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Optocoupler

Model optocoupler as LED, current sensor, and controlled current source

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Semiconductor Devices

Description

This block represents an optocoupler using a model that consists of the following components:

  • An exponential light-emitting diode in series with a current sensor on the input side

  • A controlled current source on the output side

The output-side current flows from the collector junction to the emitter junction. It has a value of CTR·Id, where CTR is the Current transfer ratio parameter value and Id is the diode current.

Use the Optocoupler block to interface two electrical circuits without making a direct electrical connection. A common reason for doing this is that the two circuits work at very different voltage levels.

    Note:   Each electrical circuit must have its own Electrical Reference block.

If the output circuit is a phototransistor, typical values for the Current transfer ratio parameter are 0.1 to 0.5. If the output stage consists of a Darlington pair, the parameter value can be much higher than this. The Current transfer ratio value also varies with the light-emitting diode current, but this effect is not modeled by the Photodiode block.

Some manufacturers provide a maximum data rate for optocouplers. In practice, the maximum data rate depends on the following factors:

  • The capacitance of the photodiode and the type of the driving circuit

  • The construction of the phototransistor and its associated capacitance

The Optocoupler block only lets you define the capacitance on the light-emitting diode. You can use the Junction capacitance parameter to add your own capacitance across the collector and emitter connections.

The Optocoupler block lets you model temperature dependence of the underlying diode. For details, see the Diode reference page.

Thermal Port

The block has an optional thermal port, hidden by default. To expose the thermal port, right-click the block in your model, and then from the context menu select Simscape block choices > Show thermal port. This action displays the thermal port H on the block icon, and adds the Thermal port tab to the block dialog box.

Use the thermal port to simulate the effects of generated heat and device temperature. For more information on using thermal ports and on the Thermal port tab parameters, see Simulating Thermal Effects in Semiconductors.

Basic Assumptions and Limitations

The Optocoupler block has the following limitations:

  • The output side is modeled as a controlled current source. As such, it only correctly approximates a bipolar transistor operating in its normal active region. To create a more detailed model, connect the Optocoupler output directly to the base of an NPN Bipolar Transistor block, and set the parameters to maintain a correct overall value for the current transfer ratio. If you need to connect optocouplers in series, use this approach to avoid the invalid topology of two current sources in series.

  • The temperature dependence of the forward current transfer ratio is not modeled. Typically the temperature dependence of this parameter is much less than that of the optical diode I-V characteristic.

  • You may need to use nonzero ohmic resistance and junction capacitance values to prevent numerical simulation issues, but the simulation may run faster with these values set to zero.

Dialog Box and Parameters

Main Tab

Current transfer ratio

The output current flowing from the transistor collector to emitter junctions is equal to the product of the current transfer ratio and the current flowing the light-emitting diode. The default value is 0.2.

Diode parameterization

Select one of the following methods for model parameterization:

  • Use I-V curve data points — Specify measured data at two points on the diode I-V curve. This is the default method.

  • Use parameters IS and N — Specify saturation current and emission coefficient.

Currents [I1 I2]

A vector of the current values at the two points on the diode I-V curve that the block uses to calculate IS and N. This parameter is only visible when you select Use I-V curve data points for the Diode parameterization parameter. The default value is [ 0.001 0.015 ] A.

Voltages [V1 V2]

A vector of the voltage values at the two points on the diode I-V curve that the block uses to calculate IS and N. This parameter is only visible when you select Use I-V curve data points for the Diode parameterization parameter. The default value is [ 0.9 1.05 ] V.

Saturation current IS

The magnitude of the current that the ideal diode equation approaches asymptotically for very large reverse bias levels. This parameter is only visible when you select Use parameters IS and N for the Diode parameterization parameter. The default value is 1e-10 A.

Measurement temperature

The temperature at which IS or the I-V curve was measured. The default value is 25 °C.

Emission coefficient N

The diode emission coefficient or ideality factor. This parameter is only visible when you select Use parameters IS and N for the Diode parameterization parameter. The default value is 2.

Ohmic Resistance Tab

Ohmic resistance RS

The series diode connection resistance. The default value is 0.1 Ω.

Junction Capacitance Tab

Junction capacitance

Select one of the following options for modeling the diode junction capacitance:

  • Fixed or zero junction capacitance — Model the junction capacitance as a fixed value.

  • Use C-V curve data points — Specify measured data at three points on the diode C-V curve.

  • Use parameters CJ0, VJ, M & FC — Specify zero-bias junction capacitance, junction potential, grading coefficient, and forward-bias depletion capacitance coefficient.

Zero-bias junction capacitance CJ0

The value of the capacitance placed in parallel with the exponential diode term. This parameter is only visible when you select Fixed or zero junction capacitance or Use parameters CJ0, VJ, M & FC for the Junction capacitance parameter. The default value is 5 pF.

Junction potential VJ

The junction potential. This parameter is only visible when you select Use parameters CJ0, VJ, M & FC for the Junction capacitance parameter. The default value is 1 V.

