents and capacitor by increasing the switching frequency, the switching lossincreases as well Converters with this type with conventional switching of transistorare often called “hard-switching” converters In order to have a breakthrough, we mayneed to consider soft-switching technique which is able to minimize or even eliminatethe cross conduction of voltage and current through the transistor This is howeverbeyond the scope of this unit of study Students who want to have further studies insoft-switching technique can go to Chapter 3 of Ang’s book [1]

Figure 5.12: Totem-pole gate driver for MOSFET.

5.5.2 Totem-pole Gate Drive

Another way to reduce the switching loss is to have a fast switching gate driver to speed

up the rate of turn-on and turn-off of transistor As we have mentioned in Section 5.3.2t=hat the MOSFET has intrinsic input capacitance across its gate and source In usualcases the switch control signal vP W M is of low current capability We may use a so-called totem-pole circuit to amplify the current of the signal It consists of one NPNtransistor and one PNP transistor connected in series, as shown in Fig 5.12

The operation is briefed as follows: To proper switch on the transistor Q3, thecontrol signal vP W M should be greater than the transistor forward biase voltage of Q1plus the gate-to-source threshold voltage of Q3

vP W M > VBE+ Vth (5.27)The base current of Q1 can be written as

Figure 5.13: Floating gate drive IR2111 Sources: International Rectifier.

Figure 5.14: Floating gate drive IR2111 functional block diagram Sources: tional Rectifier

Interna-The base-to-emitter voltage of Q2 is reverse biased and it is in off-state

To switch off Q3, we need to turn off Q1 and switch on Q2 It is achieved bydecreasing vP W M to zero Now Q2 is forward biased and the charge of gate-to-source

of Q3 will be taken away as current through emitter then collector of Q3 to ground.This is a quick discharge of the intrinsic capacitor and the transistor is turned offquickly

5.5.3 Floating Gate Drive

The totem-pole gate driver is only able to drive the transistor with reference to ground

If we need to drive a MOSFET with its source node not connecting to ground, forexample the MOSFET of buck converter, we need to have a floating gate drive toprovide voltage across gate-to-source of the MOSFET For example, the half-bridge

the capacitor will transfer its charge through the internal circuit of the chip to theintrinsic input capacitor of upper MOSFET.

[1] S Ang, and A Oliva, Power-switching converters, CRC Press, 2nd Ed., 2005.[2] N Mohan, T M Undeland, and W P Robbins, Power electronics: converters,applications and devices, John Wiley & Sons, Inc., 2nd Ed., 1995.

http://www.irf.com/product-info/datasheets/data/ir2111.pdf

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