As the saying goes, "people without foresight must have immediate worries". For electronic design engineers, before the start of the project and at the beginning of device selection, they should make full consideration and select the device most suitable for their own needs in order to ensure the success of the project.
Power MOSFET is probably one of the most commonly used devices by engineers, but do you know? As for the device selection of MOSFET, we should consider all factors, ranging from n-type or p-type and packaging type to the withstand voltage and on resistance of MOSFET. Different application requirements vary. The following article summarizes the 10 steps of MOSFET device selection. I believe you will gain a lot after reading it.
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, whether n-tube or p-tube should be selected according to the actual application. N-channel MOSFET has many models and low cost; P-channel MOSFET has few models and high cost. If the voltage at the S-pole connection end of the power MOSFET is not the reference ground of the system, the n-channel needs floating power supply drive, transformer drive or bootstrap drive, and the drive circuit is complex; The p-channel can be driven directly, which is simple to drive.
The applications of n-channel and p-channel to be considered mainly include:
(1) The fan used for cooling CPU and system in notebook computers, desktops and servers, the motor drive of printer paper feeding system, and the motor control circuit of white household appliances such as vacuum cleaner, air purifier and electric fan. These systems use full bridge circuit structure. P tube or n tube can be used on each bridge arm.
(2) The hot plug MOSFET of the 48V input system of the communication system is placed at the high end, and either P tube or n tube can be used.
(3) Two back-to-back power MOSFETs connected in series in the input circuit of notebook computer, which play the role of anti reverse connection and load switch. The n-channel needs to control the charging pump integrated and driven in the chip, and the p-channel can be directly driven.
2. Select package type
After the channel type of 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 packaging selection
Different package sizes have different thermal resistance and dissipated power. In addition to considering the heat dissipation conditions and ambient temperature of the system, such as whether there is air cooling, the shape and size limit of the radiator, and whether the environment is closed, the basic principle is to select parameters and package more general power MOSFET on the premise of ensuring the temperature rise of power MOSFET and system efficiency.
Sometimes, due to the limitations of other conditions, multiple MOSFETs need to be connected in parallel to solve the problem of heat dissipation. For example, in PFC applications, electric vehicle motor controller, module power secondary synchronous rectification of communication system, multi transistor parallel connection will be selected.
(2) Dimensional limitations of the system
Some electronic systems are subject to the size and internal height of PCB. For example, the module power supply of communication system usually adopts dfn5 * 6 and dfn3 * 3 packaging due to the limitation of height; In some ACDC power supplies, the ultra-thin design is used or due to the limitation of the shell, the power MOSFET pin of to220 package is directly inserted into the root during assembly, and the to247 package cannot be used due to the limitation of height. Some ultra-thin designs directly bend and lay the device pins flat, which will complicate the design and production process.
In the design of high-capacity lithium battery protection board, due to the extremely strict size restrictions, chip level CSP packaging is mostly used to improve the heat dissipation performance as much as possible and ensure the minimum size.
(3) Production process of the company
To220 has two kinds of packaging: bare metal packaging and all plastic packaging. The bare metal packaging has small thermal resistance and strong heat dissipation capacity, but in the production process, it needs to add insulation pendant, the production process is complex and the cost is high, while the all plastic packaging has large thermal resistance and weak heat dissipation capacity, but the production process is simple.
In order to reduce the manual process of locking screw, in recent years, some electronic systems use clips to clamp the power MOSFET in the heat sink, so there is a new packaging form that removes the perforated part on the upper part of the traditional to220, and also reduces the height of the device.
(4) Cost control
In the early days, many electronic systems used plug-in packaging. In recent years, due to the increase of labor cost, many companies began to use patch packaging. Although the welding cost of patch is higher than that of plug-in, the automation of patch welding is high, and the overall cost can still be controlled within a reasonable range. In some cost sensitive applications such as desktop motherboard and board card, DPAK packaged power MOSFET is usually used because of its low cost.
Therefore, when choosing the package of power MOSFET, we should consider the above factors in combination with our own company's style and product characteristics.
