Diode (Advanced)

The model of a diode with a piecewise linear VAC is similar to the model Diode, with the addition of a fixed junction capacity and the ability to account for charge dynamics. If the forward voltage on the diode exceeds the value specified in the parameter Forward voltage, then the diode behaves like a linear resistor with the resistance specified in the parameter On resistance. Otherwise, the diode behaves like a linear resistor with a low conductivity specified in the parameter Off conductance. At zero voltage, zero current flows through the diode.

The exponential VAC is the following relationship between the diode current and the voltage on the diode :

, when ,

, when ,

where

When , the block replaces on , which corresponds to the gradient of the diode current at and it is extrapolated linearly.

When the block replaces on , which also corresponds to the gradient and is extrapolated linearly.

Conventional electrical circuits do not reach such extreme values. The block provides this linear extrapolation to facilitate convergence when solving constraints during the simulation process.

If for the parameter Parameterization select a value Use parameters IS and N, then the diode is set as parameters Saturation current, IS and Emission coefficient, N.

If for the parameter Parameterization choose Use two I-V curve data points, then two points of measurement of voltage and current on the VAC of the diode are set, and the unit determines the values and . At the same time calculates and as follows:

,

,

where

If for the parameter Parameterization choose Use an I-V data point and IS, then the block calculates as follows:

.

If for the parameter Parameterization choose Use an I-V data point and N, then the block calculates as follows:

.

To simulate a diode with a tabulated VAC, set the parameter Diode model meaning Tabulated I-V curve. This figure shows the implementation of a variant of a diode with a tabulated VAC:

If the check box is selected Model Zener diode, then it is also possible to simulate tabular VAC for reverse displacement.

If the checkbox is not checked Model Zener diode, then the block simulates the characteristics for the reverse bias using the parameter Off conductance:

If a thermal port is used in the unit, then the junction capacity can be modeled only if for the parameter Fidelity level the value is set Include capacitance and charge dynamics.

There are three ways to enable transition capacity modeling:

where

The reverse bias voltages (defined as positive values) must satisfy the condition . This means that the capacities must satisfy the condition , since the reverse bias expands the depletion region and, consequently, reduces the capacity. Violation of these inequalities leads to an error. Voltage and must be sufficiently removed from the transition potential . Voltage there should be less transition potential , with the typical value compose 0.1 V.

The voltage-dependent junction capacitance is determined in terms of capacitor charge accumulation how:

Where

These equations are similar to those given in [2], except that the temperature dependence for and it is not modeled.

If a thermal port is used in the unit, then the junction capacity can be modeled only if for the parameter Fidelity level the value is set Include capacitance and charge dynamics.

For applications such as diodes for switching circuits, it may be important to simulate the charge dynamics of a diode. When a reverse voltage is applied to a direct-input diode, it takes time for the charge to dissipate and therefore for the diode to turn off. The time required to turn off the diode is mainly determined by the time-of-flight parameter. After switching off the diode, the remaining charge is dissipated, and the speed of this process is determined by the lifetime of the carriers.

To account for these effects, the block uses the Lauritzen and Ma model [3]. The defining equations are given below:

, (1)

, (2)

, (3)

where

This graph shows a typical reverse current characteristic for a diode.

Where

The technical specifications of the diodes show the values of the peak reverse current for the initial forward current and the constant rate of current change. The data sheet can also specify the values for the time of reverse recovery and full charge recovery.

How the block calculates and

The block calculates the flight time, , and the lifetime of the carrier, , based on the values entered for the Charge Dynamics parameter. The block uses and to solve the charge dynamics equations (1), (2) and (3).

During the initial current drop in reverse mode, the diode remains on, and the rate of current change is determined by an external test circuit.

First, the block uses equation (1) to perform this calculation:

. (4)

Then equation (4) is substituted into equation (2):

. (5)

Then equation (5) is solved for :

, (6)

where — a constant value.

When , and because the system is in a stable state.

Substituting these relations into equation (6) and solving it, we obtain .

Thus,

. (7)

At a moment in time The current is equal to , and the solder charge is equal to zero. The block substitutes these values into equation (1).

. (8)

The block expresses from equation (8) and substitutes the result into equation (7).

. (9)

Then the block expresses the time through , and .

. (10)

Consider the diode recovery process, i.e. when . The diode is inversely biased, and the junction current and charge are actually zero.

The current is determined by the equation:

, (11)

where

. (12)

Now the block relates the expression in equation (12) to the time of reverse recovery .

When The current is .

Therefore

(13)

and

. (14)

The block uses equations (9) and (14) to calculate the values and . An iterative scheme is used in the calculation, since there is an exponential term in equation (9).

