Infineon BUZ81 Power MOSFET: Datasheet Analysis and Application Circuit Design

The Infineon BUZ81 stands as a classic and enduring symbol of robustness in the realm of power MOSFETs. Designed for high-voltage, high-speed switching applications, this N-channel enhancement-mode MOSFET continues to be a reliable choice for designers seeking a proven solution. This article delves into a detailed analysis of its datasheet and outlines key considerations for its application in circuit design.

Datasheet Analysis: Deconstructing the BUZ81

A thorough examination of the BUZ81 datasheet reveals its core electrical characteristics, which are paramount for any design.

Voltage and Current Ratings: The BUZ81 is characterized by a high drain-source voltage (V_DSS) of 500V, making it suitable for off-line switch-mode power supplies (SMPS), electronic ballasts, and motor control circuits operating from rectified mains voltages. The continuous drain current (I_D) is specified at 4.5A at a case temperature (T_C) of 25°C, though this value must be derated based on thermal management conditions.

On-State Resistance (R_DS(on)): A critical parameter for efficiency, especially in switching applications, is the on-state resistance. The BUZ81 boasts a maximum R_DS(on) of just 0.9 Ω (at I_D = 2.25A, V_GS = 10V). A low R_DS(on) minimizes conduction losses, leading to cooler operation and higher overall system efficiency.

Dynamic and Switching Characteristics: The device features a low gate charge (typically 18 nC) and fast switching speeds. The turn-on delay (t_d(on)) is typically 10 ns, and the rise time (t_r) is 50 ns, enabling high-frequency operation which is essential for reducing the size of magnetic components in power supplies.

Safe Operating Area (SOA): The datasheet provides a forward-biased safe operating area (FBSOA) graph, which is crucial for ensuring the device operates within its thermal and electrical limits during switching transitions, particularly when managing inductive loads.

Application Circuit Design: A Switch-Mode Power Supply Example

A common application for the BUZ81 is in the primary side of a flyback converter, a popular topology for low-to-medium power AC/DC adapters.

Key Design Considerations:

1. Gate Driving: To achieve the fast switching speeds the BUZ81 is capable of, a dedicated gate driver IC is highly recommended. The driver must be able to source and sink sufficient current to quickly charge and discharge the MOSFET's input capacitance (C_iss), minimizing switching transition times and associated losses. A gate resistor (e.g., 10-100Ω) is used to dampen ringing and control the rise/fall time.

2. Voltage Spikes and Snubbers: When switching off an inductive load like a transformer primary, voltage spikes exceeding the V_DSS rating can occur due to leakage inductance. An RCD (Resistor-Capacitor-Diode) snubber network is often placed across the drain and source to clamp these voltage spikes and protect the MOSFET from avalanche breakdown.

3. Thermal Management: Despite its low R_DS(on), power dissipation (I²R losses plus switching losses) will generate heat. A properly sized heatsink is mandatory to maintain the junction temperature (T_J) well below the absolute maximum rating of 150°C. Thermal calculations must be performed based on the estimated power loss and the system's thermal resistance.

4. Protection Circuits: Practical designs should incorporate overcurrent protection, often implemented using a sense resistor in the source path connected to a comparator or a dedicated SMPS controller. This safeguards the MOSFET and the circuit during fault conditions like a shorted output.

A typical circuit would involve the BUZ81 with its drain connected to the transformer primary, source to ground (via a current sense resistor), and gate connected to the output of a PWM controller via the gate driver circuit.

ICGOOODFIND: The Infineon BUZ81 remains a highly capable power MOSFET for high-voltage switching. Its enduring relevance is anchored in its robust 500V voltage rating, low on-state resistance, and fast switching capability. Successful implementation hinges on careful attention to gate driving, thermal management, and protection against voltage transients, ensuring reliable and efficient performance in demanding applications like switch-mode power supplies.

Keywords:

1. Power MOSFET

2. High-Voltage Switching

3. On-State Resistance (RDS(on))

4. Gate Driver

5. Switch-Mode Power Supply (SMPS)