What are the different functions of diodes in DC and AC systems?

一, Differences in physical mechanisms: bidirectional adaptation of unidirectional conductivity
The core characteristic of a diode comes from the unidirectional conductivity of the PN junction: it conducts when forward biased (with a resistance of about tens of ohms) and turns off when reverse biased (with a resistance of megaohms). This feature needs to be functionally adapted through different methods in DC and AC systems.

DC system: static unidirectional control
In a DC system, diodes are in a stable unidirectional conducting or cutoff state. For example, in the DC bus of a photovoltaic inverter, parallel freewheeling diodes continue to conduct when the IGBT is turned off, providing a freewheeling path for the inductor current. Its DC resistance (forward voltage drop of about 0.7V) determines power loss, while the reverse recovery time (usually nanosecond level) has a relatively small impact in DC scenarios.

Communication system: dynamic cycle switching
In communication systems, diodes need to quickly switch their conduction state between the positive and negative half cycles of 50Hz/60Hz. Taking the bridge rectifier circuit as an example, four diodes conduct alternately to convert AC power into pulsating DC power. At this point, the AC resistance of the diode (which varies with frequency) and the reverse recovery time (which affects high-frequency losses) become key parameters. For example, fast recovery diodes (reverse recovery time<50ns) can reduce conduction losses by 15% in high-frequency switching power supplies.

二, Core functions in DC systems
1. Polarity protection and reverse isolation
At the input end of the DC power supply, a series connected diode can prevent the power supply from being reversed. When the polarity is reversed, the diode is subjected to reverse voltage and cut off, avoiding subsequent circuit damage. For example, the car charger uses a 1N4007 diode (reverse withstand voltage 1000V), which can withstand the transient high voltage when the car battery is reversed.

2. Continuation flow and energy release
In an inductive load circuit, diodes provide a freewheeling path for the inductor current. In the motor driver, the freewheeling diode conducts when the IGBT is turned off to prevent the back electromotive force of the inductor from penetrating the switch tube. Its reverse recovery characteristics directly affect system efficiency: ordinary diodes have a reverse recovery time of about 200ns, while Schottky diodes (without reverse recovery process) can improve efficiency by 3% -5%.

3. Voltage stabilization and voltage clamping
Zener diodes achieve precise voltage regulation in DC systems. For example, a 5.1V Zener diode connected in parallel in a 12V DC power supply can stabilize the output voltage at 5.1V ± 5%. The dynamic resistance (usually a few ohms) determines the voltage stabilization accuracy, while the power dissipation capability (such as 1W, 5W packaging) determines the application scenario.

三, Core functions in communication systems
1. Rectification and waveform conversion
Rectification is the fundamental function of diodes in AC systems. In a half wave rectification circuit, a single diode converts alternating current into pulsating direct current, with an efficiency of approximately 40.6%; The efficiency of the full wave rectification circuit (center tap transformer+two diodes) has been improved to 81.2%; The bridge rectifier circuit (four diodes) achieves full wave rectification without the need for a center tap, becoming the mainstream solution.

2. Detection and signal demodulation
In wireless communication, diodes achieve high-frequency signal detection. For example, in AM radios, diodes use unidirectional conductivity to extract audio signals, and their junction capacitance (usually a few picofarads) affects high-frequency response. Therefore, a dedicated diode for detection (such as 1N34A) needs to be selected.

3. Frequency conversion and mixing applications
In high-frequency circuits, the nonlinear characteristics of diodes enable frequency conversion. In a mixer, a double balanced structure consisting of two diodes can mix the input signal with the local oscillator signal to generate an intermediate frequency signal. The junction capacitance and series resistance determine the mixing efficiency, and surface mount diodes (such as HSMS-286x series) need to be used to reduce parasitic parameters.

四, Comparison of typical application scenarios
Functional Dimension DC System Application AC System Application
Protection function: power reverse protection, inductor surge voltage suppression, EMI filtering
Energy conversion photovoltaic cell MPPT control, battery charging AC-DC conversion, variable frequency drive
Signal processing, voltage regulation, logic gate construction, detection and demodulation, frequency mixing modulation
Typical devices include Schottky diodes, Zener diodes, fast recovery diodes, and rectifier diodes
五, Failure modes and system effects
DC system failure: risk of short circuit and thermal runaway
In DC systems, diode breakdown (especially thermal breakdown) may cause permanent short circuits. For example, the breakdown of the freewheeling diode in a photovoltaic inverter can cause the DC bus voltage to be directly applied to the IGBT collector, leading to module explosion. This type of fault requires dual protection through current limiting resistors (such as 0.1 Ω/5W) and fuses.

Communication system failure: waveform distortion and system paralysis
In communication systems, degradation of diode parameters (such as prolonged reverse recovery time) can cause distortion of the rectified waveform. In the motor driver, when the reverse recovery time of the rectifier diode increases from 50ns to 200ns, the harmonic distortion increases from 3% to 12%, causing the motor vibration to intensify. This type of fault requires predictive maintenance through online monitoring of diode junction temperature (infrared thermography).