What are the different roles of diodes in full bridge and half bridge inverters?

1, Topology differences determine the fundamental functional positioning of diodes
(1) Minimalist architecture of half bridge inverter and the role of diode core
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. Taking inductive loads as an example, when the upper bridge arm switch M1 is turned on, the current path is Vdc/2 → M1 → load → Vdc/2, and diode D1 is in the reverse cutoff state. When M1 is turned off, the reverse electromotive force generated by the load inductance forms a freewheeling circuit through D2: load → D2 → Vdc/2, which achieves two major functions:

Voltage clamp: Limit the voltage that the switch tube 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.

(2) Redundant architecture and diode function expansion 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.

2, Dynamic response of diodes in energy management mechanism
(1) Unidirectional energy flow control of half bridge inverter
In a half bridge topology, diodes form a unidirectional energy transfer network. Taking the application of photovoltaic micro inverters as an example, when the load power is less than the input power, the inductive energy storage is fed back to the DC bus through D2, and the diode needs to meet the following requirements:

Positive voltage drop ≤ 0.5V (reducing conduction loss);
Reverse recovery charge Qrr ≤ 100nC (reducing switch losses).
According to test data from a certain manufacturer, using fast recovery diodes (such as STTH3R06) can increase system efficiency by 2.3% and reduce temperature rise by 15 ℃.

(2) Bidirectional energy regulation of full bridge inverter
The full bridge structure achieves more complex energy management through a diode network:

Active clamp: In transformer coupled inverters, D1-D4 can form a clamp circuit to limit the voltage of the switch tube within a safe range;
Soft switching implementation: Combined with resonant inductors, diodes can create zero voltage switching conditions. After adopting this technology, the switching loss of a 4kW full bridge inverter was reduced by 65%.
It is particularly noteworthy that in three-phase full bridge inverters, diodes also need to undertake the function of phase to phase energy balance. When the current of a certain phase leads, the diode of the corresponding bridge arm can guide the excess energy to flow to other phases, achieving dynamic power distribution.

3, Differential requirements of control strategy for diode characteristics
(1) Simple control and diode parameter matching 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.

(2) Complex modulation and dynamic adaptation of diodes in full bridge inverters
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 improve system efficiency to 98.2% and shorten dead zone time from 500ns to 200ns.

4, Performance comparison in typical application scenarios
Parameter half bridge inverter full bridge inverter
Number of diodes 2, 4
Voltage stress Vdc/2 Vdc
Current stress 1.5 times load current 2 times load current
Low control complexity (bipolar SPWM) and high (frequency doubling SPWM)
Typical efficiency 92-95% 95-98%
Cost coefficient 1.0 1.8