Crss Mosfet

Reverse Transfer Capacitance (Crss) C rss is the dynamic capacitance between the gate and the Drain of a MOSFET; it is also referred to as Miller capacitance or transfer capacitance. This parameter has a direct impact on the dv/dt related failures/false turn-on events, so is a very significant parameter when selecting MOSFET for half bridge. Capacitance (C iss/C rss/C oss) In a MOSFET, the gate is insulated by a thin silicon oxide. Therefore, a power MOSFET has　capacitances between the gate-drain, gate-source and drain-source terminals as shown in the figure below. C iss is the input capacitance, C rss is the reverse transfer capacitance, and C oss is the output capacitance. To use a low-side MOSFET with a low gate threshold, (V th). This has three effects on the performance of the MOSFET as shown in Table 1. A low th slows down the turnV -off of the MOSFET and in doing so allows some cross-conduction, which helps to dampen the ringing of the voltage seen on the mid-point of the MOSFET bridge. Also a low gate threshold.

Ciss Coss Crss
Crss Mosfet Capacitance

In continuation of the previous discussion of Si transistor types, features and basic characteristics, we here provide an additional explanation of the characteristics of Si MOSFETs that are at present widely used as power switches.

MOSFET Parasitic Capacitance

Due to their structure, MOSFETs have a parasitic capacitance, as indicated in the diagram below. The diagram below is for an example of an N-channel MOSFET, but the situation is much the same for P-channel devices. In the power MOSFETs we are here considering that handle large amounts of power, the parasitic capacitance must be regarded as a parameter that limits the usage frequency and switching speed.

The drain and source of a MOSFET are insulated from the gate by the gate oxide film. A PN junction is formed between the drain and source with substrate intervening, and a parasitic ('body') diode is present.

The gate-source capacitance Cgs and gate-drain capacitance Cgd in the diagram below are determined by the capacitance of the gate oxide film. The drain-source capacitance Cds is the junction capacitance of the parasitic diode.

The three parameters Ciss, Coss, Crss appearing on MOSFET data sheets in general relate to these parasitic capacitances. On data sheets which provide separate descriptions of static characteristics and dynamic characteristics, these are classified as dynamic characteristics. These are important parameters affecting switching performance.

Ciss is the input capacitance, and is the capacitance obtained by totaling the gate-source capacitance Cgs and the gate-drain capacitance Cgd; it is the capacitance of the MOSFET as a whole, as seen from the input. This capacitance must be driven (charged) in order to cause the MOSFET to operate, and so is a parameter of importance when studying the drivability of an input device or input losses. Qg is the amount of charge necessary to drive (charge) Ciss.

Coss is the output capacitance, obtained by adding the drain-source capacitance Cds and the gate-drain capacitance Cgs, and is the total capacitance on the output side. If Coss is large, a current arising due to Coss flows at the output even when the gate is turned off, and time is required for the output to turn off completely.

Crss is the gate-drain capacitance Cgd itself, and is called the feedback capacitance or the reverse transfer capacitance. If Crss is large, the rise in drain current is delayed even after the gate is turned on, and the fall in current is delayed after the gate is turned off. In other words, this parameter greatly affects switching speed. Qgd is the charge amount necessary to drive (charge) Crss.

These capacitances exhibit a dependence on the drain-source voltage VDS. As indicated in the graph, there is a tendency for capacitance values to be reduced as VDS is increased.

Temperature Characteristic of MOSFET Parasitic Capacitances

Ciss, Coss and Crss change hardly at all with temperature. Hence it can be said that switching characteristics are hardly affected at all by temperature changes. Actual measurement examples are shown below.

Here we have explained parasitic capacitances, which are dynamic characteristics of MOSFETs. Next time, we will discuss device switching.

Key Points:

・MOSFETs have parasitic capacitances, which are important parameters that have an effect on switching characteristics.

・Parasitic capacitances change hardly at all with temperature, and so temperature changes exert almost no effect on switching characteristics.

MOSFET Characteristics Viewer

The Characteristics Viewer tool lets you study characteristics of a particular parameterization of a surface-potential-based MOSFET block and match the block behavior to a set of target characteristics. The tool allows you to:

Plot simulated data, using the current block parameters.
Overlay simulated data plots over tabulated target data.
Modify block parameters.
When satisfied with the results of the parameters tuning in the Characteristics Viewer, update the block parameters in the model.
Save generated parameter sets for future reuse in a different model.

Suggested Workflow

The Characteristics Viewer tool is available for surface-potential-based N-Channel MOSFET or P-Channel MOSFET blocks only. To switch to a surface-potential-based variant when you add an N-Channel MOSFET or P-Channel MOSFET block from the library, right-click the block in your model and, from the context menu, select Simscape > Block choices > Surface-potential-based. Then, when you right-click the block again, the context menu will contain the Electrical option, necessary to start the parameterization tool.

To use the MOSFET parameterization tool:

Right-click a surface-potential-based MOSFET block in your model and, from the context menu, select Electrical > Explore characteristics. A charactericViewer window opens.
Double-click Add characteristics. Specify the characteristics type (target, simulated, or both), and the desired values. Click Add to plot.
Continue adding more characteristics, as needed. The Replace plot button lets you replace previously added plots. You can also use the block, iteratively with Add characteristics, to configure your characteristics set.
Double-click Choose parameters and select the parameters of interest.
Double-click Generate plots.
Iterate between the previous two steps to tune the parameters by matching the simulation results to the target curves.
When satisfied with the results of the parameters tuning, double-click Update starting block parameters to update the block parameters in your model. Until you perform this step, the block in the original model is not affected.
You can double-click Save data to save the generated characteristics as a MAT-file, for future reuse in a different model.

