As the saying goes, "There must be near worries for people without long-term consideration." For electronic design engineers, before the start of the project, the device selection must be fully considered, and the device that best suits their needs can be selected to ensure the success of the project.

Power MOSFETs are probably one of the most used devices for engineers, but do you know? Regarding the device selection of the MOSFET, various factors should be considered, ranging from N-type or P-type, package type, to the withstand voltage and on-resistance of the MOSFET. Different application requirements are ever-changing. The following article summarizes the MOSFET device. The 10-step rule of selection is believed to be rewarding after reading.

1. The first step of power MOSFET selection: P tube, or N tube?

There are two types of power MOSFETs: N-channel and P-channel. In the process of system design, N or P is selected. It should be selected for the actual application. N-channel MOSFETs have many models and low cost. The channel MOSFETs are selected for a smaller number of models and cost. If the voltage of the S-pole connection of the power MOSFET is not the reference ground of the system, the N-channel requires a floating power supply, a transformer drive or a bootstrap drive, and the drive circuit is complicated; the P-channel can be directly driven and the drive is simple.

Applications that need to consider N-channel and P-channel are:

(1) Motors for cooling the CPU and system used by laptops, desktops, and servers, motor drives for printer paper feed systems, vacuum cleaners, air purifiers, electric fans, etc. These systems use full-bridge circuits. Structure, the upper tube of each bridge arm can use P tube, or can use N tube.

(2) The hot-swap MOSFET of the 48V input system of the communication system is placed at the high end, and the P tube or the N tube can be used.

(3) Two back-to-back power MOSFETs with series input circuit, anti-reverse connection and load switch function, use N-channel charge pump which needs to control the integrated drive inside the chip, and can be directly driven by P channel.

2, select the package type

After the channel type of the power MOSFET is determined, the second step is to determine the package. The package selection principles are as follows:

(1) Temperature rise and thermal design are the most basic requirements for selecting packages

Different package sizes have different thermal resistance and dissipated power. In addition to considering the system's heat dissipation conditions and ambient temperature, such as whether there is air cooling, the shape and size of the radiator, and whether the environment is closed, the basic principle is to ensure power. Under the premise of temperature rise of the MOSFET and system efficiency, select a more general power MOSFET with parameters and packages.

Sometimes due to other conditions, multiple MOSFETs need to be used in parallel to solve the heat dissipation problem. For example, in PFC applications, electric vehicle motor controllers, and module power supply secondary synchronous rectification of communication systems, multiple tubes are selected. Parallel way.

If multi-tube parallel connection is not possible, in addition to selecting a better-performance power MOSFET, a larger package or a new package can be used, such as changing the TO220 to a TO247 package in some AC/DC power supplies; in some communication system power supplies. , using the new package of DFN8*8.

(2) System size limit

Some electronic systems are subject to the size and internal height of the PCB. For example, the module power supply of the communication system is usually packaged in DFN5*6 or DFN3*3 due to the height limitation; in some ACDC power supplies, the ultra-thin design or due to the outer casing Limitation, the power MOSFET pins of the TO220 package are directly inserted into the root during assembly, and the height limit cannot be used in the TO247 package. Some ultra-thin designs directly bend the device pins flat, which complicates the design process.

In the design of large-capacity lithium battery protection boards, due to the extremely limited size constraints, chip-scale CSP packages are now mostly used to maximize heat dissipation while ensuring minimum size.

(3) The company's production process

The TO220 has two kinds of packages: bare metal package and full plastic package. The exposed metal package has small thermal resistance and strong heat dissipation capability. However, in the production process, it is necessary to add insulation sinks. The production process is complicated and costly, and the thermal resistance of the plastic package is high. Large, low heat dissipation, but the production process is simple.

In order to reduce the manual process of the lock screw, in recent years, some electronic systems have clamped the power MOSFET in the heat sink, which has resulted in a new package form that removes the upper part of the conventional TO220 with holes, and also reduces The height of the device.

(4) Cost control

In the early days, many electronic systems used plug-in packages. In recent years, due to the increase in labor costs, many companies have begun to switch to patch packages. Although the soldering cost of patches is higher than that of plug-ins, the automation of patch soldering is high, and the overall cost can still be controlled. A reasonable range. In some cost-sensitive applications such as desktop boards and boards, power MOSFETs in DPAK packages are often used because of the low cost of such packages.

