How do diodes and MOSFETs/IGBTs work together in inverters?

1, Functional complementarity in topology architecture
(1) Minimalist collaborative mode of half bridge inverter
The half bridge inverter adopts a dual switch dual diode structure, and the DC side forms two potential points of ± Vdc/2 through capacitor voltage division. When the upper bridge arm MOSFET (Q1) is turned on, the current path is Vdc/2 → Q1 → load → Vdc/2, and at this time, the lower bridge arm diode (D2) is in a reverse cutoff state. When Q1 is turned off, the reverse electromotive force generated by the load inductance forms a freewheeling circuit through D2: load → D2 → Vdc/2. This process achieves two core functions:

Voltage clamp: Limit the voltage that MOSFET can withstand to Vdc/2 to avoid overvoltage breakdown;
Energy feedback: Provides a release channel for inductive energy storage to prevent voltage spikes caused by sudden changes in current.
Experimental data shows that in a 1kW half bridge inverter system, the peak freewheeling current of D2 can reach 1.5 times the rated load current, and its reverse recovery time needs to be controlled within 100ns to ensure switching efficiency. The use of fast recovery diodes (such as STTH3R06) can increase system efficiency by 2.3% and reduce temperature rise by 15 ℃.

(2) Redundant collaborative architecture of full bridge inverter
The full bridge inverter adopts a four switch four diode structure, which achieves output voltage polarity reversal through the alternating conduction of two pairs of switches. Its uniqueness is reflected in:

Bipolar control: Through the combination of T1-T4 conduction, a complete voltage swing of ± Vdc can be obtained at the load end. The diodes D1-D4 not only undertake the freewheeling function, but also form an energy feedback channel;
Fault protection: When T1 and T4 are both misguided, D2-D3 can form a short-circuit protection path to prevent DC bus short circuit.
Comparative testing shows that the peak reverse voltage borne by diodes in the full bridge structure is reduced by 50% compared to the half bridge structure, but higher transient currents (up to twice the load current) need to be handled. In a three-phase full bridge inverter, diodes also need to undertake the function of phase to phase energy balance. When the current of a certain phase leads, the diodes of the corresponding bridge arm can guide the excess energy to flow to other phases, achieving dynamic power distribution.

2, Energy management mechanism in dynamic response
(1) Continuous Current Protection of MOSFET Body Diode
The body diode integrated inside MOSFET plays a key role in inverters. When the inductive load is connected to the MOSFET drain, electrical energy is immediately stored inside the load, and the reverse EMF peak generated at the moment of shutdown forms a freewheeling path through the body diode. Taking brushless DC motor drive as an example:

High frequency switching scenario: During the high-frequency switching of MOSFET Q1, the body diode D2 provides a freewheeling path for the inductor current during Q1's turn off period;
Current spike suppression: Inductance L1 exhibits high impedance to the spike current, resulting in additional current spikes when Q1 conducts. By using MOSFETs with fast body diode recovery characteristics (such as ST's SuperFREDmesh series), switch losses can be reduced by 65%, and the shell temperature can be lowered from 60 ℃ to 50 ℃.
(2) Energy feedback of IGBT anti parallel diode
As the mainstream device in high-voltage and high current scenarios, IGBT's anti parallel fast recovery diode (FRD) plays a core role in bidirectional energy flow. In a series resonant inverter:

Dead time management: During IGBT commutation in the upper and lower bridge arms, anti parallel diodes provide a path for reactive current to avoid voltage spikes caused by stray inductance in the circuit;
Resonant energy absorption: When VT1 is turned off, the stored energy in the stray inductance Lm of the line is transferred to the buffer circuit through the anti parallel diode VD1 to prevent Uce overshoot.
Experiments have shown that using high-performance fast recovery diodes (such as C3D10060E) can reduce the switching losses of IGBT modules by 40% and improve system efficiency to 98.2%.

3, Parameter matching requirements in control strategies
(1) Simple Control Adaptation of Half Bridge Inverter
The half bridge structure usually adopts bipolar or unipolar SPWM control, and the requirements for diodes are focused on static characteristics:

Reverse recovery time: trr ≤ 50ns (suitable for high-frequency switching);
Junction capacitance: Cj ≤ 100pF (reduces switch noise).
According to the selection data of a certain car inverter project, the use of ultra fast recovery diodes (such as MUR860) can reduce electromagnetic interference (EMI) by 8dB and shorten the dead zone time from 500ns to 200ns.

(2) Complex modulation adaptation of full bridge inverter
The full bridge structure supports advanced modulation technologies such as frequency doubling SPWM, which imposes higher dynamic requirements on diodes

Temperature stability: Within the range of -40 ℃~150 ℃, the forward pressure drop change rate should be ≤ 5mV/℃;
Anti avalanche capability: It needs to withstand avalanche energy at least 1.5 times the rated current.
A certain industrial motor drive case shows that using silicon carbide diodes (such as C3D10060E) can reduce the system volume by 40% and increase the power density to 3.2kW/L. Its key advantages lie in:

Reverse recovery of charge Qrr reduces by 70%;
The stability of conduction pressure drop is increased by three times in high temperature environments.