The power amplifying circuit is an amplifying circuit for the purpose of outputting a large power. Therefore, it is required to simultaneously output a large voltage and current. The pipe is working close to the limit. Generally, the load is directly driven, and the load capacity is strong.
Power MOSFETs are a class of power devices that are 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 metal, oxide (SiO2 or SiN) and semiconductor materials. The so-called power MOSFET (PowerMOSFET) refers to a device that can output a large operating current (several amps to tens of amps) for the power output stage. Power MOSFETs can be classified into enhancement type and depletion type, and can be classified into N-channel type and P-channel type according to channel division.
Switching power supply, commonly used power MOSFET. In general, MOS tube manufacturers use RDS(ON) parameters to define on-resistance; for ORing FET applications, RDS(ON) is also the most important device characteristic. The data sheet defines that RDS(ON) is related to the gate (or drive) voltage VGS and the current flowing through the switch, but for adequate gate drive, RDS(ON) is a relatively static parameter.
If designers try to develop the smallest and lowest cost power supply, the low on-resistance is even more important. In power supply design, each power supply often requires multiple ORing MOS transistors to operate in parallel, requiring multiple devices to deliver current to the load. In many cases, designers must parallel MOS transistors to effectively reduce RDS(ON). In a DC circuit, the equivalent impedance of the parallel resistive load is less than the individual impedance value of each load. For example, two parallel 2Ω resistors are equivalent to a 1Ω resistor. Therefore, in general, a low RDS(ON) MOS transistor with a large rated current allows the designer to minimize the number of MOS transistors used in the power supply.
In addition to RDS(ON), there are several MOS tube parameters that are also important to the power supply designer during the MOS tube selection process. In many cases, designers should pay close attention to the Safe Operating Area (SOA) curve in the data sheet, which also describes the relationship between drain current and drain-source voltage. Basically, SOA defines the supply voltage and current that the MOSFET can operate safely. In ORing FET applications, the primary problem is the FET current transfer capability in the "fully on state". The drain current value can actually be obtained without the SOA curve.
IRF540 is often used for flyback, and its VDSS is 100V, RDS=0.055 Euro, and ID is 22A. At the moment of turn-off, the MOSFET will withstand the maximum voltage surge. 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 it also needs to consider the quality of the power supply itself, if the power quality is not Good, you need to add some necessary protection measures 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 (electromagnetic induction law) at the moment of turn-off, and its direction is the same as the VCC direction. (Lenz's law), the maximum voltage tolerated is the sum of VCC and induced electromotive force; if it is a transformer load, the induced electromotive force caused by the leakage inductance needs to be added to the inductive load.
For the above load cases, after calculating (or measuring) the maximum voltage, leaving a margin of 20% to 30%, the required voltage MOSFET VDS value can be determined. What needs to be said here is that for better cost and more stable performance, you can choose to connect the freewheeling diode and the inductor in the inductive load to form a freewheeling circuit when the switch is turned off, and release the induced energy to protect the MOSFET. It is also possible to add an RC snubber circuit (Snubber) to suppress voltage spikes. (Note that the diode direction should not be reversed. Of course, you can also directly select a MOSFET with a large enough VDS, provided that you do not care for the cost.)
After the rated voltage is determined, the current can be calculated. However, two parameters need to be considered here: one is the continuous operating current value and the peak value of the pulse current (Spike and Surge). These two parameters determine how much the rated current value you should choose.
The FET is a new generation of amplifying components developed according to the principle of the triode. The power MOSFET FET has a negative current temperature coefficient, which can avoid the 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 the driving mode, the power MOSFET field effect transistor is a voltage type driving control element, and the design of the driving circuit is relatively simple, and the required driving 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 transistor is much smaller than that of the bipolar power transistor under startup or steady state operating conditions. The characteristics of the power FET and the bipolar power transistor are compared as follows:
1. Drive mode: The FET is voltage driven, the circuit design is relatively simple, and the driving power is small; the power transistor is current driven, the design is more complicated, the driving condition is difficult to select, and the driving condition will affect the switching speed.
2. Switching speed: FET has no minority carrier storage effect, the temperature influence is small, the switching operating frequency can reach above 150KHz; the power transistor has a minority carrier storage time to limit its switching speed, and the working 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 phenomenon, which limits the safe working area.
4. Conductor voltage: The power FET is of high voltage type, with high on-voltage and positive temperature coefficient; the power transistor has low negative voltage coefficient regardless of the withstand voltage.
5. Peak current: When the power FET is used as a switch in a switching power supply, the peak current is low during startup and steady-state operation, while the power transistor has a higher peak current during startup and steady-state operation.
6. Product cost: The cost of the power FET is slightly higher; the cost of the power transistor is slightly lower.
7. Thermal breakdown effect: The power FET has no thermal breakdown effect; the power transistor has a thermal breakdown effect.
8. Switching loss: The switching loss of the FET is small; the switching loss of the power transistor is relatively large.
In addition, power MOSFET FETs are mostly integrated with damper diodes, while bipolar power transistors mostly do not have integrated damper diodes. The damper diode in the FET can provide a reactive current path for the inductive coil of the switching power supply. Therefore, when the source potential of the FET is higher than the drain, the damper diode is turned on, but the snubber diode cannot be used in the switching power supply, and an ultra-fast diode is required in parallel. The damper diode in the FET has the same reverse recovery current as the normal diode during the turn-off process. At this time, the diode is subjected to a sharp rise in voltage between the drain and the source, and on the other hand, a reverse recovery current flows.
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