Okay, the selection of rectifier diodes is a fundamental and critical step in circuit design. The following are its core parameters, explained in logical order and importance:
Core parameters for selecting rectifier diodes:
Reverse repetitive peak voltage:
Definition: The highest peak voltage that a diode can withstand without breakdown (damage) under reverse bias. Usually labeled as' VRRM 'or' V_RRM '.
Importance: This is the primary safety parameter. When selecting, it is necessary to ensure that the VRRM is greater than or far greater than the maximum reverse peak voltage that may occur at both ends of the diode in the circuit (taking into account input voltage fluctuations, surges, spikes, and other margins). Insufficient selection can lead to instantaneous breakdown and failure of the diode.
Selection points: Calculate or measure the actual maximum reverse voltage in the circuit and multiply it by a safety factor (usually 1.5 to 2 times or even higher, depending on application reliability and risk level).
Average rectified output current:
Definition: The average forward current that a diode can continuously pass through for a long time under resistive or inductive loads and specified heat dissipation conditions. Usually labeled as' IF (AV) 'or' I2 (AV) '.
Importance: It determines the ability of diodes to handle load power. These are the main parameters of power consumption and heat generation sources.
Selection criteria: It must be greater than or significantly greater than the maximum average forward current flowing through the diode in the circuit. It is necessary to consider the environmental temperature and heat dissipation conditions, as the 'IF (AV)' provided in the data manual is usually given under specific shell temperature 'TC' or environmental temperature 'TA' and specific heat sink (if any) conditions. Under high temperature conditions, it is necessary to reduce the rating (refer to the rating curve in the data manual).
Maximum forward peak surge current:
Definition: The peak value of non repetitive forward surge current that a diode can withstand in a short period of time (usually one power frequency cycle or a few milliseconds). Usually labeled as IFSM or I-FSM.
Importance: Protect diodes from damage caused by surge currents during startup (such as charging large capacity capacitors) or unexpected short-circuit fault currents.
Selection key points: Evaluate the maximum surge current that may occur in the circuit (such as capacitor charging current, transient caused by lightning strikes or switch operations) and its duration, ensuring that the IFSM and its corresponding surge waveform (such as 8.3ms half sine wave) can cover these worst-case scenarios.
Positive pressure drop:
Definition: The voltage drop across a diode when passing a specific forward current. Usually labeled as' VF 'or' V_F ', and given under specific testing conditions (such as' IF', 'TJ'). It is a range value (minimum, typical, maximum).
Importance: Directly affects conduction loss (power consumption=VF IF) and system efficiency. Lower VF means lower heat generation and higher efficiency. At the same time, it also affects the effective output voltage of the low-voltage power supply system.
Selection points: For applications that are sensitive to efficiency or have limited heat generation (such as switch mode power supplies and battery powered equipment), priority should be given to selecting models with lower VF (such as Schottky diodes, which are usually lower than fast recovery/standard diodes) while meeting voltage and current requirements. Note that VF will increase with the increase of current and junction temperature (refer to the VF-IF curve in the data manual).
Reverse recovery time:
Definition: When a diode suddenly switches from a forward conducting state to a reverse biased state, it takes a period of time to clear the stored minority carriers and establish reverse blocking capability, which is called the reverse recovery time. Usually labeled as' trr '.
Importance: It is crucial for high-frequency switching circuits such as switching power supplies, PWM rectifiers, and inverters. An excessively long 'trr' can lead to:
Sudden increase in switch losses: During the 'trr' period, the diode conducts in reverse, generating a large reverse current 'IRRM' which, when multiplied by the reverse voltage 'VRRM', results in significant power loss.
Electromagnetic interference: Rapidly changing 'di/dt' (current change rate) can generate severe EMI.
Voltage spikes and oscillations: may interact with parasitic inductance in the circuit.
