Power electronics require simulations during the design phase, as well as simulations of the overall system.
Accurate simulations with complex regulators are only possible with verified models for each component in your system.
Your layout still plays an important role in behavior of power electronics, and post-layout simulations are needed to verify real functionality before prototyping.
A simulation of power electronics will help ensure your new prototype will pass testing.
Your new power electronics systems carry high safety requirements, especially when they operate at high voltage and current. Thermal management is also a concern in any power electronics system as components can reach very high temperatures very quickly. Then there is the matter of noise in switching regulators, which can induce unintended switching in low level downstream digital components, especially when these regulators run at high current.
What is the intrepid designer to do in order to ensure their new designs will output the correct voltage with acceptable ripple? A simulation of power electronics needs to consider any active components individually during design, and the entire system. There are some analyses to perform when evaluating a new power system, and you need a SPICE-based simulator that incorporates real component models.
What to Examine in a Simulation of Power Electronics
The answer to this question depends on the type of system you’re building. Different regulators and other power systems require different designs and different simulations to ensure components are selected properly. In general, you need to simulate the following aspects of any power electronics system:
Average output voltage/current vs. input voltage/current. The power transfer characteristics of your design can be determined by comparing the input and output voltages/currents in the system. Determining an average is as simple as calculating an integral of the output waveform over some chosen time period.
Linearity. Power electronics systems will have some range over which the output and input are related by a linear function. This should be simulated for different loads as you need to examine the smallest load (i.e., largest current) that can be connected to the system.
Ripple. PFC circuits, switching regulators, rectifiers, and other circuits will not produce a flat DC output. There will be some ripple on the output when switching or rectifying elements are present in the circuit. Ripple is normally quantified as a percentage of the average output voltage/current.
Buck or boost functionality. Switching DC-DC converters and PFC circuits have different topologies that provide buck (step-down) or boost (step-up) functionality. You’ll need to verify that the output changes in with changes in duty cycle according to the standard formulas for your converter topology.
This VRM needs to provide ultra-stable power output for a low-level processor.
What Simulations to Perform
A simulation of power electronics will generally happen in the time domain. There are some important reasons for this. First, your input and output power requirements are typically defined before you create your design, including any input/output AC frequency. You won’t need to sweep through multiple frequencies and create a transfer function, nor will you need to determine the impedance of the system itself.
This is not the case for switching regulators and other circuits that use a PWM or PFM pulse for output control. You’ll need to vary the duty cycle (for PWM and PFM control signals) and/or the modulation index (for PFM control signals) to examine how they affect the regulator’s output.
Here are some steps for successful simulation of power electronics:
Use a Controlled Source at the Input
Your source needs to be voltage-controlled or current-controlled in order to ensure your power system will mimic the behavior of a real system. Real sources will have limits on the current they can supply in a power system, and a short circuited load cannot draw infinite current. Placing too small of a load may cause the circuit to saturate prematurely, and it is important to know these limits before building a prototype.
Vary the Load with a Parametric Sweep
In order to examine power draw from a system, you’ll eventually need to vary the load component to examine the power output from the system. This can be done with a simple parameter sweep in the time domain. This type of analysis generates multiple curves showing output voltage/current over time for each load component value, providing a direct comparison of power output for various load values. You can use the same steps to examine how other components (particularly any inductors or capacitors in the system) affect power output, ripple, and linearity.
Design Load Lines for any Transistors/FETs
Don’t simply place a transistor in a circuit without simulating its load line. Your system may need to operate in either the linear or saturated regimes. In switching regulators, you need to ensure you’re working in the linear range throughout the possible range of load resistance values. Using a parametric DC sweep, you can easily create a load line for your system by varying the load and base/gate voltage. An example set of SPICE transfer curves you can use to construct a load line is shown in the following figure.
Transistor I-V curves that can be constructed for power supply transistors in PSpice.
Once the components in a power system cross into the saturation regime (i.e., once the load resistance is too low), the system will need to cross into constant current mode for safety, and the system should have current-limited output. The ideal system load line for this situation is shown below. A real load line will have some rolloff at low load resistance, and the system should abruptly cross into the current-limited regime.
Ideal system load line in a simulation of power electronics
Define PWM Sources Before Including Generator Models
It’s best to take an iterative approach to simulation of power electronics. Defining the PWM source as an independent square wave voltage source allows you to simulate the function of the circuit before you add a PWM generator. Any PWM generator circuit and other components can be added to the system as a verified component model.
From Pre to Post Layout
High current switching power systems create noise problems in other circuits, particularly switching regulators with high current output. This noise can be received as radiated EMI at the output from a system, and this received EMI can’t be examined in a SPICE-based pre-layout simulation. This is where you need to create a new layout and simulate crosstalk and noise reception. This is the other aspect involved in an accurate simulation of power electronics, and you can get a complete view of your new power systems with the right design tools.
When you need to design and run a simulation of power electronics, you’ll need to use the best PCB design and analysis software to create circuits for your new system and evaluate their functionality. The front-end design features from Cadence integrate with the powerful PSpice Simulator to create the ideal system for power electronics design. Once you’ve created a layout and are ready to examine noise and thermal behavior, Cadence has a suite of SI/PI Analysis Point Tools for post-layout verification and simulation. You’ll have all the features you need for power electronics design and optimization.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.
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