Utilizing a PCB Transmission Line Calculator Makes Design Easier

Zachariah Peterson
|  Created: January 13, 2021

Calculators are impressively helpful with your PCB designs; however, having these calculations built into your PCB design software user interface is beyond fantastic.

ALTIUM DESIGNER

Design software tools that can augment your circuit board calculations.

Power supply designers understand the complex technical details and functional requirements involved with a switched mode power supply PCB layout. The layout establishes the functional and thermal behaviors as well as the electromagnetic interference (EMI) requirements for the power supply. A good layout optimizes supply efficiency and can save resources associated with mechanical shielding and EMI filters. For the power supply designer, all this leads to reduced EMI test time and fewer PCB runs.

In contrast, a poor layout introduces problems that occur at high current levels and become obvious with large differences between input to output voltages. Common power supply problems seen with bad PCB layouts include the loss of regulation at high output currents, excessive noise on the output and switch waveforms, and circuit instability. Altium Designer offers power supply PCB layout guidelines that mitigate these and other problems.

Using those guidelines also facilitates your ability to send your product to the marketplace. Regulatory bodies such as Underwriter Laboratories and the IEC test power supplies for radiated electromagnetic interference (EMI), conducted EMI, stability, efficiency, and operating life. Altium Designer provides the circuit analysis and the information that allow you to build a switched mode power supply (SMPS) design that passes those and other tests.

Utilizing PCB Editor for Correct Trace Width and Transmission Lines

Utilizing the proper tools to find characteristic impedance, unit length, and make your transmission lines accurately can be a difficult process. Having an outside PCb transmission line calculator can save you some hassle in the future; however, having a design tool which can utilize design rules properly while also systematically placing board pieces like a conductor can be imperative.

The width of the traces has a direct impact on the capability of the power supply to minimize noise. As the high current flows through the loop and encounters trace resistance, a voltage drop occurs and radiates RF noise. As a result, the width of the trace impacts the amount of voltage drop.

Using wider traces mitigates the noise propagation because of an inversely proportional relationship that exists between the width of the trace and inductance and between trace width and resistance. Noise and its associated current travel any low resistance path in (in AC and DC circuits) and any low impedance path (in AC circuits) back to the place where generation originates.

The relationship with inductance becomes important because inductance lowers the frequency response of the loop. At lower frequencies, the loop becomes a more efficient antenna. With the loop only radiating lower frequencies, more noise energy escapes into the environment.

Along with maintaining a minimal loop circumference and wide traces, you can also use parallel capacitors and the physical characteristics of your PCB layout to decrease the overall equivalent series resistance (ESR) and equivalent series inductance (ESL) of the filter capacitor. Paralleling capacitors allows the filter capacitor to sink higher levels of ripple current while minimizing component heating. In terms of layout, you must ensure that components in the loop and each capacitor have an identical and symmetrical layout. Refining your layout in this way ensures that the parallel capacitors equally share current and heating.

Altium Designer also allows you to interactively route traces and components to curb increases in EMI. Placing the low EMI inductor, output capacitors, and output diode close to one another decreases lead inductance and resistance. In turn, the proximity of those components reduces the opportunities for radiated EMI along with the noise spikes, ringing, and resistive losses that cause voltage errors.

Use Altium Designer for the Correct Grounds

As you design your switching power supply circuit, remember that five grounds exist. Those are:

  • Input high-current source ground
  • Input high-current current loop ground
  • Output high-current rectifier ground
  • Output high-current load ground
  • Low-level control ground

Always consider the grounds separately. Your power supply circuit will become unstable if the grounds connect incorrectly. Each high-current ground serves as one leg of the current loops while representing the lowest potential return path for currents. This potential becomes the point for measuring the DC and AC signals that conduct between different points of the circuit. Because of the need to prevent noise from the high-current AC grounds escaping, the negative terminal of the appropriate filter capacitor serves as the connecting point for the high-current grounds.

The ability of the SMPS controller to precisely regulator the output voltage depends on the connection of the low-level control ground. The ground connection at ties to a point where the control IC and its associated circuitry measure the AC current, DC current, the output voltage, and other major parameters. Connecting the low-level ground to the lower side of the current sensing resistor or the output voltage divider prevents the control circuit from sensing common mode noise. The PDN Analyzer plug-in for Altium Designer provides the best resources for your DC current and voltage analysis of the circuit.

As you work with integrated circuits, input capacitors, output capacitors, and output diodes, ensure that the components connect to a ground plane. Especially when working with switching power supplies, use a ground plane on both sides of the PCB and around the high current traces. Placing a ground plane on both sides absorbs radiated EMI, reduces noise, and decreases ground loop errors. While working as electrostatic shields and dissipating radiated EMI within eddy currents, ground planes also separate the power traces and components of the power plane from the signal plane components.

Working through a ground plane in Altium Designer

Constraints and parameters for laying out a ground plane

Schematic Editor Assists with EMI Filter Layout

An SMPS operates by rapidly switching the pass units between the cutoff operating state and the saturation operating state and delivering constant power to an output load. At cutoff, high voltage exists across the pass unit but no current flows. At saturation, high current flows through the pass unit with a very small voltage drop. Because the semiconductor switch creates an AC voltage from the DC input voltage, the SMPS can either step up or step down the voltage with transformers and then filter the voltage back to DC at the output.

