Boost Converter Design and Simulation

March 10, 2020 Cadence PCB Solutions

Boost converter unit for DC-DC conversion

You can simulate this large boost converter with the right simulation tools.

 

DC-DC conversion: it’s one of those simple tasks that is made all the more complex when a converter circuit needs to be designed. Rather than simply stepping down a high voltage with a simple voltage divider network, the better choice is to use a power conversion circuit. Choosing the right converter design will help save battery life, reduce power loss to heat, and suppress EMI.

However you choose to convert between DC voltage levels, your new design should be evaluated with some simulation tools before creating your layout. This lets you qualify components used in the design and evaluate candidate replacement components. Let’s look at the basic design points for a boost converter and how simulation tools help you better understand your design.

Synchronous vs. Asynchronous Boost Converters

The two primary types of DC-DC converters used in electronics are buck converters and boost converters. A buck converter steps down the output voltage, and a boost converter steps up the output voltage. There is also the buck-boost converter, where the step-up or step-down function is controlled by adjusting the duty cycle of a PWM signal, which is used to switch a transistor in the converter circuitry.

Among boost converters, there are two different types: synchronous and asynchronous boost converters. The L-filter across the output is intended to remove any residual ripple or high frequency noise at the output from the boost converter (i.e., it is a low-pass filter). These converters use power MOSFETs driven with a PWM signal to hold charge and a stable voltage across the output capacitor. The images below show the topology of synchronous and asynchronous boost converters.

 

Synchronous boost converter

Typical synchronous boost converter.

 

Asynchronous boost converter

Typical asynchronous boost converter.

 

When the two converters are compared, the synchronous converter has higher power conversion efficiency as there is no forward voltage drop across the diode. However, the synchronous converter also passes more noise to the load as it uses two switching MOSFETs for power regulation. The synchronous boost converter will produce double the noise as the asynchronous boost converter as it uses two MOSFETs instead of one.

Choosing Components and Duty Cycle

When you’re designing a boost converter for your system from discrete and/or integrated components, you’ll need to select an appropriate PWM duty cycle. The duty cycle will determine the maximum and minimum voltage levels that can be output from a typical boost converter. The duty cycle limits can be calculated from the desired output voltage level and efficiency, as shown below.

 

 Boost converter duty cycle equations

Duty cycle limits for asynchronous and synchronous boost converters.

 

For the asynchronous converter, VD is the forward voltage drop across the diode. These equations could be solved for the output voltage in terms of the other quantities in these equations.

It is also not uncommon to find an L-filter on the input to the converter to remove conducted EMI. This is a useful way to suppress particular sources of EMI when a boost converter is used with another noisy power source. Placing an inductor on the input (before the input capacitor) would provide greater low-pass filtering on the input. Note that this additional inductor should not be placed between a full-wave rectifier and capacitor in AC-DC conversion as this will reduce the DC level at the output from the rectifier.

Boost Converter Simulation and Analysis

When you’re designing a boost converter for your system from discrete and/or integrated components, there are a number of important simulations to perform that will allow you to validate your converter’s functionality.

 

Type of analysis

Functions being simulated

Transient analysis

Examine output voltage and residual ripple on the output. To do this, use a PWM source for switching. Also, use an AC source with low amplitude and DC offset to simulate the input DC level with residual ripple.

DC sweep

Extract load lines for the switching transistors. The transistors should be operating in the linear regime during switching.

Noise analysis

Conducted and induced noise can pass to the input port of any DC-DC converter. You’ll need to check how immune the converter circuit is to input noise.

Frequency sweep

Here, it helps to examine the cut-off and roll-off of the output to better understand how residual AC noise and low frequency drift is suppressed at the output.

Parameter sweep

If you are looking to qualify the effectiveness of passive components, you can sweep through a range of component values while performing the other functions listed above.

Monte Carlo sensitivity

This is a quick check to determine how sensitive the output is to variances in component values.

 

These simulation features aren’t limited to evaluating boost converter circuits. Buck, buck-boost, flyback, forward, and other switching converter topologies can be examined with these simulations. The goals in such simulations would be the same, regardless of the topology or layout. With the right analysis tools, you can pull your design data directly from a schematic and quickly run these SPICE-based simulations. If you’re working with COTS components to support your power converter, you can even use verified component models with the right simulation tools.

When you need to build precise boost converter circuits and board layouts, you’ll need to use the best PCB design and analysis software. The design and simulation tools in PSpice Simulator for Allegro and the full suite of analysis tools from Cadence are ideal for evaluating boost converter behavior and reliability. You’ll also have access to manufacturer part search tools as you prepare to source components for your next boost converter.

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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