Lecture-4 on Power Electronics. Describes ideal and practical power switches and power diodes.

1. Semiconductor Power Switching Devices-1
(Lecture-4)
R S Ananda Murthy
Associate Professor and Head
Department of Electrical & Electronics Engineering,
Sri Jayachamarajendra College of Engineering,
Mysore 570 006
R S Ananda Murthy Semiconductor Power Switching Devices-1

2. Static i-v Characteristics of Ideal Switch
+ -
(a)
+ -
(b)
ON
OFF
Fully
Controlled
ON
OFF
Fully
Controlled (c)
We use power semiconductor devices as switches in
converters.
R S Ananda Murthy Semiconductor Power Switching Devices-1

3. Properties of Ideal Switch
When switch is OFF, i = 0 and −∞ ≤ v ≤ +∞ which implies
that PON = 0
When switch is ON, v = 0 and −∞ ≤ i ≤ +∞ which implies
that POFF = 0
It should be possible to easily turn the switch ON and OFF
by applying an appropriate control signal.
The power terminals of the switch should be electrically
isolated from the control terminals.
R S Ananda Murthy Semiconductor Power Switching Devices-1

4. Properties 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.
Should be able to withstand infinite temperature which
means that its power handling capability is infinite, i.e.,
PHmax → ∞.
Should be able to withstand infinite value of di/dt during
turn ON and infinite value of dv/dt during turn OFF.
R S Ananda Murthy Semiconductor Power Switching Devices-1

5. Static i-v Characteristics of Practical Switch
−V1 +V2
Limit
Power
Limit
Power
III
II I
IV
v
i
+ Ito
toOFF State close
v − axis
i − axis
ON State close
− I
The switch is assumed to be bilateral while drawing this.
R S Ananda Murthy Semiconductor Power Switching Devices-1

6. Classification of Power Semiconductor Devices
Bipolar voltage blocking
Pulse triggered (latched)
Continuously triggered
Unilateral
Bilateral
Semi−controlled Fully−controlledUncontrolled
Unidirectional
Power Semicondutor Switching Devices
Unipolar voltage blocking
R S Ananda Murthy Semiconductor Power Switching Devices-1

7. Classification of Power Semiconductor Devices
Device Control Input Controllability Conduction Blocking
Diode Nil Uncontrolled Forward Reverse
SCR Pulse Can be turned on Forward Bipolar
LASCR Pulse Can be turned on Forward Bipolar
TRIAC Pulse Can be turned on Bilateral Bipolar
BJT Continuous Fully-controlled Forward Forward
MOSFET Continuous Fully-controlled Forward Forward
IGBT Continuous Fully-controlled Forward Forward
SITH Continuous Fully-controlled Forward Forward
GTO Pulse Fully-controlled Forward Bipolar
MCT Pulse Fully-controlled Forward Forward
SITH – Static Induction Thyristor; MCT – MOS Controlled Thyristor.
R S Ananda Murthy Semiconductor Power Switching Devices-1

8. Power Losses in a Switch
t
p = vi
t
p = vi
i
Turn−on time Turn−off time
v
i
v
The total average power loss over a time interval T is
PD = PON +POFF +PG +PSW ≈ PON +PSW =
1
T
ˆ T
0
pdt
as POFF +PG is negligible as compared to PON +PSW .
R S Ananda Murthy Semiconductor Power Switching Devices-1

9. Static Characteristics Power Diodes
Reverse
leakage
current
VBR
VF
( )
Forward
conduction
drop
A K
Circuit symbol
+ −
i
v
A K
Structure
p i n
i
v
i
v
Linear Approximation
i
v
Ideal Characteristics
Avalanche
breakdown
VF = Vγ +IF RF where RF is the ON-state bulk resistance.
R S Ananda Murthy Semiconductor Power Switching Devices-1