Therefore, when selecting the package of the power MOSFET, it is necessary to combine the characteristics of the company and the characteristics of the product to comprehensively consider the above factors.

3. Select pressure-resistant BVDSS

In most cases, it seems that choosing the withstand voltage of a power MOSFET is the easiest thing for many engineers because the input voltage of the designed electronic system is relatively fixed. The company selects some item numbers from specific suppliers. The rated voltage is also fixed. For example, in laptop adapters and mobile phone chargers, the input is 90 ~ 265V AC, the primary is usually 600V or 650V power MOSFET; notebook computer motherboard input voltage 19V, usually choose 30V power MOSFET, no need for any consideration.

The breakdown voltage BVDSS of the power MOSFET in the datasheet has certain test conditions, different values ​​under different conditions, and BVDSS has a positive temperature coefficient, which should be considered in combination with practical factors in practical applications.

It is often mentioned in many sources and documents that if the maximum peak voltage of the VDS of the power MOSFET in the system is greater than BVDSS, even if the spike voltage lasts only a few or tens of ns, the power MOSFET will enter an avalanche and cause damage.

Unlike triodes and IGBTs, power MOSFETs are resistant to avalanche, and the avalanche energy of many large semiconductor power MOSFETs is fully tested and 100% detected on the production line, which is a guaranteed measurement in the data. The avalanche voltage usually occurs in 1.2-1.3 times BVDSS, and the duration is usually μs or even ms. Then the peak pulse voltage that lasts only a few or dozens of ns and is much lower than the avalanche voltage is not correct. The power MOSFET is damaged.

Why in the actual design, in the most extreme case, the maximum VDS voltage of the power MOSFET must be lower than BVDSS, but also have a certain derating, such as 5%, 10%, or even 20% derating?

The reason is: to ensure the productivity of the electronic system, and the reliability in mass production.

The design of any electronic system, the actual parameters will have a certain range of variation, and sometimes it is difficult to ensure that multiple extreme situations come together, causing problems with the system, especially under high temperature conditions, power devices and other systems. The drift of the temperature coefficient of the component creates some unimaginable problems, and the derating and design margin can minimize the damage caused under these extreme conditions.

4. Select VTH from the driving voltage

The driving voltages of the power MOSFETs of different electronic systems are not the same. The AC/DC power supply usually uses a driving voltage of 12V, and the motherboard DC/DC converter of the notebook uses a driving voltage of 5V. Therefore, different threshold voltages should be selected according to the driving voltage of the system. VTH power MOSFET.

The threshold voltage VTH of the power MOSFET in the datasheet also has certain test conditions, with different values ​​under different conditions, and VTH has a negative temperature coefficient. Different driving voltages VGS correspond to different on-resistances. In practical applications, temperature changes should be considered, both to ensure that the power MOSFET is fully turned on, and to ensure that the spikes coupled to the G-pole during the turn-off process are guaranteed. There is no false trigger to create a through or short circuit.

5, select the on-resistance RDSON, note: not current

Many times engineers care about RDSON because RDSON is directly related to conduction loss. The smaller the RDSON, the smaller the conduction loss of the power MOSFET, the higher the efficiency, and the lower the temperature rise. Similarly, engineers use existing components in the previous project or in the material library as much as possible, and there is not much consideration for the true selection of RDSON. When the temperature rise of the selected power MOSFET is too low, the larger components of RDSON will be used for cost reasons; when the temperature rise of the power MOSFET is too high and the efficiency of the system is low, the smaller components of RDSON will be used. Or adjust by optimizing the external drive circuit, improving the way of heat dissipation, and so on.

If it is a brand new project, there is no previous project to follow, then how to choose the RDSON of the power MOSFET? Here is a method for everyone: power allocation method.

When designing a power system, the known conditions are: input voltage range, output voltage / output current, efficiency, operating frequency, drive voltage, and of course other technical indicators related to power MOSFET are mainly these parameters. Proceed as follows:

(1) Calculate the maximum loss of the system based on the input voltage range, output voltage/output current, and efficiency.

(2) The stray loss of the power loop, the static loss of the non-power loop component, the static loss of the IC and the drive loss, and a rough estimate, the empirical value can account for 10% to 15% of the total loss. If the power loop has a current sampling resistor, calculate the power consumption of the current sampling resistor. The total loss is subtracted from these losses, and the remainder is the power loss of the power device, transformer or inductor.

The remaining power loss is distributed to the power device and the transformer or inductor in a certain proportion. If not, the average number of components is distributed, so that the power loss of each MOSFET is obtained.

