MOSFET Applications & Failure Analysis in Inverters

1. Comparison of Half-Bridge, Push-Pull and Full-Bridge Inverter Circuits (DC-AC Stage)

1.1 Half-Bridge Inverter

Structure: Consists of 2 MOSFETs and 2 voltage-dividing capacitors. It features a simple circuit and low cost.

Working Principle: The two MOSFETs conduct alternately. The capacitors act as an auxiliary DC bus, generating alternating positive and negative voltages across the load to form AC power.

Features:

• Its voltage utilization rate is half that of a full-bridge circuit, resulting in lower output power under the same input condition.
• Suitable for low-power inverters and low-voltage & low-current applications.
• MOSFETs endure high voltage stress, so it is rarely used in high-voltage and high-power scenarios.

1.2 Push-Pull Inverter

• Structure: Composed of 2 MOSFETs and a center-tapped transformer.
• Working Principle: The two MOSFETs turn on in turn and supply power to two separate windings of the transformer respectively, inducing alternating voltage on the secondary side.
• Features:

Simple drive circuit with built-in isolation via the transformer.
Each MOSFET withstands twice the DC bus voltage, making it vulnerable to breakdown under high voltage.
Prone to transformer magnetic bias. Mainly applied to the front-stage DC-DC boost circuit, and seldom used as a standalone DC-AC inverter.

1.3 Full-Bridge (H-Bridge) Inverter (Mainstream Solution)

• Structure: Equipped with 4 MOSFETs, which work in two diagonal pairs.
• Working Principle: Diagonally paired MOSFETs switch synchronously, enabling bidirectional current flow through the load with the highest voltage utilization efficiency.
• Features:

Covers a wide power range, applicable to both low-power and high-power inverters.
Low voltage requirement for MOSFETs with well-distributed current.
Slightly complicated drive design. It is the standard topology for sine wave inverters.

2. Common Causes of MOSFET Damage in Inverters

2.1 Overvoltage Breakdown (Most Common)

• Causes: DC bus voltage surge, back EMF, lightning strike, or voltage spikes caused by load short-circuit. The voltage exceeds the MOSFET’s rated drain-source voltage (\(V_{DSS}\)).
• Symptoms: Permanent short-circuit between drain and source terminals; immediate device burnout or blown fuse once powered on.
• High-risk Scenarios: Startup/shutdown of inductive loads (motors, inductors) and grid backfeed.

2.2 Overcurrent Burnout

• Causes: Output short-circuit, overload, excessive inrush current at power-on; uneven current sharing among parallel-connected MOSFETs leading to overload on individual devices.
• Symptoms: Excessively hot component body, cracking of the device package and burned pins.

2.3 Damage Caused by Abnormal Gate Drive
Causes:

• Insufficient or unstable gate drive voltage makes the MOSFET operate in the linear region, which sharply increases on-resistance and eventually leads to thermal burnout.
• Insufficient dead time causes shoot-through (simultaneous conduction of upper and lower MOSFETs on the same arm), resulting in direct short-circuit of the DC bus and instantaneous heavy current that burns out the devices.
• Improper selection of gate resistance or electrostatic damage to the gate terminal.

2.4 Thermal Failure from Excessive Switching Loss

Causes: Excessively high switching frequency, mismatched switching speed of MOSFETs generate massive heat during switching; insufficient heat sink or blocked heat dissipation channels.
Symptoms: Continuous overtemperature under full load gradually degrades device performance and eventually causes thermal breakdown.

2.5 Failure of Freewheeling & Snubber Circuits

Causes: Damaged freewheeling diodes or RC snubber circuits fail to clamp voltage spikes generated by inductive loads, leading to repeated impact on MOSFETs.

3. Basic Device Selection & Protection Guidelines

• Voltage Rating: Reserve a safety margin of 1.5 ~ 2 times the maximum operating voltage.
• Gate Drive: Set reasonable dead time, adopt matched gate resistors, and connect Zener diodes at the gate to prevent overvoltage.
• Heat Dissipation: Equip high-power designs with adequate heat sinks and thermal grease.
• Surge Protection: Add RC snubbers, varistors and fast recovery diodes to suppress voltage spikes.
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