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Guidance: Working Principle of Power Field Effect Transistor (MOSFET). Power MOSFET, also known as power field effect transistor, is a kind of unipolar voltage control device, which not only has the ability of self-turning off, but also has low driving power.
Power Mosfet, also known as power field effect transistor (EFT), is a kind of unipolar voltage control device. It not only has the ability of self-switching, but also has the characteristics of low driving power, high switching speed, no secondary breakdown and wide safe working area. Because of its easy driving and switching frequency up to 500 kHz, it is especially suitable for high frequency power electronic devices, such as DC/DC conversion, switching power supply, portable electronic equipment, aerospace and automotive electronic and electrical equipment. However, because of its small current, heat capacity and low voltage withstand, it is generally only suitable for low-power power electronic devices.
Structure and Working Principle of Power Mosfet
There are many kinds and structures of power mosfet. They can be divided into P channel and N channel according to the conductive channel. They can also be divided into depletion type and enhancement type. In power electronic devices, N-channel enhancement is mainly used. The conductive mechanism of mosfet is the same as that of low power insulated gate MOS transistor, but its structure is quite different. Low power insulated gate MOS transistor is a device formed by primary diffusion. Its conductive channel is parallel to the chip surface and transversely conductive. Most of the power mosfets adopt vertical conductive structure, which improves the voltage and current resistance of the devices. According to the vertical conductive structure, it can be divided into two kinds: V-groove V VVMOSFET and double-diffusion VDMOSFET. The power mosfet(EFT) is composed of thousands of small MOSFETs with a multi-unit integrated structure. The surface diagram of a cell of the N-channel enhanced double-diffusion power mosfet is shown in Fig. 1 (a). Electrical symbols, as shown in Figure 1 (b).
The power mosfet has three terminals: drain D, source S and gate G. When the drain is connected to the positive power supply and the source is connected to the negative power supply, the voltage between the gate and the source is 0, the channel is not conductive, and the tube is cut off. If a forward voltage UGS is added between the gate and the source, and the UGS is greater than or equal to the opening voltage UT of the tube, the tube opens and the current ID flows between the drain and the source. The larger the UGS exceeds UT, the stronger the conductivity and the larger the drain current.
The static characteristics of power MOSFET mainly refer to the output characteristics and transfer characteristics. The main parameters corresponding to the static characteristics are drain breakdown voltage, drain rated voltage, drain rated current and gate open voltage.
- (1) Output characteristics
The output characteristic is the volt-ampere characteristic of the drain. The characteristic curve is shown in Fig. 2 (b). As can be seen from the figure, the output characteristics are divided into three regions: cut-off, saturated and unsaturated. The concepts of saturation and unsaturation are different from GTR. Saturation means that the ID of drain current does not increase with the increase of UDS, that is to say, it remains basically unchanged. Unsaturated means that the ID changes linearly with the increase of UDS when UCS is fixed.
- (2) Transfer characteristics
The transfer characteristic represents the transfer characteristic curve of voltage UGS between drain current ID and gate source, as shown in Figure 2 (a). The transfer characteristics can indicate the amplification capability of the device and are similar to the current gain beta in GTR. Because power MOSFET is a voltage-controlled device, it is expressed by the parameter of transconductance. Transconductance is defined as
The UT in the figure is the open voltage. Only when UGS = UT, the conductive channel will appear and the drain current ID will be generated.
Main Parameter of Power Mosfet
- (1) Leakage breakdown voltage BUD-BUD is not the limiting parameter of device breakdown, it is larger than the leakage voltage rating. BUD increases with the increase of junction temperature, which is just contrary to GTR and GTO.
- (2) The drain rated voltage UD-UD is the nominal rated value of the device.
- (3) The drain current ID and IDM-ID are rated parameters of the drain DC current; IDM is the amplitude of the drain pulse current.
- (4) Gate opening voltage UT-UT, also known as threshold voltage, is the gate-source voltage of power MOSFET, which is the intersection of the characteristic curve of transfer characteristics and the transverse axis. The applied gate voltage should not be too high, otherwise the device will break down.
- (5) Transconductance gm-gm is a parameter to characterize the gate control capability of power MOSFET.
Dynamic Characteristics and Main Parameters of power Mosfet
Dynamic characteristics mainly describe the time relationship between input and output, which affects the switching process of devices. Because the device is unipolar and conducts electricity by most carriers, it has fast switching speed and short switching time, generally in the order of nanoseconds. The dynamic characteristics of power MOSFET are shown in Figure 3.
The dynamic characteristics of the power MOSFET are tested with the circuit shown in Figure 3 (a). In the figure, up is a rectangular pulse voltage signal source; RS is the internal resistance of the signal source; RG is the gate resistance; RL is the drain load resistance; RF is used to detect the drain current.
The switching process waveform of Power MOSFET is shown in Figure 3 (b).
Power MOSFET start-up process: Because power MOSFET has input capacitance, when the rising edge of pulse voltage up comes, the input capacitance has a charging process, and the gate voltage uGS increases exponentially. When the uGS rises to the turn-on voltage UT, the conductive channel begins to form and the drain current iD appears. From the up front time to uGS = UT, and the beginning of iD, this period is called the opening delay time TD (on). Thereafter, iD increases with the rise of uGS, and the period from UT of uGS to uGSP of gate voltage near saturation area of power MOSFET is called rise time tr. So the turn-on time of power MOSFET is on = TD (on) +tr
Power MOSFET’s turn-off process: When up signal voltage drops to zero, the charge stored on the gate input capacitor discharges through resistors RS and RG, which decreases the gate voltage exponentially. When the voltage drops to uGSP, iD begins to decrease. This time is called turn-off delay time TD (off). Thereafter, the input capacitance continues to discharge, the uGS continues to decline, and the iD also continues to decline. When the uGS < SPAN > T, the conductive channel disappears, iD = 0. This period is called the descent time tf. In this way, power MOSFET’s turn-off time toff = TD (off) +tf
From the above analysis, we can see that in order to increase the switching speed of the device, the switching time must be reduced. When the input capacitance is fixed, the switching speed can be accelerated by reducing the internal resistance RS of the driving circuit.
