19985 energy conversion and drives · web view1.2.3 the thyristor (silicon controlled rectifer,...

19985 Energy Conversion and Drives

Power Electronics Section 1 - Devices

1.1 Background

Power electronics is the application of semi-conductor devices to the controlor utilisation of electrical energy.

Why use power electronics?

Readily accessible electrical power sources are usually limited to the ‘mains’ –single-phase or three-phase, 50/60 Hz, ‘constant’ voltage, sinusoidal supplies.

If economically justifiable, special sources may be introduced eg dc generators, battery banks, high frequency generators, but this is unusual

The diverse applications of electrical energy often mean that one single type of power source is not suited to all applications eg motor drives, heating, electro-production (eg electrolysis, plating), and all ‘electronic’ equipment as used for telecommunications, computers etc.

This leads to the concept of drawing power from the readily available ac supply and then modifying it to suit each application.

“Modifying” includes changing the frequency, waveform and voltage level –the most common examples are:

RECTIFICATION - converting ac to dc

INVERSION - converting dc to ac ( usually after rectification!)

Applications of rectifiers:

a) d.c. power supplies for all electronic systemsb) d.c. motor drivesc) electrochemistry (eg chlorine production, aluminium smelting,

electroplating, battery charging)

Applications of inverters:

a) a.c. motor drivesb) “portable” a.c. supplies using battery sourcec) “standby” supplies in event of mains failure using battery sources (and non-interruptable supplies)d) frequency changing.


1.2 Power semiconductor devices

All semi-conductor devices owe their “external” characteristics (eg voltage / current relationships) to the physics of charge motion within them. The fundamental conductivity property of silicon - the most common semi-conductor material for power devices - can be controlled by ‘doping’ the purecrystal with ‘impurity’ elements such as phosphorus, antimony and arsenic (valency 5) or aluminium, boron, gallium or indium (valency 3) to produce the n-type and p-type semi-conductors which have much higher conductivity due to surplus ‘free’ electrons (n-type) or surplus ‘holes’ (p-type).

By combining p-type and n-type regions in a single silicon crystal, with varyingdegrees of doping level and in various configurations, a wide range of devicescan be created. Each device will have external characteristics which derivedirectly from its structure, and each will have its strengths and weakness inregard to its usefulness as a power control element.

The principle power semi-conductor devices are:

DIODES

THYRISTORS (SCR) – STANDARD ASYMMETRIC

GATE-TURN-OFF (GTO) MOS-CONTROLLED (MCT)

TRANSISTORS – BIPOLAR (BJT) FIELD-EFFECT (MOSFET) INSULATED-GATE BIPOLAR (IGBT)

The devices may be grouped into categories under significant behavioural characteristics, as follows:

uncontrolled conduction controlled conduction

DIODES

continuous control

start-up control only

TRANSISTORS THYRISTORS

In all cases significant conduction is possible in one direction only, theproperty which allows them to carry out the control of power flow.


1.2.1 The Diode

The diode is the simplest device, consisting of a single p-n junction.

Conduction from A to K (p to n) is possible by the external circuit making A positive to K - this narrows the depletion layer and allows energetic carriers to cross the junction. Reverse bias widens the layer and prevents significant charge movement except for minority carriers (holes in the n, electrons in the p), released by thermal ionisation

Fig. 1.1 The diode symbol and static v - i characteristic[ (c) is idealised ]

Figure 1.1 shows the familiar diode circuit symbol, static v-i characteristic andthe ideal diode characteristic. The STATIC characteristic reveals theunidirectional conduction property and the highly non-linear nature of the device. Under steady or slowly changing values of voltage and current this relationship is valid.

1.2.2 The Bi-Polar Junction Transistor (BJT)

The power transistor is a larger version of the familiar bipolar junction transistor, a three layer device with two p-n junctions. Its symbol andcommon-emitter static characteristics are shown in figure 1.2.

With no external connections, the two p-n junctions will have set up internal fields opposing diffusion. By connecting external sources which forward bias the BASE-EMITTER junction and reverse bias the BASE-COLLECTORjunction, majority carries from the emitter enter the base region where most can be swept into the collector by the applied bias, but a small fraction flow tothe base connection.


