Selection of MOS switching transistor

Selection of MOS switching transistor

In general, MOSFETs commonly used for high-end driving require a gate voltage greater than the source voltage when conducting, while MOSFETs for high-end driving require the source voltage to be the same as the drain voltage (VCC) when conducting, so the gate voltage should be 4V or 10V greater than VCC. If you want to obtain a voltage greater than VCC in the same system, you need a dedicated boost circuit. Many motor drivers integrate charge pumps, and it is important to choose appropriate external capacitors to obtain sufficient short-circuit current to drive MOS transistors.

MOS transistor is voltage driven, theoretically, as long as the gate voltage reaches the turn-on voltage, it can conduct DS, and any resistance in the gate string can conduct. But if a high switching frequency is required, the gate to ground or VCC can be regarded as a capacitor. For a capacitor, the larger the resistance of the string, the longer the time for the gate to reach the conduction voltage, and the longer the MOS is in a semi conductive state. In the semi conductive state, the internal resistance is large, and the heat generation will also increase, which is very easy to damage the MOS. Therefore, at high frequencies, the resistance of the gate to ground string not only needs to be small, but generally requires a pre driver circuit.

Types and structures of MOS transistors

MOSFET is a type of FET (the other being JFET) that can be manufactured in either enhancement or depletion mode, with a total of four types: P-channel or N-channel. However, in practical applications, only enhancement mode N-channel MOS and enhancement mode P-channel MOS are used, so NMOS or PMOS are usually referred to as these two types. For these two types of enhanced MOS transistors, the most commonly used is NMOS - because of its low on resistance and ease of manufacturing, NMOS is generally used in applications such as switching power supplies and motor drives. There is parasitic capacitance between the three pins of the MOS transistor, which is not what we need, but due to manufacturing process limitations. The existence of parasitic capacitance makes it more troublesome to design or select driving circuits, but it cannot be avoided. We will introduce it in detail later. There is a parasitic diode between the drain and source of a MOS transistor, called a body diode, which is important for driving inductive loads such as motors. By the way, body diodes only exist in individual MOS transistors and are usually not present inside integrated circuit chips.

MOS transistor conduction characteristics

The meaning of conduction is to act as a switch, equivalent to closing the switch.

The characteristic of NMOS is that Vgs will conduct when it is greater than a certain value, which is suitable for use in situations where the source is grounded (low-end driving), as long as the gate voltage reaches 4V or 10V. The characteristic of PMOS is that Vgs will conduct when it is less than a certain value, making it suitable for situations where the source is connected to VCC (high-end driving). However, although PMOS can be conveniently used as a high-end driver, due to its high on resistance, high price, and limited replacement options, NMOS is still commonly used in high-end drivers.

Loss of MOS switching transistor

Whether it is NMOS or PMOS, there is a conduction resistance present after conduction, and the current will consume energy on this resistance, which is called conduction loss. Choosing MOS transistors with low on resistance will reduce conduction losses. The current on resistance of low-power MOS transistors is generally around tens of milliohms, and there are also several milliohms. MOS does not complete its conduction and cutoff in an instant. The voltage across the MOS has a decreasing process, and the current flowing through it has an increasing process. During this period, the loss of the MOS transistor is the product of the voltage and current, called the switching loss. Usually, the switching loss is much greater than the conduction loss, and the faster the switching frequency, the greater the loss. The product of voltage and current at the moment of conduction is very large, resulting in significant losses. Shortening the switching time can reduce the loss during each conduction; Reducing the switching frequency can decrease the number of switches per unit time. Both methods can reduce switch losses.

