How to understand the working principle of high-power mos tube?

The power amplifier circuit is an amplifier circuit for the purpose of outputting larger power. Therefore, it is required to output a larger voltage and current at the same time. The high-power mos tube works close to the limit state. Generally, the load is directly driven, and the load capacity is stronger.

　　

　　 High-power MOSFET is a type of power device that is more commonly used. "MOSFET" is the abbreviation of English MetalOxideSemicoductorFieldEffectTransistor, translated into Chinese is "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 relatively large operating current (a few amperes to tens of amperes) and is used in a power output stage. Power MOSFET can be divided into enhancement type and depletion type, according to the channel can be divided into N-channel type and P-channel type.

　　

　　SOT-223 package MOS tube

　　

　　 For switching power supply, power MOSFET is commonly used. Generally speaking, MOS tube manufacturers use RDS (ON) parameters to define the on-resistance; for ORing FET applications, RDS (ON) is also the most important device characteristic. The data sheet defines RDS(ON) related to 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 try to develop the smallest size and lowest cost power supply, low on-resistance is even more important. In power supply design, each power supply often requires multiple ORing MOS transistors to work in parallel, and multiple devices are needed to deliver current to the load. In many cases, designers must connect MOS transistors in parallel to effectively reduce RDS(ON). In a DC circuit, the equivalent impedance of parallel resistive loads is smaller than the individual impedance of each load. For example, two 2Ω resistors connected in parallel are equivalent to a 1Ω resistor. Therefore, generally speaking, a low RDS (ON) value MOS transistor with a large rated current allows designers to minimize the number of MOS transistors used in the power supply.

　　

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

　　

The high-power MOSFET will withstand the maximum voltage shock at the moment of turning 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. The quality of the power supply is not good, and some necessary protection measures need to be added in the front stage; if it is an inductive load, the voltage will be much larger, because the inductor will generate an induced electromotive force (the law of electromagnetic induction) at the moment of turning off, and its direction is The direction of VCC is the same (Lenz‘s law), and the maximum voltage that 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, leaving a margin of 20%~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 inductor in parallel to the inductive load to form a freewheeling loop when it is turned off, releasing the induced energy to protect the MOSFET, if necessary , Can also add RC snubber circuit (Snubber) to suppress voltage spikes. (Note that the diode direction should not be reversed. Of course, you can also directly choose a MOSFET with a large enough VDS, provided that you do not 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 spike (Spike and Surge). These two parameters determine how much rated current you should choose.

　　

　　SOT-23N package MOS tube

　　

The field effect tube is a new generation of amplifying element developed based on the principle of the triode. The power MOSFET field effect tube has a negative current temperature coefficient, which can avoid the thermal instability and secondary breakdown of its work, and is suitable for high power and high current Application under working conditions. From the perspective of drive mode, power MOSFET field effect tube is a voltage-type drive control element. The design of the drive circuit is relatively simple and the required drive power is small. Using the power MOSFET field effect as the power switch in the switching power supply, the peak current of the power MOSFET field effect tube is much smaller than that of the bipolar power transistor under the start-up or steady-state working conditions. The characteristic comparison between power FET and bipolar power transistor is as follows:

　　

　　1. Drive mode: The FET is driven by voltage, the circuit design is relatively simple, and the drive power is small; the power transistor is driven by current, the design is more complicated, and the selection of driving conditions is difficult, and the driving conditions will affect the switching speed.

　　

　　2. Switching speed: FET has no minority carrier storage effect, and the temperature influence is small. The switching frequency can reach 150KHz or more; the power transistor has minority carrier storage time to limit its switching speed, and the operating frequency generally does not exceed 50KHz.

　　

　　3. Safe working area: The power FET has no secondary breakdown, and the safe working area is wide; the power transistor has a secondary breakdown, which limits the safe working area.

　　

　　4. Conductor voltage: The power FET is a high-voltage type, with a higher turn-on voltage and a positive temperature coefficient; regardless of the withstand voltage of the power transistor, the conductor voltage is lower and has a negative temperature coefficient.

　　

　　 5. Peak current: When the power FET 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 the power transistor is higher during startup and steady-state operation.

　　

　　 6. Product cost: The cost of power FET is slightly higher; the cost of power transistor is slightly lower.

　　

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

　　

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

　　

　　MOS tube

　　

　　 In addition, most high-power MOSFETs have integrated damping diodes, while most bipolar power transistors do not have integrated damping diodes. The damping diode in the field effect tube can provide a reactive current path for the inductive coil of the switching power supply. Therefore, when the source potential of the field effect tube is higher than the drain, the damping diode is turned on, but this damping diode cannot be used in a switching power supply, and an extra fast diode is required in parallel. The damping diode in the field effect tube has a reverse recovery current in the turn-off process like a general diode. At this time, on the one hand, the diode bears the voltage rising sharply between the drain and the source, and on the other hand, a reverse recovery current flows.