Fig. 1.2 The npn bipolar junction transistor (BJT) symbol and common emitter characteristic [ (c) is idealised ]

The fraction of IE which reaches the collector, to become Ic, is substantially constant, and is called the current transfer ratio

= IIc

E

For power transistors is typically 0.9-0.98

In power applications most transistors are connected in the “common emitter” configuration, which results in the iC/vCE characteristics of figure 1.2. In thisarrangement the significant “external characteristic” is the relation between IB

- the controlling current - and Ic, the controlled current.

, the current gain

Rearranging the equations in and hFE we get

hFE 1

and is thus in the range 10-50 for power devices.

To achieve a higher hFE value, the Darlington configuration can be used, figure 1.3, in which either a monolithic construction or two or three discretetransistors are arranged as shown, the effective hFE being the product the individual values.


Fig. 1.3 Darlington configurations

In power electronic applications the power transistor should be in the “CUT-OFF” state or “SATURATED ON” state, behaving as nearly as possible as a switch. The current gain indicates the minimum base current required toensure saturation for a given collector current.

Example

The power transistor in figure 1.4 has a current gain of 20. What is the minimum value of base current to ensure saturation? Neglect VCE.

Fig. 1.4

Neglecting VCE : IC = 100/10 = 10A

But IC/IB = 20 IB = IC/20 = 10/20 = 0.5A

1.2.3 The Thyristor (Silicon Controlled Rectifer, SCR)


The thyristor, or SCR, is a 4-layer, 3 junction device with a diode-like behaviour, but with the addition of a control electrode called the gate. Forward breakdown at a very low VAK (~1V) is initiated by injecting currentinto the gate. This starts an internal positive feedback mechanism whichcauses conduction from anode to cathode to be self sustaining. Figure 1.5 shows the symbol, and v-i characteristics.

Fig. 1.5 Thyristor symbol and static v-I characteristics [ (c) is idealised ]

Once conduction is initiated, anode current IA will continue to flow without gatecurrent as long as IA has exceeded a value called the LATCHING CURRENT,and will be self-sustaining as long as IA remains above a value called theHOLDING CURRENT. If IA falls below this level the positive feedback (orregenerative) action fails to sustain the conduction and the thyristor returns tothe blocking state.

1.2.4 The Gate Turn-Off Thyristor (GTO)


The GTO is a modified p-n-p-n device which can be turned off as well as turned on by gate current injection. To turn the device off the gate current direction is reversed. The circuit symbol and v-I characteristics are shown infigure 1.6.

The gate current required to turn on a GTO is higher than that of a standard thyristor and the latching current is also higher. The gate current required to turn off is MUCH higher, of the order of 20 to 30% of the forward anode current! The rate of rise of this turn-off gate current must be withinmanufacturer's specifications to minimise overall switching time.

Fig. 1.6 GTO symbol and v-i characteristics [ (c) is idealised ]

1.2.5 The Power MOSFET

The power MOSFETis a development of the field effect transistor (FET), in which the conductivity of the device is controlled, not by current at the control input, but by electric field i.e. voltage, between gate source. Its symbol and v-i characteristics are shown in figure 1.7.

Fig. 1.7 MOSFET symbol and v-I characteristicsThis important difference is due to the inclusion of a non-conducting metal


oxide layer between the gate terminal and the semiconductor body. A positive potential difference between the gate and the source creates an electric field which draws electrons into a channel between the n type drain and source regions, allowing conduction between them.

The doping pattern of the MOSFET reveals that the device presents a simple pn junction to negative VDS and so behaves like a diode. This “internal” diode has rather slow reverse recovery characteristics however, and is of limited practical use.

The switching characteristics of the power MOSFET are extremely fast, due to the absence of recombination processes (only electrons carry charge in the n-channel type, and holes in the p-channel). The switching times are generally governed by the ability of the gate circuit to charge and discharge the gate-source capacitance. Turn-on and turn-off times of 0.1s or less are readily achievable.

The power MOSFET has effectively a source-drain resistance, resulting in a current-dependent forward voltage drop (cf relatively constant VCE for a BJT). This will be typically in the range of 1 to 3 volts at rated current. Recentadvances in semiconductor device manufacture have resulted in high current(hundreds of amps) versions with on-state resistance of only a few milliohms.

