Power Diode World

Ideally, diodes should possess zero reverse recovery time, but achieving this may escalate manufacturing costs.

The diodes are generally classified based on the turn-OFF times.

The reverse recovery time (\(t_{rr}\) ) is a measure of the speed at which the diode can switch (turn-OFF) and therefore gives an indication on the external switching frequencies that may be used.

Based on this, the diodes are classified as follows:

1. Rectifier diodes (\(t_{rr}\) in microsecond range).

2. Fast recovery diodes (\(t_{rr}\) 200–500 ns).

3. Ultra fast recovery diodes (\(t_{rr}\) 30–200 ns).

4. Schottky diode (metal semiconductor junction –\(t_{rr}<\) 30 ns)

Reverse Recovery Time: Typically 25\(\mu\)s

Used in low-speed applications where recovery time is not critical (e.g., diode rectifiers and converters for low-input frequency up to 1 kHz applications and line-commutated converters).

Current Ratings: From less than 1 A to several thousands of amperes.

Voltage Ratings: From 50 V to around 5 kV.

Manufacturing: Generally manufactured by diffusion.

Alloyed Types: Used in welding power supplies.

Cost-Effectiveness and Ruggedness: Alloyed types are most cost-effective and rugged.

Ratings of Alloyed Types: Can go up to 1500 V, 400 A.

Recovery Time: Normally less than 5 \(\mu\)s.

Applications: Used in DC-DC and DC-AC converter circuits where recovery speed is critical.

Current Ratings: From less than 1 A to hundreds of amperes.

Voltage Ratings: From 50 V to around 3 kV.

Manufacturing for High Voltage Ratings (Above 400 V):

• Made by diffusion.

• Recovery time controlled by platinum or gold diffusion.

Manufacturing for Low Voltage Ratings (Below 400 V):

• Epitaxial diodes provide faster switching speeds.

• Epitaxial diodes have a narrow base width, resulting in recovery times as low as 50 ns.

Elimination of Charge Storage Problem: Achieved by setting up a barrier potential with a metal-semiconductor contact.

Rectifying Action: Depends on majority carriers only, minimizing excess minority carriers and recombination.

Recovery Effect: Due solely to the self-capacitance of the semiconductor junction.

Recovered Charge: Much less than that of an equivalent pn-junction diode.

Dependence on Reverse di/dt: Largely independent due to junction capacitance.

Forward Voltage Drop: Relatively low.

Leakage Current: Higher than that of a pn-junction diode, inversely proportional to conduction voltage.

Maximum Allowable Voltage and Current Rating: Generally limited to 100 V and 1-400 A.

Ideal Applications: High-current, low-voltage DC power supplies.

Various parameters in diode datasheets are crucial for selecting the right diode for specific applications.

Calculating these parameters beforehand is essential for ensuring proper functionality and reliability.

Important Parameters:

• Forward Voltage Drop (\(V_F\)): Voltage drop across the diode when forward biased.

• Reverse Leakage Current (\(I_R\)): Current flowing in reverse direction when the diode is reverse biased.

• Maximum Forward Current (\(I_F\)): Maximum continuous current the diode can handle in forward direction.

• Peak Forward Surge Current (IFSM): Maximum transient current the diode can withstand for a short duration.

• Maximum Reverse Voltage (\(V_R\)): Maximum voltage the diode can withstand in reverse bias without breakdown.

• Reverse Recovery Time (\(t_{rr}\)): Time taken by the diode to switch from conducting in the forward direction to blocking in the reverse direction.

• Power Dissipation (\(P_{d}\)): Maximum power the diode can dissipate without damage.

• Temperature Coefficient (\(\alpha V\)): Rate of change of forward voltage with temperature.

• Junction Capacitance (\(C_j\)): Capacitance across the diode junction.

• Operating Temperature Range: Temperature range within which the diode operates efficiently.

• Package Type: Physical packaging of the diode (e.g., SMD, through-hole).

• Reverse Recovery Charge (\(Q_{rr}\)): Total charge stored in the diode during the reverse recovery time.

• Forward Voltage Variation with Current (\(\Delta VF\)): Change in forward voltage drop with changing forward current.

• Reverse Recovery Energy (ERR): Energy dissipated during reverse recovery.

• Reverse Breakdown Voltage (\(V_{BR}\)): Voltage at which the diode starts conducting heavily in reverse bias.

• Diode Type: Whether it’s a standard diode, Schottky diode, Zener diode, etc.

• For sinusoidal currents, average, RMS, and maximum values are interrelated and widely understood. However, this relationship does not hold for non-sinusoidal currents.

• In non-sinusoidal cases, average, RMS, and peak values must be calculated based on the specific current and voltage waveforms of the diodes in the circuit.

• In high-power circuits, it’s crucial to ensure that the chosen diode ratings exceed the actual currents in the circuit for reliable operation.

• Detailed datasheets may present parameters using nomographs, providing another method for diode selection based on these graphical representations.