University of Technology Lecture Note 2 Electrical Engineering Department Introduction to PSDs Electrical Engineering Division Page 1 of 10 EG 405: Power Electronics Dr. Oday A. Ahmed
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
Vo Vo Vs Vs Vs Vo
Mode I: S → ON Mode II: S→ OFF Vs Vo= Vs for T=ton V = 0 for T=t o off
T: Switching cycle Vo
Vs Average output voltage is less than DC input Fig.2 ton toff T voltage Vs
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 PON=0 University of Technology Lecture Note 2 Electrical Engineering Department Introduction to PSDs Electrical Engineering Division Page 2 of 10 EG 405: Power Electronics Dr. Oday A. Ahmed
■ When switch is ON, V =0 and -∞≤ I ≤+∞ which implies that PON=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 tON=0, tOFF=0 and PSW=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!
University of Technology Lecture Note 2 Electrical Engineering Department Introduction to PSDs Electrical Engineering Division Page 3 of 10 EG 405: Power Electronics Dr. Oday A. Ahmed
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 I2R loss when on and some leakage when off (very small). A real switch needs a finite ton for ON switching and toff 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:
University of Technology Lecture Note 2 Electrical Engineering Department Introduction to PSDs Electrical Engineering Division Page 4 of 10 EG 405: Power Electronics Dr. Oday A. Ahmed
Fig.5
The average switching power loss is therefore proportional to the switching frequency fs and is given in watt as: = × 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)=Vsw*isw. Also, determine the average power dissipated in one switching cycle. Solution:
University of Technology Lecture Note 2 Electrical Engineering Department Introduction to PSDs Electrical Engineering Division Page 5 of 10 EG 405: Power Electronics Dr. Oday A. Ahmed
ton toff
Ts- ton- toff
Ts