MOSFET banks. Series & Parallel

Series (left) and parallel (right) arrangements of N-MOSFETs.

Can MOSFETs be used in Series connections?

Yes, MOSFETs can be connected in series to handle higher voltages, essentially allowing the total drain-source voltage (\(V_{DS}\)) to be shared across multiple devices. This configuration is used to increase the overall voltage rating beyond what a single MOSFET can handle, though it requires careful design to ensure voltage balancing during switching.

Key Considerations for Series Connection:

• Voltage Sharing: To prevent one MOSFET from bearing the entire voltage (due to variations in leakage current), voltage-balancing components—such as resistors or capacitors—must be used in parallel with each MOSFET.
• Gate Driving: Both (or all) MOSFETs must be switched ON and OFF simultaneously. This is often achieved through isolated gate drivers.
• Applications: Series MOSFETs are typically used for high-voltage switching, in push-pull arrangements, or as bidirectional switches when placed in "anti-series".
• Challenges: If not balanced correctly, unequal voltage distribution during turn-on/off transitions can lead to component failure.

While series connections increase the voltage capacity, they increase the total on-resistance (\(\text{R}_{\text{DS(on)}}\)), leading to higher conduction losses compared to a single device. 

Can MOSFETs be used in Parallel connections?

Yes, MOSFETs can be connected in parallel to increase current capacity, decrease total on-resistance (\(R_{DS(on)}\)), and distribute heat. While this is common in high-power applications, it requires careful layout to ensure even current sharing and to avoid oscillations

Key Considerations for Parallel MOSFETs:

• Gate Resistors: Use individual gate resistors (\(R_{g}\)) for each MOSFET to prevent parasitic oscillations.
• Current Sharing: While MOSFETs have a positive temperature coefficient that aids in balancing, variations in threshold voltage (\(V_{GS(th)}\)) can cause current imbalance.
• Thermal Coupling: Paralleled devices should be kept at similar temperatures.
• Layout: Keep wiring symmetric and stray inductance low to avoid switching imbalances.

Deep dive

MOSFET banks are configured in series or parallel to increase power handling capabilities beyond what a single device can offer, effectively managing higher voltages, higher currents, or both. Paralleling is commonly used to increase current capability, while series connections are utilized for higher voltage ratings.

Parallel MOSFET Banks (Current Sharing)

More common practice. Parallel connections involve connecting source-to-source and drain-to-drain for multiple MOSFETs, allowing the total load current to be distributed among them.

• Purpose: To reduce total on-resistance ($R_{DS(on)}$.
• Advantages: MOSFETs have a positive temperature coefficient of $R_{DS(on)}$.
• Key Considerations:
• Current Imbalance: Discrepancies in $V_{th}$ (threshold voltage) and $R_{DS(on)}$ can cause uneven current sharing, especially during switching transitions.
• Gate Oscillation: Parallel MOSFETs can form a high-Q resonant circuit with stray inductance, resulting in parasitic oscillation.
• Mitigation: Use matched devices from the same production lot and include a small individual gate resistor for each MOSFET to dampen oscillation.
• Layout:.Maintain symmetrical PCB layouts for connections to ensure equal parasitic inductance.

Series MOSFET Banks (Voltage Sharing)

Least common practice. Series connections involve connecting the drain of one MOSFET to the source of the next, allowing the total voltage to be divided among them

• Purpose: To handle voltages that exceed the rating of a single device.
• Advantages: Spreads switching losses and heat dissipation across multiple devices.
• Key Considerations:
• Voltage Balancing: Static and dynamic voltage sharing is critical to prevent one device from experiencing overvoltage.
• Driving Requirements: Each MOSFET in a series string typically requires an isolated gate driver, as the source terminals are not at a common potential.
• Protection: Diodes are often added to block voltage spikes.

Summary Comparison

Feature	Parallel	Series
Main Goal	Higher Current	Higher Voltage
Connection	Source-Source, Drain-Drain	Drain-Source Chain
Self-Balancing	Yes (via $R_{DS(on)}$)	No (Requires balancing circuits)
Main Challenge	Parasitic Oscillation	Complex Gate Driving

Design Tip:

For optimal performance, minimize the PCB trace inductance in the layout, as parasitics in series/parallel circuits can lead to strong oscillations.

Benefits	Series	Parallel
Electrical	Spreads switching loss across multiple MOSFETs	Increases the total current that can be delivered to a load
Thermal	Spreads heat dissipation across multiple MOSFETs	Spreads heat dissipation across multiple MOSFETs