3. Select bvdss
In most cases, it seems that selecting the withstand voltage of 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 material numbers of specific suppliers, and the product rated voltage is also fixed. For example, in laptop adapters and mobile phone chargers, the input is 90 ~ 265V AC, and the primary usually selects 600V or 650V power MOSFET; The input voltage of notebook computer motherboard is 19v, and 30V power MOSFET is usually selected, which does not need any consideration at all.
The breakdown voltage bvdss of power MOSFET in the data sheet has certain test conditions, has different values under different conditions, and bvdss has positive temperature coefficient. These factors should be considered comprehensively in practical application.
It is often mentioned in many materials and literatures that if the maximum peak voltage of VDS of power MOSFET in the system is greater than bvdss, even if the peak pulse voltage lasts only a few or dozens of NS, the power MOSFET will enter avalanche and be damaged.
Unlike triodes and IGBTs, power MOSFETs have the ability to resist avalanche. Moreover, the avalanche energy of power MOSFETs of many large semiconductor companies is fully inspected and 100% detected on the production line, that is, this is a guaranteed measurement value in the data. The avalanche voltage usually occurs at 1.2 ~ 1.3 times of bvdss, and the duration is usually μ s. Even ms level, the peak pulse voltage lasting only a few or dozens of NS and far lower than the avalanche voltage will not damage the power MOSFET.
Why in the actual design, it is required that the maximum VDS voltage of power MOSFET must be lower than bvdss in the most extreme cases, and there must be a certain derating, such as 5%, 10%, or even 20%?
The reason is to ensure the producibility of the electronic system and the reliability in mass production.
In the design of any electronic system, the actual parameters will have a certain variation range. Sometimes it is difficult to ensure that multiple extreme situations meet together, which will cause problems to the system, especially under the condition of high temperature, the drift of temperature coefficient of power devices and other components of the system will produce some unimaginable problems, Derating and design margin can minimize the problem of damage under these extreme conditions.
4. Vth selected by driving voltage
The threshold voltage Vth of power MOSFET in the data sheet also has certain test conditions. It has different values under different conditions, and Vth has negative temperature coefficient. Different driving voltages VGS correspond to different on resistances. In practical application, the change of temperature should be considered. It is necessary to ensure that the power MOSFET is fully turned on and that the spike pulse coupled to the G pole will not be triggered by mistake, resulting in direct or short circuit.
5. Select the on resistance RDSON. Note: it is not current
Engineers often care about RDSON because RDSON is directly related to on loss. The smaller the RDSON, the smaller the on loss, higher efficiency and lower temperature rise of power MOSFET. Similarly, engineers try to use the existing components in the previous project or material warehouse as much as possible, and do not think much about the real selection method of RDSON. When the temperature rise of the selected power MOSFET is too low, components with larger RDSON will be used for cost consideration; When the temperature rise of power MOSFET is too high and the efficiency of the system is low, it will be adjusted by using smaller RDSON components, or by optimizing the external drive circuit and improving the way of heat dissipation.
If it is a new project and there is no previous project to follow, how to select the RDSON of power MOSFET? Here is a method: power allocation method.
When designing a power supply system, the known conditions are: input voltage range, output voltage / output current, efficiency, working frequency, driving voltage, of course, there are other technical indexes and power MOSFET related parameters. The steps are as follows:
(1) Calculate the maximum loss of the system according to the input voltage range, output voltage / output current and efficiency.
(2) The stray loss of power circuit, the static loss of non power circuit elements, the static loss of IC and driving loss are roughly estimated. The empirical value can account for 10% ~ 15% of the total loss. If the power circuit has a current sampling resistance, calculate the power consumption of the current sampling resistance. The total loss minus the above losses, the remaining part is the power loss of power devices, transformers or inductors.
The remaining power loss is distributed to power devices, transformers or inductors in a certain proportion. If it is uncertain, it is evenly distributed according to the number of components, so as to obtain the power loss of each MOSFET.
(3) The power loss of MOSFET is allocated to the switching loss and on loss in a certain proportion. If it is uncertain, the switching loss and on loss are equally allocated.
(4) The maximum allowable on resistance is calculated from the on loss of MOSFET and the RMS current flowing through. This resistance is the RDSON of MOSFET at the highest operating junction temperature.