Selection points: In power frequency rectification (50/60Hz), standard rectifier tubes (trr in microseconds) are usually sufficient. In high-frequency switching applications (kHz to MHz), fast recovery diodes (trr below several hundred nanoseconds) or ultrafast recovery diodes (trr below several tens of nanoseconds), and even Schottky diodes (theoretically trr close to zero, but with junction capacitance issues) must be selected. When selecting, attention should be paid to the trr value and corresponding testing conditions (such as IF, di/dt).
Maximum working junction temperature:
Definition: The maximum safe temperature that the PN junction inside a diode can withstand. Usually labeled as' Tj 'or' TJ (max) '.
Importance: Determine the reliability and upper limit of the lifespan of the diode. Long term work at excessively high junction temperatures can accelerate component aging and even lead to thermal runaway failure. The actual working junction temperature Tj is determined by the power consumption (VF IF), ambient temperature Ta, and thermal resistance R θ JA/JC.
Key selection points: It is necessary to ensure that under the worst operating conditions (maximum IF, maximum VF, maximum Ta), the calculated actual working junction temperature Tj is much lower than TJ (max) (with sufficient margin). This usually requires thermal design in conjunction with thermal resistance parameters.
Other important considerations (closely related to core parameters):
Packaging and Thermal Resistance:
Thermal resistance (junction to environment/junction to shell): labeled as' R θ JA '/' R θ JC '. It measures the ease of heat transfer from the junction to the environment or shell. The lower the value, the stronger the heat dissipation ability. This is a key parameter for thermal design and ensuring that the junction temperature does not exceed the standard. When selecting, it is necessary to choose the appropriate packaging and calculate the thermal resistance based on the expected heat dissipation conditions (natural convection, air cooling, heat sink).
Packaging form: determines the installation method, heat dissipation capacity, current carrying capacity, and voltage isolation capacity (such as bolt type vs patch type). Need to match the circuit board space, heat dissipation requirements, and process.
Reverse leakage current:
The tiny current (` IR `) flowing under the maximum reverse voltage. Usually at the microampere or nanoampere level. It will significantly increase at high temperatures. Attention should be paid to low-power or high impedance circuits such as precision sample and hold.
Avalanche energy:
Annotated as' EAS'. Indicates the single pulse energy that a diode can absorb during avalanche breakdown (non-destructive). For applications that may withstand high voltage surges (such as lightning induction), selecting diodes with specified 'EAS' values (known as avalanche diodes) can provide additional protection margin.
Summary of selection logic:
Fixed voltage: Firstly, determine the required VRRM (with added safety margin) based on the maximum reverse voltage of the circuit.
Fixed current: Determine the required 'IF (AV)' based on the maximum average current of the load, taking into account heat dissipation conditions and temperature derating. Assess surge current demand (IFSM).
Fixed speed: Select the appropriate type of trr based on the operating frequency (standard tube for power frequency, fast recovery/ultrafast recovery/Schottky for high frequency).
Excellent efficiency/heat generation: Compare the VF and choose the model with lower conduction loss while meeting the first three criteria.
Temperature verification: Combining VF, IF, thermal resistance R θ, and ambient temperature Ta, calculate the actual working junction temperature Tj to ensure Tj<TJ (max) and leave a margin.
Check packaging: Choose the appropriate packaging form based on heat dissipation requirements, space limitations, and installation process.
Check special requirements: If there is a need for surge resistance, pay attention to 'EAS'; If there is a low leakage requirement, pay attention to 'IR'.
Key Tip:
The data manual is fundamental: the specific values and testing conditions of all parameters must be based on the official data manual of the selected model.
Application scenarios determine weights: Different applications have different focuses on parameters (such as switching power supplies that are most concerned with trr and VF); Pay attention to VRRM and IF (AV) in power supply; Pay attention to 'IR' for small signals.
Margin design: All key parameters (especially VRRM, IF (AV), Tj) must have sufficient design margin to improve system reliability and lifespan.
Simulation and testing: In complex or high demand applications, circuit simulation and actual testing should be conducted to verify the rationality of selection.
By systematically evaluating these core parameters and their interrelationships, and combining them with specific application scenarios and requirements, reliable and cost-effective diodes can be selected for rectifier circuits.