Pulse-width modulated (PWM) switching power supplies operate either in forward-mode or boost mode. Forward-mode supplies have an L-C filter at the output that creates a DC output voltage from the volt-time average of the output obtained from the filter. To control the volt-time average of the signal, the switching power supply controller changes the duty cycle of the input rectangular voltage.

Boost mode supplies connect an inductor directly across the input voltage source when the power switch turns on. The inductor current increases from zero and reaches its peak simultaneously with the turning off the power switch. An output rectifier clamps the inductor output voltage and prevents the voltage from exceeding the supply output voltage. When energy stored in the core of the inductor passes to the output capacitor, the switched terminal of the inductor falls back to the level of the input voltage.

EMI filters within the SMPS suppress high frequency noise caused by the high frequency currents conducted within the DC input and output wiring. You can use the integrated component pricing and availability and the data sheet links found within Libraries and the Altium Content Vault to select the filters that give the best results.

Altium Designer also allows you to select the appropriate design rules to layout the filters so that switching energy does not couple around the filters to traces located on the other side of the filter.

Along with using PCB Editor to establish a good layout, you can use Schematic Editor to place the EMI filters close to the point where the signal exits the enclosure. In addition, good layout practices combined with the easy-to-use wiring and smart paste to maintain a straight layout and consistent spacing of the EMI circuit to prevent inductive coupling between the input and output traces.

Altium Designer Assists with Power Supply Routing

Switching power supplies conduct high frequency noise until the noise frequency reaches approximately 100 times the switching frequency. Then, the noise frequency falls at a rate of -20 to -40 dB per decade. When working with the power supply layout, keep traces that handle high switching currents short, direct, and thick. Manufacturers recommend a minimum thickness of 15 mils per Ampere for the traces. You can easily access routing commands through the Altium Designer Active Bar and place objects in the schematic, PCB, Draftsman, and library documents.

Switching power supply circuits consist of a power switch loop and output rectifier loops. When laying out the power supply, pay specific attention to circumference of the loops and the length and width of traces. Keeping the loop circumference small eliminates the possibility of the loop working as a low frequency noise antenna. From the perspective of circuit efficiency, the wider traces also provide additional heat sinking for the power switch and rectifiers.

Because switching regulators operate with “on” and “off” power states, large current pulses with sharp edges flow within the switching power supply circuit and—as a result—create EMI. Each “on” and “off” power state causes power components to conduct and create the current loop. Good power supply layout requires the layout of the loops defined by the currents. You can use the active route routing engine to achieve human routing results and arrange your components to allow the switching current loops to conduct in the same direction. With the current loops conducting in the same direction, the control circuitry couples to specific spots in the layout. As a result, the magnetic field cannot reverse along the traces located between the two half-cycles and generate radiated EMI.

Altium Designer’s tight routing capabilities

Capabilities of smart routing features are endless

Layout Help with SMPS AC Voltage Nodes

Depending on the SMPS configuration, the AC voltage nodes exist at the drain of the power MOSFET or the collector of a BJT and the anodes of the output rectifiers. Each of these nodes can have high AC voltages. As an example, the peak-to-peak AC voltage found at the MOSFET drain can measure one to two times the input voltage. With the drain bolted to a heatsink through an insulator, the earth-grounded heat sink provides a path for capacitively coupled noise. You can use the PCB layout tools found in Altium Designer to place susceptible signals on the same side rather than underneath a noisy AC node. In addition, you can cross-hatch any ground planes located under the node to eliminate the noise.

Surface mount environments have smaller values of capacitance but can couple noise into sensitive signals. Because of those factors, your layout also needs to address the possibility of capacitive coupling of the AC node voltages into heat sinks or adjacent ground planes. When laying out a surface mount PCB design, make the nodes large enough to serve as heat sinks for either the power switch or rectifier. Some multilayer designs increase the thermal mass of the design by making all the layers below the AC node identical to the AC node and connecting the layers with plated-through holes.

Altium Designer Allows You to Cut Complexity

Designing the layout for a switched-mode power supply may seem daunting and overwhelming at times. Altium Designer provides the tools that break the complexity of power supply down into easily understood tasks. The unified structure seen with Altium Designer allows to select the right layout, apply effective routing, and establish design rules.

Using Altium Designer can improve your efficiency and relieve your headache from plug-and-practice CAD tool approaches. A unified design environment with the user interface built for the designers and design teams to be more compatible with the entire electronic production process is invaluable. Try Altium Designer today.

About Author

About Author

Zachariah Peterson has an extensive technical background in academia and industry. He currently provides research, design, and marketing services to companies in the electronics industry. Prior to working in the PCB industry, he taught at Portland State University and conducted research on random laser theory, materials, and stability. His background in scientific research spans topics in nanoparticle lasers, electronic and optoelectronic semiconductor devices, environmental sensors, and stochastics. His work has been published in over a dozen peer-reviewed journals and conference proceedings, and he has written 1000+ technical blogs on PCB design for a number of companies. He is a member of IEEE Photonics Society, IEEE Electronics Packaging Society, and the American Physical Society, and he currently serves on the INCITS Quantum Computing Technical Advisory Committee.

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