1.2.6 The Insulated Gate Bipolar Transistor (IGBT)

This is a device which combines the best features of MOSFET and bipolar transistors. It may be regarded as a monolithic Darlington arrangement of FET input transistor and bipolar output transistor. This gives it control characteristics like a MOSFET - voltage controlled, fast switching, no gate current (hence no gate power) in the steady state - and forward conduction characteristics like a bipolar transistor - low saturation voltage. For these reasons it has become the favoured device in many applications where its relatively limited rating is suitable. Its circuit symbols and v-I characteristicsare shown in figure 1.8. It has switching times between the BJT and theMOSFET, of the order of 1s. It is controlled like a MOSFET.


Fig. 1.8 IGBT symbols and v-I characteristics

1.2.7 The MCT (MOS-Controlled Thyristor)

This new device is to the thyristor family what the IGBT is to the transistor family – a thyristor-like conduction characteristic and a MOS-type control characteristic. It can be turned on or off by a voltage applied to the isolated gate and has good switching speed and low on-state voltage.

Fig. 1.9 MCT symbols and v-I characteristics


As its ratings rise with progressive development then it seems likely that it will take over many of the functions of standard and GTO thyristors. Its symbol and v-I characteristics are shown in figure 1.9

1.3 Switching characteristics

Semiconductor devices switch very rapidly from ON to OFF and vice-versa, but not instantaneously. Each of the devices examined has its specific switching behaviour and typical voltage and current waveforms during the transition, but they all share certain common features

all diodes and thyristors conduct reverse current briefly when turning off all controllable switches exhibit a short time delay between the control

signal being applied and the transition starting all controllable switches take time to change state; when turning OFF the

voltage will start to rise across the switch before the current starts to fall; when turning ON the current starts to rise before the voltage starts to fall

The most important consequence of this behaviour are

there is a limit to the frequency at which the devices can be switched energy is dissipated within the device every time it switches

1.4 Comparison of Power Semiconductor Devices

Each device has its pros and cons, and the application will often determine the most appropriate. Where device ratings permit a choice, then economics, circuit complexity, reliability etc are all involved in the decision process.

As a general rule, conventional RECTIFIER applications do not require controlof both switch-on and switch-off and so DIODES and THYRISTORS are usedin these cases.

INVERSION and DC/DC CONVERSION applications require control of bothswitch-on and switch-off, and so BJT, GTO, MCT, MOSFET and IGBT areused in these cases. There are also special designs of RECTIFIERS, calledswitching rectifiers, where control of both turn-on and turn-off is required. For such applications it is helpful to see their relative merits with respect to themost important parameters, as shown in the table below.


1.5 Control requirements for turn-on and turn-off

1.5.1 Diode - not applicable

1.5.2 BJT

Turn-on: a fast rising (<1s) base current reaching a value which will ensure saturation at the level of collector current; maintained throughout conduction period; the base-emitter voltage isaround 1V only.

Turn-off: a fast falling (<1s) base current, reversing for a few s to speed recombination and hence blocking capability. For large BJT’s with low hFE the gate current may be 10’s of amps. TheDARLINGTON configuration can reduce this to a few amps, withthe auxiliary base drive transistor either separate or monolithic(same silicon).

1.5.3 THYRISTOR (also TRIAC and ASCR)

Turn-on: a fast rising (<1s) current injected between gate and cathode at a p.d. of a few volts; current has to persist until the anode to cathode current has risen beyond the LATCHING CURRENT value; gate current can then be removed; gate current typically50 to 500 mA.

1.5.4 GTO Thyristor.

Turn-on: similar requirements to the THYRISTOR, but a somewhat higher gate current level.

Turn-off: a current of 1 5 1 3to of the anode current must be extracted from the gate by reverse biasing the gate-cathode junction. The rise of this current should be rapid but will be limited by the gate drive circuit inductance. When the gate current reaches the required fraction of the anode current, the anode current beginsto fall, as does the gate current (since fewer carriers are available in the gate region of the device).

1.5.5 MOSFET

Turn-on: a fast rising (<<1s) voltage of typically 5-10V applied between G and S (VGS positive) with a low enough source impedance to supply the G-S capacitance charging current of around an amp for fast dVGS/dt. VGS must be maintained during conduction, but gate current is zero except when switching.