_ Alternatives to direct instruction _

In addition, the block allows you to set the time for reverse recovery. directly, it supports three alternative parameterizations. The block can define from any of the parameters:

The relationship between the stretching coefficient of the reverse recovery time and expressed by the equation

.

The reverse recovery time should be longer than . Its typical value is .

Therefore, the typical value is equally 3. Meaning There should be more 1.

Reverse Recovery charge — this is the integral over the time of the reverse current from the moment the current goes into negative value to its reverse decline to zero.

Initial charge at the moment of time expressed by the following equation:

. (15)

Integrating equation (11) gives a charge between time points and inf. This charge is equal to .

Thus, the total charge of the reverse reduction is determined by the equation

. (16)

Rearranging equation 16 to solve and substituting the result into equation 14 gives the equation expressing in the values :

.

Alternatively, the block calculates using the energy of reverse recovery . This equation defines the voltage curve across the diode.:

, (17)

where the maximum reverse voltage of the diode.

If , which is a common condition for a reverse recovery test circuit, the unit calculates the maximum reverse voltage of the diode as:

.

Since the drop-off time is small, the block assumes that the diode current drop is linear.:

. (18)

Then equation (18) is substituted into equation (5):

. (19)

To obtain the total accumulated charge, equation (19) is solved:

, (20),

where — current gradient.

When , then the peak return current is:

. (21)

Now the block substitutes equation (21) into equation (20):

. (22)

Finally, the block solves equation (22) to obtain reverse recovery energy:

. (23)

By default for the block Diode (Advanced) The dependence on temperature is not modeled, and the device is modeled at the temperature for which the block parameters are set. An exponential VAC diode contains several options for modeling the dependence of diode current and voltage on temperature during the simulation process. The temperature dependence of the junction capacitance is not modeled, since its effect is much smaller.

When the temperature dependence is turned on, the determining equation of the diode remains the same. Measurement temperature value , is replaced by the simulation temperature . Saturation current, , becomes a function of temperature according to the following equation:

,

where

Relevant values and they depend on the type of diode and the semiconductor material used. The default values for specific types of materials and diodes reflect the approximate behavior when the temperature changes. The section shows the default values for common types of diodes.

In practice, setting the values is required to simulate the exact behavior of a particular diode. and . Some manufacturers indicate these configured values in their technical data sheets, where you can view the corresponding values. Otherwise, improved estimates can be determined for using a specific current-voltage data point at a higher temperature in the data sheet. To do this, the block provides the possibility of parameterization. It also allows you to directly set the saturation current at a higher temperature. .

If the final reverse breakdown voltage is set , the value of the inverse modulated by the temperature coefficient of reverse breakdown (set using the parameter Reverse breakdown voltage temperature coefficient dBV/dT):

. (24)

You can use the perfect switching option with blocks for switching circuits.:

The diode in these blocks can be modeled internally, or using a separate block. Diode (Advanced).

To use the perfect switching option, set the parameter Fidelity level meaning Ideal switching.

Reverse recovery losses are one of the main sources of heat loss in diodes. The diode dissipates energy each time it is switched off, switching from a conductive state to an open circuit state.

With perfect switching, the unit does not use a physical charge model. The block represents the losses created by the charge during reverse recovery as instant losses.

The unit applies reverse recovery losses by increasing the transition temperature by an amount equal to the reverse recovery losses divided by the total heat capacity of the transition.

If for the parameter Reverse recovery loss model the value is set Tabulated loss, the value of the parameter Reverse recovery loss table, Erec(Tj, If) defines the dissipated energy as a function of the junction temperature and forward current immediately before the switching event. The shutdown voltage linearly scales the losses relative to Turn-off voltage when measuring recovery loss, Vrec. The table uses the delayed values of current and voltage. To use a value close to instantaneous in the search table, set the parameter Filter time constant for voltage and current values the value is less than the fastest switching period.

If for the parameter Reverse recovery loss model the value is set Fixed loss, the value of the parameter Reverse recovery loss determines the energy dissipated during each shutdown event. If you select an option Scale reverse recovery loss with current and voltage, then the block scales this loss value linearly with respect to the on-current and off-voltage. To use zoom values close to instantaneous values, set Filter time constant for voltage and current values by a value less than the fastest switching period.

A thermal port can be used to simulate the effect of the generated heat and the temperature of the device.:

To simulate a Zener diode (semiconductor zener diode) conducting in the forward and reverse biased directions, check the box Model Zener diode and specify the final value for the parameter Reverse breakdown voltage.

When selecting this option, you can simulate the reverse VAC for the diode block by specifying the parameter values. Reverse voltages, Vr(Tj,Ir), Reverse currents, Ir(Tj,Vr), Reverse voltages, Vr and Reverse currents, Ir.