10. Reverse Recovery Characteristics of Power Diodes
IR
t1
t2
t3
VF
IF
QRR
IRM
t 3
t 2
SF =
VR
IRM
dt
diQRR
QRR
2
di/dt
trr =
dt
di
< 0
Diode with snubber
Turn OFF
starts here
turn OFF
ends here
v
t
t
Area =
RM
V
- E
0.25 IRM
= 2
0
Reverse Recovery Charge
Reverse Recovery Time
is the time taken by the
minority carriers in the diode
to recombine when the diode
is reverse biased
R S Ananda Murthy Semiconductor Power Switching Devices-1

11. Important Specifications of Power Diodes
Reverse Blocking Voltage (VRRM).
Forward Average Current (IF(AVE)).
Forward RMS Current (IF(RMS)).
Surge Current Rating (IFSM).
Maximum On-state Voltage Drop (VFM).
I2t Rating.
Reverse Recovery Time (trr ).
The maximum allowable junction and case temperatures
(θJM and θCM).
Junction-to-case and case-to-sink thermal resistances
(RθJC and RθCS).
R S Ananda Murthy Semiconductor Power Switching Devices-1

12. Types of Power Diodes
1 Standard Recovery Diodes
2 Fast Recovery Diodes
3 Schottky Diodes
4 Silicon Carbide Diodes.
R S Ananda Murthy Semiconductor Power Switching Devices-1

13. Standard Recovery Diodes
trr ≈ 25µs.
Leakage current in the OFF-state is of the order of few µA.
Have lower ON-state voltage drop.
Available with ratings of several kV and kA.
Typically used in rectifiers at power frequencies i.e., at 50
Hz or 60 Hz.
R S Ananda Murthy Semiconductor Power Switching Devices-1

14. Fast Recovery Diodes
trr ≤ 5µs
Have relatively higher ON-state voltage drop.
Typically used in D.C.-to-D.C. converters and inverters
operating at higher frequencies as freewheeling diodes.
R S Ananda Murthy Semiconductor Power Switching Devices-1

15. Schottky Diodes
A K
Barrier Metal
n−type silicon layer
Heavily doped
silicon substrate
A K
Barrier Metal
n−type silicon layer
Heavily doped
silicon substrate
n+
trr is typically around few ns.
Have very low ON-state voltage drop of the order of
0.15-0.45 V and consequently very low PON.
Typically available in voltage ratings in the range 50-200 V.
Used in circuits having very low output voltages like
switched mode power supplies.
R S Ananda Murthy Semiconductor Power Switching Devices-1

16. Silicon Carbide Diodes
These are Schottky diodes constructed from silicon
carbide.
Have very low power loss.
Have extremely fast switching behavior with ultra-low trr .
Can operate at junction temperatures > 225◦ C.
Switching behavior is independent of ON-state forward
current and temperature.
Have much lower reverse leakage current than silicon
Schottky diodes, and higher reverse voltage rating.
Very expensive as compared to Si Schottky diodes.
R S Ananda Murthy Semiconductor Power Switching Devices-1

17. Some Diode Packages
Axial Pack Plastic Pack Plastic Pack
Stud Type Stud Type Disc Type
Source: www.irf.com
R S Ananda Murthy Semiconductor Power Switching Devices-1

18. Some Diode Packages
Stud type diodes (Source: www.china-rectifier.com)
R S Ananda Murthy Semiconductor Power Switching Devices-1

19. Some Diode Packages
Disc type diodes (Source: www.china-rectifier.com)
R S Ananda Murthy Semiconductor Power Switching Devices-1

20. Some Diode Rectifier Modules
Single-phase Diode Bridge Module
(Source: www.china-rectifier.com)
R S Ananda Murthy Semiconductor Power Switching Devices-1

21. Some Diode Rectifier Modules
Three-phase Diode Bridge Module
(Source: www.china-rectifier.com)
R S Ananda Murthy Semiconductor Power Switching Devices-1

22. Next Lecture...
In the next lecture we will discuss some more power
semiconductor switching devices used in power electronics.
Thank You.
R S Ananda Murthy Semiconductor Power Switching Devices-1