POWER SEMICONDUCTOR DIODES

Objectives

1. Identify different important features and steady-state and switching characteristics of power semiconductor diodes
2. Understand the importance of forward and reverse recovery times
3. Know the applications of different power semiconductor diodes in Power Electronic Circuits
4. Get idea on characteristics of a commercial diode

Power Diode: Features
Power diodes have larger power-, voltage-, and current handling capabilities than that of ordinary signal diodes.
Power diodes have an additional lightly doped drift region (n-) which helps to increase the voltage capability of the device.
However, the frequency response or switching speed is low compared to that of signal diodes.
Applications: 1) as uncontrolled rectifiers to convert AC to fixed DC voltages and 2) as freewheeling diodes to provide a path for the current flow in inductive loads.

I-V Characteristics (at steady state) of Power Diode

Forward-biased region, where v_D > 0. I_D is very small if v_D < V_TH (threshold or cut-in or turn-on or knee voltage). I_D is large if v_D >V_TH. The diode conducts fully and I_D is limited externally.

Reverse-biased region, where v_D < 0. The reverse leakage current, I_R (or I_S) is very small, in the range of micro- to milli-ampere due to the flow of the thermally generated carriers.

Breakdown region, where v_D < -V_BR (breakdown voltage). V_BR is avalanche or zener voltage, where avalanche breakdown begins to occur. The reverse current increases rapidly with a small change in reverse voltage beyond V_BR. If the current is limited external, the diode is not spoiled as long as the power dissipation (I_D*V_BR) < P_D,max.

Comparisons of Power and Signal Diodes
VTH

Power diodes: ~1V
Signal diodes: ~0.7V

Forward i-v Characteristics

Power diodes: linear
Signal diodes: exponential

Ron (ohmic resistance)

Power diodes: >0Ω
Signal diodes: ~0Ω

previous Control Characteristics Devices
next Switching Characteristics of Power Diodes

Control Characteristics Devices

Desired characteristics in controllable switches
The characteristics of ideal controllable switch are as below:

1. No power loss when ON
2. No power loss when OFF
3. No power loss during turning ON or OFF
4. Little power required to turn it ON or OFF
5. Adequate voltage and current ratings
6. Low Turn-on and Turn-off times

However, power semiconductor switches are not ideal. The following characteristics in a controllable switches are desirable:

1. Small leakage current in the off state.
2. Small on-state voltage drop.
3. Short turn-on and turn-off times.
4. Large forward- and reverse-voltage-blocking capability. (Reverse-voltage-blocking capability is not required in certain cases).
5. High on-state current rating.
6. Positive temperature coefficient of on-state resistance.
7. Small control power required to switch the device.
8. Capability to withstand rated voltage and rated current simultaneously while switching.
9. Large dv/dt and di/dt ratings.

previous Power Semiconductor Devices-Diode
next POWER SEMICONDUCTOR DIODES

Power Semiconductor Devices-Diode

Topics to be covered

1. Classification of power semiconductor devices
2. Rating of power semiconductor devices
3. Device symbol and characteristics
4. Control characteristics of few power semiconductor devices
5. Desirable characteristics of power semiconductor devices
6. DETAILS OF POWER DIODES

Classification of Power Electronic Devices
Broad classifications of Power Electronic devices are as follows:
(A)Based on family

1. DIODES: PN Junction Diodes, Schottky dodes
2. TRANSISTOR: Power BJT (Bipolar Junction Transistor), MD (Monolithic Darlington), Power MOSFET (Metal Oxide Semiconductor Field Effect Transistor), IGBT (Insulated Gate Bipolar Transistor)
3. THYRISTORS: SCR (Silicon Controller Rectifier), GTO (Gate Turn Off Thyristor), MCT (MOS-Controlled Thyristor) etc.

(B)Based on controllability

1. Uncontrolled Devices such as diodes: on and off sates controlled by the power circuit.
2. Pulse Driven Devices such as SCR and TRIAC:latched on by a control signal but must be turned off by the power circuit.
3. Controllable Devices: Two types which are turned on and off by control signals. Current-Driven Devices: BJT, MD, GTO
   

Voltage-Driven Devices: MOSFET, IGBT, MCT.

All devices are operated in the switching mode which has the advantages of high efficiency and high power capability.
(C) Based on material used
Classification of state-of-the-art power semiconductors

Device Symbols and Characteristics

next Control Characteristics Devices