“What is a MOSFET? What does it do? A MOSFET is a field effect transistor that can be widely used in analog and digital circuits. It is also one of the most frequently utilized devices by engineers, so it is very necessary to understand the key indicators of MOSFET.
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What is a MOSFET? What does it do? A MOSFET is a field effect transistor that can be widely used in analog and digital circuits. It is also one of the most frequently utilized devices by engineers, so it is very necessary to understand the key indicators of MOSFET.
This article outlines some of the key MOSFET specifications, how these are stated on the datasheet, and a clear picture of what you need to understand these specifications. Like most Electronic devices, MOSFETs are also affected by operating temperature. So it is important to know the test conditions under which the mentioned metrics are applied. It’s also critical to figure out if the metrics you see in the “Product Brief” are “maximum” or “typical” values, because some datasheets don’t make it clear.
Voltage level
The primary characteristic that determines a MOSFET is its drain-source voltage, VDS, or “drain-source breakdown voltage,” which is the highest voltage a MOSFET can withstand without damage with a gate-to-source short circuit and a drain current of 250µA . VDS is also referred to as the “absolute maximum voltage at 25°C”, but it is important to remember that this absolute voltage is temperature dependent, and data sheets usually have a “VDS temperature coefficient”. You also need to understand that the maximum VDS is the DC voltage plus any voltage spikes and ripples that may be present in the circuit. For example, if you use a 30V device on a 30V supply with a 100mV, 5ns spike, the voltage will exceed the absolute maximum limit of the device and the device may go into avalanche mode. In this case, the reliability of the MOSFET cannot be guaranteed.
At high temperatures, the temperature coefficient changes the breakdown voltage significantly. For example, some N-channel MOSFETs in the 600V voltage class have a positive temperature coefficient, and near the maximum junction temperature, the temperature coefficient will make these MOSFETs behave like 650V MOSFETs. Design rules for many MOSFET users require a 10% to 20% derating factor. In some designs, considering that the actual breakdown voltage is 5%~10% higher than the rated value at 25°C, a corresponding useful design margin will be added in the actual design, which is very beneficial to the design.
Equally important to proper MOSFET selection is understanding the role of the gate-to-source voltage VGS during turn-on. This voltage is the voltage at which the MOSFET is fully turned on for a given maximum RDS(on) condition. This is why the on-resistance is always tied to the VGS level, and it is only at this voltage that the device is guaranteed to be on. An important design consequence is that you cannot fully turn on the MOSFET with a voltage lower than the minimum VGS used to achieve the RDS(on) rating. For example, to drive a MOSFET fully on with a 3.3V microcontroller, you need a MOSFET that can turn on at VGS = 2.5V or lower.
On-resistance, gate charge, and “figure of merit”
The on-resistance of a MOSFET is always determined at one or more gate-source voltage conditions. The maximum RDS(on) limit can be 20% to 50% higher than the typical value. The maximum RDS(on) limit usually refers to the value at a junction temperature of 25°C, and at higher temperatures, the RDS(on) can be increased by 30% to 150%, as shown in Figure 1. Since RDS(on) varies with temperature and the minimum resistance value cannot be guaranteed, current sensing based on RDS(on) is not a very accurate method.
On-resistance is very important for both N-channel and P-channel MOSFETs. In switching power supplies, Qg is a key selection criterion for N-channel MOSFETs used in switching power supplies because Qg affects switching losses. These losses have two effects: one is the transition time that affects the turn-on and turn-off of the MOSFET; the other is the energy required to charge the gate capacitance during each switch. One thing to keep in mind is that Qg depends on the gate-source voltage, i.e. switching losses can be reduced with lower Vgs.
As a quick way to compare MOSFETs ready for use in switching applications, designers often use a singular formula that includes RDS(on) for conduction losses and Qg for switching losses: RDS(on) x Qg. This “Factor of Merit” (FOM) summarizes the performance of the device and can be used to compare MOSFETs with typical or maximum values. To ensure accurate comparisons in the device, you need to be sure that the same VGS is used for RDS(on) and Qg, and that the typical and maximum values are not accidentally mixed in the publications. A lower FOM will give you better performance in switching applications, but this is not guaranteed. The best comparison results are obtained only in an actual circuit, which may require fine-tuning of the circuit for each MOSFET in some cases.
Rated Current and Power Dissipation
Based on different test conditions, most MOSFETs have one or more continuous drain currents in the datasheet. You’ll want to take a close look at the datasheet to see if this rating is at the specified case temperature (eg TC = 25°C), or ambient temperature (eg TA = 25°C). Which of these values is most relevant will depend on the device characteristics and application (see Figure 2).
For small surface mount devices used in handheld devices, the most relevant current levels may be at 70°C ambient temperature, for larger devices with heat sinks and forced air cooling, at TA = 25°C The rating may be closer to reality. For some devices, the die can handle more current at its maximum junction temperature than the current level specified by the package. In some data sheets, this “die-limited” current level is a Additional supplemental information on the current rating can give you an idea of the robustness of the die.
A similar situation is considered for continuous power dissipation, which depends not only on temperature but also on-time. Suppose a device operates continuously for 10 seconds with PD=4W at TA=70℃. The factors that make up a “continuous” time period vary depending on the MOSFET package, so you’ll want to use the normalized thermal transient impedance graph from the datasheet to see what the power dissipation looks like after 10 seconds, 100 seconds, or 10 minutes. As shown in Figure 3, the thermal resistance coefficient of this dedicated device after a 10-second pulse is about 0.33, which means that after about 10 minutes, once the package reaches thermal saturation, the heat dissipation capability of the device is only 1.33W instead of 4W, although in In the case of good cooling, the heat dissipation capacity of the device can reach about 2W.