MOS transistor driver

Compared with bipolar transistors, it is generally believed that conducting MOS transistors does not require current, as long as the GS voltage is higher than a certain value. This is easy to achieve, but we still need speed. In the structure of MOS transistors, it can be seen that there is parasitic capacitance between GS and GD, and the driving of MOS transistors is actually the charging and discharging of the capacitance. Charging a capacitor requires a current, as the capacitor can be viewed as a short circuit at the moment of charging, resulting in a relatively large instantaneous current. The first thing to pay attention to when selecting/designing MOS transistor drivers is the size of the instantaneous short-circuit current that can be provided. When choosing MOSFETs, there are two main types: N-channel and P-channel. In power systems, MOSFETs can be seen as electrical switches. When a positive voltage is applied between the gate and source of an N-channel MOSFET, its switch becomes conductive. When conducting, current can flow from the drain to the source through the switch. There is an internal resistance between the drain and source, called the conduction resistance RDS (ON). It must be clear that the gate of MOSFET is a high impedance terminal, therefore, a voltage must always be applied to the gate, which is the resistance to ground connected to the gate in the circuit diagram described later. If the gate is suspended, the device will not function as designed and may turn on or off at inappropriate times, resulting in potential power loss in the system. When the voltage between the source and gate is zero, the switch closes and the current stops flowing through the device. Although the device has been 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 design is to decide whether to use N-channel or P-channel MOSFETs. In typical power applications, when a MOSFET is grounded and the load is connected to the mains voltage, the MOSFET constitutes a low side switch. In low-voltage side switches, N-channel MOSFETs should be used to consider the voltage required for turning off or conducting devices. When MOSFET is connected to the bus and load ground, a high voltage side switch is required. Usually, P-channel MOSFETs are used in this topology due to considerations for voltage driving.

Step 2: Determine the rated current

The second step is to select the rated current of the MOSFET, which depends on the circuit structure. The rated current should be the maximum current that the load can withstand under all conditions. Similar to the voltage situation, designers must ensure that the selected MOSFET can withstand this rated current, even when the system generates peak currents. The two current scenarios considered are continuous mode and pulse spike. In continuous conduction mode, MOSFET is in steady state, and current continuously flows through the device. Pulse spike refers to a large amount of surge (or spike current) flowing through a device. Once the maximum current under these conditions is determined, simply select the device that can withstand this maximum current. After selecting the rated current, it is also necessary to calculate the conduction loss. In practical situations, MOSFETs are not ideal devices because there is electrical energy loss during the conduction process, which is called conduction loss. MOSFET acts like a variable resistor when 'conducting', determined by the device's RDS (ON) and significantly changing with temperature. The power consumption of the device can be calculated by Iload2 × RDS (ON), and since the on resistance changes with temperature, the power consumption will also vary proportionally. The higher the voltage VGS applied to the MOSFET, the smaller the RDS (ON), and vice versa, the higher the RDS (ON). For system designers, this is where trade-offs need to be made depending on the system voltage. For portable designs, it is relatively easy (and common) to use lower voltages, while for industrial designs, higher voltages can be used. Please note that the RDS (ON) resistance will slightly increase with the current. Various electrical parameter changes regarding the RDS (ON) resistance can be found in the technical data sheet provided by the manufacturer.

Step 3: Determine the thermal requirements

The next step in selecting MOSFETs is to calculate the cooling requirements of the system. Designers must consider two different scenarios, namely the worst-case scenario and the real situation. It is recommended to use the worst-case scenario calculation result, as this result provides a greater safety margin and ensures that the system will not fail. There are some measurement data that need to be noted on the MOSFET data sheet, such as the thermal resistance between the semiconductor junction of the packaged device and the environment, as well as 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 x power dissipation]). According to this equation, the maximum power dissipation of the system can be solved, which is defined as I2 x RDS (ON). Since the designers have determined the maximum current to be passed through the device, the RDS (ON) at different temperatures can be calculated. It is worth noting that when dealing with simple thermal models, designers must also consider the thermal capacity of the semiconductor junction/device casing and the casing/environment, which requires that the printed circuit board and packaging do not immediately heat up. Usually, a PMOS transistor will have parasitic diodes that prevent the source and drain terminals from being reversed. For PMOS, its advantage over NMOS is that its turn-on voltage can be 0, and the voltage difference between DS voltages is not significant. However, the conduction condition of NMOS requires VGS to be greater than the threshold, which will inevitably result in the control voltage being greater than the required voltage, causing unnecessary trouble.