Electric field effect transistor (FET) is a voltage-controlled device, which hardly inputs current in static state. But in the switching process, the input capacitor needs to be charged and discharged, so a certain driving power is still needed. The faster the working speed is, the greater the driving power is needed.
(1) There are capacitors CGS, CGD and CDS between the three poles of Power MOSFET. Usually, the manufacturer provides input capacitor CiSS, common source output capacitor CoSS and reverse transfer capacitor CrSS when the drain source is open. The relationship between them is
- CiSS = CGS + CGD
- CoSS = CGD + CDS
- CrSS = CGD
The input capacitance mentioned above can be approximately replaced by CiSS.
(2) Leakage source voltage rise rate – the dynamic characteristics of devices are also limited by the leakage source voltage rise rate. Excessive du/dt may lead to poor circuit performance and even damage of devices.
Safety Workspace of Power Mosfet
- Forward bias safe workspace – Forward bias safe workspace, as shown in Figure 4. It is surrounded by four boundary limits, i. e. maximum drain source voltage limit line I, maximum drain current limit line II, drain source on-state resistance line III and maximum power limit line IV. Four cases are shown in the figure: DC, pulse width 10 ms, 1 ms, 10 mu s. Compared with GTR safe working area, it has two obvious differences: 1) there is no PSB limit line for secondary breakdown power because of no secondary breakdown problem; 2) Because of its large on-state resistance and large on-state power consumption, it is not only limited by the maximum drain current, but also by on-state resistance.
- Switch Safety Workspace – Switch Safety Workspace is the limit range of device operation, as shown in Fig. 5. It is determined by the maximum peak current IDM, the minimum drain breakdown voltage BUDS and the maximum junction temperature TJM, beyond which the device will be damaged.
- Conversion Safety Workspace – Because of the high frequency of power FETs, they are often in the process of conversion, and there are parasitic equivalent diodes in the devices, which affect the conversion of the tubes. In order to limit the reverse recovery charge of the parasitic diode, it is sometimes necessary to define a transition safe working area.
In practical applications, the safe working area should be left with a certain degree of affluence.
Driving and Protection of Power Mosfet
- Electric field effect transistor (EFT), the driving circuit of EFT, is a unipolar voltage-controlled device with fast switching speed. However, with the existence of inter-pole capacitance, the greater the power of the device, the greater the inter-pole capacitance. In order to improve its switching speed, it is required that the driving circuit must have a high enough output voltage, a high voltage rise rate and a small output resistance. In addition, a certain gate driving current is needed.
The gate current can be calculated by the following formula when it is turned on: IGon = CiSSuGS / T r= (GGS + CGD) uGS / tr, and when it is turned off, the gate current can be calculated by the following formula: IGoff = CGDuDS / tf.
IGon = CiSSuGS / t r = (GGS + CGD) uGS / TR is the main basis for selecting turn-on driving elements, and IGoff = CGDuDS / TF is the main basis for selecting turn-off driving elements.
In order to satisfy the requirement of driving signal of power mosfet (EFT), dual power supply is usually used, and direct coupling or isolator coupling can be used between the output and the device.
A discrete element drive circuit for power mosfet is shown in Fig. 6. The circuit consists of input photoelectric isolation and signal amplification. When the input signal UI is 0, the photocoupler cuts off, the operational amplifier A outputs low level, the transistor V3 turns on, and the driving circuit outputs about negative 20V driving voltage, which turns off the power mosfet. When the input signal UI is positive, the optocoupler is turned on, the operational amplifier A outputs high level, the transistor V2 is turned on, and the driving circuit outputs about 20V voltage, which makes the power mosfet open.
There are many kinds of integrated driving circuits for MOSFET. Here are a few of them.
IR2130 is a 28-pin integrated drive circuit manufactured in the United States. It can drive a MOSFET with a voltage not higher than 600V. It contains over-current, over-voltage and under-voltage protection. The output can directly drive six MOSFETs or IGBTs. Single power supply, maximum 20V. It is widely used in three-phase MOSFET and IGBT inverter control.
IR2237/2137 is an integrated driving circuit manufactured in the United States, which can drive MOSFET for 600V and 1200V lines. Its protective performance and electromagnetic interference suppression ability are stronger, and it has soft start function. Using three-phase gate driver integrated circuit, it can suppress short circuit between lines and ground fault, and use soft shutdown function to suppress short circuit to cause high peak voltage. The short-circuit state of high-end MOSFET and IGBT can be sensed by using unsaturated detection technology. In addition, the internal soft shutdown function, after three-phase synchronous processing, even if the phenomenon of fast current disconnection caused by short circuit occurs, will not appear too high transient surge overvoltage, while equipped with a variety of integrated circuit protection functions. When a fault occurs, the fault signal can be output.
TLP250 is a dual-in-line 8-pin integrated driving circuit made in Japan. It contains a light emitting diode and an integrated photodetector. It has the characteristics of input and output isolation, short switching time, small input current and large output current. Suitable for driving MOSFET or IGBT.
- 2. The protective measure of power mosfet(EFT) – the insulation layer of EFT is easily broken down, which is its fatal weakness, and the grid source voltage should not exceed.