The power amplifier circuit is an amplifier circuit for the purpose of outputting a higher power. Therefore, it is required to output a large voltage and current at the same time. The pipe is working close to the limit state. Generally direct drive load, load capacity to be strong.

Power MOSFETs are more commonly used as a class of power devices. "MOSFET" is the English MetalOxideSemicoductorFieldEffectTransistor acronym, translated into Chinese is "metal oxide semiconductor field effect transistor." It is made of metal, oxide (SiO2 or SiN) and semiconductor materials made of three kinds of materials. The so-called power MOSFET (PowerMOSFET) is that it can output a large operating current (several to several tens of security), for power output stage devices. Power MOSFET can be divided into enhanced and exhausted, according to the channel points can be divided into N-channel and P-channel type.

Do switching power supply, commonly used power MOSFET. In general, MOS transistor manufacturers use RDS (ON) parameters to define the on-resistance; for ORing FET applications, RDS (ON) is also the most important device characteristics. 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 sufficient gate drive, RDS (ON) is a relative static parameter.

If designers try to develop the smallest, lowest cost power, low on-resistance is even more important. In the power supply design, each power supply often requires multiple ORing MOS tubes to work in parallel, requiring multiple devices to deliver current to the load. In many cases, the designer must parallelize the MOS tube to effectively reduce RDS (ON). In the DC circuit, the equivalent impedance of the parallel resistive load is less than the individual impedance of each load. For example, two parallel 2Ω resistors correspond to a 1Ω resistor. Therefore, in general, a low RDS (ON) value of the MOS tube, with a large current, you can let the designer to minimize the number of MOS tubes used in the power supply.

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

Flyback often used IRF540, the VDSS is 100V, RDS = 0.055 Europe, ID is 22A. MOSFET in the off moment, will withstand the maximum voltage shock, the maximum voltage with the load has a great relationship: if it is resistive load, that is, from the VCC side of the voltage, but also need to consider the quality of the power supply itself, if the power quality is not Good, need to add some necessary protection measures in the front; if it is inductive load, then bear the voltage will be much larger, because the inductor will produce an instantaneous electromotive force (electromagnetic induction law), the direction and VCC direction (Lenz's law), the maximum voltage to bear the sum of VCC and induced electromotive force; if the transformer load, then in the emotional load on the basis of the need to add leakage inductance caused by induced electromotive force.

For the above load conditions, after calculating (or measured) the maximum voltage, and then leave 20% to 30% of the margin, you can determine the required voltage rating of the VDS value. What needs to be said here is that, in order to better cost and more stable performance, you can choose in the inductive load on the parallel freewheeling diode and the inductor in the off when the composition of the freewheeling circuit, the release of energy to protect the MOSFET, if necessary , You can also add RC snubber circuit (Snubber) to suppress the voltage spike. (Note that the direction of the diode should not be reversed, of course, you can also choose a large enough VDS MOSFET, provided that you do not care cost.

After the rated voltage is determined, the current can be calculated. But here we need to consider two parameters: one is the continuous working current value and the pulse current spikes (Spike and Surge), these two parameters determine how much you should choose the rated current value.

Field effect transistor is based on the principle of the transistor to develop a new generation of amplification components, power MOSFET FET has a negative current temperature coefficient, it can avoid the work of thermal instability and secondary breakdown, suitable for high power and high current Application under working conditions. Power MOSFET FET from the drive mode point of view, are voltage-type drive control components, drive circuit design is relatively simple, the required drive power is very small. Using the power MOSFET field effect as the power switch in the switching power supply, the peak current of the power MOSFET FET is much smaller than that of the bipolar power transistor under the starting or steady-state operating conditions. The characteristics of the power field effect transistor and the bipolar power transistor are compared as follows:

1. Drive mode: FET is the voltage drive, the circuit design is relatively simple, the drive power is small; power transistor is the current drive, the design is more complex, difficult to drive conditions, drive conditions will affect the switching speed.

2. Switching speed: No effect of the carrier effect of the carrier, the temperature impact is small, the switching frequency of up to 150KHz or more; power transistor has a small carrier storage time limit its switching speed, the operating frequency is generally not more than 50KHz.

3. Safe working area: power FET no secondary breakdown, safe working area wide; power transistor secondary breakdown phenomenon, limiting the safe working area.

4. Conductor voltage: power FET is a high voltage type, high on-voltage, a positive temperature coefficient; power transistor regardless of the level of voltage, the conductor voltage is low, with a negative temperature coefficient.

5. Peak current: power FET in the switching power supply used as a switch, the start and steady state work, the peak current is low; and power transistor in the start and steady state work, the peak current is high.

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

7. Thermal breakdown effect: power field effect tube without thermal breakdown effect; power transistor with thermal breakdown effect.

8. Switching losses: FET switching losses are small; power transistor switching losses are relatively large.

In addition, the power MOSFET FETs are mostly integrated with damping diodes, while bipolar power transistors are mostly integrated damping diodes. Damping diodes in the field effect transistor can provide reactive current path for the switching power supply inductive coil. Therefore, when the source potential of the FET is higher than the drain, the damping diode turns on, but the damping diode can not be used in the switching power supply, and another super fast diode is required in parallel. The damping diode in the field effect transistor has a reverse recovery current as in the case of the turn-off. At this point the diode on the one hand to withstand the drain - the sharp rise between the voltage between the source, on the other hand there is reverse recovery current flow.