What electrical parameters should be considered when selecting diodes?

一, Basic electrical parameters: determine the basic performance of the device
1. Positive voltage drop (VF)
Forward voltage drop is the voltage difference between the anode and cathode when a diode is conducting, which directly affects circuit efficiency. The typical value of silicon diodes is 0.6-0.7V, while Schottky diodes can be as low as 0.2-0.4V. In low voltage and high current scenarios (such as DC-DC converters), reducing VF by 0.1V can increase efficiency by 2-3%. For example, in a 5V/3A output circuit, using a Schottky diode with VF=0.3V (such as 1N5819) can reduce power consumption by 12W compared to a regular silicon diode (VF=0.7V).
2. Maximum rectified current (IF)
This parameter defines the maximum average current that a diode can pass through under long-term stable operation, determined by the PN junction area and heat dissipation conditions. For example, the 1N4007 rectifier diode has a rated IF of 1A, but its actual peak current can reach 30A (non repetitive pulse). When selecting, it is necessary to consider:
Continuous working current: a margin of 20-30% should be left
Pulse working current: reference IFSM (non repetitive surge current) parameters
Heat dissipation design: TO-220 package has more than 5 times higher heat dissipation capacity than SOD-123 package
3. Reverse breakdown voltage (VBR) and maximum reverse operating voltage (VRM)
VBR is the critical voltage for diode reverse breakdown, while VRM typically takes 60-80% of VBR as the safe operating zone. For example, in a 220V AC rectifier circuit, diodes with VRM ≥ 600V (such as 1N4007 with VRM=1000V) need to be selected. Attention should be paid to special scenarios:
Transient overvoltage: Consider matching the VBR of TVS diode with the clamp voltage (VC)
High voltage application: PIN structure fast recovery diode can withstand thousands of volts of reverse voltage
4. Reverse leakage current (IR)
IR reflects the reverse cutoff capability of diodes, and for every 25 ℃ increase in temperature, IR increases by approximately 10 times. In high-voltage detection circuits, excessive IR can lead to measurement errors. For example, the 2AP germanium detector diode can achieve an IR of 100 μ A at VR=50V, while the silicon-based 1N4148 has an IR of<0.1 μ A under the same conditions.
二, Dynamic characteristic parameters: affecting high-frequency and switching performance
5. Reverse recovery time (trr)
TRR is the transition time of a diode from conduction to cutoff, which is crucial for the efficiency of switching power supplies. The traditional rectifier diode TRR can reach hundreds of nanoseconds, while fast recovery diodes (such as FR107) can shorten it to 50ns, and Schottky diodes can even lower it to a few nanoseconds. In a 500kHz switching power supply, using a fast recovery diode with trr=20ns can improve efficiency by more than 5% compared to ordinary diodes.
6. Junction capacitance (Cj)
Cj is composed of diffusion capacitors and barrier capacitors, which directly affect the integrity of high-frequency signals. In RF circuits, excessive Cj can cause signal attenuation and phase distortion. For example:
1N4148 small signal switch diode Cj=4pF (@ VR=0V)
HSMS-286x series Schottky diode Cj<0.6pF, suitable for GHz level applications
Varactor diodes can achieve continuous Cj variation by adjusting the reverse voltage, which is used for tuning circuits
7. Maximum operating frequency (fm)
Fm is jointly determined by trr and Cj, with a typical value range of:
Rectifying diode:<1kHz
Fast recovery diode: 10kHz-1MHz
Schottky diode: above 100MHz
Variable capacitance diode: capable of reaching GHz level
三, Extreme parameter: Ensure safe operation of the device
8. Non Repetitive Surge Current (IFSM)
IFSM defines the maximum pulse current that a diode is allowed to withstand within a 10ms period, typically 5-20 times that of IF. Key verification is required in scenarios such as motor start-up and capacitor charging:
1N5408 rectifier diode IFSM=200A (@ 10ms)
The actual surge energy needs to be calculated using the formula: E=I ² RMSt (where I is the surge current and t is the duration)
9. Junction temperature (Tj) and thermal resistance (R θ JA)
Tj is the highest temperature inside the chip, usually not exceeding 150 ℃ for silicon tubes. The thermal resistance R θ JA reflects the heat dissipation capability, for example:
SOD-123 packaging: R θ JA ≈ 300 ℃/W
TO-220 package (with heat sink): R θ JA<10 ℃/W
The actual junction temperature can be calculated using the formula Tj=Ta+P × R θ JA (where Ta is the ambient temperature and P is the power consumption).
10. Power dissipation (PD)
PD defines the maximum power consumption of a diode under specific heat dissipation conditions, which needs to be matched with the actual circuit power consumption. For example:
The PD of 1N4007 in free air is 1W
Under forced air cooling conditions, it can be increased to 3W
Actual power consumption needs to be calculated using P=VF × IF, with a 50% margin left
四, Special application parameters: key to scenario based selection
11. Voltage stabilization parameters (VZ, RZ)
Zener diodes need to pay attention to:
Stable voltage (VZ): accuracy can reach ± 1%, ± 2%
Dynamic Resistance (RZ): Reflects voltage stabilization performance, typical value 0.1-100 Ω
Voltage temperature coefficient: For example, the 2DW7C type voltage regulator has a temperature coefficient of+0.07%/℃
12. ESD protection parameters (TVS diode)
Transient voltage suppression diode needs to be verified:
Breakdown voltage (VBR): slightly higher than the operating voltage of the circuit
Clamp voltage (VC): the protective voltage at a specified pulse current
Peak Pulse Power (PPP): For example, the PPP of SMAJ5.0A TVS is 400W (@ 8/20 μ s waveform)
五, Selection Methodology: Four Step Method for Parameter Matching
Scenario definition: Clearly define the application type (rectification/switching/voltage regulation/protection)
Parameter filtering: Select models based on core parameters (VF/IF/VRM/trr)
Derating design: voltage/current at 80% rated value, temperature at 50% margin
Verification testing: Measure key parameters such as VF, IR, trr, etc. through actual circuit measurements
Typical case:
In the 48V/10A switching power supply output rectifier circuit, the selection steps are as follows
Determine requirements: VF<0.5V, IF ≥ 15A, VRM ≥ 60V, trr<50ns
Initial selection model: MBR2060CT Schottky diode (VF)= 0.45V@10A , IF=20A,VRM=60V,trr=10ns)
Thermal verification: Calculate Tj=25 ℃+(0.45V × 10A × 0.05 ℃/W)=47.5 ℃ (using copper substrate for heat dissipation)
Actual testing: VF=0.47V was measured under full load conditions, with a temperature rise of 22 ℃, which meets the design requirements