Grading coefficient M

The coefficient that quantifies the grading of the junction. This parameter is only visible when you select Use parameters CJ0, VJ, M & FC for the Junction capacitance parameter. The default value is 0.5.

Reverse bias voltages [VR1 VR2 VR3]

A vector of the reverse bias voltage values at the three points on the diode C-V curve that the block uses to calculate CJ0, VJ, and M. This parameter is only visible when you select Use C-V curve data points for the Junction capacitance parameter. The default value is [ 0.1 10 100 ] V.

Corresponding capacitances [C1 C2 C3]

A vector of the capacitance values at the three points on the diode C-V curve that the block uses to calculate CJ0, VJ, and M. This parameter is only visible when you select Use C-V curve data points for the Junction capacitance parameter. The default value is [ 3.5 1 0.4 ] pF.

Capacitance coefficient FC

Fitting coefficient that quantifies the decrease of the depletion capacitance with applied voltage. This parameter is only visible when you select Use C-V curve data points or Use parameters CJ0, VJ, M & FC for the Junction capacitance parameter. The default value is 0.5.

Temperature Dependence Tab

Parameterization

Select one of the following methods for temperature dependence parameterization:

  • None — Simulate at parameter measurement temperature — Temperature dependence is not modeled, or the model is simulated at the measurement temperature Tm1 (as specified by the Measurement temperature parameter on the Main tab). This is the default method.

  • Use an I-V data point at second measurement temperature T2 — If you select this option, you specify a second measurement temperature Tm2, and the current and voltage values at this temperature. The model uses these values, along with the parameter values at the first measurement temperature Tm1, to calculate the energy gap value.

  • Specify saturation current at second measurement temperature T2 — If you select this option, you specify a second measurement temperature Tm2, and saturation current value at this temperature. The model uses these values, along with the parameter values at the first measurement temperature Tm1, to calculate the energy gap value.

  • Specify the energy gap EG — Specify the energy gap value directly.

Current I1 at second measurement temperature

Specify the diode current I1 value when the voltage is V1 at the second measurement temperature. This parameter is only visible when you select Use an I-V data point at second measurement temperature T2 for the Parameterization parameter. The default value is 0.029 A.

Voltage V1 at second measurement temperature

Specify the diode voltage V1 value when the current is I1 at the second measurement temperature. This parameter is only visible when you select Use an I-V data point at second measurement temperature T2 for the Parameterization parameter. The default value is 1.05 V.

Saturation current, IS, at second measurement temperature

Specify the saturation current IS value at the second measurement temperature. This parameter is only visible when you select Specify saturation current at second measurement temperature T2 for the Parameterization parameter. The default value is 1.8e-8 A.

Second measurement temperature

Specify the value for the second measurement temperature. This parameter is only visible when you select either Use an I-V data point at second measurement temperature T2 or Specify saturation current at second measurement temperature T2 for the Parameterization parameter. The default value is 125 C.

Energy gap parameterization

This parameter is only visible when you select Specify the energy gap EG for the Parameterization parameter. It lets you select a value for the energy gap from a list of predetermined options, or specify a custom value:

  • Use nominal value for silicon (EG=1.11eV) — This is the default.

  • Use nominal value for 4H-SiC silicon carbide (EG=3.23eV)

  • Use nominal value for 6H-SiC silicon carbide (EG=3.00eV)

  • Use nominal value for germanium (EG=0.67eV)

  • Use nominal value for gallium arsenide (EG=1.43eV)

  • Use nominal value for selenium (EG=1.74eV)

  • Use nominal value for Schottky barrier diodes (EG=0.69eV)

  • Specify a custom value — If you select this option, the Energy gap, EG parameter appears in the dialog box, to let you specify a custom value for EG.

Energy gap, EG

Specify a custom value for the energy gap, EG. This parameter is only visible when you select Specify a custom value for the Energy gap parameterization parameter. The default value is 1.11 eV.

Saturation current temperature exponent parameterization

Select one of the following options to specify the saturation current temperature exponent value:

  • Use nominal value for pn-junction diode (XTI=3) — This is the default.

  • Use nominal value for Schottky barrier diode (XTI=2)

  • Specify a custom value — If you select this option, the Saturation current temperature exponent, XTI parameter appears in the dialog box, to let you specify a custom value for XTI.

Saturation current temperature exponent, XTI

Specify a custom value for the saturation current temperature exponent, XTI. This parameter is only visible when you select Specify a custom value for the Saturation current temperature exponent parameterization parameter. The default value is 3.

Device simulation temperature

Specify the value for the temperature Ts, at which the device is to be simulated. The default value is 25 C.

Ports

The block has the following ports:

+

Electrical conserving port associated with the diode positive terminal

-

Electrical conserving port associated with the diode negative terminal

C

Electrical conserving port associated with the transistor collector terminal

E

Electrical conserving port associated with the transistor emitter terminal

References

[1] G. Massobrio and P. Antognetti. Semiconductor Device Modeling with SPICE. 2nd Edition, McGraw-Hill, 1993.

[2] H. Ahmed and P.J. Spreadbury. Analogue and digital electronics for engineers. 2nd Edition, Cambridge University Press, 1984.

See Also

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