Power supplies come in many forms, both as highly integrated modules that plug into an assembly, or as a dedicated circuit on a PCB. Designs for many new areas place important requirements on power systems that a designer must meet to ensure EMC compliance, as well as sufficient power delivery within the system’s required frequency range. Adding to the complexity is cost and form factor, both of which are constantly being driven smaller.
For designers that want to dominate the coming market for electronics design, as well as companies that want to best serve market needs, a refresher on advanced power supply designs may be needed. Newer power systems are pushing the limits in terms of noise reduction and EMC needs, and it’s up to PCB designers and systems designers to create products that can meet these needs. For designers that want to stay competitive in this important field, keep reading to see some of the major challenges in power supply design.
Modern Challenges in Power Supply Design
Power supply designers must understand something about all areas and aspects of power systems within the following list:
- Power supply topologies (isolated, switching, multi-phased)
- Power distribution architectures (digital, analog, and mixed-signal)
- EMC challenges and fixes in PCB layouts for power supplies
- Control strategies that reduce noise and provide precise power tracking
Higher Power Density Topologies
Some newer applications, such as charging and power for electric vehicles, involve power supply topologies that involve high power delivery to a load. These are generally switching regulators, and they may be isolated to ensure safety of the end user. Some of the advanced power supply designs that can delivery highly stable power include:
- Bridge topologies: High-power bridge converters will drive a transformer on one side of a full-bridge or half-bridge arrangement. These monodirectional converters can implement a complex feedback scheme in an isolated system with high current delivery.
- Bi-directional topologies: These topologies involve high power delivery or sinking in the power supply. These converters are typically half-bridge on each side of a transformer with a PWM driving stage on each side of the system.
- Resonant converter: These topologies may be bridge topologies, but they use resonances in a specific LC resonance to maximize power delivery at a particular switching frequency.
- Multi-phase converters: These appear in RF systems but they can be used in general to provide low-ripple power by mimicking a switching converter operating at a higher frequency.
These power supply topologies can have highly integrated controllers that need to be implemented in a very low-noise layout and assembly. These power topologies may require a specific control methodology to reduce losses and output noise.
Control strategies in a power system refer to three aspects of system design:
- Accurately tracking the power output
- Increasing power conversion efficiency
- Increasing stability without noise increases
Typically these are implemented with a mix of signal processing, feedback, and control mechanisms that regulate the switching action and noise in a modern power converter.
- Zero-voltage switching and soft switching: this is used to prevent switching in the sub-threshold region of a MOSFET, where the channel has non-negligible resistance and loss.
- PFC circuits: these circuits control switching at the input rectification stage to eliminate noise at the system input.
- Multi-phase design: this uses multiple switching phases so that the power supply has lower ripple but without running at higher PWM frequency.
The feedback required in these designs requires high precision current-sense resistors and carefully designed power sense stages (e.g., with a current sense amplifier). When feedback is present, an optocoupler may be needed to transfer the output signal back into the input side for measurement and execution of the control strategy.
Power Routing in a PCB
Power supplies that are built into a PCB, rather than as separate units, must be laid out in conjunction with the other components on a PCB such that noise is minimized.
The PDN architecture in many digital and mixed-signal PCBs attempts to build a structure with multiple rails to provide power to processors and ASICs operating at different logic levels. Within each section of a PDN, it is common to see strategies used to isolate power provided to different components, but without guaranteeing galvanic isolation. This is common in digital designs that must provide stable power at multiple core logic levels, as well as a stable reference voltage for clocks (integrated PLL) in the main processor components.
The typical system architecture for power distribution involving multiple rails is shown below.
Example power architecture that a power supply may need to accommodate integrated into a device’s PCB.
The design choices implemented above are meant to balance high power density with stable power, as well as the potential need to isolate noise-sensitive analog rails (PLL, ADC/DAC, etc.) from the noisier digital rails. In addition, the PCB stackup and layer will be big determinants of noise suppression as they provide decoupling needed to isolate each section of the PDN.
Working With Modern FETs
Newer power supply designs are operating with newer semiconductor materials, namely silicon carbide (SiC), gallium arsenide (GaAs), or gallium nitride (GaN). These semiconductor materials provide higher power density with lower losses due to their lower on-state channel resistance. They can also be operated at higher voltages than silicon in smaller packages thanks to lower breakdown voltage. Finally, GaN is becoming a material of choice for high-frequency power applications, such as in power supplies for mmWave systems.
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