As manufacturing technology develops and advances, system designers must keep pace with technological developments in order to select the most appropriate electronic components for their designs. MOSFET is a basic component in electrical systems, and engineers need to have an in-depth understanding of its key characteristics and specifications to make the right choice. This article will discuss how to select the correct MOSFET based on RDS(ON), thermal performance, avalanche breakdown voltage, and switching performance specifications.
There are two major types of MOSFETs: N-channel and P-channel. In power systems, MOSFETs can be thought of as electrical switches. When a positive voltage is applied between the gate and source of an N-channel MOSFET, its switch turns on. When turned on, current can flow from drain to source through the switch. There is an internal resistance between the drain and source, called on-resistance RDS(ON). It must be understood that the gate of the MOSFET is a high-impedance terminal, so a voltage must always be applied to the gate. If the gate is left floating, the device will not operate as designed and may turn on or off at inappropriate times, causing potential power loss in the system. When the voltage between source and gate reaches zero, the switch closes and current stops flowing through the device. Although the device is turned off at this time, there is still a small current, which is called leakage current, or IDSS.
The first step in choosing the right device for your design is deciding whether to use an N-channel or P-channel MOSFET. In a typical power application, when a MOSFET is connected to ground and the load is connected to the mains voltage, the MOSFET forms the low-side switch. In the low-side switch, N-channel MOSFETs should be used due to considerations of the voltage required to turn the device off or on. When the MOSFET is connected to the bus and load to ground, a high-side switch is used. P-channel MOSFETs are usually used in this topology, also due to voltage drive considerations.
To select the right device for your application, you must determine the voltage required to drive the device and the easiest way to do it in your design. The next step is to determine the required voltage rating, or the maximum voltage the device can withstand. The higher the voltage rating, the higher the cost of the device. According to practical experience, the rated voltage should be greater than the mains voltage or bus voltage. This will provide sufficient protection so that the MOSFET will not fail. When selecting a MOSFET, it is necessary to determine the maximum voltage that can be tolerated from drain to source, that is, maximum VDS. It is important to know that the maximum voltage a MOSFET can withstand changes with temperature. Designers must test voltage variations over the entire operating temperature range.
The rated voltage must have enough margin to cover this variation range to ensure that the circuit will not fail. Other safety factors that design engineers need to consider include voltage transients induced by switching electronics such as motors or transformers. Rated voltages vary for different applications; typically, 20V for portable devices, 20-30V for FPGA power supplies, and 450-600V for 85-220VAC applications.
The second step is to choose the current rating of the MOSFET. Depending on the circuit configuration, this rated current should be the maximum current that the load can withstand under all circumstances. Similar to the voltage situation, the designer must ensure that the selected MOSFET can withstand this current rating, even when the system generates current spikes. The two current conditions considered are continuous mode and pulse spike. In continuous conduction mode, the MOSFET is in a steady state, where current flows continuously through the device. A pulse spike refers to a large surge (or spike current) flowing through the device. Once the maximum current under these conditions is determined, it is simply a matter of selecting a device that can handle this maximum current.
After selecting the rated current, the conduction loss must also be calculated. In actual situations, MOSFET is not an ideal device because there is power loss during the conduction process, which is called conduction loss. A MOSFET behaves like a variable resistor when "on", which is determined by the device's RDS(ON) and changes significantly with temperature. The power loss of the device can be calculated by Iload2×RDS(ON). Since the on-resistance changes with temperature, the power loss will also change proportionally. The higher the voltage VGS applied to the MOSFET, the smaller RDS(ON) will be; conversely, the higher RDS(ON) will be. For the system designer, this is where the tradeoffs come in depending on the system voltage. For portable designs, it is easier (and more common) to use lower voltages, while for industrial designs, higher voltages can be used. Note that the RDS(ON) resistance will rise slightly with current. Variations in various electrical parameters of the RDS(ON) resistor can be found in the technical data sheet provided by the manufacturer.
Technology has a significant impact on device characteristics, as some technologies tend to increase RDS(ON) when increasing maximum VDS. For such a technology, if you intend to reduce VDS and RDS(ON), you have to increase the chip size, thereby increasing the matching package size and related development costs. There are several technologies in the industry trying to control the increase in chip size, the most important of which are channel and charge balancing technologies.
