A Cheaper Alternative to Relays When Driving 240 VDC loads — 8-MOSFET

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Driving high voltage direct current (DC) loads can be a challenging task, especially when considering the cost and reliability of traditional solutions such as electromechanical relays. In many industrial and commercial applications, devices operating at 240 VDC or higher are common, and finding cost-effective and reliable switching solutions is crucial. This article introduces an alternative approach using a configuration of eight metal-oxide-semiconductor field-effect transistors (MOSFETs) to replace relays in driving 240 VDC loads.

Understanding the Challenge

When working with high voltage DC loads, traditional electromechanical relays have been the go-to solution for decades. However, relays have several limitations, including:

  1. Limited Lifespan: Relays are mechanical devices with moving parts, which can wear out over time, leading to failures and the need for frequent replacements.
  2. Arcing and Contact Erosion: The switching action of relays can cause arcing and contact erosion, further reducing their lifespan and reliability.
  3. Relatively High Cost: High-voltage relays capable of handling 240 VDC or higher can be expensive, especially when multiple relays are required in a system.
  4. Bulky Size: Relays have a relatively large physical footprint, making them less suitable for compact designs or applications with space constraints.

To address these limitations, solid-state switching solutions have gained popularity in recent years. One such solution involves the use of power MOSFETs, which offer several advantages over relays, including increased reliability, faster switching speeds, and higher energy efficiency.

The 8-MOSFET Configuration

The proposed 8-MOSFET configuration is a solid-state switching solution designed to replace relays in driving 240 VDC loads. This configuration utilizes eight N-channel power MOSFETs arranged in a specific manner to handle the high voltage and current requirements while providing reliable and cost-effective operation.

Circuit Diagram

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The circuit diagram above illustrates the 8-MOSFET configuration. Here’s a breakdown of the components:

  1. Q1-Q8: Eight N-channel power MOSFETs capable of handling the required voltage and current ratings.
  2. R1-R8: Gate resistors to control the switching speed and prevent oscillations.
  3. D1-D8: Flyback diodes to suppress inductive kickback and protect the MOSFETs.
  4. C1-C8: Gate capacitors to stabilize the gate voltage and prevent false triggering.
  5. VCC: Control voltage supply (typically 12-15 VDC) for driving the MOSFET gates.
  6. GND: Ground reference.
  7. Load: The 240 VDC load to be driven by the circuit.

Operating Principle

The 8-MOSFET configuration works by splitting the high voltage across multiple MOSFETs, effectively reducing the voltage stress on each individual device. By connecting the drain terminals of the MOSFETs in series and the source terminals in parallel, the configuration can handle the desired voltage and current levels.

When the control voltage (VCC) is applied to the gates of the MOSFETs through the gate resistors and capacitors, the devices turn on, allowing current to flow through the load. Conversely, when the control voltage is removed, the MOSFETs turn off, effectively cutting off the current to the load.

The flyback diodes (D1-D8) are essential components in this configuration. They provide a path for the inductive energy stored in the load to dissipate when the MOSFETs turn off, preventing potentially damaging voltage spikes from occurring across the devices.

Advantages over Relays

The 8-MOSFET configuration offers several advantages over traditional electromechanical relays when driving 240 VDC loads:

  1. Increased Reliability: MOSFETs are solid-state devices without moving parts, significantly reducing the risk of mechanical failures and increasing overall reliability.
  2. Longer Lifespan: MOSFETs have a much longer lifespan compared to relays, reducing the need for frequent replacements and maintenance.
  3. No Arcing or Contact Erosion: Since MOSFETs operate purely electronically, there is no arcing or contact erosion, further enhancing reliability and longevity.
  4. Faster Switching Speeds: MOSFETs can switch much faster than relays, enabling precise control and responsiveness in applications that require rapid switching.
  5. Higher Energy Efficiency: MOSFETs have lower power dissipation and heat generation compared to relays, resulting in improved energy efficiency.
  6. Compact Size: The 8-MOSFET configuration can be implemented in a relatively small footprint, making it suitable for space-constrained applications.
  7. Cost-Effective: By utilizing readily available and low-cost power MOSFETs, the 8-MOSFET configuration can provide a more cost-effective solution compared to high-voltage relays, especially when multiple switching channels are required.

