What is the function of diodes in inverter bridge circuits?

一, Bridge Circuit Topology: Energy Channels Constructed by Diodes
The bridge circuit consists of four switching devices (such as IGBT, MOSFET) and four diodes, forming a symmetrical "H" - shaped structure. According to the type of switching device, it can be divided into fully controlled bridge circuits (such as IGBT bridges) and semi controlled bridge circuits (such as thyristor bridges), but regardless of the type, diodes play a critical role.

Diode configuration in full bridge topology
In a fully controlled full bridge inverter, each switching device (such as IGBT) is connected in reverse parallel with a diode. For example, in an H-bridge composed of four N-channel MOSFETs, diodes D1-D4 are connected in reverse parallel with Q1-Q4, forming bidirectional current channels. When Q1 and Q4 are conductive, current flows from the positive pole of the DC side through Q1, load, and Q4 back to the negative pole; When Q1 is turned off and Q2 is turned on, the load current flows through D2 to avoid voltage spikes.
The role of diodes in half bridge topology
A half bridge circuit consists of two switching devices and two capacitors, with diodes mainly used for clamping in this scenario. For example, in photovoltaic inverters, the half bridge topology clamps the DC side voltage within a safe range through diodes to prevent damage to the switching devices due to overvoltage.
二, The core function of a diode: from freewheeling to energy feedback
1. Continuous current protection: suppresses the back electromotive force of inductive loads
When the inverter drives inductive loads (such as motors and transformers), the load current lags behind the voltage changes. At the moment when the switching device is turned off, the energy of the load magnetic field will form a high-voltage peak through the back electromotive force (EMF), which may break down the switching device. At this point, the reverse parallel diodes provide a freewheeling path for the load current, clamping the back electromotive force within a safe voltage range.

Case: In asynchronous motor drive, the stator winding can be equivalent to a series connection of resistance and inductance. When the IGBT is turned off, the winding current flows through the reverse parallel diode to prevent voltage spikes from damaging power devices. Experimental data shows that the failure rate of switching devices in inverters without freewheeling diodes is more than three times that of systems with diodes.

2. Energy feedback: achieving bidirectional flow of reactive power
The voltage type inverter requires a parallel capacitor on the DC side to provide a channel for reactive energy feedback from the AC side to the DC side. When the polarity of the output voltage is opposite to that of the current (such as in the inductive current stage of a resistive load), the reverse parallel diodes conduct, allowing reactive energy to be fed back to the DC side capacitor through the diodes, avoiding energy accumulation and voltage rise.

Comparative analysis: The DC side of the current source inverter is connected in series with a large inductor, and the reactive energy is buffered by the inductor without the need for feedback diodes; Voltage type inverters must rely on diodes to construct energy feedback channels, otherwise the system will collapse due to reactive power accumulation.

3. Dead zone compensation: Eliminating current distortion caused by switch delay
To prevent direct short-circuit of the bridge arm, the inverter control needs to insert a dead time (usually 1-5 μ s). During this period, the switching devices are all in the off state, but the load current still needs to flow. Reverse parallel diodes automatically conduct during dead time, maintaining current continuity and avoiding distortion of the output voltage waveform.

Experimental data: In a 10kW photovoltaic inverter without dead zone compensation diode, the output voltage THD (total harmonic distortion) reaches 8%; After introducing diodes, THD decreased to below 3%, significantly improving power quality.

三, Typical application scenarios: from industrial drive to new energy grid connection
1. Industrial frequency converter: high-precision motor control
In industrial frequency converters, bridge circuits achieve variable frequency speed regulation through PWM modulation. In this scenario, diodes need to withstand high-frequency switching stress (usually above 20kHz), so ultrafast recovery diodes (such as SiC diodes) are required. Their reverse recovery time can be shortened to less than 10ns, which is 10 times higher than traditional silicon-based diodes and significantly reduces switching losses.

Case: After replacing silicon-based devices with SiC diodes, the system efficiency of a rolling mill frequency converter in a certain steel enterprise increased from 96% to 98.5%, and the annual power savings reached 2 million kWh.

2. Photovoltaic inverter: Maximum Power Point Tracking (MPPT)
In photovoltaic grid connected inverters, the bridge circuit needs to achieve DC to AC conversion while maximizing power generation efficiency through MPPT algorithm. In this scenario, diodes need to balance low forward voltage drop and high withstand voltage capability. For example, using Schottky diodes can reduce the forward voltage drop from 0.7V to 0.3V, thereby reducing power loss.

Data comparison: In a 100kW photovoltaic inverter, using Schottky diodes can increase annual power generation by 12000 kWh and shorten the investment payback period by 6 months compared to ordinary diodes.

3. Electric vehicle motor controller: high-density power conversion
The electric vehicle motor controller needs to achieve high power density conversion in a limited space. The diodes in bridge circuits need to withstand high current densities (usually above 200A/cm ²), so a crimped diode module is required to achieve low thermal resistance connection through silver sintering technology, ensuring stable operation of the device at high temperatures of 150 ℃.

Technological breakthrough: The latest motor controller of a certain car company adopts a crimped SiC diode module, with a power density of 50kW/L, which is three times higher than traditional silicon-based solutions, and the system efficiency has exceeded 98.5%.