The first step is to make a selection of MOSFETs, which come in two main types: N-channel and P-channel. In power systems, MOSFETs can be thought of as electrical switches. When a positive voltage is added between the gate and source of an N-channel MOSFET, its switch conducts. During conduction, current can flow through the switch from the drain to the source. There exists an internal resistance between the drain and the source called the on-resistance RDS(ON). It must be clear that the gate of a MOSFET is a high impedance terminal, so a voltage is always added to the gate. This is the resistance to ground that the gate is connected to in the circuit diagram presented later. If the gate is left dangling, the device will not operate as designed and may turn on or off at inopportune moments, resulting in potential power loss in the system. When the voltage between the source and gate is zero, the switch turns off and current stops flowing through the device. Although the device is turned off at this point, there is still a small current present, which is called leakage current, or IDSS.
Step 1: Choose N-channel or P-channel
The first step in selecting the correct device for a design is to decide whether to use an N-channel or P-channel MOSFET. in a typical power application, when a MOSFET is grounded and the load is connected to the trunk voltage, that MOSFET constitutes the low voltage side switch. In a low voltage side switch, an N-channel MOSFET should be used due to the consideration of the voltage required to turn off or turn on the device. When the MOSFET is connected to the bus and the load is grounded, the high voltage side switch is to be used. A P-channel MOSFET is usually used in this topology, again for voltage drive considerations.
Step 2: Determine the current rating
The second step is to select the current rating of the MOSFET. Depending on the circuit structure, this current rating should be the maximum current that the load can withstand under all circumstances. Similar to the case of voltage, the designer must ensure that the selected MOSFET can withstand this current rating, even when the system is generating spike currents. The two current cases considered are continuous mode and pulse spikes. This parameter is based on the FDN304P tube DATASHEET as a reference and the parameters are shown in the figure:
In continuous conduction mode, the MOSFET is in steady state, when current flows continuously through the device. Pulse spikes are when there is a large amount of surge (or spike current) flowing through the device. Once the maximum current under these conditions has been determined, it is simply a matter of directly selecting a device that can withstand this maximum current.
After selecting the rated current, you must also calculate the conduction loss. In practice, the MOSFET is not the ideal device, because in the conductive process there will be power loss, which is called conduction loss. MOSFET in the "on" like a variable resistance, determined by the device's RDS (ON), and with the temperature and significant changes. The power dissipation of the device can be calculated from Iload2 x RDS(ON), and since the on-resistance varies with temperature, the power dissipation varies proportionally. The higher the voltage VGS applied to the MOSFET, the smaller the RDS(ON) will be; conversely the higher the RDS(ON) will be. For the system designer, this is where the tradeoffs come into play depending on the system voltage. For portable designs, it is easier (and more common) to use lower voltages, while for industrial designs, higher voltages can be used. Note that the RDS(ON) resistance rises slightly with current. Variations in the various electrical parameters of the RDS(ON) resistor can be found in the technical data sheet supplied by the manufacturer.
Step 3: Determine Thermal Requirements
The next step in selecting a MOSFET is to calculate the thermal requirements of the system. The designer must consider two different scenarios, the worst case and the true case. The calculation for the worst-case scenario is recommended because this result provides a greater margin of safety and ensures that the system will not fail. There are also some measurements to be aware of on the MOSFET data sheet; such as the thermal resistance between the semiconductor junction of the packaged device and the environment, and the maximum junction temperature.
The junction temperature of the device is equal to the maximum ambient temperature plus the product of thermal resistance and power dissipation (junction temperature = maximum ambient temperature + [thermal resistance × power dissipation]). From this equation the maximum power dissipation of the system can be solved, which is by definition equal to I2 x RDS(ON). Since the personnel have determined the maximum current that will pass through the device, RDS(ON) can be calculated for different temperatures. It is important to note that when dealing with simple thermal models, the designer must also consider the heat capacity of the semiconductor junction/device case and the case/environment; i.e., it is required that the printed circuit board and the package do not warm up immediately.
Usually, a PMOSFET, there will be a parasitic diode present, the diode's function is to prevent the source-drain reverse connection, for PMOS, the advantage over NMOS is that its turn-on voltage can be 0, and the voltage difference between the DS voltage is not much, while the NMOS on condition requires that the VGS be greater than the threshold, which will lead to the control voltage is inevitably greater than the required voltage, and there will be unnecessary trouble. PMOS is chosen as the control switch for the following two applications:
The junction temperature of the device is equal to the maximum ambient temperature plus the product of thermal resistance and power dissipation (junction temperature = maximum ambient temperature + [thermal resistance × power dissipation]). From this equation the maximum power dissipation of the system can be solved, which is by definition equal to I2 x RDS(ON). Since the designer has determined the maximum current that will pass through the device, RDS(ON) can be calculated for different temperatures. It is important to note that when dealing with simple thermal models, the designer must also consider the heat capacity of the semiconductor junction/device case and the case/environment; i.e., it is required that the printed circuit board and the package do not warm up immediately.
Usually, a PMOSFET, there will be a parasitic diode present, the diode's function is to prevent the source-drain reverse connection, for PMOS, the advantage over NMOS is that its turn-on voltage can be 0, and the voltage difference between the DS voltage is not much, while the NMOS on condition requires that the VGS be greater than the threshold, which will lead to the control voltage is inevitably greater than the required voltage, and there will be unnecessary trouble. PMOS is chosen as the control switch for the following two applications:
Looking at this circuit, the control signal PGC controls whether or not V4.2 supplies power to P_GPRS. This circuit, the source and drain terminals are not connected to the reverse, R110 and R113 exist in the sense that R110 control gate current is not too large, R113 control the gate of the normal, R113 pull-up to high, as of PMOS, but also can be seen as a pull-up on the control signal, when the MCU internal pins and pull-up, that is, the output of the open-drain when the output is open-drain, and can not drive the PMOS off, at this time, it is necessary to external voltage given pull-up, so resistor R113 plays two roles. It will need an external voltage to give the pull-up, so resistor R113 plays two roles. r110 can be smaller, to 100 ohms can also.
Post time: Apr-18-2024
