如何彻底读懂并理解MOSFET的Datasheet

2-40 APPENDIX A Estimating MOSFET Parameters from the Data Sheet (Equivalent Capacitances, Gate Charge, Gate Threshold Voltage, Miller Plateau Voltage, Internal Gate Resistance, Maximum Dv/Dt) In this example, the equivalent CGS, CGD, and CDS capacitances, total gate charge, the gate threshold voltage and Miller plateau voltage, approximate internal gate resistance, and dv/dt limits of an IRFP450 MOSFET will be calculated. A representative diagram of the device in a ground referenced gate drive application is pictured below. VDRV D S G RHI RGATE RG,I CGD CGS CDS RLO IRFP450 ID VDS,off The following application information are given to carry out the necessary calculations: VDS,OFF=380V the nominal drain-to-source off state voltage of the device. ID=5A the maximum drain current at full load. TJ=100°C the operating junction temperature. VDRV=13V the amplitude of the gate drive waveform. RGATE=5Ω the external gate resistance. RLO=RHI=5Ω the output resistances of the gate driver circuit. A1. Capacitances The data sheet of the IRFP450 gives the following capacitance values: Using these values as a starting point, the average capacitances for the actual application can be estimated as: Administrator 高亮 2-41 Equations: Numerical Example: offDS, specDS, specOSS,aveOSS, offDS, specDS, specRSS,aveRSS, V V C2C V V C2C ⋅⋅= ⋅⋅= 369pF 380V 25V720pF2C 174pF 380V 25V340pF2C aveOSS, aveRSS, =⋅⋅= =⋅⋅= The physical capacitor values can be obtained from the basic relationships: aveRSS,aveOSS,DS RSSISSGS aveRSS,GD CCC CCC CC −= −= = 195pF174pF369pFC 2260pF340pF2600pFC 174pFC DS GS GD =−= =−= = Notice that CGS is calculated from the original data sheet values. Within one equation, it is important to use capacitor values which are measured under the same test conditions. Also keep in mind that CGS is constant, it is not voltage dependent. On the other hand, CGD and CDS capacitors are strongly non-linear and voltage dependent. Their highest value is at or near 0V and rapidly decreasing as the voltage increases across the gate-to-drain and drain-to-source terminals respectively. A2. Gate charge The worst case gate charge numbers for a particular gate drive amplitude, drain current level, and drain off state voltage are given in the IRFP450 data sheet. Correcting for a different gate drive amplitude is simple using the typical Total Gate Charge curve as illustrated on the left. Starting from the 13V gate-to-source voltage on the left hand side, find the corresponding drain-to- source voltage curve (interpolate if not given exactly), then read the total gate charge value on the horizontal axes. If a more accurate value is required, the different gate charge components must be determined individually. The gate-to-source charge can be estimated from the curve on the left, only the correct Miller plateau level must be known. The Miller charge can be calculated from the CRSS,AVE value obtained in A1. Finally, the over drive charge component – raising the gate-to-source voltage from the Miller plateau to the final amplitude – should be estimated from the graph on the left again. 13V 122nC Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 2-42 A3. Gate threshold and Miller plateau voltages As it was already shown in A2, and will be demonstrated later, several MOSFET switching characteristic are influenced by the actual value of the gate threshold and Miller plateau voltages. In order to calculate the Miller plateau voltage, one possibility would be to use the gate-to-source threshold voltage (VTH) and transconductance (gfs) of the MOSFET as listed in the data sheet. Unfortunately, the threshold is not very well defined and the listed gfs is a small signal quantity. A more accurate method to obtain the actual VTH and Miller plateau voltages is to use the Typical Transfer Characteristics curves of the data sheet. From the same temperature curve, pick two easy to read points and note the corresponding drain currents and gate-to-source voltages. Select the drain current values to correspond to vertical grid lines of the graph, that way the currents can be read accurately. Then follow the intersections to the horizontal axes and read the gate-to-source voltages. Starting with the drain currents will result in higher accuracy because the gate-to-source voltage is on a linear scale as opposed to the logarithmic scale in drain current. It is easier to estimate Vgs1 and Vgs2 on the linear scale therefore the potential errors are much smaller. For this example, using the 150°C curve: 5.67VV 20AI 4.13VV 3AI GS2 D2 GS1 D1 = = = = The gate threshold and Miller Plateau voltages can be calculated as: ( ) ( ) ( ) K IVV VV IK II IVIV V VVKI VVKI LOAD THMillerGS, 2 THGS1 D1 D1D2 D1GS2D2GS1 TH 2 THGS2D2 2 THGS1D1 += − = − ⋅−⋅ = −⋅= −⋅= ( ) 4.413V 3.169 5A3.157VV 3.169 3.157V4.13V 3AK 3.157V 3A20A 3A5.67V20A4.13VV MillerGS, 2 TH =+= = − = = − ⋅−⋅ = ID1 ID2 VGS1 VGS2 Typical