In fact, we can divide MOSFET selection into four steps.
Step 1: Choose N-channel or P-channel
The first step in choosing the right device for a design is deciding whether to use an N-channel or P-channel MOSFET. In a typical power application, when a MOSFET is grounded and the load is connected to the mains voltage, the MOSFET constitutes a low-side switch. In low-side switches, N-channel MOSFETs should be used because of the voltage required to turn off or turn on the device. High-side switches are used when the MOSFET is connected to the bus and the load to ground. P-channel MOSFETs are typically used in this topology, also for 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 implement 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 provides sufficient protection so that the MOSFET does not fail. For the selection of a MOSFET, the maximum voltage that can be experienced from drain to source, or maximum VDS, must be determined. It is important to know that the maximum voltage that a MOSFET can withstand varies with temperature. Designers must test the voltage variation over the entire operating temperature range. The rated voltage must have sufficient margin to cover this variation and ensure that the circuit does not fail. Other safety factors that design engineers need to consider include voltage transients induced by switching electronics such as motors or transformers. Voltage ratings vary for different applications; typically, 20V for portable devices, 20-30V for FPGA power supplies, and 450-600V for 85-220VAC applications.
Step 2: Determine the rated current
The second step is to choose the current rating of the MOSFET. Depending on the circuit configuration, this current rating should be the maximum current the load can withstand under all conditions. Similar to the case of voltage, the designer must ensure that the selected MOSFET can withstand this current rating, even when the system generates current spikes. The two current cases considered are continuous mode and pulsed spikes. In continuous conduction mode, the MOSFET is in steady state, where current flows continuously through the device. A pulse spike is when there is a large surge (or peak current) flowing through the device. Once the maximum current under these conditions is determined, simply select the device that can handle this maximum current.
After selecting the rated current, the conduction loss must also be calculated. In practice, MOSFETs are not ideal devices because there is power loss during conduction, which is called conduction loss. A MOSFET acts like a variable resistor when it is “on”, determined by the RDS(ON) of the device and varies significantly with temperature. The power dissipation of the device can be calculated from Iload2×RDS(ON), and since the on-resistance varies with temperature, the power dissipation also varies proportionally. The higher the voltage VGS is applied to the MOSFET, the smaller the RDS(ON) will be; otherwise the RDS(ON) will be higher. For the system designer, this is where the trade-off comes 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. Various electrical parameter variations for RDS(ON) resistors 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 the maximum VDS. For such a technology, if the VDS and RDS(ON) are to be reduced, then the die size must be increased, thereby increasing the package size and associated development costs associated with it. Several techniques exist in the industry to try to control the increase in wafer size, chief among them are channel and charge balancing techniques.
In trench technology, a deep trench is embedded in the wafer, usually reserved for low voltages, to reduce the on-resistance RDS(ON). To reduce the effect of maximum VDS on RDS(ON), an epitaxially grown pillar/etched pillar process was used during development. For example, Fairchild has developed a technology called SuperFET that adds an additional manufacturing step for RDS(ON) reduction.
This focus on RDS(ON) is important because as the breakdown voltage of standard MOSFETs increases, RDS(ON) increases exponentially and leads to an increase in die size. The SuperFET process turns the exponential relationship between RDS(ON) and die size into a linear relationship. In this way, SuperFET devices can achieve ideal low RDS(ON) at small die sizes, even with breakdown voltages up to 600V. The result is a wafer size reduction of up to 35%. And for the end user, this means a significant reduction in package size.
Step 3: Determine Thermal Requirements
The next step in choosing a MOSFET is to calculate the thermal requirements of the system. Designers have to consider two different scenarios, worst-case and real-world. It is recommended to use the calculation result for the worst case, because this result provides a larger safety margin to ensure that the system does not fail. There are also some measurements to be aware of on the MOSFET’s data sheet; such as the thermal resistance between the semiconductor junction of the packaged device and ambient, 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 +[热阻×功率耗散]). The maximum power dissipation of the system can be solved from this equation, which is by definition equal to I2×RDS(ON). Since the designer has determined the maximum current that will pass through the device, the RDS(ON) at different temperatures can be calculated. 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/ambient; that is, the requirement that the printed circuit board and package do not heat up immediately.
Avalanche breakdown means that the reverse voltage on a semiconductor device exceeds the maximum value, and a strong electric field is formed 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 perform avalanche tests on their devices, calculate their avalanche voltages, or test the robustness of the devices. There are two methods for calculating the rated avalanche voltage; one is statistical method and the other is thermal calculation. Thermal calculations are widely used because they are more practical. Many companies have provided the details of their device testing, such as Fairchild Semiconductor provides “Power MOSFET Avalanche Guidelines” (Power MOSFET Avalanche Guidelines – can go to the Fairchild website to download). In addition to computing, technology also plays a big role in the avalanche effect. For example, an increase in die size increases avalanche resistance and ultimately improves device robustness. For the end user, this means a larger package in the system.
Step 4: Determine Switch Performance
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 each time they switch. The switching speed of the MOSFET is thus reduced and the device efficiency is also reduced. To calculate the total loss of the device during switching, the designer must calculate the loss during turn-on (Eon) and the loss 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 the switching performance.
Based on the importance of switching performance, new technologies are constantly being developed to solve this switching problem. An increase in chip size increases gate charge; this in turn increases device size. To reduce switching losses, new technologies such as channel thick bottom oxide have emerged, aiming to reduce gate charge. The above is the MOSFET analysis, I hope it can help you.
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