MOSFETs are used in many electronic devices. If you do not know what a MOSFET is, an N-channel Metal Oxide Semiconductor Field Effect Transistor (MOSFET) allows electric current to flow through it from the Drain electrode to the Source electrode when a positive voltage is applied between both electrodes.
A MOSFET’s Gate electrode is used to control the flow of electric current by capacitive coupling with the conductive channel that forms between Drain and Source electrodes, as well as boosting or reducing voltage using a linear regulator.
MOSFETs can be used in many different circuits. When implemented correctly, they can be used as amplifiers, switches, and digital to analog converters. They can also be used to handle high power loads such as motor control.
One of the most common mistakes in using MOSFETs is not properly controlling the Gate voltage and current. This can lead to unreliable or malfunctioning devices. In some cases, the Gate voltage can be directly controlled by a low power circuit which ensures that the MOSFET never dissipates more than a certain amount of current, leaving the current handling capacity of the device unscathed. Although this is possible, it can get quite complex and expensive.
It is usually more cost effective to use a gate driver which allows for simple circuitry that will boost the voltage and current supplied by the microcontroller, while protecting it from damage.
A gate driver is basically an integrated circuit (IC) which boosts the voltage supply by using another power supply or capacitor. This prevents any damage or failure in the low power source.
How to choose the right MOSFET
To select a MOSFET for a given circuit, two sets of requirements must be met: operating requirements, which ensure proper operation, and performance requirements, which minimize device losses as much as possible.
First, let’s go over the operational requirements.
The MOSFET must be able to handle the peak current of the circuit while keeping in mind that the current rating decreases when the temperature increases. The MOSFET’s Drain-Source voltage must be able to withstand the maximum voltage supplied. This voltage rating also varies with temperature.
The MOSFET’s threshold voltage must be less than the input voltage as well as the maximum gate-source voltage specified. The MOSFET must operate within the confines of its SOA (Safe Operating Area). This is where the performance criteria come into play.
The junction temperature, breakdown voltage, and maximum drain current all contribute to the SOA. Losses must be kept to a minimum for a specific thermal junction for the MOSFET to function properly.
Conduction and switching losses are the two types of losses.
- Switching Losses. A MOSFET contains a number of internal capacitors that play an important role in the switching process. The values of these capacitors can be determined by consulting the datasheet and employing the following equations:
Cgd = Crss
Cgs = Ciss – Crss
Cds = Coss – Crss
Another parameter to consider is the total gate charge. This, along with the switching frequency, will provide you with the current required to charge the MOSFET’s gate capacitance. The greater the gate charge, the greater the dissipation loss. However this can be mitigated with a driver gate with high output peak current. The drain-source voltage, drain current, switching frequency, and rise and fall times all have an effect on switching losses.
- Conduction Losses. The second type of loss is conduction loss. The MOSFET has a low resistance drain-source. This is one of the most important variables. It must be kept as low as possible.
These two kinds of losses are dissipated as heat, which raises the junction temperature. The maximum junction temperature (usually 150-200 degrees Celsius) that the MOSFET can withstand is specified by the manufacturer. For a given maximum junction temperature, there is a maximum power dissipation.
Gate Driver
A MOSFET driver IC converts TTL or CMOS logical signals to higher voltage and current in order to quickly and completely switch the gate of a MOSFET. A microcontroller’s output pin is typically sufficient to drive a small-signal logic level MOSFET. But, it’s a different story when it comes to driving larger, high voltage MOSFETs.
The gate capacitance of high voltage MOSFETs is higher. Digital signals are designed to power small loads (on the order of 10-100pF). This is significantly less than the many MOSFETs, which can have thousands of pF.
Furthermore, the gate voltage of these MOSFETs is higher. A maximum of 3.3V or 5V from a pulse modulated signal delivered by a microcontroller is not sufficient for the MOSFET. To fully turn on the MOSFET, typically 8-12V is required.
MOSFET drivers are built to handle this back current. When selecting a driver, say, servo drive, make sure that its output voltage capability matches the MOSFET’s turn on voltage.
A servo drive, by the way, is an electrical amplifier that regulates the current/voltage output to the motor in servomechanisms.
Finally, a switch in a power conversion circuit must be made up of a MOSFET and a gate driver. The MOSFET must be chosen in such a way that it can operate in the circuit while causing the least amount of loss. To quickly and completely switch the gate of the MOSFET, a driver gate with high output peak current must be used. If the temperature in the system exceeds the maximum junction temperature specified by the manufacturer, a heat sink should be installed.
