Fast Recovery Diode: Working Principle and Key Parameter Analysis

What is a Fast Recovery Diode?

A Fast Recovery Diode(FRD/MUR) is a specially designed semiconductor device that is characterized by an extremely short reverse recovery time. These diodes perform exceptionally well in high-frequency power electronic circuits, not only possessing excellent forward recovery characteristics (low transient voltage and short recovery time), but also having outstanding reverse recovery characteristics (short recovery time, less recovery charge, and obvious soft recovery characteristics).

1.Core Feature Analysis

1.1 Characteristics of Activation

The fast soft recovery diode undergoes a unique transient stage during the activation process:

1. High-voltage transient phenomenon: A relatively high transient voltage drop (UFF) occurs at the beginning of activation, and then gradually stabilizes to the normal operating voltage.
2. Inductance effect manifestation: This characteristic makes the diode unable to immediately respond to changes in forward current.

The figure below shows the curve of diode voltage drop over time, where UFF is the forward peak voltage and trr is the forward recovery time.

The figure below shows the waveform of the diode's turn-on current, and the rate of current increase is represented by diF/dt.

1.2 Key parameters affecting:

1. Forward peak voltage (UFF)
2. Forward recovery time (trr)
3. Current rise rate (diF/dt)

Therefore, when the forward voltage rise rate exceeds 50A/µs, high-voltage diodes will exhibit more significant transient voltage drops, and this characteristic is particularly crucial in buffer circuit design.

1.3 Influencing factors include:

1. Internal structure design of the device
2. Lead length
3. Packaging material (especially magnetic material)

Design suggestion: The higher the current rise rate (di/dt), the greater the peak voltage (UFF), and the forward recovery time also extends accordingly.

2. Analysis of turn-off characteristics in depth

All PN junction diodes store charges in the form of minority carriers when conducting forward. This phenomenon brings two key effects:

1. Forward advantage: Minority carrier injection realizes charge modulation, reducing forward voltage drop (VF)
2. Reverse challenge: During turn-off, these stored charges need to be cleared, forming a reverse recovery process

Reverse recovery time (trr) is the time required for the diode to completely restore its blocking capability from the conducting state. During this process, the diode is equivalent to a short-circuit state.

3. Detailed explanation of key performance parameters

Importance of the softening coefficient (S)

The softening coefficient is the core parameter describing the reverse recovery characteristics:

Mathematical expression: S = (tb/ta)

Physical meaning: Reflects the rate at which reverse recovery current disappears from its maximum value (IRM)

Key discovery: The rate of reverse recovery current decline (di/dt) directly affects:

1. Reverse peak voltage (URM) size
2. System oscillation degree
3. Device safety and reliability
4. Design formula: URM = L × (di(rec)/dt)

Here, L represents the total inductance of the circuit, and this voltage must be lower than the rated value of the device.

Guide for selecting reverse recovery time (trr)

trr is the decisive parameter for diode selection:

• Ordinary diodes: approximately 25 µs (suitable for low-frequency circuits below 1 kHz)
• Fast recovery diodes: < 5 µs (suitable for chopping, inverter circuits)
• Ultra-fast recovery diodes: particularly suitable for absorption circuit applications

Optimization direction

The optimization of the softness factor depends on multiple design factors:

1.Minor carrier lifetime control technology

2.Base region width design

3.Diffusion concentration distribution optimization

4.Component structure innovation

5.Key structural parameter adjustment

SHYSEMI advice: By retaining more residual charges in the remaining base region after the space charge region expansion and prolonging the residence time, the softening coefficient can be effectively improved, achieving better soft recovery characteristics.

By deeply understanding these principles and parameters, engineers can select the most suitable fast soft recovery diode for high-frequency power electronic applications, optimize circuit performance and improve system reliability.