Turn-off: rapid removal of VGS , falling in typically <<1s, the low source impedance permitting rapid discharge of the G-S capacitance.

1.5.6 IGBT and MCT

Turn-on and turn-off control are similar to the MOSFET, since both the IGBTand MCT have an FET stage as their control input.

1.6 Practical power semiconductors – achieving reliability.

Like all components, power semiconductors have limits to their capabilities, and exceeding these limits may result in catastrophic failure. Semiconductor devices are less tolerant of stresses beyond their design capabilities than are passive components like resistors, capacitors, etc. Therefore their design limits must be carefully considered when choosingdevices for an application.

There are basically three ways of killing a semiconductor - too many volts, toomany amps, too many degrees. This is complicated by the interaction among these variables (v*i P degrees!) and the fact that not only must theirabsolute values be considered but also the rates of rise of v and i.

1.6.1 Current and di/dt limits

Each device has an absolute maximum current which should not be exceeded, related to the current density allowable within the semiconductor.

Each device also has a number of usable current ratings, dictated by thethermal limit of maximum junction temperature produced by the I²R heating.Due to thermal capacity, these ratings depend on current duration.

Transistors (BJT, IGBT and MOSFET) have a CONTINUOUS CURRENT RATING (DC) and a SHORT TIME or PULSED CURRENT RATING(specified duration).

Diodes and thyristors have AVERAGE CURRENT RATING, RMS CURRENTRATING and SURGE CURRENT RATING. The average is usually based on ahalf-cycle of 50/60Hz current, while the RMS and surge ratings aredetermined by “I²t”, a measure of the heat energy input to the device.All devices have a limit to the allowable rate of rise of current, due to a combination of power dissipation (i.e. temperature rise) limit and the need to ensure uniform current density. Inductance in series with any device will help to limit di/dt. Added inductors and associated resistors/diodes are called turn-on snubbers.

Diodes and standard thyristors can be protected against sustainedovercurrents by fast-acting fuses. The characteristics of fuses are described in


terms of "pre-arcing" time (or I2t) and total let-through I2t, quantities whichdepend on the fuse rating, the circuit voltage and the "prospective" circuitcurrent – ie the current which would be reached if the fuse did not rupture.Protection of diodes or thyristors by fuses requires the careful co-ordination ofthe fuse characteristic with the I2t capability of the device as specified by themanufacturer. As a rule of thumb, however, fuse protection against shortcircuits will usually be achieved if the rms current rating of the fuse is lessthan the average current rating of the diode or thyristor.

Transistors cannot be protected by fuses, but their turn-off capability means that “active protection” current monitoring systems can be devised which can switch off a transistor before its current rises excessively. Alternatively, "crow-bar" protection can be applied, in which a fuse-protected thyristor is triggered into creating a short-circuit to by-pass the transistor. The thyristor is rated towithstand the fuse let-through I2t and so is not sacrificed. This is the favouredapproach for GTOs, where any attempt to switch off excessive current cancause device failure.

1.6.2 Voltage and dv/dt limits

All devices have an absolute maximum reverse voltage or forward off-state voltage withstand, limited by electric field considerations and the production of field-induced ionisation leading to breakdown. The maximum voltage limitmust not be exceeded, even transiently.

Thyristors (all types) are limited in respect of rate of rise of forward off-state voltage to avoid spurious turn-on by internal capacitive current, and rate of rise of reverse voltage during turn-off, to prevent the device reverting to aconducting state due to the presence of unrecombined carriers.

Transient over-voltage limiting can be provided by connecting non-linear voltage suppression devices such as Zener diodes or varistors across the thyristor, while dv/dt limiting can be provided by turn-off snubbers – iecombinations of resistors, capacitors and diodes which absorb energy andcontrol the voltage across the device. Fig.1.10 shows a typical turn-off snubber applied to a GTO, and the effect on the transient voltage and current.

Fig. 1.10 Turn-off snubber applied to GTO


Transistors are less sensitive to dv/dt in regard to causing spurious conduction, but controlling dv/dt during turn-off will ensure that the powerdissipation during switching is minimised.

1.6.3 Series and parallel combinations of devices for enhanced capability

Some applications may require voltage or current ratings beyond the present limits of a single device. In these circumstances current "rating" can be enhanced by connecting devices in parallel while voltage "rating" can beraised by series connection.

