What is Overdrive Voltage in MOSFET?

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The overdrive voltage of a MOSFET, often denoted as VOV or VGS​ – Vth, is defined as the difference between the gate-source voltage (VGS​) and the threshold voltage (Vth​).

Mathematically:

VOV = VGS − Vth

Where:

  • VGS​ is the voltage between the gate and source terminals.
  • Vth​ is the threshold voltage, the minimum VGS​ required for the MOSFET to turn on and enter the conduction region.

Significance of Overdrive Voltage

  1. In the saturation region, where a MOSFET operates as a current source, the overdrive voltage helps define the operating point of the device.
  2. In the linear region, it defines how strongly the MOSFET is turned on.
  3. Overdrive voltage controls the strength of the inversion layer, determining the channel current.

MOSFET Operating Regions

The MOSFET operates in three regions depending on VGS, Vth and VDS :

1. Cut-off Region: When VGS < Vth, the MOSFET is OFF, and no current flows between drain and source.

2. Linear Region (Triode): When VGS > Vth and VDS < VOV, the MOSFET behaves like a resistor, and the drain current is approximately linear with VDS.

The current is given by:

ID = kn [( VGS − Vth ) VDS − V2DS / 2]

Where:

kn = μnCoxW / L is the MOSFET process transconductance parameter.

3. Saturation Region: When VGS > Vth and VDS ≥ VOV, the MOSFET behaves like a current source, and the current becomes constant and independent of VDS.

The drain current in saturation is:

ID = kn / 2 ( VGS − Vth )2 = ( kn / 2 ) V2OS

This shows that the current is directly proportional to the square of the overdrive voltage VOV.

Example Problem

Question: Calculate Overdrive Voltage and Drain Current

A MOSFET has the following parameters:

  • Vth ​= 1V
  • VGS = 3V
  • VDS = 5V
  • kn = 200 μA / V2

Step 1: Calculate Overdrive Voltage

VOV = VGS − Vth = 3V − 1V = 2V

Step 2: Check the Operating Region

Since VDS = 5V and VOV = 2V, the condition VDS ≥ VOV holds, meaning the MOSFET operates in the saturation region.

Step 3: Calculate Drain Current

Using the saturation current equation:

ID = ( kn / 2 ) V2OV = ( 200μA / V2 ) / 2 x (2V)2

ID = 100μA / V2 x 4V2 = 400μA

Thus, the drain current is 400μA.

Can Overdrive Voltage be Negative?

For enhancement-mode MOSFETs (which are the most common):

  • VGS​ needs to be greater than Vth​ for the MOSFET to turn on.
  • A negative overdrive voltage would imply that VGS is less than Vth​, which means the MOSFET would be in the cutoff region and not conducting any current. In this case, the MOSFET is effectively turned off.

Thus, for a MOSFET to be on and conducting current, VOV​ (i.e., VGS − Vth​) must be positive.

Does Overdrive Drain the Battery?

Yes, overdrive voltage in a MOSFET can contribute to battery drain in certain situations, especially in circuits where MOSFETs are used for power management or amplification.

However, the exact impact of overdrive on battery life depends on how the MOSFET is being used and its operating conditions.

→ How Overdrive Voltage Affects Battery Drain:

The overdrive voltage VOV​ = VGS​ − Vth​ influences the amount of current flowing through the MOSFET. This, in turn, affects power dissipation and can contribute to battery drain:

1. Higher Overdrive → Higher Drain Current:

When the overdrive voltage VOV is increased (i.e., VGS is higher than Vth ​):

  • Drain current increases: The MOSFET allows more current to flow through it. In circuits where the MOSFET is connected to a battery, this increase in current will draw more power, leading to faster battery discharge.

ID = ( kn / 2 ) V2OS

  • Power Dissipation: Higher current flowing through the MOSFET leads to greater power dissipation, typically in the form of heat, which wastes battery power.

2. Power Consumption in Different Regions:

  • In the saturation region: Power consumed is mainly due to the current through the MOSFET. The more the overdrive voltage, the higher the current, which can drain the battery faster.
  • In the linear region: The MOSFET operates as a variable resistor, and the power consumption is proportional to both the current and the voltage across the MOSFET.

→ The power dissipated by the MOSFET is given by:

PMOSFET = ID ⋅ VDS

Key Takeaways

  • Overdrive voltage controls the degree of MOSFET conduction in both the linear and saturation regions.
  • In the saturation region, the drain current ID​ depends on the square of the overdrive voltage.
  • A higher overdrive voltage means the MOSFET is “more on,” allowing greater drain current to flow.

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