Series connecting low voltage MOSFETs is a technique often employed in various electronic applications to achieve higher voltage ratings, handle larger currents, or improve overall system performance. As a supplier of low voltage MOSFETs, I’ve encountered numerous inquiries regarding the proper way to series connect these components. In this blog, I’ll share in – depth insights into the process, challenges, and best practices for series connecting low voltage MOSFETs. Low Voltage Mosfet
Understanding Low Voltage MOSFETs
Before delving into series connection, it’s crucial to understand what low voltage MOSFETs are. Low voltage MOSFETs typically have a breakdown voltage (VDS) in the range of a few volts to around 100 volts. They are widely used in applications such as battery – powered devices, power management circuits, and low – voltage motor control. These MOSFETs offer advantages like low on – resistance (RDS(on)), fast switching speeds, and high efficiency.
Reasons for Series Connecting Low Voltage MOSFETs
There are several compelling reasons to series connect low voltage MOSFETs. Firstly, when the required voltage rating of a circuit exceeds the breakdown voltage of a single low voltage MOSFET, series connection can be used to achieve a higher overall voltage rating. For example, if you have a 24V application but your MOSFETs have a breakdown voltage of only 12V, connecting two MOSFETs in series can handle the 24V without risking breakdown.
Secondly, series connection can help in current sharing. In high – current applications, a single MOSFET may not be able to handle the entire current load. By connecting multiple MOSFETs in series, the current can be distributed among them, reducing the stress on each individual device and improving reliability.
Challenges in Series Connecting Low Voltage MOSFETs
However, series connecting low voltage MOSFETs is not without its challenges. One of the main issues is voltage sharing. In an ideal scenario, when MOSFETs are connected in series, the voltage across each MOSFET would be evenly distributed. But in reality, due to variations in the MOSFETs’ parameters such as threshold voltage (Vth), on – resistance (RDS(on)), and gate – source capacitance (Cgs), the voltage across each MOSFET may not be equal.
Uneven voltage sharing can lead to over – stress on one or more MOSFETs, which can cause premature failure. For instance, if one MOSFET has a slightly lower on – resistance than the others in a series connection, it will carry more current and thus have a higher voltage drop across it, potentially exceeding its breakdown voltage.
Another challenge is the switching behavior. When MOSFETs are connected in series, the gate – drive signals need to be carefully synchronized. Any delay in the switching of one MOSFET compared to the others can result in high voltage spikes and increased power dissipation, which can also damage the MOSFETs.
Steps to Series Connect Low Voltage MOSFETs
1. Selecting Compatible MOSFETs
The first step in series connecting low voltage MOSFETs is to select MOSFETs with similar electrical characteristics. Look for MOSFETs from the same batch or with closely matched datasheet parameters, especially Vth, RDS(on), and Cgs. This helps to minimize the differences in their performance and improve voltage sharing.
2. Voltage Balancing Circuits
To ensure even voltage sharing across the series – connected MOSFETs, voltage balancing circuits are often used. One common approach is to use resistors in parallel with each MOSFET. These resistors, known as bleeder resistors, help to equalize the voltage across the MOSFETs by providing a path for leakage currents.
The value of the bleeder resistors should be chosen carefully. A too – high resistance value may not be effective in voltage balancing, while a too – low resistance value can cause excessive power dissipation. As a general rule, the resistance value should be in the range where it can shunt the leakage current effectively without drawing too much current from the circuit.
3. Gate – Drive Design
Proper gate – drive design is essential for series – connected MOSFETs. The gate – drive signals should be synchronized to ensure that all MOSFETs turn on and off at the same time. This can be achieved using a dedicated gate – driver circuit or by using a single gate – driver with proper isolation techniques.
Isolation is important to prevent interference between the gate – drive signals of different MOSFETs. Opto – couplers or transformer – based isolation can be used to isolate the gate – drive circuits of each MOSFET.
4. Thermal Management
Since series – connected MOSFETs may dissipate significant power, proper thermal management is crucial. Ensure that the MOSFETs are mounted on a suitable heatsink to transfer the heat away effectively. Thermal pads or thermal grease can be used to improve the thermal contact between the MOSFETs and the heatsink.
Testing and Verification
After series connecting the low voltage MOSFETs, it’s important to test and verify the circuit. Use a high – voltage probe to measure the voltage across each MOSFET to ensure that the voltage sharing is within an acceptable range. Monitor the temperature of the MOSFETs during operation to make sure that they are not overheating.
You can also perform dynamic tests by switching the MOSFETs at different frequencies and loads to check for any abnormal behavior such as voltage spikes or excessive power dissipation.
Application Examples
Battery – Powered Inverters
In battery – powered inverters, series – connected low voltage MOSFETs can be used to handle the high – voltage output required for AC power conversion. By connecting multiple MOSFETs in series, the inverter can operate at a higher voltage while still using low voltage MOSFETs, which are often more cost – effective and have better performance characteristics.
Electric Vehicle (EV) Charging Stations
EV charging stations require high – current and high – voltage capabilities. Series – connecting low voltage MOSFETs can be a viable solution to achieve the desired voltage and current ratings. The series – connected MOSFETs can handle the charging currents and voltages safely while ensuring efficient power conversion.
Conclusion
Series connecting low voltage MOSFETs is a valuable technique for achieving higher voltage ratings and better current handling in electronic circuits. However, it comes with challenges such as voltage sharing and synchronized switching that need to be carefully addressed. By selecting compatible MOSFETs, using voltage balancing circuits, proper gate – drive design, and effective thermal management, you can successfully series connect low voltage MOSFETs.
Transistor If you’re involved in an application where series – connected low voltage MOSFETs could be beneficial, I encourage you to reach out. As a trusted supplier of low voltage MOSFETs, I can offer high – quality products and technical support to help you with your projects. Whether you need more information on the right MOSFETs for your specific requirements or assistance in implementing a series – connection solution, I’m here to help. Contact me for a detailed discussion and to explore how we can work together to meet your needs.
References
- "MOSFET Power Electronics Handbook" by Ned Mohan
- "Application Notes on Low Voltage MOSFETs" by major semiconductor manufacturers
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