For successful series operation it is necessary that the devices share equally the reverse voltage or forward blocking voltage imposed by the circuit. In the steady-state this could be achieved by connecting a resistor across each device. The resistor value is determined by the range of reverse leakage current (ie the range of reverse resistance) expected in the string of devices. The current through the resistors will be calculated to be sufficiently greater than the highest leakage value so that the resultant leakage+resistor current is nearly the same for all devices.

Under transient conditions, eg during switch-off, the voltage across each device in a series string will additionally depend on turn-off time, stray capacitance and stored charge, quantities which will also vary from device to device, resulting in unbalanced voltage share.

To equalise transient voltage sharing, a capacitor could be connected across each device. In fact the conventional R-C or R-C-D turn-off snubber performs the equalising function satisfactorily. To ensure that variation in turn-on times does not cause excessive voltage stress on the slowest devices, an inductive turn-on snubber is always connected to slow the current rise and allow all devices to achieve the full forward conductive state in good time. The design of drive circuits for series-connected strings will pay particular attention to fast rise-time and drive energy to ensure that turn-on delays are minimised.

For successful parallel operation it is essential that the devices share equally the circuit current. This can be realised by

using matched devices forced sharing

Matched devices are ones with sufficiently similar on-state voltage/current characteristics and turn-on times that they will naturally share current equally.Where this cannot be realised then forced sharing is possible by either

a resistor in series with each device – the value will be high enough to mask the variation in effective on-state resistance.

or


inductive coupling – mutual coupling between windings in series with each current path induces voltages which oppose any tendency for the current values to diverge.

The resistor method is simpler and cheaper but incurs energy losses. The induction approach is more expensive and becomes prohibitively so as the number of parallel devices rises.

1.6.4 Electrical noise (rfi) and EMC.

Radio frequency interference (rfi) is the catch-all name for all the unwantedhigh frequency signals which emanate by radiation or conduction from anycircuit in which high values of di/dt or dv/dt occur. Many power electroniccircuits fit the bill, even mundane rectifiers.

Electro-magnetic compatibility (EMC) describes the compliance of anyelectrical or electronic equipment with agreed standards of rfi emission orsusceptibility.

Conducted rfi is usually effectively attenuated by filtering. A typical L-C filterfor equipment likely to produce conducted rfi is shown in figure 1.11.

Fig.1.11 L-C mains filter

Radiated rfi is minimised by design and manufacture. Good design practice includes minimum loop lengths of circuits with fast di/dt, use of ground planesin pcb layout, capacitive decoupling, the use of twisted pairs for interwiringand use of snubbers as close to the switching devices as possible. Sensitive equipment can be shielded by enclosure in steel, copper or metal sheet, thechoice depending on the frequency range of the rfi.


1.7 Cooling of Semiconductor Devices

The physical size of power semiconductors is small relative to the amount of power which they may have to dissipate. Since natural heat loss depends on surface area and temperature gradient, the small size of semiconductor devices implies high temperatures could be reached - and they will be if you’re not careful!

Power semiconductor devices dissipate power as heat

a) when they are conducting steadilyb) when they are switching

In both cases, power p = v * i

In a) v is small and i may be quite large but relatively slowly changing, so p is relatively steady too.

In b) both v and i are changing very rapidly over a wide range and so p is changing rapidly too.

The critical parameter is the temperature of the p-n junction(s) within the device. Exceeding the maximum temperature causes rapid deterioration of performance and possibly catastrophic failure. Typical maximum junction temperatures vary from 125C for thyristors to as high as 200C for transistors. The actual temperature reached can be calculated by a thermal model of the device, treating the power dissipation p = v * i as a thermal source at the junction(s) and the heat flow rate (in Watts) obeying the basic heat transfer equation:-

[Rate of thermal energy input]=[rate of increase of stored thermal energy]+[rate of energy loss]

where K1 = thermal capacity in Joules/C (or J/K); = temperature difference = 1 - 2 R = thermal resistance of path between 1 and 2.


This is analogous to an electric circuit supplied by a current i (p) and consisting of a capacitor C (K) and resistor R ( R) producing a voltage v above a reference potential, as in figure 1.12.

