SPICE Simulations with Switched-Mode Power Supplies

May 8, 2019 Cadence PCB Solutions

Examining what kind of issues exist with switch mode power supplies

When I am working through a puzzle, I like to take it very incrementally. Fine-tuning each minute area before attempting to tackle the larger image or challenge at-hand ensures that I have the greatest chance of success in completing the puzzle. Simulation programs feel quite similar to me: always trying to modify each parameter in your model so that the results are as close to perfect as possible. When you work with the right simulation package, making pieces fit into a puzzle can be easier than you think.

Working with a SPICE simulator is a great way to glean insights from the behavior of complicated circuits in the time domain, or even in the frequency domain. As SPICE simulators are normally applicable to lower frequency devices, they can be used to examine the behavior of switched-mode power supplies. This helps verify design in these power supplies and helps designers identify potential power integrity problems in the devices they are used to power.

Linear vs. Switched-Mode Power Supplies

These two types of power supplies are used to convert an input AC voltage into a steady DC voltage. The methods used to output a DC voltage in each type of power supply are different, although both methods can be analyzed with SPICE-based simulations. A linear power supply uses a rectifier to generate a DC voltage, while a switched-mode power supply uses switching MOSFETs to output a DC voltage during each undulation in the input AC signal.

The MOSFETs in a switched-mode power supply are switched by the driving AC signal, and the DC output is regulated and smoothed using pulse width modulation (PWM). This outputs a high DC voltage, which is then stepped down using a small ferrite transformer and set of transistors. The entire process requires the MOSFETs switch at high frequency, typically on the order of hundreds of kHz. The low DC voltage is then converted to a DC output with another set of diodes, capacitors, and inductors.

These power supplies contain some ripple voltage in their output, although the ripple voltage arises in different ways for each type of power supply. In a linear power supply, the ripple voltage arises due to the RC time constant of the output rectifier circuit. In the switched power supply, ripple arises due to repeated charging/discharging of the output capacitor, and the shape of the ripple waveform depends on the value of the output resistance and capacitance.

Unregulated power supplies, both linear and switched-mode, output a significant ripple waveform that is superimposed on top of the intended DC output. For this reason, linear and switched-mode power supplies will include a regulator circuit that is intended to maintain the output voltage at a specified value. In a switched-mode power supply, regulation is applied by adjusting the pulse width of the high-frequency waveform using a feedback circuit. The feedback circuit monitors the output voltage and adjusts the on-off ratio of a PWM signal as needed.

SPICE Simulations for Switched-Mode Power Supplies

What kind of issues exist with switched-mode power supplies? The greater complexity of these power supplies and the switching action itself presents some potential signal and power integrity problems. The goal in a simulation is to verify that the design minimizes these potential problems. With a SPICE-based simulator, you can examine changes in the ripple waveform by adjusting component values in the power supply circuitry.

 

Interior of a switched-mode power supply

Proper simulation can ensure power supplies maintain as anticipated

 

It is important to note that ripple can never be removed completely, although it can be reduced to the point that it falls within the input voltage specifications for your device. Ideally, you should try to minimize the ripple output from a switching power supply as this will help you maintain power integrity in your PCB. This then affects the level of filtration required to remove ripple from the power supply output. The goal is to reduce ripple on the output voltage so low that it is unmeasurable.

The important component values that affect the shape and amplitude of the ripple waveform are the output capacitance and resistance. Changing these values iteratively in a SPICE simulator allows you to examine the ripple waveform for different combinations of these two components. The same idea applies when designing a regulator for a switched-mode power supply.

If you are designing a regulator to reduce ripple, a SPICE-based simulation can be used to tune component values in the regulator circuit for a switched-mode power supply. Ideally, you can determine the right combination of resistance and capacitance, as well as the best regulator design, that will minimize ripple on the output voltage.

EMI from Switched-Mode Power Supplies

The switching action of the MOSFETs draws a strong burst of current, which generates a strong magnetic field. The switching action generates a strong magnetic field at the switching frequency, as well as at higher harmonics. These fields can induce a signal in nearby traces on a PCB. While digital components have better immunity to EMI than analog components, the switching action in a power supply can induce signals that are strong enough to interfere with digital signals.

MOSFETs in these power supplies typically take ~50 ns to switch between the ON and OFF states. Even for a power supply outputting a modest current of ~2 A and traces with typical parasitic inductance of ~10 nH, the voltage spike induced in this trace will be ~1 V. This is sufficient to cause involuntary switching in some logic families.

When designing a switched-mode power supply, you cannot calculate this induced signal directly unless you include an equivalent circuit for traces connected to the power supply output, which complicates the analysis. It is better to use a simple calculation for the signal that can be induced due to switching action and ripple. This can then be used to examine signal integrity in digital and analog downstream components.

 

Waveform measurement with an oscilloscope

Try to get that ripple waveform as small as possible

 

Whether you are investigating what kind of issues exist with switch mode power supplies or any other type of circuit, the right circuit simulation and analysis package can help you diagnose potential signal problems and inform your design.

No matter the complexity of your designs, PSpice uses data directly from your schematic and/or PCB to analyze the behavior of your circuits. PSpice technology provides the ultimate conducive environment for advanced system-design and packaging to help designers experiment and create solutions in power electronics.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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