Design Considerations

When implementing the 8-MOSFET configuration, there are several design considerations to keep in mind:

Component Selection

  1. MOSFETs: Choose N-channel power MOSFETs with appropriate voltage and current ratings for the application. Consider factors such as drain-source breakdown voltage (VDS), continuous drain current (ID), and on-resistance (RDS(on)).
  2. Gate Resistors: Select gate resistor values that balance the switching speed and potential oscillations. Typical values range from a few ohms to tens of ohms.
  3. Flyback Diodes: Use fast-recovery diodes with a reverse voltage rating higher than the maximum expected voltage across each MOSFET. The diode’s current rating should match or exceed the expected load current.
  4. Gate Capacitors: Choose gate capacitors with appropriate voltage ratings and capacitance values to stabilize the gate voltage and prevent false triggering due to noise or transients.

Heat Dissipation

MOSFETs generate heat during switching and conduction, which needs to be properly dissipated to ensure reliable operation. Depending on the load current and switching frequency, adequate heat sinking or cooling solutions may be required. Proper thermal management is crucial to prevent overheating and ensure optimal performance and longevity.

Gate Drive and Control

The 8-MOSFET configuration requires a control voltage (VCC) to drive the MOSFET gates. This control voltage can be sourced from a dedicated power supply or generated using appropriate circuitry. Ensure that the control voltage is within the recommended operating range specified by the MOSFET manufacturer.

Additionally, consider implementing appropriate protection mechanisms, such as overcurrent protection, overvoltage protection, and short-circuit protection, to safeguard the circuit from potential faults and damage.

Layout and PCB Design

Proper layout and printed circuit board (PCB) design are essential for the successful implementation of the 8-MOSFET configuration. Follow best practices for high-frequency and high-voltage PCB design, such as minimizing trace lengths, using appropriate trace widths, and ensuring proper grounding and shielding. Pay attention to potential sources of noise and interference, and incorporate appropriate filtering and decoupling techniques as necessary.


The 8-MOSFET configuration can be applied in various industrial, commercial, and residential applications where reliable and cost-effective switching of high-voltage DC loads is required. Some potential applications include:

  1. Industrial Automation: Controlling high-voltage DC motors, solenoids, and other actuators in manufacturing processes.
  2. Renewable Energy Systems: Switching high-voltage DC loads in solar and wind energy systems.
  3. Telecommunications: Switching high-voltage DC loads in telecommunications equipment and infrastructure.
  4. Transportation: Controlling high-voltage DC loads in electric vehicles, trains, and other transportation systems.
  5. Building Automation: Switching high-voltage DC loads in building management systems, lighting control, and HVAC systems.
  6. Test and Measurement Equipment: Switching high-voltage DC loads in various testing and measurement applications.

FAQs (Frequently Asked Questions)

1. Can the 8-MOSFET configuration handle AC loads or only DC loads?

The 8-MOSFET configuration is specifically designed to handle high-voltage direct current (DC) loads. It is not suitable for alternating current (AC) loads without additional circuitry to rectify the AC input.

2. What is the maximum voltage and current rating of this configuration?

The maximum voltage and current rating of the 8-MOSFET configuration depend on the specific MOSFETs and other components used in the design. Typically, this configuration can handle voltages up to several hundred volts (e.g., 240 VDC, 400 VDC) and currents ranging from a few amps to tens of amps, depending on the MOSFET specifications.

3. How does the switching speed of the 8-MOSFET configuration compare to electromechanical relays?

The 8-MOSFET configuration offers significantly faster switching speeds compared to electromechanical relays. MOSFETs can switch in nanoseconds or microseconds, while relays typically have switching times in the range of milliseconds to tens of milliseconds. This faster switching capability makes the 8-MOSFET configuration suitable for applications that require rapid and precise control.

4. Can the 8-MOSFET configuration be used in parallel for higher current handling?

Yes, multiple 8-MOSFET configurations can be connected in parallel to increase the overall current handling capability. However, proper current sharing and balancing techniques must be implemented to ensure even distribution of current across the parallel configurations.

5. What are the potential drawbacks or limitations of the 8-MOSFET configuration?

While the 8-MOSFET configuration offers several advantages over relays, it also has some potential drawbacks or limitations:

  • Complexity: The circuit design and implementation are more complex compared to using a single relay.
  • Gate Drive Requirements: Proper gate drive circuitry and control voltage supply are required, adding complexity to the overall system.
  • Heat Dissipation: Adequate heat sinking or cooling solutions may be necessary, especially in high-current or high-frequency switching applications.
  • Voltage Limitation: The maximum voltage rating is limited by the available MOSFET specifications and the number of MOSFETs in the configuration.

Despite these limitations, the 8-MOSFET configuration remains a viable and cost-effective alternative to relays in many applications, offering improved reliability, longer lifespan, and faster switching capabilities.