Diode ＜Types of Diodes＞

Structure	Symbol	Applications・ Characteristics
		Used for rectification (i.e. primary side of the power supply block) Mainly 1A-class and higher, high breakdown voltage (400V/600V)

Rectifier diodes, as their name implies, are designed to rectify common AC frequencies. Rectification primarily involves conversion from AC to DC and may include high voltages and currents. Conversion efficiency can vary greatly depending on the operating frequency and conditions. As such, different types are offered, including low V_F(forward voltage), high-speed switching, and low-noise models.

Structure	Symbol	Applications･Characteristics
		Ideal for a variety of switching applications Switching speed：Short reverse recovery time trr

These diodes provide switching operation. Supplying voltage in the forward direction will cause current to flow (ON). Conversely, applying voltage in the reverse direction will stop current flow. Switching diodes are normally characterized by shorter reverse recovery times (trr), resulting in better switching performance.

Switch ON	Switch OFF
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What is Reverse Recovery Time (trr)?

The reverse recovery time trr refers to the time it takes for the switching diode to turn completely OFF from an ON state. Generally, electrons cannot be stopped immediately after operation turns OFF, resulting in some current flow in the reverse direction.The higher this leakage current is, the greater the loss. However, reverse recovery time can be shortened through heavy metal diffusion, material optimization, or by developing FRDs (Fast Recovery Diodes) that suppress ringing after recovery.

Structure	Symbol	Applications･Characteristics
		Used for secondary power supply rectification Low VF(low loss),large IR Fast switching speed

Unlike conventional diodes that provide diode characteristics through a PN (semiconductor-semiconductor) junction, Schottky barrier diodes utilize a Schottky barrier consisting of a metal-semiconductor junction.This results in much lower V_F characteristics (forward voltage drop) compared with a PN junction diodes, enabling faster switching speeds. However, one drawback is larger leakage current (I_R), making countermeasures necessary to prevent thermal runaway.

SBDs, which are often used for secondary power supply rectification, feature characteristics that can vary greatly depending on the type of metal used. ROHM offers a broad lineup of industry-leading SBDs that utilize a variety of different metals.

• RB**1 series low V_F type
• RB**0 series low I_R type
• ROHM offers the RB**8 series of ultra-low-I_R diodes for automotive applications

Thermal Runaway

Schottky barrier diodes are susceptible to excessive heat generation during large current flow. As a result, the combination of high heat with increasing I_R (leakage current) can cause both the case and ambient temperature to rise. Consequently, implementing incorrect thermal design may result in the amount of heat generated to exceed the amount dissipated, which can lead to increasing heat generation and leakage current and eventually result in damage. This phenomenon is referred to as "thermal runaway."

Structure	Symbol	Applications・Characteristics
		Used in constant voltage circuits Protects ICs from damage due to surge currents and ESD Generates a constant voltage when voltage is supplied in the reverse direction

Zener diodes are typically used in constant voltage circuits to ensure constant voltage even if the current fluctuates or as protection elements against surge currents and ESD. Unlike standard diodes that are employed in the forward direction, Zener diodes are designed to be used in the reverse direction. The reverse breakdown voltage of a Zener diode is referred to as the Zener voltage V_Z, and the current value at this time is called the Zener current (I_Z). In recent years, with the continuing miniaturization and increasing performance of electronic devices comes a need for more advanced protection devices, bringing about the emergence of TVS (Transient Voltage Suppression) diodes.

Basic Structure	Symbol	Applications and Features
*In the case of single direction		Protects later-stage ICs from unexpected surges caused by static electricity and power supply fluctuations. Typically utilizes the operating principle of constant voltage (Zener) diodes

An acronym for Transient Voltage Suppressor, TVS is designed for overvoltage and ESD protection.

TVS Diode Operation

TVS diodes are devices used to protect downstream ICs from unexpected overvoltages and surges caused by static electricity and power supply fluctuations.
In contrast to rectifier and Schottky barrier diodes that use forward characteristics, TVS diodes (like Zeners) take advantage of the reverse characteristics.
By placing the TVS diode in parallel with the IC as shown in the figure below, the diode operates normally in the OFF state, consuming only a certain amount of leakage current.
When an overvoltage such as surge occurs, the IC turns ON and clamps the overvoltage by allowing the pulse current to be consumed on the TVS side, protecting the subsequent stage IC.

TVS Diode Polarity (Unidirectional and Bidirectional)

The polarity of the TVS diode is a parameter related to circuit quality.
While unidirectional TVS is used as a protection element in LH and other single-pole circuits, it is not suitable for protecting bipolar circuits.
In contrast, bidirectional TVS can protect both the positive and negative poles, making it suitable for protecting data lines in bipolar circuits, CAN, and the like.
Bidirectional TVS can also be used to protect single-pole circuits.

Differences Between TVS and ZD

While TVS and ZD both take advantage of the reverse characteristics of the diode, ZD is mainly utilized for constant voltage applications, so the Zener voltage（V_Z）is specified in the low current range of 5mA to 40mA where the voltage is stable.
What’s more, it is typically used in the ON state.
For TVS, it is important to have breakdown voltage in the event sudden overvoltage such as surge is applied while also operating in the normally OFF state to prevent interference with the IC’s drive voltage.
As a result, there are two specified voltages for standoff（V_RWM）and breakdown（V_BR）, which are voltages that never cause breakdowns.
And since overvoltage protection is the main application, high current in the range of several A to tens of A are listed as part of the protection characteristics.

Structure	Symbol	Applications・Characteristics
		High-frequency switching→Suitable for mobile phones and the like Variable resistor element for AGC(※) and attenuator circuits ※AGC: Automatic Gain Control The diode (internal) capacitance (Ct)is very low

A high resistivity I-type semiconductor is utilized to provide significantly lower diode capacitance (C_t). As a result, PIN diodes act as a variable resistor with forward bias, and behave as a capacitor with reverse bias. High frequency characteristics (low capacitance ensures minimal effect of signal lines) make them suitable for use as variable resistor elements in a wide variety of applications, including attenuators, high-frequency signal switching (i.e. mobile phones requiring an antenna), and AGC circuits.

Forward Voltage	Reverse Voltage
Variable resistance with forward bias	Capacitor with reverse biase

What is the Diode Capacitance (C_t)

The amount of accumulated charge internally when supplying a reverse bias is called the diode capacitance (C_t). An electrically neutral depletion layer is formed by filling the intrinsic layer - created between P and N layers - with charge carriers (holes and electrons). The depletion layer acts as a parasitic capacitor, with a capacitance proportional to the PN junction area and inversely proportional to the distance d. The distance is determined by the concentration of the P and N layers. Supplying a voltage to the diode will increase the depletion layer and decrease C_t.The required Ct will vary depending on the application.

［When supplying a reverse voltage］