In the data sheet, the RDSON of the power MOSFET is marked with determined test conditions and has different values under different defined conditions. The test temperature is TJ = 25 ℃, and the RDSON has a positive temperature coefficient. Therefore, according to the highest operating junction temperature of the MOSFET and the RDSON temperature coefficient, the corresponding RDSON at the temperature of 25 ℃ is obtained from the above calculated value of the RDSON.
(5) Select the power MOSFET with appropriate model from the RDSON at 25 ℃, and trim it down or up according to the actual RDSON parameters of the MOSFET.
Through the above steps, the model and RDSON parameters of power MOSFET are preliminarily selected.
In many materials and literatures, the maximum current of the system is often calculated, and then the derating is carried out. The device is selected from the current value of the power MOSFET data sheet. This method is wrong.
The current of power MOSFET is a calculated value, and it is based on TC = 25 ℃, and the switching loss is not considered. Therefore, this method is too far from the actual application and has no reference value. In some applications with high current impact requiring short-circuit protection, the maximum drain pulse current value and its duration in the data sheet will be checked, which is not directly related to the selection of RDSON.
6. Select switch characteristics: CRSs, coss, CISS; Qg、Qgd、Qoss
The switching loss of power MOSFET is mainly related to these switching characteristic parameters. QG affects the driving loss, which is not consumed in the power MOSFET, but in the driving IC. The larger the QG, the greater the driving loss.
After selecting the model of power MOSFET based on RDSON, these switching characteristic parameters can be found in the data table, and then the switching loss can be calculated according to these parameters.
7. Thermal design and verification
According to the data sheet of the selected power MOSFET and the working state of the system, the conduction loss and switching loss are calculated. The maximum junction temperature of the MOSFET is calculated from the total power loss and the working ambient temperature to check whether it is within the design range. All conditions are based on the worst conditions, and then adjusted according to the calculation results.
If the total loss is too large and greater than the allocated power loss, it is necessary to re select the power MOSFET of other models. You can check other models larger or smaller than the RDSON of the selected power mosfe, and check the total power loss again. The above process usually needs to cooperate with steps 5 and 6, and finally determine the model matching the design after repeated verification for several times, Until the design requirements are met.
Sometimes, due to the limitation of product model, the product with appropriate parameters cannot be found. The following methods can be adopted:
(1) Multi tube parallel connection is used to solve the problems of heat dissipation and temperature rise.
(2) Redistribute the power loss, and allocate more power consumption to transformers or inductors and other power components. 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 radiator.
(4) Other factors, such as adjusting the working frequency, changing the circuit structure, etc. for example, PFC adopts staggered structure, LLC or other soft switching circuits.
8. Check diode characteristics
In bridge circuits such as full bridge, half bridge, LLC and buck circuits, there is a problem of reverse recovery of internal parasitic diodes. The simplest method is to use the power MOSFET with internal fast recovery diode. If there is no internal fast recovery diode, the reverse recovery characteristics of internal parasitic diodes should be considered: IRRM, qrr, TRR, trr1 / trr2. For example, TRR should be less than 250ns, These parameters affect the voltage spike, efficiency and reliability of shutdown. For example, in the starting and short circuit of LLC, the system enters capacitive mode. If the reverse recovery performance of diode is poor, it is easy to damage the upper and lower tubes. If the controller has capacitive mode protection function, this factor is not considered.
9. Avalanche energy and UIS, DV / DT
Avalanche energy and test conditions refer to the following articles, which are very detailed. In addition to flyback and some motor-driven applications, this simple voltage clamped avalanche will not occur in most structures. In many applications, the combined action of DV / DT, over temperature and large current during diode reverse recovery will produce dynamic avalanche breakdown damage. For relevant contents, please refer to the article.
10. Other parameters
The size of internal RG, the problem of load switch and hot plug working in linear region, SOA characteristics, EMI related parameters, etc., Read the full text, the original title: what is the difficulty in MOSFET selection? The 10 step rule teaches you to do it step by step
The source of the article: [micro signal: weixin21ic, WeChat official account: 21IC electronic network]
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