Types of Power Diodes

3 types of diodes:

1. standard or general-purpose diodes
2. fast-recovery diodes
3. Schottky diodes

General-purpose diodes: typically t_rr ~ 25μs, used for low-speed applications where recovery time is not critical (f < 1kHz), IF = less than 1A to several thousands of amperes, V_RB = 50V to ~5kV. Types: diffusion and alloyed. Alloyed rectifiers are cost-effective and rugged (for welding machines) with up to 300A and 1000V.

Fast-recovery diodes: normally t_rr < 5μs, used for applications where the speed of recovery is often critical importance, I_F = less than 1A to several hundreds of amperes, V_RB = 50V to ~3kV. Types: diffusion and epitaxial. Epitaxial diodes: <400V and as low as 50ns.

Schottky diodes: Advantages: low forward voltage drop, 0.3 to 0.4 V typically. Disadvantages: reverse leakage current > that of a pn junction diode and reverse biased breakdown voltage < 100V. In fact, a Schottky diode with relatively low forward voltage has relatively high leakage current, and vice versa. I_F = 1 - 100A. Suitable for low voltage applications.

Freewheeling Diodes
If an attempt is made to open the switch shown in the Figure below, the energy stored in the inductor will arc across the contacts of the switch, and could cause damage to the circuit components. Freewheeling diodes are placed across inductive loads to provide a path for the release of energy stored in the load when the load voltage drops to zero. Diode Dm is the freewheeling diode. The circuit operation is divided into two modes. Mode 1, which begins when the switch is closed, and Mode 2, which begins when the switch is opened.

Typical Few Data Sheet of Diodes
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Switching Characteristics of Power Diodes

Voltage and current waveforms for a power diode driven by currents with a specified di_F/dt and a specified di_R/dt.

1. Finite times are required for fully turn-on (t₁+t₂) and turn-off (t₃+t₄+t₅).
2. This is due to the excess carriers, space charges, and the bulk semiconductor material.

Two Types of Reverse Recovery Characteristics
t_rr = reverse recovery time, measured as the time between the initial zero crossing of the diode current to the time when this current reaches 25% of the peak reverse current. I_RR = maximum reverse current t_a = time between zero crossing and the maximum reverse current and it is due to the charge stored in the depletion region of the junction t_b = time between maximum reverse current IRR and 25% of the of the maximum reverse current IRR and is due to charge stored in the bulk semiconductor material The reverse recovery time is measured from the initial zero crossing from forward conduction to reverse blocking condition of the diode current to 25% of the maximum reverse current I_RR. Its magnitude depends on:

1. junction temperature
2. rate of fall of forward current
3. forward current prior to commutation

From the graph
 
Reverse Recovery Charge
This is the amount of charge carriers that flow across the diode in the reverse direction due to changeover from forward conduction to reverse blocking condition. Its value is determined from the area enclosed by the path of the reverse recovery current (Recall Δ Q = (Δ I)(Δ t)). That is

or   or
From above equations we get




Ideally diodes should not have a reverse recovery time, and it is possible to construct such a diode. However, the manufacturing cost of such a diode would be quite high for such a feature which in most cases has minor consequences
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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

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