How Power MOSFETs Achieve Higher Efficiency in Power Electronics

Modern electronic applications require high switching frequencies to achieve corresponding high efficiency. Power MOSFETs are an important component in power-intensive applications. They offer a relatively low gate charge, making them well suited for medium and high-power use cases. This lower gate charge reduces driving current requirements, enabling the high frequencies and higher efficiencies. 

This article explores the benefits of medium-power MOSFETs in various applications, their limitations, and considerations for making selections. 

 

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The Role of Power MOSFETs in Electronic Applications

Power MOSFETs are often categorized by their breakdown voltage. Unlike high- and very-high-power MOSFETs, which feature breakdown voltage ranges of 400 - 650 V and >700 V, respectively, medium-power MOSFETs’ breakdown voltages range from 30 V to 350 V, offering low gate charge and on-resistance down to 2.6 mΩ (30 V). As a result, many designers incorporate medium-power MOSFETs into the design of their power systems. 

Power MOSFETS that have lower voltage breakdown ranges support higher switching speeds. An example of this is ROHM's 6th generation family of power MOSFETs. They are commonly available in both n-channel and p-channel versions, and offer switching frequencies of up to 100 kHz. Integrating power MOSFETs into PCBs is critical for high switching speeds and efficiencies at lower voltages when compared to other semiconductor devices like thyristors and IGBTs, significantly reducing energy losses and reverse recovery time.

 

Benefits and Limitations of Power MOSFETs

Power MOSFETs offer several benefits to a broad range of applications. These benefits include:

• Low cost
• Compact size for easy integration with power electronics
• Enhanced switching speed
• Operation at high switching frequencies (reduces energy losses)
• Simple gate-driving circuit
• Thermally stable due to non-negative coefficient of power
• Low on-resistance (helps to limit power loss)
• No requirement of extra circuitry for commutation 

However, power MOSFETs are limited by their blocking capacity, which is non-symmetrical. This protects them from forward voltage surges, but also leaves them vulnerable to reverse voltages. Consequently, they require an extra diode for reverse voltage surge protection. 

 

Power MOSFETs in Industrial Applications

Power MOSFETs are commonly used in applications with voltage requirements within the 350 V threshold. Their low ON resistance quality is particularly attractive for most applications. They reduce power dissipation, ensuring reduced costs, size, and required cooling, which leads to a comprehensive improvement in electronic power systems. Some industrial applications that utilize power MOSFETs include load switching, DC/DC converters, power supplies, and low voltage motor control.

 

Factors to Consider When Choosing Power MOSFETs

The following are essential considerations for selecting power MOSFETs for high-power applications:

• Channel type
• Max drain-to-source voltage
• Drain-to-source resistance
• Package/case
• Gate voltage threshold
• Maximum DC drain current
• Gate charge

 

Channel Type

This refers to the nature of the construction of silicon incorporated into the device. An n-channel power MOSFET turns ON with a positive voltage on the gate relative to the source while a p-channel power MOSFET turns ON with a negative gate-source voltage. Knowing the position of the device on the system informs the designer’s decision of which type would be more appropriate.

 

Maximum Drain-to-Source Voltage

This rating is assigned after considering the device’s capacity to block voltage applied to it when it is switched off. A general guideline most designers follow involves selecting the part with a voltage rating that is double the expected voltage applied to the drain. This is because short voltage spikes above the input voltage are common in MOSFET-integrated electrical systems.

 

Drain-to-Source Resistance

This critical parameter impacts the heat generated by the semiconductor device while conducting electricity. Designers need to consider the corresponding RDS(on) values at specific source breakdown voltage (VGS) and operating temperatures before selecting the ideal power MOSFET for their applications.

 

Package/Case

The MOSFET package/case must be selected based on the design’s thermal and mechanical requirements. Also, board space and physical layout make designers prefer some devices over others since it affects their thermal performance in high current or power dissipating designs.

 

Gate Voltage Threshold

This threshold determines the voltage at which the power MOSFET solution will begin to conduct. Therefore, a lower gate voltage threshold allows a MOSFET to turn on for full current conduction more quickly. By considering the output voltages of the controlling system MCU and gate driver, designers can then select the most suitable power MOSFET for their applications.

 

Maximum DC Drain Current

This is the maximum current the device can tolerate at a specific operating temperature. By referring to the safe operating area curves in datasheets, system designers can determine the desired current for their power MOSFET application.

 

Gate Charge

The amount of charge required to turn the device full-on is known as the gate charge (Qg). Lower power MOSFETs have a lower value for Qg which leads to higher efficiencies in switching operations. 

 

ROHM Power FETs for Power Electronic Applications

ROHM is an industry-leading manufacturer of ICs, semiconductors, and related components. ROHM’s power MOSFETs are particularly suitable for DC/DC converter and load switching applications. They are available in a broad range of high-power and miniature packages (HSOP8, HSMT8, and HUML2020L8) with gate charges ranging from 5.8 to 36 nC. ROHM’s low on-resistance, RoHS compliant, and lead and halogen-free product portfolio with state-of-the-art packaging and protections for safety and maximum reliability allow designers to discover the right solutions for high-efficiency and reliable applications.