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 C GS , C GD , and C DS capacitances, total gate charge, the gate threshold voltage and Miller plateau voltage, approximate internal gate resistance, and dv/dt limits of an IRFP450MOSFET will be calculated. A reprentative diagram of the device in a ground referenced gate drive application is pictured below.
V The following application information are given to carry out the necessary calculations:V DS,OFF =380V the nominal drain-to-source off state voltage of the device.I D =5A the maximum drain current at full load.T J =100°C the operating junction temperature.
V DRV =13V the amplitude of the gate drive waveform.R GATE =5Ωthe external gate resistance.
R LO =R HI =5Ω
the output resistances of the gate driver circuit.
A1.Capacitances
The data sheet of the IRFP450 gives the following capacitance values:
Using the values as a starting point, the average capacitances for the actual application can be estimated as:
Equations:
Numerical Example:
off
DS,spec DS,spec OSS,ave OSS,off DS,spec DS,spec RSS,ave RSS,V V C 2C V V C 2C ⋅
⋅=⋅⋅=369pF 380V
25V
720pF 2C 174pF 380V 25V
340pF 2C ave OSS,ave RSS,=⋅
⋅==⋅⋅=The physical capacitor values can be obtained from the basic relationships:
ave
RSS,ave OSS,DS RSS ISS GS ave RSS,GD C C C C C C C C −=−==195pF班长管纪律小窍门
174pF 369pF C 2260pF 340pF 2600pF C 174pF
C DS GS G
D =−==−==Notice that C GS is calculated from the original data sheet values. Within one equation, it is important to u capacitor values which are measured under the same test conditions. Also keep in mind that C GS is constant, it is not voltage dependent. On the other hand, C GD and C DS capacitors are strongly non-linear and voltage dependent. Their highest value is at or near 0V and rapidly decreasing as the voltage increas across the gate-to-drain and drain-to-source terminals respectively.
A2.Gate charge
The worst ca 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 C RSS,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
A3.Gate threshold and Miller plateau voltages
As it was already shown in A2, and will be demonstrated later, veral 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 u the gate-to-source threshold voltage (V TH ) and transconductance (g fs ) of the MOSFET as listed in the data sheet.
Unfortunately, the threshold is not very well defined and the listed g fs is a small signal quantity. A more accurate method to obtain the actual V TH and Miller plateau voltages is to u the Typical Transfer Characteristics curves of the data sheet.
赢利模式
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 interctions to the horizontal axes and read the gate-to-source voltages.Starting with the drain currents will result in higher accuracy becau the gate-to-source voltage is on a linear scale as oppod to the logarithmic scale in drain current. It is easier to estimate Vgs1 and Vgs2on the linear scale therefore the potential errors are much smaller.
For this example, using the 150°C curve:5.67V
V 20A I 4.13V V 3A I GS2D2GS1D1====The gate threshold and Miller Plateau voltages can be calculated as:
()()()K
I V V V V I K I I I V I V V V V K I V V K I LOAD
TH
Miller GS,2
TH GS1D1
D1
D2D1
GS2D2GS1TH 2
TH GS2D22
TH GS1D1+=−=
−⋅−⋅=−⋅=−⋅=
() 4.413V 3.169
5A
3.157V V 3.1693.157V
4.13V 3A
濑尿虾的做法K 3.157V
3A 20A 3A
5.67V 20A 4.13V V Miller GS,2
TH =+
==−==−⋅−⋅=
I D1
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I D2
V GS1
V GS2
Typical Transfer Characteristics
The values correspond to 150°C junction temperature, becau the 150°C curve from the Typical Transfer Characteristics was ud. 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:()TC
C 150T ∆V J ADJ ⋅°−=()0.35V
C V 0.007C 150C 100∆V ADJ +=÷øöçè
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°−⋅°−°=A4.Internal gate resistance
Another interesting parameter is the internal gate mesh resistance (R G,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. Conquently, 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.
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The most reliable method to determine R G,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 t to R S -C S or if it is available R S -C S -L S equivalent circuit to yield the component values of the equivalent gate resistor, R G,I , the MOSFET’s input capacitance, C ISS and the ries parasitic inductance of the device, all connected in ries.
For this example, the equivalent component values of an IRFP450 were measured by an HP4194impedance analyzer. The internal gate resistance of the device was determined as R G,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 ris rapidly. Fundamentally, the turn-on is caud 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 C GD and C GS capacitors. Bad on the capacitor values the gate-to-source voltage can be calculated as:
GD GS GD
DS GS C C C V V +⋅=If V GS <V TH , the MOSFET stays off. The maximum drain-to-source voltage to ensure this can be estimated by:
GD
GD
GS
TH MAX DS,C C C V V +⋅≈This mechanism provides full protection against dv/dt induced turn-on in low voltage applications,independent of the internal gate resistor and the external drive impedances.
For higher voltage applications, it is desirable to determine the natural dv/dt limit of the MOSFET. This characteristic corresponds to the maximum dv/dt the device can withstand without turning on in an ideal situation where the external drive impedance is zero. This is signified by the shorted gate-source connection in the schematic diagram on the right.
The turn-on is initiated by the voltage drop
across R G,I due to the charge current of C GD .硬实力
Accordingly, the natural dv/dt limit can be calculated by:
GD
I G,TH LIMIT -N C R V dt dv
⋅=
This number is significant in evaluating the suitability of a device for a specific application where the turn-off dv/dt is forced by other components in the circuit. The applications include synchronous rectifiers, resonant mode and soft-switching power converters.
The third calculation describes the resulting dv/dt limit of the drain-to-source voltage waveform bad on the parasitic components of the MOSFET device and the characteristics of the gate drive circuit.To avoid turn-on, the gate-to-source voltage must stay below the turn-on threshold voltage:
()GD
LO GATE I G,TH LIMIT C R R R V dt dv
⋅++=
It is important to emphasize again that the threshold voltage of the MOSFET transistor changes signi
ficantly with temperature. Therefore, the effect of high junction temperature must be taken into effect. For the particular example using the IRFP450 type transistor at 100°C operating junction temperature the calculations yield the following limitations:
Ca 1. No dv/dt induced turn-on takes place below the drain-to-source voltage of:
()GD GD GS
ADJ TH MAX DS,C C C ∆V V V +⋅+=()26.82V 340pF 2600pF
0.35V 3.157V V MAX DS,=⋅+=Ca 2. The natural dv/dt limit of the IRFP450 is:
GD
I G,ADJ TH
LIMIT -N C R ∆V V dt dv
⋅+=µs
kV
c盘格式化6.4
340pF 1.6Ω0.35V 3.157V dt dv LIMIT -N =⋅+=Ca 3. The in-circuit dv/dt limit including the effect of the driver’s output impedance is:
()GD LO GATE I G,ADJ TH LIMIT C R R R ∆V V dt dv ⋅+++=
()µs
V
889340pF 5Ω5Ω1.6Ω0.35V 3.157V dt dv LIMIT =⋅+++=