(3) The power loss of the MOSFET is allocated to the switching loss and the conduction loss according to a certain ratio. If not, the switching loss and the conduction loss are evenly distributed.

(4) Calculate the maximum allowable on-resistance by the MOSFET conduction loss and the rms current flowing through it. This resistor is the RDSON of the MOSFET at the highest operating junction temperature.

The RDSON of the power MOSFET in the datasheet is marked with certain test conditions and has different values ​​under different defined conditions. The tested temperature is: TJ=25°C, RDSON has a positive temperature coefficient, so according to the highest working junction temperature of the MOSFET The RDSON temperature coefficient is calculated from the above RDSON to obtain the corresponding RDSON at 25 °C.

(5) Select the appropriate power MOSFET from the 25°C RDSON and trim it down or upward according to the RDSON actual parameters of the MOSFET.

Through the above steps, the model and RDSON parameters of the power MOSFET are initially selected.

In many materials and literature, the maximum current of the system is often calculated, and then derating is performed. The current value of the power MOSFET data table is used to select the device. This method is wrong.

The current of the power MOSFET is a calculated value, and it is based on TC=25°C, and the switching loss is not considered. Therefore, the difference between this method and the actual application is too large and has no reference value. In some applications where large current surges require short-circuit protection, the maximum drain pulse current value and its duration in the data sheet are checked. This is not directly related to the selection of RDSON.

6, select the switch characteristics: Crss, Coss, Ciss; Qg, Qgd, Qoss

Power MOSFETs generate switching losses during switching, and switching losses are primarily related to these switching characteristics. QG affects the drive loss, which is not consumed in the power MOSFET and is consumed in the driver IC. The larger the QG, the greater the drive loss.

After selecting the model of the power MOSFET based on RDSON, these switch characteristic parameters can be found in the data sheet, and then the switching loss is calculated based on these parameters.

7. Thermal design and calibration

According to the selected power MOSFET data table and the operating state of the system, calculate its conduction loss and switching loss, calculate the maximum junction temperature of the MOSFET from the total power loss and the operating ambient temperature, and check whether it is within the design range. All conditions are based on the worst conditions and are then adjusted accordingly.

If the total loss is too large and greater than the allocated power loss, then re-select other types of power MOSFETs, you can view other models larger or smaller than the RDSON of the selected power MOSFE, and check the total power loss again. The above process usually has to cooperate with steps 5 and 6, after several iterations of verification, and finally determine the model that matches the design until the design requirements are met.

Sometimes the product with the correct parameters can not be found due to the limitation of the product model. The following methods can be used:

(1) Use multi-tube parallel connection to solve the problem of heat dissipation and temperature rise.

(2) Redistribute power loss, transformers or inductors, and other power components allocate more power. When changing the power distribution, it is also necessary to ensure that the temperature rise of other components meets the system design requirements.

(3) If the system allows, change the way of heat dissipation or increase the size of the heat sink.

(4) Other factors, adjust the working frequency, change the circuit structure, etc., such as PFC adopts staggered structure, adopts LLC or other soft switching circuit.

8, check the diode characteristics

In bridge circuits such as full-bridge, half-bridge, LLC, and BUCK circuit down-tubes, there is the problem of reverse recovery of internal parasitic diodes. The simplest method is to use a power MOSFET with a fast recovery diode inside, if the internal is not For fast recovery diodes, consider the reverse recovery characteristics of the internal parasitic diode: Irrm, Qrr, trr, trr1/trr2, such as trr less than 250ns, these parameters affect the voltage spike, efficiency, and reliability of the shutdown, as in During the startup and short circuit of the LLC, the system enters the capacitive mode. If the diode reverse recovery performance is poor, it is easy to cause the upper and lower tubes to pass through and be damaged. If the controller has capacitive mode protection, you don't have to consider this factor.

9, avalanche energy and UIS, dv/dt

The avalanche energy and test conditions refer to the following article, which is detailed in detail. In addition to flyback and some motor-driven applications, most of these structures do not suffer from this simple voltage-clamping avalanche. In many applications, the combined effects of dv/dt, over-temperature, and large current during diode reverse recovery are dynamic. Avalanche breakdown damage, related content can refer to the article.

10, other parameters

The size of the internal RG, load switch and hot-swap problems in the linear region, SOA characteristics, and EMI-related parameters, and so on.

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