MOSFET Device Selection of The 3 Major Rules

MOSFET device selection to consider all aspects of factors, from small to choose N-type or P-type, package type, large to MOSFET voltage, on-resistance, etc., different application requirements vary. The following article summarizes the MOSFET device selection of the 3 major rules, I believe that after reading you will have a great deal.

1. Power MOSFET selection step one: P-tube, or N-tube?

There are two types of power MOSFETs: N-channel and P-channel, in the process of system design to select the N-tube or P-tube, to the actual application specific to choose, N-channel MOSFETs to choose the model, low cost; P-channel MOSFETs to choose the model less, high cost.

If the voltage at the S-pole connection of the power MOSFET is not the reference ground of the system, the N-channel requires a floating ground power supply drive, transformer drive or bootstrap drive, drive circuit complex; P-channel can be directly driven, drive simple.

Need to consider the N-channel and P-channel applications are mainly

a. Notebook computers, desktops and servers used to give the CPU and system cooling fan, printer feeding system motor drive, vacuum cleaners, air purifiers, electric fans and other home appliances motor control circuit, these systems use full-bridge circuit structure, each bridge arm on the tube can use P-tube, can also use N-tube.

b. Communication system 48V input system of hot-plug MOSFETs placed at the high end, you can use P-tubes, you can also use N-tubes.

c. Notebook computer input circuit in series, play the role of anti-reverse connection and load switching two back-to-back power MOSFETs, the use of N-channel need to control the chip internal integrated drive charge pump, the use of P-channel can be directly driven.

2. Selection of package type

Power MOSFET channel type to determine the second step to determine the package, package selection principles are.

a. Temperature rise and thermal design is the most basic requirements for selecting the package

Different package sizes have different thermal resistance and power dissipation, in addition to considering the thermal conditions of the system and the ambient temperature, such as whether there is air cooling, heat sink shape and size restrictions, whether the environment is closed and other factors, the basic principle is to ensure the temperature rise of the power MOSFET and system efficiency, the premise of selecting parameters and package more general power MOSFET.

Sometimes due to other conditions, the need to use multiple MOSFETs in parallel to solve the problem of heat dissipation, such as in PFC applications, electric vehicle motor controllers, communications systems, such as the module power supply secondary synchronous rectification applications, are selected in parallel with multiple tubes.

If multi-tube parallel connection cannot be used, in addition to selecting a power MOSFET with better performance, in addition, a larger size package or a new type of package can be used, for example, in some AC/DC power supplies TO220 will be changed to TO247 package; in some communication system power supplies, the new DFN8*8 package is used.

b. Size limitation of the system

Some electronic systems are limited by the size of the PCB and the height of the interior, such as the module power supply of communication systems due to the height of the restrictions usually use DFN5 * 6, DFN3 * 3 package; in some ACDC power supply, the use of ultra-thin design or due to the limitations of the shell, assembly TO220 package power MOSFET pins directly into the root, the height of the restrictions can not use TO247 package.

Some ultra-thin design directly bend the device pins flat, this design production process will become complex.

In the design of large-capacity lithium battery protection board, due to the extremely harsh size restrictions, most now use chip-level CSP package to improve thermal performance as much as possible, while ensuring the smallest size.

c. Cost control

Early many electronic systems using plug-in package, these years due to increased labor costs, many companies began to switch to SMD package, although the welding cost of SMD than plug-in high, but the high degree of automation of SMD welding, the overall cost can still be controlled in a reasonable range. In some applications such as desktop motherboards and boards that are extremely cost-sensitive, the power MOSFETs in DPAK packages are usually used because of the low cost of this package.

Therefore, in the selection of power MOSFET package, to combine their own company’s style and product features, taking into account the above factors.

3. Select the on-state resistance RDSON, note: not current

Many times engineers are concerned about RDSON, because RDSON and conduction loss is directly related, the smaller the RDSON, the smaller the power MOSFET conduction loss, the higher the efficiency, the lower the temperature rise.

Similarly, engineers as far as possible to follow the previous project or the existing components in the material library, for the RDSON of the real selection method does not have much to consider. When the temperature rise of the selected power MOSFET is too low, for cost reasons, will switch to RDSON larger components; when the temperature rise of the power MOSFET is too high, the system’s efficiency is low, will switch to RDSON smaller components, or by optimizing the external drive circuit, improve the way to adjust the heat dissipation, etc..

If it is a brand new project, there is no previous project to follow, then how to select the power MOSFET RDSON?Here is a method to introduce to you: power consumption distribution method.

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

a. According to the input voltage range, output voltage / output current, efficiency, calculate the maximum loss of the system.

b. Power circuit spurious losses, non-power circuit components static losses, IC static losses and drive losses, to make a rough estimate, the empirical value can account for 10% to 15% of the total losses.

If the power circuit has a current sampling resistor, calculate the power consumption of the current sampling resistor. Total loss minus these losses above, the remaining part is the power device, transformer or inductor power loss.

The remaining power loss will be allocated to the power device and transformer or inductor in a certain proportion, and if you are not sure, the average distribution by the number of components, so that you get the power loss of each MOSFET.

c. The power loss of the MOSFET is allocated to the switching loss and conduction loss in a certain proportion, and if uncertain, the switching loss and conduction loss are allocated equally.

d. By the MOSFET conduction loss and the RMS current flowing, calculate the maximum allowable conduction resistance, this resistance is the MOSFET at the maximum operating junction temperature RDSON.

Data sheet in the power MOSFET RDSON marked with a defined test conditions, in different defined conditions have different values, the test temperature: TJ = 25 ℃, RDSON has a positive temperature coefficient, so according to the highest operating junction temperature of the MOSFET and RDSON temperature coefficient, from the above RDSON calculated value, to get the corresponding RDSON at 25 ℃ temperature.

e. RDSON from 25 ℃ to select the appropriate type of power MOSFET, according to the actual parameters of the MOSFET RDSON, down or up trim.

Through the above steps, the preliminary selection of the power MOSFET model and RDSON parameters.

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