Electronic Design

Do We Need Another Spice? Yes, And Here's Why

Analog circuit simulation has been inseparable from analog IC design. Spice simulators are the only way to test circuitry prior to integration onto a chip. Furthermore, the Spice simulation permits measurements of currents and voltages that are virtually impossible to do any other way. The success of these analog circuit simulators has spread circuit simulation to board-level circuit design. It's easier in many cases to simulate rather than breadboard, and the ability to analyze the circuit in the simulation for performance and problems speeds the design of well-understood, robust circuits.

Given the number of commercially available Spice simulators, why should a new simulator be written from scratch? Be-cause certain analog functions are extremely difficult to simulate with com-mercially available Spice simulators.

Switch-mode power supplies have fast, high-frequency switching square waves as well as slow overall loop response. This means simulations must run for thousands to hundreds of thousands of cycles in order to see the overall response of a switching regulator. Commercially available Spices simply take too long for this to be a useful simulation method. Simulation times for a switch-mode power supply must be in minutes, not hours, for a simulator to be useful.

There have been analog circuit simulators that have shown some success in speeding up switch-mode power-supply simulation. But they've done so at a cost of making overly simplifying assumptions that don't allow arbitrary control logic or fully simulate the complexity of the switching waveforms.

A new Spice with integrated logic primitives that perform the switch-mode control is a better answer. It can provide fast simulation times, yield detailed waveforms, and still allow the flexibility for arbitrary circuit modifications.

We have shown that it's possible to develop a new Spice simulator specifically for modeling board-level switching regulator systems. Incorporated into this new Spice are circuit elements to model practical board-level components. Capacitors and inductors can be modeled with series resistance and other parasitic aspects of their behavior without using subcircuits or internal nodes.

Also, a simulation circuit element was developed for power MOSFETs that accurately exhibits their usual gate charge behavior without using subcircuits or internal nodes. Reducing the number of nodes the simulator needs to solve significantly reduces the computation required for a given simulation without compromising the accuracy or detail of the switching waveforms. Another benefit of these new simulation devices is that convergence problems are easier to avoid since they, like the board-level component they model, have finite conductivity at all frequencies.

Modern switch-mode power supplies include controller logic with multiple modes of operation. For example, devices may change from pulse-switch modulation to burst-mode or to cycle skipping, depending on the circuit's operation. An original new mixed-mode compiler and simulator were written into this program. With it, designers can realistically model products in a computationally fast manner.

Over 500 devices are modeled into this fully functional Spice simulator. The program is freely downloadable from Linear's Web site. Included are demonstration files that let designers watch step-load re-sponse, start-up, and transient behavior on a cycle-by-cycle basis. Included with the Spice is a full-featured schematic-entry program for entering new circuits.

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