Configuring For DC Power Supply Limitations
During the 1940s, the United States Navy relied on teletype machines powered by the “REC-30 rectifier” dc power supply for communication. The 100-pound, two-foot-wide switching power supply used a large autotransformer that could accept several input voltages with an output of 400 VAC for the mercury-vapor thyratron tubes. The tubes functioned as rectifiers that rectified and regulated the ac voltage into a 120 vdc output filtered by a network of capacitors and inductors.
DC Power Supplies Are Not Created Equal
DC power supplies establish the dc output voltages needed for a wide variety of consumer and industrial devices. While the concept of a dc power supply may seem simple, two basic types of dc power supplies--linear and switched mode--respond to the requirements of those devices. While linear power supplies conduct current, switched mode power supplies convert dc to a switched signal. Rectifiers in the switched mode power supply produce the dc output voltages.
In terms of physical size, linear power supplies are usually larger and heavier. The two types of power supplies also differ in how designs address electromagnetic interference (EMI), power handling, and regulation. Although linear power supplies continue to work for some applications, most devices utilize switched mode power supplies (SMPS). A switched mode supply rectifies and filters an ac input voltage to obtain the dc output voltages.
Switched Mode Power Supplies Include Two Types
Most SMPS follow a pulse-width modulated (PWM) approach operating in either 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.
Choose Components That Match the Power Supply Specifications
While vendors offer a wide range of passive and active components, DC power supplies require components that match the specifications needed to achieve good power supply stability. For example, inductors require a higher temperature rating to withstand the elevated operating temperature caused by resistance in the SMPS transformer winding.
Schottky diodes, thyristors, and MOSFETs used as bridge rectifiers must handle peak and output currents of the supply as well as the level of voltage drops. In addition, circuits must control MOSFET switching to prevent short circuits at the circuit input. Any dc power supply design must incorporate appropriate components in the output circuits that prevent reverse voltages and currents.
Maintaining accurate, reliable SPICE models of the above-mentioned components is paramount for any designer working through the validation and verification of their circuit. Utilizing a library of over 34,000 accurate component models, PSpice can ensure that your circuit is optimized for both usability and yield before going to production.
Start with the Layout
The complex technical details and functional requirements involved with dc power supplies challenge design teams. In any design, the layout establishes the functional and thermal behaviors as well as the EMI requirements for the power supply. A good layout optimizes supply efficiency.
A poor layout introduces problems at high current levels and large differences between input to output voltages. Other common power supply problems associated with poor PCB layouts include the loss of regulation at high output currents, excessive noise on the output and switch waveforms, and circuit instability.
Enclosed PCB power supply layout.
When laying out an SMPS power supply, PCB designers must control the circumference of the power switch and output rectifier loops and the length and width of traces. Maintaining small loop circumferences eliminates the possibility of the loop working as a low frequency noise antenna. Wider traces also provide additional heat sinking for the power switch and rectifiers.
Switching regulators operate with “on” and “off” power states. Each “on” and “off” power state causes power components to conduct and create the current loop. As a result, large current pulses with sharp edges flow within the switching power supply circuit can create EMI. Good power supply layout defines the layout of the loops by the current flow. With the current loops conducting in the same direction, the control circuitry couples to specific spots in the layout. With this approach, the magnetic field cannot reverse along the traces located between the two half-cycles and generate radiated EMI.
Good PCB designs also 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. The use of parallel capacitors allows the filter capacitor to sink higher levels of ripple current while minimizing component heating.
Pay Attention to Power Supply Traces
When working with a DC power supply layout, keep traces that handle high switching currents short, direct, and thick. The width of the traces has a direct impact on the capability of the power supply to minimize noise as well as the amount of voltage drop. As the high current flows through the loop and encounters trace resistance, a voltage drop occurs and radiates RF noise.
Using wider traces decreases the noise propagation because of two factors. An inversely proportional relationship that exists between the width of the trace and inductance. 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. Another inversely proportional relationship also exists between trace width and resistance. Noise and its associated current travel any low resistance path back to the place where generation originates.
By analyzing the voltage gain at a range of angles, and by managing the phase angle of the circuit, PSpice can help you plan out and simulate for the proper trace width in your circuit designs. This is of the utmost importance when heading into layout, without proper trace width, your design can easily short or fail to meet power requirements.
Trace management techniques are imperative when it comes to properly routing a power supply.
Establish the Correct Grounds
Switching power supplies typically rely on the following separate grounds at the input, output, and for control.
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
Any power supply circuit will become unstable if the grounds connect incorrectly. For an SMPS, each high-current ground serves as one leg of the current loops while representing the lowest potential return path for currents.
All components in a dc power supply must 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. A DC power supply with a ground plane on both sides absorbs radiated EMI, reduces noise, and decreases ground loop errors. The ground planes function as electrostatic shields and dissipating radiated EMI within eddy currents. In addition, ground planes also separate power plan traces and components of the power plane from the signal plane components.
The suite of design and analysis tools from Cadence are more than capable of working through the challenges of any power supply designs. PSpice, your simulation solution, will be alongside you with every model, simulation, and tolerance expectancy necessary to generate the utmost confidence in your design’s function.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.