Transfer Characteristics Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 2-43 These values correspond to 150°C junction temperature, because the 150°C curve from the Typical Transfer Characteristics was used. Due to the substantial temperature coefficient of the threshold voltage, the results have to be corrected for the 100°C operating junction temperature in this application. The gate threshold voltage and the Miller plateau voltage level must be adjusted by: ( ) TCC150T∆V JADJ ⋅°−= ( ) 0.35VC V0.007C150C100∆VADJ +=� � � � � � ° −⋅°−°= A4. Internal gate resistance Another interesting parameter is the internal gate mesh resistance (RG,I), which is not defined in the data sheet. This resistance is an equivalent value of a distributed resistor network connecting the gates of the individual MOSFET transistor cells in the device. Consequently, the gate signal distribution within a device looks and behaves very similar to a transmission line. This results in different switching times of the individual MOSFET cells within a device depending on the cells distance from the bound pad of the gate connection. The most reliable method to determine RG,I is to measure it with an impedance bridge. The measurement is identical to the ESR measurement of capacitors which is routinely carried out in the lab. For this measurement the source and drain terminals of the MOSFET are shorted together. The impedance analyzer should be set to RS-CS or if it is available RS-CS-LS equivalent circuit to yield the component values of the equivalent gate resistor, RG,I, the MOSFET’s input capacitance, CISS and the series parasitic inductance of the device, all connected in series. For this example, the equivalent component values of an IRFP450 were measured by an HP4194 impedance analyzer. The internal gate resistance of the device was determined as RG,I=1.6Ω. The equivalent inductance was measured at 12.9nH and the input capacitance was 5.85nF. A5. dv/dt limit MOSFET transistors are susceptible to dv/dt induced turn-on only when their drain-to-source voltage rises rapidly. Fundamentally, the turn-on is caused by the current flowing through the gate-drain capacitor of the device and generating a positive gate-to-source voltage. When the amplitude of this voltage exceeds the gate-to-source turn-on threshold of the device, the MOSFET starts to turn-on. There are three different scenarios to consider. First, look at the capacitive divider formed by the CGD and CGS capacitors. Based on these capacitor values the gate-to-source voltage can be calculated as: GDGS GD DSGS CC CVV + ⋅= If VGS> CBST, the bootstrap capacitor can be recharged to the full VDRV level. Usually, CDRV is an order of magnitude larger capacitance than CBST. When selecting the value of the low side bypass capacitor, primarily the steady state operation should be considered. Accordingly, BST,1DRV C10C ⋅≈ , which requires CDRV = 2.2µF. 2-48 APPENDIX D Coupling Capacitor and Transient Settling Time Calculation In this example the coupling capacitor and gate-to-source resistor value of an AC coupled gate drive circuit will be calculated. The design goal is to provide a 3V negative bias for the MOSFET during its off time. The application circuit is shown below: RGS VCC OUT GND VDRV PWM controller CC +VDRV VDRV-VCL 0V -VCL -VCLCDRV VC + - VIN The following application information is given: dVIN/dt=200V/ms the maximum dv/dt of the input voltage during power up, limited by the combined effect of the inrush current limiting circuit and the input energy storage capacitor. CGD,0=1nF the maximum gate-to-drain capacitance of the MOSFET read from the data sheet at 0V drain-to-source voltage (worst case start-up condition). VTH=2.7V the gate-to-source turn-on threshold @ TA,MAX. VDRV=15V the supply voltage of the PWM controller, i.e. the gate driver’s bias voltage. fDRV=100kHz the switching frequency. DMAX=0.8 maximum duty ratio, limited by the PWM controller to reset the transformer. VCL=3V the negative bias amplitude. ∆VC=1.5V maximum allowable ripple of the coupling capacitor. QG=80nC total gate charge of the MOSFET . τ=100µs transient time constant for the coupling capacitor voltage (VC). This is the start-up time constant as well to establish the initial value of VC. The design starts by determining the maximum value of the gate pull down resistor. During power-up, RGS must be low enough to keep the MOSFET off. When the voltage rises across the drain-source terminal, the CGD capacitor is charged and a current proportional to dVIN/dt flows through RGS. The MOSFET stays off if the voltage drop across RGS remains below the gate threshold. Therefore, the maximum allowable RGS value is: dt dVC VR IN GD,0 TH MAXGS, ⋅ = 13.5kΩ s V2000001nF 2.7VR MAXGS, = ⋅ = 2-49 The next step is to find the common solution for the required time constant and ripple voltage. The two equations are: D(D)VDVf∆V fQC RC CDRVDRVC DRVG C GSC τ τ τ ⋅+⋅−⋅⋅ ⋅⋅ =