Fig. 1.12

The heat flow from the “heat source” in a device passes through distinct thermal resistance regions:

1. the junction to the body or case

2. the case to a) the surroundingsor b) the heat sink

and if b) then

3. the heatsink to the surroundings

The “surroundings” are assumed to be at an invariant temperature (ambient) and this constitutes an “infinite heat sink” (an absorber of thermal energy with no rise in temperature). The physical “heat sink” is an object of low thermalresistance which efficiently transfers heat from the case to the ambient – typically an aluminium extrusion of large surface area. Thermal resistancevalues for heatsinks vary from around 10K/W for small ones of a few cm linear dimensions to less than 0.3K/W for large ones.

Each region is associated with its own thermal capacity, giving rise to the general heat flow “circuit” model below:

where subscripts are j – junction; c – case;

s – sink; a - ambient


For steady or quasi-steady thermal input conditions (dc or 50/60 Hz variation) the thermal capacity effects can be ignored and a “steady state” model used, with thermal resistances only.

In this case the heat flow equation simplifies to

p= / R

where = temperature rise.

The heat sink temperature S will be given by

p=(S-A)/RSA

and the junction temperature J by

p=(J - A)/RJA

where RJA= RJC+RCS+RSA ,and so on.

Hence, knowing p and ambient temperature, the temperature at any region may be calculated.


Example 1

A power diode carries repeated half cycles of sinusoidal current with a peak value of 60A. If the forward voltage drop VAK = 1.2V, calculate the maximum junction temperature of the diode, neglecting thermal capacity. The thermal data are as below:

RJC = 0.3K/W ; RCS = 0.2K/W ; RSA = 0.8K/W ; ambient temperature = 25C.

power dissipation p = v * i = 1.2 i pmax = 1.2 imax = 1.2 * 60 = 72W.

J - A = p * R J = 25 + 72(0.3 + 0.2 + 0.8) = 118.6C


Example 2.

A power MOSFET is switching a direct current of 50A at a frequency of 2kHz with a duty factor of 50% (equal on and off periods). The switch-on and switch-off voltage-current trajectories are as shown.

Calculate :

(a) the turn-on and turn-off energy losses(b) the average switching power loss(c) the on-state power loss(d) the average junction and heatsink temperatures. Assume linear voltage and current variations with time during switching. On-state VDS = 2V; RJC = 0.3K/W; RCS = 0.2K/W; RSA = 0.8K/W; ambient = 20C.While switching, energy loss If v and i vary linearly with time over interval T, then

v =(V/T)*t and i = I – (I/T)*t during turn-off

and v = V – (V/T)*t and i = (I/T)*t during turn-on

where V and I are the steady-state off-state voltage and on-state current respectively.


eg turn-off:

(Repeat for turn-on and show that the same result is obtained)

(a) For turn-on: Switching interval T, when both v and i exist (are non-zero), is 2s ; V=50V ; I=20A; eon = VIT/6 = (50*20*2*10-6)/6 = 0.333mJ

For turn-off: Switching interval T is 5s; V=100V; I=50A;

eoff = VIT/6 = (100*50*5*10-6)/6 = 4.167mJ

(b) Total switching energy loss per switching cycle (once on, once off)

e = 0.333 + 4.167 = 4.5 mJ

average power loss = 4.5mJ/cycle * switching cycles/second = 4.5 * 10-3 * 2 * 103

= 9W.

(c) On-state loss = VDE * I = 2 * 50 = 100W for continuous conduction.For a 50% duty factor, the device conducts for only 50% of the time and so the average conduction power losses will be reduced to 50% of the continuous value.

on-state loss = 100W * 0.5 = 50W.

(d) Thermal model:


Total average power loss p = 9+50 = 59W Rise in junction temperature J = p * (thermal resistance between junction and ambient) =59 * (0.3+0.2+0.8) =76.7 K

J = 76.7K + 20C = 96.7C.

Rise in heatsink temperature S = p * RSA = 59 * 0.8 = 47.2K S = 47.2K + 20C = 67.2C

References for further reading.

Power Electronics – Devices, Drivers, Applications and Passive Components; 2nd ed; B W Williams (Macmillan)

Power Electronics: Converters, Applications and Design; 3rd ed; N Mohan, T M Undeland, W P Robbins (Wiley)


19985 energy conversion and drives · web view1.2.3 the thyristor (silicon controlled rectifer,...

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