Some brief summaries on power MOSFET applications, you deserve them

What is a power MOSFET? What is its role? Engineers engaged in the power electronics industry are certainly no strangers to power MOSFETs. Today, the editor has summarized some MOSFET application experience for you. It is very classic and practical, don’t miss it!

Power MOSFET is a commonly used type of power device. "MOSFET" is the abbreviation of English MetalOxideSemicoductorFieldEffectTransistor, which is translated into Chinese as "metal oxide semiconductor field effect transistor". It is a device made of three materials: metal, oxide (SiO2 or SiN) and semiconductor. The so-called power MOSFET (PowerMOSFET) refers to a device that can output a large operating current (several amps to tens of amps) and is used in the power output stage. Power MOSFET can be divided into enhancement type and depletion type, and can be divided into N-channel type and P-channel type according to the channel.

For switching power supplies, power MOSFETs are commonly used. Generally speaking, MOS tube manufacturers use the RDS(ON) parameter to define on-resistance; for ORing FET applications, RDS(ON) is also the most important device characteristic. The data sheet defines RDS(ON) as a function of the gate (or drive) voltage VGS and the current flowing through the switch, but for sufficient gate drive, RDS(ON) is a relatively static parameter.

If designers are trying to develop the smallest, lowest-cost power supply, low on-resistance is doubly important. In power supply design, each power supply often requires multiple ORing MOS tubes to work in parallel, and multiple devices are required to deliver current to the load. In many cases, designers must connect MOS tubes in parallel to effectively reduce RDS(ON). In a DC circuit, the equivalent impedance of parallel resistive loads is less than the individual impedance value of each load. For example, two 2Ω resistors in parallel are equivalent to a 1Ω resistor. Therefore, generally speaking, a MOS tube with a low RDS(ON) value and a large rated current allows designers to minimize the number of MOS tubes used in the power supply.

In addition to RDS(ON), there are several MOS tube parameters that are also very important to power supply designers in the selection process of MOS tubes. In many cases, designers should pay close attention to the safe operating area (SOA) curve on the data sheet, which describes both drain current and drain-source voltage. Basically, SOA defines the supply voltage and current at which the MOSFET can operate safely. In ORing FET applications, the primary issue is the FET's ability to deliver current in the "fully on" state. In fact, the drain current value can be obtained without the SOA curve.

When doing flyback, IRF540 is often used, its VDSS is 100V, RDS=0.055 ohm, and ID is 22A. The MOSFET will withstand the maximum voltage impact at the moment it is turned off. This maximum voltage has a lot to do with the load: if it is a resistive load, it is the voltage from the VCC terminal, but the quality of the power supply itself needs to be considered. If the power quality is not good Good, some necessary protection measures need to be added to the front stage; if it is an inductive load, the voltage it withstands will be much larger, because the inductor will generate an induced electromotive force (law of electromagnetic induction) at the moment of shutdown, and its direction is the same as the direction of VCC (Lenz's law), the maximum voltage it can withstand is the sum of VCC and induced electromotive force; if it is a transformer load, the induced electromotive force caused by leakage inductance needs to be added to the inductive load.

For the above load conditions, after calculating (or measuring) the maximum voltage, and leaving a margin of 20% to 30%, the required rated voltage VDS value of the MOSFET can be determined. What needs to be said here is that for better cost and more stable performance, you can choose to connect a freewheeling diode and an inductor in parallel on the inductive load to form a freewheeling loop when turned off, releasing the induced energy to protect the MOSFET. If necessary , an RC snubber circuit (Snubber) can also be added to suppress voltage spikes. (Be careful not to reverse the direction of the diode. Of course, you can also directly choose a MOSFET with a large enough VDS, provided you don't care about the cost.)

After the rated voltage is determined, the current can be calculated. But there are two parameters to consider here: one is the continuous operating current value and the pulse current peak value (Spike and Surge). These two parameters determine the rated current value you should choose.

The field effect transistor is a new generation amplification component developed based on the principle of the triode. The power MOSFET field effect transistor has a negative current temperature coefficient, which can avoid thermal instability and secondary breakdown of its operation and is suitable for high power and high current. Application under working conditions. From the perspective of drive mode, power MOSFET field effect transistors are voltage-type drive control components. The design of the drive circuit is relatively simple and the required drive power is very small. Using a power MOSFET field effect transistor as the power switch in a switching power supply, under startup or steady-state operating conditions, the peak current of the power MOSFET field effect transistor is much smaller than that of a bipolar power transistor.

The characteristics comparison between power field effect transistors and bipolar power transistors are as follows:

1. Driving method: The field effect transistor is voltage driven, the circuit design is relatively simple, and the driving power is small; the power transistor is current driven, the design is complex, and the selection of driving conditions is difficult, and the driving conditions will affect the switching speed.

2. Switching speed: Field effect transistors have no minority carrier storage effect, have little temperature impact, and the switching operating frequency can reach more than 150KHz; power transistors have minority carrier storage time that limits their switching speed, and the operating frequency generally does not exceed 50KHz.

3. Safe working area: Power field effect transistors have no secondary breakdown and have a wide safe working area; power transistors have secondary breakdown, which limits the safe working area.

4. Conductor voltage: Power field effect transistors are high-voltage types with high conduction voltage and positive temperature coefficient; power transistors have low conductor voltage and negative temperature coefficient regardless of their withstand voltage.

5. Peak current: When a power field effect transistor is used as a switch in a switching power supply, the peak current is lower during startup and steady-state operation; while the peak current of a power transistor is higher during startup and steady-state operation.

6. Product cost: The cost of power field effect transistors is slightly higher; the cost of power transistors is slightly lower.

7. Thermal breakdown effect: Power field effect transistors have no thermal breakdown effect; power transistors have thermal breakdown effects.

8. Switching loss: The switching loss of field effect transistors is very small; the switching loss of power transistors is relatively large.

In addition, most power MOSFET field effect transistors have integrated damping diodes, while most bipolar power transistors do not have integrated damping diodes. The damping diode in the field effect transistor can provide a reactive current path for the inductive coil of the switching power supply. Therefore, when the source potential of the field effect transistor is higher than the drain, the damping diode conducts, but this damping diode cannot be used in a switching power supply, and an additional ultra-fast diode needs to be connected in parallel. The damping diode in the field effect transistor has the same reverse recovery current as the general diode during the turn-off process. At this time, the diode is subjected to the sharply rising voltage between the drain and the source on the one hand, and reverse recovery current flows on the other hand. The above is the relevant analysis of power MOSFET, I hope it can help you.