What is a half bridge?

It is widely utilised in various applications, including power supplies, motor drives, and inverters, due to its simplicity and efficiency. Understanding the half bridge's structure, operation, and applications is crucial for engineers working in fields related to electrical design and power management.

Structure of a half bridge

A half bridge consists of two semiconductor switches, typically MOSFETs or IGBTs, connected in series across a DC power supply. The midpoint of these switches is connected to the load, which could be a resistor, inductor, or transformer, depending on the application. Each switch is accompanied by a freewheeling diode, allowing current to circulate when the switches are in their off state. Additionally, capacitors are often placed across the power supply to stabilise the voltage and filter out noise.

The basic structure includes:

• Two switches (S1 and S2): These are power transistors like MOSFETs or IGBTs.
• Two diodes (D1 and D2): These are placed in parallel with the switches to handle the freewheeling current.
• A DC power supply: Provides the input voltage.
• A load: Connected to the midpoint between the switches.

Operation of a half bridge

The operation of a half bridge involves alternating the switching states of S1 and S2. This creates a pulsating output voltage at the midpoint, which is then applied to the load. The switches are controlled such that when one is on, the other is off, ensuring that both are never on simultaneously to prevent short-circuiting the power supply. This alternation results in a square wave or pulse-width modulated (PWM) waveform, depending on the control scheme used.

Key operational aspects include:

• Switching sequence: S1 and S2 are turned on and off alternately.
• Dead time: A brief period when both switches are off to avoid shoot-through (simultaneous conduction causing a short circuit).
• Output voltage: The output is a bipolar waveform that oscillates between positive and negative values relative to the DC supply midpoint.

Applications of half bridge circuits

The half bridge is highly versatile and finds use in a variety of applications:

• DC-DC converters: In power supplies, half-bridge converters efficiently step down or step up DC voltages.
• Motor drives: Half bridges are employed in controlling the speed and direction of motors by modulating the voltage and current supplied.
• Inverters: Used in converting DC to AC, particularly in uninterruptible power supplies (UPS) and renewable energy systems like solar inverters.
• Induction Heating: Half bridges generate high-frequency AC signals necessary for induction heating applications.

Advantages and disadvantages

Advantages

• Simplicity: The half bridge has a straightforward design, making it easier to implement and control.
• Efficiency: It offers high efficiency due to reduced switching losses, especially when MOSFETs are used.
• Cost-effective: Fewer components are required compared to full-bridge configurations, reducing overall cost.

Disadvantages

• Limited output power: Half bridges are typically less powerful than full-bridge configurations, which can be a limitation in high-power applications.
• Voltage stress: The switches endure higher voltage stress, requiring components that can handle such conditions.

Conclusion

The half bridge is a critical component in modern power electronics, offering a balanced trade-off between simplicity, efficiency, and cost. Its applications are vast, spanning from power supplies to motor drives, making it an indispensable tool for electrical design engineers. Understanding its operation, benefits, and limitations allows engineers to harness its potential effectively, ensuring optimal performance in their applications.

(Image credit By Cyril BUTTAY - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=854051)