Introduction to MOSFETs

Metal-oxide-semiconductor field-effect transistors, or MOSFETs, are voltage-controlled semiconductor devices widely used for switching and amplifying electronic signals. MOSFETs have revolutionized the electronics industry due to their high input impedance, fast switching speeds, and excellent power efficiency compared to bipolar junction transistors (BJTs).

MOSFETs are divided into two main categories based on their channel type:

1. N-Channel MOSFETs
2. P-Channel MOSFETs

N-Channel MOSFETs have electrons as the majority charge carriers, while P-Channel MOSFETs have holes as the majority charge carriers. This article focuses on the IRLB8721, an N-Channel 30V logic level power MOSFET.

Key Features of the IRLB8721

The IRLB8721 is a popular choice for various applications due to its impressive features:

• Logic level gate drive (4.5V to 20V)
• Low on-resistance (RDS(on)) of 7.9 mΩ at VGS = 10V
• High continuous drain current (ID) of 62A
• Fast switching speeds (td(on) = 18 ns, td(off) = 42 ns)
• Low gate charge (Qg) of 48 nC
• TO-220AB package

These features make the IRLB8721 suitable for high-current, high-efficiency applications such as:

• DC-DC converters
• Motor drivers
• Power supplies
• Battery management systems
• Automotive electronics

Understanding MOSFET Parameters

To effectively use the IRLB8721 in your designs, it is essential to understand key MOSFET parameters:

1. Drain-Source Breakdown Voltage (BVDSS)

BVDSS represents the maximum voltage that can be applied between the drain and source terminals without causing an avalanche breakdown. For the IRLB8721, BVDSS is 30V.

2. Drain Current (ID)

ID is the maximum continuous current that the MOSFET can conduct through its drain terminal. The IRLB8721 has a continuous drain current rating of 62A.

3. On-Resistance (RDS(on))

RDS(on) is the resistance between the drain and source terminals when the MOSFET is fully on. A lower RDS(on) results in lower power dissipation and higher efficiency. The IRLB8721 has an RDS(on) of 7.9 mΩ at VGS = 10V.

4. Gate Threshold Voltage (VGS(th))

VGS(th) is the minimum gate-source voltage required to turn the MOSFET on. For the IRLB8721, VGS(th) ranges from 1.0V to 2.5V.

5. Gate Charge (Qg)

Qg represents the amount of charge required to turn the MOSFET on fully. A lower Qg allows faster switching and reduces power losses. The IRLB8721 has a Qg of 48 nC.

Here is a table summarizing the key parameters of the IRLB8721:

Parameter	Value
BVDSS	30V
ID	62A
RDS(on)	7.9 mΩ @ VGS = 10V
VGS(th)	1.0V to 2.5V
Qg	48 nC

IRLB8721 Application Circuits

1. High-Side Switch

The IRLB8721 can be used as a high-side switch to control the power supply to a load. In this configuration, the MOSFET is placed between the positive power supply and the load. When a positive gate-source voltage is applied, the MOSFET turns on, allowing current to flow through the load.

2. Low-Side Switch

In a low-side switch configuration, the IRLB8721 is placed between the load and ground. When the MOSFET is turned on, it provides a low-resistance path for the current to flow from the load to ground.

3. H-Bridge Motor Driver

An H-Bridge is a circuit configuration used to control the direction and speed of a DC motor. It consists of four MOSFETs arranged in an H-shape, with the motor connected between the two legs of the H. The IRLB8721 can be used as the low-side switches in an H-Bridge motor driver.

IRLB8721 PCB Layout Considerations

Proper PCB layout is crucial for optimal MOSFET performance and reliability. Here are some guidelines to follow when designing a PCB with the IRLB8721:

1. Minimize the trace lengths between the MOSFET terminals and the power source, load, and gate driver to reduce parasitic inductances and resistances.

2. Use a large, low-impedance ground plane to provide a stable reference for the MOSFET and minimize voltage spikes during switching.

3. Place the gate resistor close to the MOSFET gate terminal to prevent oscillations and ringing.

4. Use a Kelvin connection for the source terminal to accurately sense the source voltage and avoid voltage drops caused by high currents.

5. Provide adequate copper area for the drain and source terminals to dissipate heat effectively.

Thermal Management

Proper thermal management is essential to ensure the IRLB8721 operates within its safe operating area (SOA) and maintains a long lifespan. The MOSFET’s power dissipation can be calculated using the following equation:

PD = RDS(on) × ID^2

Where:
– PD is the power dissipation in watts (W)
– RDS(on) is the on-resistance in ohms (Ω)
– ID is the drain current in amperes (A)

To prevent the MOSFET from overheating, you can:

1. Use a heatsink to dissipate the generated heat efficiently.
2. Ensure proper airflow around the MOSFET and heatsink.
3. Monitor the MOSFET’s temperature using a temperature sensor and implement thermal shutdown protection if necessary.

IRLB8721 vs. Other MOSFETs

When selecting a MOSFET for your application, it is important to compare the IRLB8721 with other available options. Here is a comparison table of the IRLB8721 and two other popular N-Channel MOSFETs:

Parameter	IRLB8721	IRLZ44N	IRF3205
BVDSS	30V	55V	55V
ID	62A	47A	110A
RDS(on)	7.9 mΩ	22 mΩ	8.0 mΩ
VGS(th)	1.0V to 2.5V	1.0V to 2.0V	2.0V to 4.0V
Qg	48 nC	63 nC	185 nC

As evident from the table, the IRLB8721 offers a lower RDS(on) and Qg compared to the IRLZ44N, making it more suitable for high-efficiency applications. However, the IRF3205 provides a higher current capacity, which may be advantageous in certain high-power applications.

Frequently Asked Questions (FAQ)

1. What is the maximum gate-source voltage (VGS) for the IRLB8721?

The maximum VGS for the IRLB8721 is 20V. Applying a higher voltage may damage the MOSFET.

2. Can the IRLB8721 be used in parallel to increase current capacity?

Yes, multiple IRLB8721 MOSFETs can be connected in parallel to increase the overall current capacity. However, ensure that the MOSFETs are properly matched and have equal gate-source voltages to prevent current imbalances.

3. What is the purpose of the gate resistor in MOSFET circuits?

The gate resistor helps to control the MOSFET’s switching speed, minimize oscillations and ringing, and protect the gate from voltage spikes. A typical gate resistor value ranges from 10Ω to 100Ω.

4. How do I select the appropriate heatsink for the IRLB8721?

To select the appropriate heatsink, calculate the MOSFET’s power dissipation and determine the maximum allowable temperature rise. Use the heatsink’s thermal resistance (in °C/W) to ensure it can effectively dissipate the generated heat.

5. Can the IRLB8721 be used in a synchronous buck converter?

Yes, the IRLB8721 is well-suited for use as the low-side switch in a synchronous buck converter due to its low RDS(on) and fast switching speeds. However, ensure that the MOSFET is properly driven and protected against shoot-through currents.

Conclusion

The IRLB8721 is a versatile and high-performance N-Channel 30V logic level power MOSFET suitable for a wide range of applications. Its low on-resistance, high current capacity, and fast switching speeds make it an excellent choice for power electronics designers seeking efficiency and reliability.

By understanding the MOSFET’s key parameters, application circuits, PCB layout considerations, and thermal management techniques, you can effectively incorporate the IRLB8721 into your designs and achieve optimal performance.

Remember to compare the IRLB8721 with other available MOSFET options to select the best fit for your specific application requirements.