3 MOSFET device selection rules that will teach you how to become a device selection wizard

As saying goes, "people who don't think about long term should think about short term." Electronics design engineers before starting a project, at beginning of device selection, must carefully consider everything and choose most suitable one. devices for your needs Only in this way can success of project be guaranteed.

The power MOSFET is probably one of most used devices by engineers, but did you know about it? When it comes to choosing MOSFET devices, there are various factors to consider, ranging from choice of N-type or P-type, package type, to MOSFET on-state withstand voltage and resistance, etc. The requirements of different applications are constantly changing. The following article briefly describes MOSFET devices. 10 step model selection rule, I think you will gain a lot by reading it.

1. First step in choosing a power MOSFET: P tube or N tube?

There are two types of power MOSFETs: N-channel and P-channel. During system design process, choice of N-channel or P-channel should be selected according to actual application. There are many types of N-channel MOSFETs and low cost; P-channel MOSFETs have fewer models and high cost.

If voltage of S terminal of power MOSFET is not system ground reference, N-channel must be controlled by a floating power supply, a transformer, or a starter, and control circuit is complicated. ; P-channel can be controlled directly, and drive is simple.

Main applications where N-channel and P-channel need to be considered:

(1) Fans used in laptops, desktops and servers to cool CPU and system, printer paper feed system drive, and motor control circuits of household appliances such as vacuum cleaners, air purifiers, and electric fans.Bridge circuit structure, top tube each arm of bridge can use a P tube or an N tube.

(2) A hot swappable 48V input system MOSFET for communication system is placed at top end, and either P or N lamps can be used.

(3) Two back-to-back power MOSFETs connected in series in laptop's input circuit, used as anti-reverse connection and load switch, use an N-channel to drive an on-chip charging pump. , and use P-channel to control it directly.

2. Select package type

After determining channel type of power MOSFET, second step is to determine package. The principles for choosing a case are as follows:

(1) Temperature rise and thermal design are main requirements for packaging selection

Different size kits have different thermal resistance and power dissipation. In addition to system heat dissipation conditions and ambient temperature, such as whether there is air cooling, shape and size of heatsink, and environment is closed, basic principle is to ensure temperature rise and efficiency of power MOSFET system, select power MOSFET with more general parameters and package.

Sometimes, due to limitation of other conditions, it is necessary to use multiple MOSFETs in parallel to solve problem of heat dissipation, such as in PFC applications, electric vehicle motor controllers, secondary synchronous rectification of module power supplies in communication. systems, etc., method of connecting multiple handsets in parallel will be selected.

If it is not possible to use parallel connection with multiple lamps, in addition to choosing a high power MOSFET with better performance, you can use a larger package or a new package, such as replacing TO220 package with TO247 package in some AC/DC power supplies. power supply, in some communication systems The power supply adopted new DFN8*8 package.

(2) System Size Limit

Some electronic systems are limited by size and internal height of printed circuit board. For example, communication system module power supply usually adopts DFN5*6 and DFN3*3 chassis due to height limitation; In some ACDC power supplies, ultra-thin design or Due to package limitations, power MOSFET leads of TO220 package are inserted directly into root during assembly, and TO247 package cannot be used due to height restrictions.

Some ultra-slim designs bend pins of device and lay flat, making manufacturing process more difficult.

Due to extremely strict size restrictions in development of large capacity lithium battery protection boards, most of them now use chip-level CSP packages to maximize heat dissipation efficiency in a minimal size.

(3) Cost control

In past, many electronic systems used pluggable enclosures. Due to rising labor costs in recent years, many companies have begun to switch to SMD packages. Although cost of welding SMD is higher than that of plug-in, automation of SMD welding is high, and overall cost can be controlled within reasonable limits. In some extremely cost sensitive applications such as motherboards and boards fordesktop PCs, power MOSFETs in DPAK packages are commonly used due to low cost of this package.

Therefore, when choosing a power MOSFET package, above factors should be considered in combination with company's style and product characteristics.

3. Select RDSON with resistance, note: not current

Engineers often care about RDSON because RDSON is directly related to conduction losses. The smaller RDSON, lower conduction loss of power MOSFET, higher efficiency and lower temperature rise.

Similarly, engineers go out of their way to use existing components from previous designs or material libraries, with little regard for actual RDSON selection method. When temperature rise of selected power MOSFET is too low, high RDSON components will be used instead for cost reasons; when high power MOSFET temperature rise is too high and system efficiency is low, components with smaller RDSON will be used instead or adjusted by optimizing external drive circuit, improving heat dissipation method, etc.

If this is a new project and there is no previous project, how to choose RDSON power MOSFET? Here's a method for everyone: power distribution method.

When designing a power system, following conditions are known: input voltage range, output voltage/output current, efficiency, operating frequency, control voltage and, of course, other technical indicators, and power of MOSFETs is mainly related to these parameters. The steps are:

(1) Calculate maximum system losses according to input voltage range, output voltage/output current, and efficiency.

(2) Power stray loss, non-power static loss, IC static loss, and drive loss are estimates, and empirical value can be 10% of total loss ~15 %.

If there is a current sampling resistor in power loop, calculate power consumption of current sampling resistor. These losses are subtracted from total losses, and remainder is power loss of power device, transformer or inductor.

Distribute remaining power loss to power devices and transformers or inductors in a certain proportion. If you're not sure, spread it evenly across number of components so you can get power loss of each MOSFET.

(3) Divide MOSFET power loss into switching losses and conduction losses according to a certain ratio. If you're not sure, split switching losses and conduction losses equally.

(4) Calculate maximum allowable on-state resistancebased on conduction loss of MOSFET and RMS current flowing through it. This resistance represents RDSON of MOSFET at highest operating junction temperature.

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

(5) Select a suitable model power MOSFET based on RDSON at 25°C, and reduce or reduce it according to actual RDSON value of MOSFET.

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