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Introduction
The following six points must be considered when selecting a MOSFET.
・ How many V withstand voltage is required?
-At what V should the gate be driven?
・ How many mΩ should the ON resistance be set to?
・ At what Hz should the switching frequency be driven?
・ What is the drain current and how long does it flow?
・ How many watts will the peak power reach in what period?
・ What is the package size?
Details are explained below.
Selection of withstand voltage
When switching the MOSFET, ringing may occur when the MOSFET is turned off due to the influence of the inductor component of the wiring.
For the withstand voltage of the MOSFET, it is necessary to select a product with a voltage higher than the peak voltage of the ringing.
It is a good idea to start by selecting a MOSFET with a withstand voltage rating that is twice the drain voltage of the MOSFET, and then consider whether to use a higher withstand voltage MOSFET or a lower withstand voltage MOSFET based on the evaluation results of the actual equipment.
Selection of gate drive voltage
A MOSFET with a higher gate voltage can have a lower on-resistance. However, I think that most ICs are driven by a voltage of 5 V or less on the board.
Therefore, it is necessary to consider whether to prepare a high voltage for driving the gate of the MOSFET or to drive it with a logic level of 5 V or less. Of course, it is possible to drive a logic-level drivable MOSFET with a higher gate voltage, but there is a risk of false turn-on of the MOSFET due to the lower gate threshold.
Selection of on-resistance
The MOSFET on-resistance is a parameter that affects the conduction losses of the MOSFET. The lower the on-resistance, the smaller the conduction loss of the MOSFET, but for MOSFETs of the same product generation, the lower the on-resistance, the higher the cost.
Selection of switching frequency
The MOSFET gate voltage rise time can be estimated from the aforementioned gate drive current and gate charge characteristics.
From these relationships, MOSFETs with higher gate drive current and lower gate charge can switch faster.
However, since the MOSFET gate charge and on-resistance have a trade-off relationship, reducing the gate charge will increase the on-resistance. It is necessary to select a MOSFET with the optimum gate charge and on-resistance from the balance between switching loss and conduction loss.
Consideration of drain current
If the MOSFET is to be used as a load switch in which current flows in the always-on state, it is necessary not to exceed the continuous drain current specified in the datasheet.
However, since the MOSFET is a part used as a switching element, ON/OFF operations occur. There is a transition time between V DS and ID during turn-on and turn-off, during which the relationship between V DS and ID must be within the safe region of the SOA graph given in the datasheet.
Examination of peak power
In the case of using a MOSFET for switching, calculate the junction temperature under the usage conditions from the transient thermal resistance graph described in the data sheet from the power pulse width and duty ratio of the MOSFET, and ensure that the rated junction temperature is not exceeded. Must be used conditionally.
Also note that the calculation of the peak temperature rise must be done using the peak power of the pulse, not the average power. Power pulse waveforms such as trapezoidal, triangular, and partial sine waves can be calculated by approximating square pulses with the same total energy.
Selection of package size
It is necessary to select the package size of the MOSFET according to the substrate size of the application and heat dissipation restrictions.
In recent silicon power MOSFET package trends released by MOSFET vendors, many lineups of surface-mount DFN packages of 1 cm square or less for 150 V or less have been released.
In addition, many MOSFETs for high-power applications such as SiC MOSFETs use the TO-247 package, and there is a trend toward an increase in the lineup of some surface mount types.
lastly
Recently, along with the rapid electrification of automobiles and motorcycles, robots such as automated guided vehicles (AGV) that work together with humans have appeared in the industrial field. Many MOSFETs are used in actuators and high power power circuits used in these applications.
However, since these circuits handle a large amount of power, the loss also increases. As a result, the heat generated by the component increases, so a MOSFET with a large package must be selected to improve heat dissipation.
However, large package MOSFETs also add weight to the component, which negatively impacts the overall weight of the application.
As automobiles, motorcycles, and AGVs become heavier, fuel consumption worsens, so there is also demand for smaller and lighter weight products.
In order to solve these problems, it is necessary to select the optimal parts for MOSFET loss and package shape. Please feel free to contact us using the inquiry form below to discuss the optimal parts selection.
Also, if you want to know the basic terms and knowledge about MOSFETs, please refer to the article below.
https://www.macnica.co.jp/business/semiconductor/articles/onsemi/142988
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