Okay, regarding the issues that need to be noted in the use of
MOSFETs (metal oxide semiconductor field-effect transistors), this is a very comprehensive and important topic. As one of the most essential switching devices in modern electronic circuits, its performance advantages are obvious, but if used improperly, it can easily lead to failure. The following will systematically explain the key issues that need to be paid attention to when applying MOS transistors from multiple dimensions.
1、 Electrical parameter limit: an absolutely unbreakable red line
The data manual of any MOS transistor clearly specifies its absolute maximum rated value, which is the theoretical limit that the device can withstand. Once exceeded, it will cause permanent damage.
1. Leakage source voltage Vds: This is one of the most important parameters. When selecting MOS transistors, it is necessary to ensure that their Vds rating is much higher than the maximum voltage that may occur in the circuit, and to leave sufficient margin (for example, more than 1.5 times the actual maximum voltage). Excessive Vds can cause avalanche breakdown, instantly burning down the device. It should be noted that the anti peak voltage generated when inductive loads (such as motors and relay coils) are turned off in the circuit is the main culprit of Vds overvoltage.
2. Drain current Id: The data manual usually provides two values: continuous current and pulse current. Continuous current is limited by the thermal resistance of the chip and package, ensuring that the junction temperature of the MOS transistor does not exceed the maximum value during continuous conduction. Pulse current reflects the short-term overload capability of the chip. In practical design, it is necessary to calculate the actual power consumption when the MOS transistor is conducting, and evaluate its current carrying capacity based on heat dissipation conditions, rather than simply looking at the nominal value.
3. Gate source voltage Vgs: The gate of MOS transistor is isolated by a very thin layer of silicon dioxide insulation, which is very fragile. The maximum Vgs value of the vast majority of MOS transistors is around ± 20V. Once exceeded, the insulation layer will be broken down, forming a permanent short circuit that is usually irreparable. Gate overvoltage may come from driver circuit faults, power fluctuations, or electrostatic discharge.
2、 Static conduction characteristics: the determining factor of conduction loss
1. Conducting resistance Rds (on): This is a key parameter at a specific Vgs and junction temperature, directly determining the power consumption in the conducting state (P=I ² Rds (on)). It should be noted that Rds (on) has a positive temperature coefficient, meaning that as the junction temperature increases, Rds (on) will also increase. This feature is beneficial for automatic current sharing when multiple MOS transistors are connected in parallel, but it also means that in practical work, their conduction loss and heat generation will be higher than the values measured at room temperature.
3、 Dynamic switching process: the core challenge of high-frequency applications
At the moment of turn-on and turn off, MOS transistors experience a state of both voltage and current, resulting in significant switching losses. This is the main reason for the heating of MOS transistors in high-frequency switching power supplies.
1. Switching speed and losses: In order to improve efficiency, we hope that the switching process can be as fast as possible to reduce switching losses. But this will bring another problem: the rate of change of voltage and current (dv/dt and di/dt) is too high.
2. Miller effect: During the shutdown process, when Vds starts to rise from low voltage to bus voltage, a huge current is generated through the gate drain capacitor Cgd to "charge" the gate. This current flows through the driving resistor, hindering the decrease of gate voltage and creating a plateau period (Miller plateau). The Miller effect significantly prolongs the turn off time, increases turn off losses, and places higher demands on the current output capability of the driving circuit. If the driving capability is insufficient, it may even cause the device to overheat and burn out due to the slow switching process.
4、 Parasitic parameters and oscillation problems
MOS transistors are not ideal devices as they contain parasitic capacitance (Cgs, Cgd, Cds) and parasitic inductance (source lead inductance) inside.
1. Gate oscillation: The driving circuit (including the output of the driving chip, gate resistance, and PCB wiring) and the gate input capacitance of the MOS transistor will form an LC resonant circuit. If the layout is improper and the lead inductance is too large, it can easily cause high-frequency oscillation. This oscillation can cause additional switching losses and even damage the gate by exceeding the limit of ± 20V Vgs. Adding a suitable gate resistance (Rg) is the most effective way to suppress oscillations, although it slows down the switching speed, a balance must be struck between speed and stability.
2. Source parasitic inductance: For switch circuits, especially half bridge and full bridge topologies, the source of the lower transistor is not directly grounded, but is connected to the sampling resistor or ground through a lead wire. This parasitic inductance will interact with the driving current, generating a negative feedback voltage that offsets some of the driving voltage, resulting in a decrease in actual Vgs and slowing down the turn-on speed, increasing turn-on losses. Therefore, the PCB layout of high current switch circuits must be as short and thick as possible to reduce parasitic inductance.
5、 Body diode and reverse recovery
The parasitic diode integrated inside the MOS transistor is an important inherent structure. When the MOS transistor conducts in reverse (source high, drain low), the diode will naturally conduct. However, the reverse recovery characteristics of this diode are usually poor.
In a bridge circuit (such as a half bridge), when the upper tube is turned off and the lower tube is turned on, the body diode of the lower tube will lead the continuous current, and then the lower tube is driven to turn on. At the moment when the lower tube is turned on, its body diode is in a conducting state, and forcibly turning it off (reverse recovery) will produce a large reverse recovery current peak. This peak will:
-Increase the opening loss and current stress of the lower tube.
-Generate severe electromagnetic interference.
-The formation of voltage spikes on parasitic inductance may pose a threat to device safety.
Therefore, in applications that require frequent use of body diodes for freewheeling, it is necessary to carefully evaluate their reverse recovery characteristics or choose silicon carbide (SiC) MOSFETs with better reverse recovery characteristics.
6、 Heat dissipation and reliability
The failure of MOS transistors ultimately manifests as thermal failure. The junction temperature Tj is the core indicator.
1. Thermal resistance: It is necessary to understand the concept of thermal resistance from the chip junction to the environment (R θ ja) and from the junction to the housing (R θ jc). After calculating power consumption, combined with thermal resistance and ambient temperature, the junction temperature can be estimated, which must be ensured to be below the maximum allowable junction temperature (usually 150 ° C or 175 ° C) and have sufficient margin.
2. Heat dissipation design: A good heat sink, thermal conductive silicone grease, and reasonable air duct or water cooling design are the basis for ensuring the long-term reliable operation of high-power MOS tubes. When PCB layout, copper foil should be fully utilized to dissipate heat for MOS tubes with low power consumption.
7、 Static Electricity Protection (ESD)
The gate of MOS transistor is extremely fragile and easily broken down due to static electricity. Although most modern power MOSFETs integrate ESD protection devices internally, caution still needs to be exercised:
-Use conductive foam or aluminum foil to wrap the pins during storage and transportation.
-Wear an anti-static wristband during operation, and use anti-static pads on the workbench.
-When soldering with an electric soldering iron, it must be reliably grounded, and it is best to use an anti-static soldering station.
summary
In summary, the safe use of MOS transistors is a system engineering that requires comprehensive consideration from multiple aspects, including selection (voltage, current, Rds (on)), driver circuit design (driver voltage, driver current, gate resistance), PCB layout (reducing parasitic inductance and resistance), heat dissipation management (thermal resistance calculation and heat sink), and electrostatic protection. A deep understanding of its static characteristics and dynamic switching process, and reverence for the absolute maximum rated value in the data manual, is the key to avoiding "mysterious" damage and designing efficient and reliable products.
