SIMPLIS vs. SPICE: Which is better for simulating power circuits?
One simulates up to 50× faster than the other, and both will give you comparable accuracy. Which engine would you choose for your power supply circuit? Sounds like a trick question, but it isn’t. Before answering that question, though, let’s first consider why we simulate in the first place. By establishing the desired outcome from simulation, we can objectively compare the differences between the two engines in question, SIMPLIS and SPICE.
Most engineers will agree that simulation is about increasing the chance that your design will work when it’s in production. Saving time is great, too, since you’re always under time-to-market pressures. And, ultimately, you want to save some money in the process.
Face it, redesigning and re-spinning PCBs is expensive. Back in the day, we would design boards, test them in the lab, debug, redesign, and repeat the whole cycle again and again and again, until we got it right. Often, this process delayed product introduction—and, if you were unlucky, it would allow competitors to slip their products into customers’ reach first. There would also be the unfortunate times when not all the problems were caught, and thousands (or millions) of products were built before customers found problems that needed to be corrected.
Decades ago, the semiconductor industry recognized that it could only be successful by completely simulating IC designs before fabricating the first wafers. Maxim, for instance, measures simulation success by the number of its ICs that go to production with “first silicon.” The power management industry, however, has been late to the simulation party. Power supplies and converters are notoriously difficult to simulate, and the simulation solutions optimized for ICs are not necessarily the best tools for power conversion simulation.
Testing the health of your power converter design
Given that simulation saves rework, time, and, ultimately, money, what simulations do you need to run? To establish the health and robustness of a power converter design, either by simulation or lab measurement, there is a set of “vital sign” tests analogous to medical vital signs used to quickly establish a human’s overall health. Based on industry practices, Maxim’s EE-Sim design generation and simulation environment defines these tests to be:
- Load step
- AC loop
- Steady state
- Line transient
- Start up
Load step is arguably the most important, akin to taking a patient’s pulse. Just like a person’s pulse will change with exercise, the output voltage of a power converter will change when it is exercised by a change in the load current. Load-step simulation measures how much the output voltage changes and how quickly it recovers. If the feedback circuit is not properly designed, the converter can overshoot or undershoot too far, ring excessively, recover too slowly, or break into oscillation. The trained eye can qualitatively judge the effectiveness of the control loop by inspecting the load-step transient response graph. For a more complete picture of the “health” of the control loop, there’s the AC loop analysis method.
AC loop analysis looks at the control loop in the frequency domain, enabling direct measurement of the control-loop bandwidth and phase margin (going back to our medical example, this step is like taking a patient’s blood pressure). AC analysis, also known as small signal, Bode, or frequency-response analysis, requires specialized equipment such as the AP Instruments AP300, Omicron Bode 100, or Agilent 4194A or 4195A, which are not commonly found in the lab. When available, a Bode analyzer injects a signal into the control loop and then measures the signal at various points in the control loop to establish the gain and phase shift between two signals. The signal is swept over a frequency range and the gain and phase response are plotted on a log scale. Since this analysis may not be available in the lab, being able to simulate it is especially valuable.
Steady-state analysis is arguably an oxymoron for switched-mode power conversion; equilibrium analysis would be a better description. With a converter in equilibrium, every switching cycle looks just like every other switching cycle, kind of like a patient’s respiratory rate. If the cycles are not identical, the converter may be oscillating. Steady-state operation can actually be observed during load-step tests by zooming in for a closer inspection of the waveforms between the load steps. Using a separate steady-state analysis is actually just a convenience.
Line transient is another way to disturb the control loop and observe its recovery. The input voltage is quickly stepped between two values while observing the output voltage. There are some applications specifically sensitive to line-transient performance (audio, for example), but most of the time this test is less important compared to load step.
Start-up looks specifically at what happens when input voltage is first applied (or an enable pin is asserted). The output voltage should smoothly ramp up relatively slowly with little or no overshoot as it reaches the regulated value. Typically, before turning on the power converter for the first time, control-system health should be verified by simulating load step, AC loop, and then start up before heading to the lab.
Efficiency analysis establishes converter power losses which lead to an estimate of component temperature rise (just like taking a patient’s temperature). If losses are high, efficiency will be low and the design produces excessive heat. Note that if the converter is oscillating, component stresses will be higher, power losses will be higher, and efficiency will be lower.
Before any of these tests have meaning, the converter must be stable and not oscillating. Therefore, the tests that directly establish the control system stability and responsiveness are the most important: load step and AC loop analysis.