In trench technology, a deep trench is embedded in the wafer, usually reserved for low voltages, to reduce the on-resistance RDS(ON). In order to reduce the impact of maximum VDS on RDS(ON), an epitaxial growth column/etching column process was used during the development process. For example, Fairchild Semiconductor has developed a technology called SupeRFET that adds additional manufacturing steps for RDS(ON) reduction. This focus on RDS(ON) is important because as the breakdown voltage of a standard MOSFET increases, RDS(ON) increases exponentially and leads to an increase in die size. The SuperFET process changes the exponential relationship between RDS(ON) and wafer size into a linear relationship. In this way, SuperFET devices can achieve ideal low RDS(ON) in small die sizes, even with breakdown voltages up to 600V. The result is that wafer size can be reduced by up to 35%. For end users, this means a significant reduction in package size. Sawmin Semiconductor is currently working hard to tackle the problem and plans to launch trench technology products in the second quarter of 2009.
The next step in selecting a MOSFET is to calculate the thermal requirements of the system. Designers must consider two different scenarios, the worst-case scenario and the real-world scenario. It is recommended to use the worst-case calculation result because this result provides a larger safety margin and ensures that the system will not fail. There are also some measurement data that need attention on the MOSFET data sheet; such as the thermal resistance between the semiconductor junction of the packaged device and the environment, and the maximum junction temperature.
The junction temperature of the device is equal to the maximum ambient temperature plus the product of thermal resistance and power dissipation (junction temperature = maximum ambient temperature + [thermal resistance × power dissipation]). According to this equation, the maximum power dissipation of the system can be solved, which is equal to I2×RDS(ON) by definition. Since the designer has determined the maximum current that will pass through the device, RDS(ON) can be calculated at different temperatures. It is worth noting that when dealing with simple thermal models, designers must also consider the thermal capacity of the semiconductor junction/device case and case/environment; this requires that the printed circuit board and package do not heat up immediately.
Avalanche breakdown means that the reverse voltage on the semiconductor device exceeds the maximum value and forms a strong electric field to increase the current in the device. This current will dissipate power, increase the temperature of the device, and possibly damage the device. Semiconductor companies will conduct avalanche testing on devices, calculate their avalanche voltage, or test the robustness of the device. There are two methods for calculating rated avalanche voltage; one is statistical method and the other is thermal calculation. Thermal calculation is widely used because it is more practical. Many companies provide details of their device testing, such as PHILIS, ST, etc. Samwin also provides similar instructions for users' reference. In addition to computing, technology also has a large influence on the avalanche effect. For example, an increase in die size increases avalanche resistance and ultimately increases device robustness. For end users, this means using larger packages in the system.
The final step in selecting a MOSFET is to determine the switching performance of the MOSFET. There are many parameters that affect switching performance, but the most important are gate/drain, gate/source and drain/source capacitance. These capacitors create switching losses in the device because they are charged every time they switch. The switching speed of the MOSFET is therefore reduced, and the device efficiency is also reduced. To calculate the total losses in a device during switching, the designer must calculate the losses during turn-on (Eon) and the losses during turn-off (Eoff). The total power of the MOSFET switch can be expressed by the following equation: Psw=(Eon+Eoff)×switching frequency. The gate charge (Qgd) has the greatest impact on switching performance.
Based on the importance of switching performance, new technologies are constantly being developed to solve this switching problem. Increasing chip size increases gate charge; this increases device size. In order to reduce switching losses, new technologies such as channel thick bottom oxidation have emerged, aiming to reduce gate charge. For example, the new technology SupeRFET can minimize conduction losses and improve switching performance by reducing RDS(ON) and gate charge (Qg). In this way, MOSFETs can cope with high-speed voltage transients (dv/dt) and current transients (di/dt) during switching, and can even operate reliably at higher switching frequencies.
By understanding the types of MOSFETs and understanding and determining their important performance characteristics, designers can select the correct MOSFET for a specific design. Since MOSFETs are one of the most fundamental components in electrical systems, choosing the right MOSFET plays a key role in the success of the entire design.
