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Model photodiode as parallel controlled current source and exponential diode
Sensors
The Photodiode block represents a photodiode as a controlled current source and an exponential diode connected in parallel. The controlled current source produces a current Ip that is proportional to the radiant flux density:
where:
DeviceSensitivity is the ratio of the current produced to the incident radiant flux density.
If you select Specify measured current for given flux density for the Sensitivity parameterization parameter, the block calculates this variable by converting the Measured current parameter value to units of amps and dividing it by the Flux density parameter values.
If you select Specify current per unit flux density for the Sensitivity parameterization parameter, this variable is defined by the Device sensitivity parameter value.
RadiantFluxDensity is the incident radiant flux density.
To model dynamic response time, use the Junction capacitance parameter to include the diode junction capacitance in the model.
The exponential diode model provides the following relationship between the diode current I and the diode voltage V:
where:
q is the elementary charge on an electron (1.602176e–19 Coulombs).
k is the Boltzmann constant (1.3806503e–23 J/K).
N is the emission coefficient.
IS is the saturation current, which is equal to the Dark current parameter value.
T is the temperature at which the diode parameters are specified, as defined by the Measurement temperature parameter value.
When
, the block replaces
with
, which matches the gradient
of the diode current at
and extrapolates
linearly. When
, the block replaces
with
, which also matches the gradient
and extrapolates linearly. Typical electrical circuits do not reach
these extreme values. The block provides this linear extrapolation
to help convergence when solving for the constraints during simulation.
When you select Use dark current and N for the Diode parameterization parameter, you specify the diode in terms of the Dark current and Emission coefficient N parameters. When you select Use dark current plus a forward bias I-V data point for the Diode parameterization parameter, you specify the Dark current parameter and a voltage and current measurement point on the diode I-V curve. The block calculates N from these values as follows:
where:
VF is the Forward voltage VF parameter value.
.
IF is the Current IF at forward voltage VF parameter value.
The exponential diode model provides the option to include a junction capacitance:
When you select Fixed or zero junction capacitance for the Junction capacitance parameter, the capacitance is fixed.
When you select Use parameters CJO, VJ, M & FC for the Junction capacitance parameter, the block uses the coefficients CJO, VJ, M, and FC to calculate a junction capacitance that depends on the junction voltage.
When you select Use C-V curve data points for the Junction capacitance parameter, the block uses three capacitance values on the C-V capacitance curve to estimate CJO, VJ and M and uses these values with the specified value of FC to calculate a junction capacitance that depends on the junction voltage. The block calculates CJO, VJ and M as follows:
where:
VR1, VR2, and VR3 are the values in the Reverse bias voltages [VR1 VR2 VR3] vector.
C1, C2, and C3 are the values in the Corresponding capacitances [C1 C2 C3] vector.
It is not possible to estimate FC reliably from tabulated data, so you must specify its value using the Capacitance coefficient FC parameter. In the absence of suitable data for this parameter, use a typical value of 0.5.
The reverse bias voltages (defined as positive values) should satisfy VR3 > VR2 > VR1. This means that the capacitances should satisfy C1 > C2 > C3 as reverse bias widens the depletion region and hence reduces capacitance. Violating these inequalities results in an error. Voltages VR2 and VR3 should be well away from the Junction potential VJ. Voltage VR1 should be less than the Junction potential VJ, with a typical value for VR1 being 0.1 V.
The voltage-dependent junction is defined in terms of the capacitor charge storage Qj as:
For
:
For
:
where:
These equations are the same as used in [2], except that the temperature dependence of VJ and FC is not modeled. This model does not include the diffusion capacitance term that affects performance for high frequency switching applications.
The Photodiode block has the following limitations:
When you select Use dark current plus a forward bias I-V curve data point for the Diode parameterization parameter, choose a voltage near the diode turn-on voltage. Typically this will be in the range from 0.05 to 1 Volt. Using a value outside of this region may lead to a poor estimate for N.
This block does not model temperature-dependent effects. SimElectronics™ simulates the block at the temperature at which the component behavior was measured, as specified by the Measurement temperature parameter value.
You may need to use nonzero ohmic resistance and junction capacitance values to prevent numerical simulation problems, but the simulation may run faster with these values set to zero.
Select one of the following methods for sensitivity parameterization:
Specify measured current for given flux density — Specify the measured current and the corresponding flux density. This is the default method.
Specify current per unit flux density — Specify the device sensitivity directly.
The current the block uses to calculate the device sensitivity. This parameter is only visible when you select Specify measured current for given flux density for the Sensitivity parameterization parameter. The default value is 25 µA.
The flux density the block uses to calculate the device sensitivity. This parameter is only visible when you select Specify measured current for given flux density for the Sensitivity parameterization parameter. The default value is 5 W/m2.
The current per unit flux density. This parameter is only visible when you select Specify current per unit flux density for the Sensitivity parameterization parameter. The default value is 5e-06 m2*A/W.
Select one of the following methods for diode model parameterization:
Use dark current plus a forward bias I-V data point — Specify the dark current and a point on the diode I-V curve. This is the default method.
Use dark current and N — Specify dark current and emission coefficient.
The current at the forward-biased point on the diode I-V curve that the block uses to calculate IS and N. This parameter is only visible when you select Use dark current plus a forward bias I-V data point for the Diode parameterization parameter. The default value is 0.08 A.
The corresponding voltage at the forward-biased point on the diode I-V curve that the block uses to calculate IS and N. This parameter is only visible when you select and Use dark current plus a forward bias I-V data point for the Diode parameterization parameter. The default value is 1.3 V.
The current through the diode when it is not exposed to light. The default value is 5e-09 A.
The temperature at which the I-V curve or dark current was measured. The default value is 25 °C.
The diode emission coefficient or ideality factor. This parameter is only visible when you select Use dark current and N for the Diode parameterization parameter. The default value is 3.
The series diode connection resistance. The default value is 0.1 Ω.
Select one of the following options for modeling the 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.
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 60 pF. When you select Fixed or zero junction capacitance for the Junction capacitance parameter, a value of zero omits junction capacitance.
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.
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 [ 45 30 6 ] pF.
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.
The grading coefficient. 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.
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.
The block has the following ports:
Physical port representing incident flux.
Electrical conserving port associated with the diode positive terminal.
Electrical conserving port associated with the diode negative terminal.
[1] MH. Ahmed and P.J. Spreadbury. Analogue and digital electronics for engineers. 2nd Edition, Cambridge University Press, 1984.
[2] G. Massobrio and P. Antognetti. Semiconductor Device Modeling with SPICE. 2nd Edition, McGraw-Hill, 1993.
Diode, Light-Emitting Diode, Optocoupler
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