Electrical Engineering Division Page 1 of 10 Power semiconductor devices Power semiconductor devices constitute the heart of modern power electronic apparatus. The main function of the power semiconductor devices (PSD) in the power converter system are used as on/off switches to control the energy transfer between the source and the load. The Basic representation of the power semiconductor device can be represented as a traditional switch as shown in Fig.1. Fig.1 Figure 2 below shows a simple feature of using PSD as a switch in converting energy between the source and the load. S S S V s V o V s V o V s V o Mode I: S ON V o = V s for T=t on V s Mode II: S OFF V o = 0 for T=t off T: Switching cycle V o Fig.2 V s t on T t off Average output voltage is less than DC input voltage V s The difference between ideal switch and practical switch As mentioned before, PSDs use as switches in PE converters. The switch in practical has different features in practical over its features in ideal case. This can be recognize as described below: Ideal Switch If the PSD considered working as ideal switch then it should be: When switch is OFF, i =0 and - v + which implies that P ON =0
Electrical Engineering Division Page 2 of 10 When switch is ON, V =0 and - I + which implies that P ON =0 It should be possible to easily turn the switch ON and OFF by applying an appropriate control signal. Control Signal Features of Ideal Switch The power required to keep the switch in a particular state, or to switch it ON/OFF should be infinitesimally small. Should be able to change state instantaneously which implies that t ON =0, t OFF =0 and P SW =0 (see Fig.3) Should be able to withstand infinite temperature that means that its power handling capability is infinite. Requires very low thermal impedance from internal junction to ambient, RJA=0, so that it transmits heat Fig.3 easily to the ambient Should be able to withstand infinite value of di=dt during turn ON and infinite value of dv=dt during turn OFF. Current limitless when on-either direction. No limit on amount of voltage across switch when off (Blocking voltage infinite (forward or reverse)). In real life, there exists nothing like that!
Electrical Engineering Division Page 3 of 10 Practical Switch Although Semiconductor Industry has produced amazing devices, the real world switch is not ideal. Limited conduction current when the switch on, Limited blocking voltage when the switch is in the off Limited switching speed that caused by the finite turn-on and turn-off times Real world (Semiconductor) switches are charge driven Finite, nonzero on-state and off-state resistances (There is a I 2 R loss when on and some leakage when off (very small). A real switch needs a finite t on for ON switching and t off for OFF switching (see Fig.5). These finite switching times have two major consequences: Limits the highest repetitive switching frequencies possible Introduce additional power dissipation in the switches themselves. Depending on the nature of the current and voltage waveforms during the transition, the peak power can reach a relatively large magnitude. The energy dissipation in the switch is equal to the area shown under the power waveform. Fig.4 The total energy J dissipated in the switch in one switching cycle T is given by the sum of the areas under the power waveform during ton and toff, as shown in the figure 5:
Electrical Engineering Division Page 4 of 10 Fig.5 The average switching power loss is therefore proportional to the switching frequency f s and is given in watt as: P = f J Including the ON and OFF power losses to the switching losses, the total average power loss over a time interval T is: Total power loss= conduction loss+ off-state loss + (turn-off loss+ turn-on loss) Example 1: Derive an expression for instantaneous power p(t)=v sw *i sw. Also, determine the average power dissipated in one switching cycle. Solution:
Electrical Engineering Division Page 5 of 10 t on t off T s - t on - t off T s t I t I + I 0 t t i = I t t (T t ) t T I t I + I (T t ) t T t t V t V + V 0 t t v = V t t (T t ) t (T t ) V t V + V (T t ) t T The instantaneous power is equal to p(t)=vsw*isw. By assuming the Ioff and VON = 0, then:
Electrical Engineering Division Page 6 of 10 p(t) = I V t (t t )t 0 t t p (t) = p(t) = V I t t (T t ) p(t) = I V t (T t ) (t T ) (T t ) t T t The average switching power loss over one cycle can be given as: p = 1 p(t)dt T P = 1 T p(t) dt + p(t) dt + p(t) dt P = I V 6T t + t + I V T (T t + t ) Hence, P = I V f 6. t + t P sw = Switching power loss in watt I ON = Forward Current in Amps. V OFF = Blocking voltage in volt f s = switching frequency in Hz. (t ON +t Off )= switching time in second P = (I V )f. (T t + t P c = Conduction power loss in watt I ON = Maximum Forward Current in Amps. V ON = Maximum Forward voltage in volt f s = switching frequency in Hz. T s - (t ON +t Off )= Conduction time in second
Electrical Engineering Division Page 7 of 10 For Practical Switches, you have to know that: Conduction loss : I 2 R loss, R being the resistance of the switch R On = 0 for ideal switch. R On = its value based on the voltage rating of the switch and its structure. For a good switch, R on should be small. Nevertheless, its not ignored. Voltage blocking High voltage can be sustained across the switch during OFF state. Surely, it depends on device dimension. V B =V off OFF Terminals of a PSD Control terminal Control Signal Common terminal Control signal from drive circuit must be connected between the control terminal and a fixed power circuit terminal (therefore called common terminal) Note: The difference between PSD and low-power semiconductor device Large voltage in the off state, High current capability in the on state, and operate at lower switching speeds
Electrical Engineering Division Page 8 of 10 Important Switch Parameters 1. Breakdown voltage rating 2. On-resistance/on-state resistance and specific on-resistance 3. Forward conduction drop 4. Forward blocking capability 5. Reverse blocking capability 6. Rise time 7. Fall time A good way to get more familiar with these things is to look at datasheets of power devices. They can be found on the website of device manufactures like International Rectifier, Infineon Technologies, Ixys Corporation etc. Classification of PSD NOTE; in general, the power semiconductor devices classified into three switch types: Uncontrolled: Diodes (a switch controlled by the power circuit) Semi-controlled: Thyristors, SCR (can be switched on by a signal but only the power circuit can turn it off) Fully Controlled: Transistors, GTO, SIT, MCT (can be switched on and off by a signal)
Electrical Engineering Division Page 9 of 10 Device Choices Which of the power switches is chosen will be determined by cost, availability of rating to suit the requirement and the ease with which it can be turned 'on' and 'off'. MOSFETS and IGBTs have the simplest driving requirements; they are voltage controlled and the gate current is virtually zero during the 'on' period. MOSFETS and IGBTs lack the reverse blocking capability which make the SCR, TRIAC and GTO so suitable for AC mains power applications. GTOs are most suited for high-power applications requiring forced commutation. IGBTs are potential replacements for MOSFETS and BJTs, respectively, in low and medium power applications. Power-handling capability Frequency-handling capability Choice of power switch based on each switch specifications is depicted in the table below:
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