Simulating Buck-Boost Converter Designs
When I was learning about transformers, I realized that these important devices were silently enabling the world of modern technology and are all around us. AC/AC and AC/DC conversion are certainly important tasks in electronics, but DC/DC conversion is one of those tasks that is less obvious to many.
There are many ways to convert between DC voltage levels, and each circuit has varying levels of complexity. Working with a buck-boost converter provides certain advantages over simpler DC/DC conversion methods like using a voltage divider. A converter will inevitably appear as part of a DC power supply, and simulations can help you tune the component values you need to use in your converter circuit.
Types of DC/DC Converters
Perhaps the simplest DC/DC conversion circuit is a voltage divider. This circuit is entirely linear and steps down an input voltage to a lower value with the same polarity, and the excess electrical energy is dissipated as heat. However, a voltage divider is not an ideal circuit for DC/DC conversion. First, the load connected to the voltage divider will significantly affect the output voltage unless the load is much larger than the dividing resistor.
With an unregulated power supply, the output DC voltage will contain some ripple superimposed on a DC voltage, which can then be removed with a regulator circuit. This regulator circuit appears as part of on overall DC/DC converter. A voltage divider used to DC/DC conversion will step down voltage but will still allow ripple to propagate to the output unless further filtration is applied or unless this output is passed into a regulator. A regulator circuit is really the best choice as it naturally provides filtration.
There are two types of regulator circuits for DC power supplies: linear regulators and switching regulators. Switching regulators include the buck converter, boost converter, or a merger of the two: a buck-boost converter. Buck-boost converters are available as ICs that can be mounted on PCBs. These converters output a range of voltages with significant current and low noise, making them useful for battery-operated devices, industrial systems, and automotive applications.
One example of a switching DC power supply
Buck, Boost, and Buck-Boost Converters
Buck converters and boost converters operate using a switching transistor with a feedback mechanism, similar to a linear regulator. Rather than acting as a varistor, a switching converter uses switching to modulate the current produced from an inductor and control its direction with one or more diodes. These switching regulator offers significantly improved efficiency when compared with linear regulators.
A buck-boost converter simply combines the functionality of a buck or a boost converter, and the desired mode is chosen by applying pulse-width modulation (PWM) to the switching signal. This converts an input DC voltage to the opposite polarity. This converter can either step up or step down the output voltage, depending on the duty cycle of the square wave applied to the switching transistor. If the duty cycle is greater than 50% (boost mode), the converter steps up the output voltage, while the converter steps down the output voltage (buck mode) when the duty cycle is less than 50%.
More advanced buck-boost converters will include a feedback mechanism to regulate the duty cycle against phase noise, providing a more stable output voltage. This feedback mechanism can include a comparator/relaxation oscillator circuit that compensates noise in the switching waveform and further suppresses noise in the output.
Simulating a Buck-Boost Converter
When working with a SPICE-based circuit simulator during converter design, your goal should be to iterate through possible component values in various portions of the converter. One goal is to examine how each component helps reduce ripple, ultimate producing a flat linear output from the power supply. You can also examine how the duty cycle applied to the switching signal affects the output waveform.
When your simulation package includes SPICE models for various ICs, you can include these models in your simulation and examine the performance provided by various components. This allows you to determine the right mix of components for your particular converter and experiment with different feedback mechanisms.
The essential parts in a power supply
Note that a buck-boost converter contains at least one equivalent RLC circuit, both due to the components in the design and due to parasitic capacitance and inductance. This makes a buck-boost converter prone to ringing when the regulator signal switches. You’ll need to choose the right mix of components in your circuit such that the natural frequency of each circuit does not match the switching frequency or its higher order harmonics.
Finally, the switching IC (usually an FET or MOSFET) in a buck-boost converter can generate significant noise, which appears as conducted EMI in the output signal. Although these ICs will switch at the same frequency as the control signal, this generates higher order harmonics in addition to the fundamental frequency. A circuit simulator that includes IC models will allow you to examine these higher order harmonics and determine the level of filtration required to remove conducted EMI.
Noise resulting from switching can also appear in nearby circuit traces or sensitive analog circuitry as radiated EMI. At high enough frequency and current, this noise can be strong enough to cause involuntary switching in nearby digital circuits. It is important to note that radiated EMI cannot be considered directly in a circuit simulator. If you want to examine how radiated EMI affects nearby circuits, you’ll need to use a 3D field solver.
You can examine the time-domain and frequency domain behavior of a buck-boost converter and more complex power electronics when you work with OrCAD PSpice Simulator from Cadence. This unique package is specifically adapted to complex PCB designs, and you can build models to simulate and analyze the behavior of circuits in your schematic and/or PCB. This package also comes with 34000+ models for various semiconductor devices. Many semiconductor manufacturers also develop PSpice models for their parts.
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