Add and Manage Characteristics

You start the MOSFET parameters tuning process by specifying the desired set of target characteristics:

In the charactericViewer window, double-click Add characteristics.
The Characteristics window opens.
Enter Plot number. This number defines the number of the figure that the characteristic will be plotted on. It allows you to add multiple characteristics to the same figure, for overlaying characteristics on top of each other. However, the figure will comprise one xy-axis only.
Specify the Characteristic type:
- Target only — The plot will contain data that you specified, in terms of both input and output values. No simulation will be performed in this case. The data will simply be added to the appropriate plot.
- Simulated only — The plot will contain data that is a result of a simulation over the input bias conditions that you specify.
- Target and simulated — The plot will contain both types of data. This option is useful if you are trying to adjust parameters for the model to fit data that you have extracted from a datasheet.
Select Sweep type, which defines the x-axis variable for the resultant plot:
- V_GS — Sweep over the gate-source voltage.
- V_DS — Sweep over the drain-source voltage.
- I_D — Sweep over the drain current. Normally, the drain current is not a typical input for a characteristic sweep.
If the Characteristic type is Simulated only, specify Sweep range. This is a vector of values indicating the range for the swept variable. Only the minimum and maximum values of this vector are utilized by the tool, since the exact sample points for the output data are determined by the variable-step simulation.
If the Characteristic type is Target only or Target and simulated, specify Sweep values. This is a vector of values for the swept variable at which the output is sampled for the target data. As an example, for an I_D-V_DS characteristic extracted from a datasheet, the vector would contain the V_DS values corresponding to the sampled I_D values in the target curve.
Select Step type to define the second independent input bias condition. The choices are the same as for Sweep type. For example, if an I_D-V_DS curve is defined as being at a constant V_GS, choose V_GS for Step type.
Use Step values to specify the values for the stepped variable. For example, if an I_D-V_DS curve is desired for V_GS values of 0 and 10V, set Step type to V_GS and Step values to [0 10].
Select Output type, which defines the output measurement for the characteristic. This is the y-axis variable for the resultant plot. The available values are: V_GS, V_DS, I_D, C_GG, C_GD, C_DG, and C_DD. The capacitances C_GG, C_GD, C_DG, and C_DD are defined according to their terminals. To relate these quantities to the datasheet parameters of Ciss, Crss and Coss, note that C_GG = Ciss, C_DD = Coss, and C_GD = Crss at V_GS = 0.
V_GS is not a good choice as an output for the surface-potential-based MOSFET model. This value is provided in anticipation of using this tool for other device types.
If the Characteristic type is Target only or Target and simulated, specify Output values. This is the target data that you want to plot in the figures. Provide this data as an m-by-n matrix, where m is the size of Step values and n is the size of Sweep values.
Click Add to plot to add the characteristic specification to the appropriate Plot number.
Continue adding more characteristics, as needed.
The Replace plot button lets you replace previously added plots. You can also use the block, iteratively with Add characteristics, to configure your characteristics set.

Choose Parameters and Generate Plots

After you have specified the desired set of target characteristics, the next step is to define the parameters for the MOSFET block:

In the charactericViewer window, double-click Choose parameters.
The Tuner window opens. It contains a series of sliders on different tabs, according to which feature of the MOSFET characteristics is most impacted by the specific parameter:
- The VT tab displays parameters that primarily impact the threshold voltage (gamma and phib2ref).
- The parameters on the DC tab primarily affect the DC characteristics.
- The parameters on the AC tab primarily affect the MOSFET dynamics.
- The parameters on the T tab affect temperature scaling.
- The parameters on the FIXED tab are generally fixed at some particular value that is not easy to derive from the displayed characteristics, such as the simulation temperature and the gate resistance (which is often indicated directly on datasheets).
- The EXTRAS tab contains other parameters, which impact the characteristics in ways similar to parameters that already appear on other tabs. For example, Rsref (the series resistance associated with the source) operates similarly to betaref from the DC tab. As a result, it is not always possible to disentangle these two effects.
Use the sliders on the appropriate tabs of the Tuner dialog.
You can modify the min and max values, as needed, because they simply define the range over which the various sliders work. These values have no meaning for the underlying model parameters. Changing a min or max value automatically updates the slider range, without needing to click or .
After adjusting the sliders, generate the plots to see how close the simulation data is to the target data. In the charactericViewer window, double-click Generate plots.
Iterate between tuning the parameters and generating plots until the simulation results match the target curves.

Save the Results

Once you are satisfied with the results of the parameters tuning:

Ciss Coss Crss

Double-click Update starting block parameters to update the block parameters in your model. Until you perform this step, the block in the original model is not affected.
Note
For this step to work, the original model must stay open while you are tuning the parameters.
You can also double-click Save data to save the generated characteristics as a MAT-file, for future reuse in a different model. Specify the file name for saving the data. Inside the file, all the data is saved in an object named parameterHelper.
To apply the parameters stored in this object to another MOSFET block, select the MOSFET block in a model and, in the MATLAB^® Command Window, type:
This command applies the parameter values to the block defined by the handle gcbh.
You can also use a string instead of the block handle, for example:
To inspect the parameters directly, type parameterHelper.parameters.values for the values (stored as character vectors) or parameterHelper.parameters.names for the names.

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