Overview
When a diode is driven from a voltage source through a series resistance, the forward voltage is not known in advance. The diode current follows the Shockley diode equation, while the series resistor adds a linear voltage-current relation. Together they form a transcendental equation for the diode voltage.
This calculator uses the temperature-dependent saturation current model commonly used in SPICE. The equation can be solved analytically with the Lambert W function. However, for very large arguments the exponential term may exceed the numerical range of double precision, so the computation automatically switches to a Newton-Raphson iteration.
Forward voltage Vf calculator
This calculator analyzes the forward voltage of a diode with a series resistor to the input voltage.
In the high current range exceeding several tens of mA, the results may differ significantly from the calculations.
Formulas used
q : Elementary charge 1.602176634 × 10-19 [C] (exact)
Eg : Energy gap [eV]
N : Emission coefficient
T0 : Reference temperature [K] at which Is(T0) is specified.
T : Junction temperature [K] (= 273.15 + Tj).
XTI : Saturation current temperature exponent. Usually equal to 3 for junction diodes, 2 for Schottky barrier diodes.
Temperature-dependent saturation current model:
The forward voltage can be expressed in closed form using the Lambert W function:
where:
For large Vin (Vin > 220 N VT), the following Newton-Raphson method is used:
Appendix
Forward current as a function of forward voltage; typcal values
RESOURCES (External links)
- The SPICE Home Page (Berkeley)
- NIST — Fundamental Physical Constants
- Cree, XPEB, Xlamp XP-E2 series — High-power lighting-class LEDs
- Diodes Incorporated, 1N5819HW — 1.0A surface mount schottky barrier rectifier
- Hewlett Packard, Surface Mount Microwave Schottky Mixer DIodes Technical Data HSMS-8101 Single (OBSOLETED)
- Nexperia, BAV70 — High-speed switching double diode
- Onsemi, MMBD301L — 200 mA, 30 V, Schottky Diode UHF
- Onsemi, MMBD770 — 200 mA, 70 V, Schottky Diode UHF
- OSRAM, SFH4045N — 63 mW/sr (If = 70 mA), λpeak = 950 nm, IR chip LED
- Rohm, RB400D — 40V, 500mA, SOT-346, Schottky Barrier Diode
- Rohm, BAV70HY — 80V, 215mA, SOT-23, Cathode Common, Switching Diode
- Rohm, CSL1901 series — 0603-size LEDs Optimized for Low-current Low-light Chip Applications
- Rohm, SML-D12x8 series — Chip LEDs (Mono-color type)
- Rohm, RBR3LB30B — 3 A, 30 V, DO-214AA(SMBP), Low VF Schottky Barrier Diode
- Rohm, RBR3LB40C — 3 A, 40 V, DO-214AA(SMBP), Low VF Schottky Barrier Diode
- Rohm, RBR3LB60B — 3 A, 60 V, DO-214AA(SMBP), Low VF Schottky Barrier Diode
- STMicroelectronics, BAR43, 30 V, 100 mA, Vf 0.33 V @ 2 mA SMD General purpose Signal Schottky Diode
- Toshiba, 1SS154 — High frequency Schottky barrier diode, SPICE PARAMETER
- Toshiba, 1SS193 — Switching diode
- Vishay, SS34-E3, 3 A, 40 V, DO-214AB(SMC), Vf 0.5 V @ 3 A Surface-Mount Schottky Barrier Rectifier
SEE ALSO
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An implementation of Lambert W function and exponent of Lambert W function in C99
A technical note on the closed-form solution of the diode equation and practical implementation of the Lambert W function.