Spectrum Software
Industrial Strength Simulation




Spectrum Software has released Micro-Cap 11, the eleventh generation of our SPICE circuit simulator.

For users of previous Micro-Cap versions, check out the new features available in the latest version. For those of you who are new to Micro-Cap, take our features tour to see what Micro-Cap has to offer.




Constant Power Load Macro


In some simulations, it can be useful to have a load that draws a constant power in the circuit. Applications that use a load such as this can be found when simulating mains, SMPS, DC/DC converters, or other power circuits. To model a constant power load, the simplest method is to use the nonlinear function current source (NFI). The obvious equation to define the NFI with would be similar to the following:

I = Power/V(I)

where I is the current produced by the source, Power is the specified constant power, and V(I) is the voltage across the current source. While this equation would work fine in certain configurations, it can often cause convergence issues should the voltage across the current source become small since an excessively large current would be generated. The solution is to adapt the above equation so that when the applied voltage across the load becomes too small, the impedance that the current source represents clips at a minimum value. The macro circuit below shows one technique for modeling the constant power load.

Constant Power Load macro

The macro circuit has two parameters that are passed through to it: Power and Rlow. The Power parameter defines the constant power value in Watts that the load will draw. The Rlow parameter specifies the minimum resistance that the load will clip at should the voltage across the load become too small.

The macro circuit consists of just a single NFI source. The VALUE attribute of the source has been defined as:

1/(Rlow/V(G1) + V(G1)/Power)

where V(G1) represents the voltage across the current source. At voltages near zero, the equation reduces to:

I = V(G1)/Rlow

so that the current source represents an impedance with a value of Rlow. At larger voltages, the equation reduces to:

I = Power/V(G1)

which is the basic constant power equation. In this case, the current out of the source will be adjusted depending on the voltage across the load so that a constant power is generated.

The circuit below shows the use of the constant power load macro. The circuit is a current mode buck converter that was derived by Christophe Basso. The constant power load macro is the X2 device at the Out node. The parameters for the macro were defined as:


The load will generate a constant 50W in its normal operating range and will clip at 10uohms when the voltage across the load becomes too small.

Current mode buck converter with constant power load

The transient analysis of this schematic is displayed below. The simulation has been run for 250us. Two waveforms have been plotted.

The top waveform displays the voltage at node Out. After the initial transient, the output voltage settles down into the 5V range.

The bottom waveform displays the power dissipated at the load. The expression used to plot the power at the load is:


The X2.G1 entry refers to the G1 current source within the X2 macro which is the constant power load macro. As expected, the waveform shows that a constant 50W was dissipated throughout the simulation.

Current mode buck converter with constant power load analysis

1) "Modeling Constant Power Loads", Microsim Tutorial Including Application Notes and Design Ideas, Microsim, 1995.

2) "Switch-Mode Power Supplies: SPICE Simulations and Practical Designs", Christophe Basso, McGraw Hill, 2008.

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