LTspice: Simple Idealized Diode

2017-05-02

LTspice semiconductor diode models are essential for simulations, especially when you want to see results that include breakdown behavior and recombination current. However, as complete as the semiconductor diode model is in LTspice, there are times when you need a simple “idealized diode” model to quickly simulate, for example, an active load, a current source or a current limiting diode. To assist, LTspice provides a representation of an idealized diode model.

To use of this idealized model in LTspice, insert a .model statement for a diode (D) with a unique name and define one or more of the following parameters: Ron, Roff, Vfwd, Vrev or Rrev.

.model MyIdealDiode D(Ron=1 Roff=1Meg Vfwd=1 Vrev=2)

The idealized diode model in LTspice has three linear regions of conduction: on, off and reverse breakdown. The forward conduction and reverse breakdown can further be specified with current limit parameters Ilimit and revIlimit.

.model MyIdealDiode D(Ron=1 Roff=1Meg Vfwd=1 Vrev=2 Ilimit=1 RevIlimit=1

Furthermore, to smooth the switch between the off and conducting states the parameters epsilon and revepsilon can also be defined.

.model MyIdealDiode D(Ron=1 Roff=1Meg Vfwd=1 Vrev=2 Ilimit=1 RevIlimit=1 Epsilon=1 RevEpsilon=1)

A quadratic function is also used between the off and on state such that the idealized diode IV curve is continuous in value and slope, so that the transition occurs over a voltage specified by the value of epsilon and revepsilon.

Once you have inserted your .model statement in your schematic you can edit the diode symbol’s Value in the component attributes (Ctrl + Right Click) to match the name you specified in your statement. For more information on LTspice diode models, please refer to the help topics (F1).

Just for fun, in the circuit example below an idealized diode model is used to simulate a MOSFET’s RDS(ON) in an otherwise nonsynchronous step-down controller. By using an idealized diode model instead of the traditional Schottky diode, the conduction losses of synchronous rectification can be easily compared.