his is the first of a series of articles devoted to the design, test, and EMC emissions evaluation of 1- and 2-layer PCBs that contain AC/DC and/or DC/DC converters and employ different ground techniques. In this introductory article, we present a top-level block diagram description of the design problem under research. The subsequent articles will be devoted to the specific parts of the design, and subsequently to the RF emissions performance of the PCB assembly. This is a research in progress. The goal of this study is to evaluate the impact of different grounding strategies and the tradeoff with other design constraints that designers often face.
This article is organized as follows. Section 2 presents the top-level functional block diagram with the EMC considerations. Section 3 is devoted to the individual functional blocks. In Sections 4 and 5, several grounding schemes for 1-layer and 2-layer boards, respectively, are shown. Section 6 provides a brief outline of the next article.
The board will be capable of accepting either an AC or DC input. The AC to DC conversion will take part in Partition A of the board (not drawn to scale). The DC to DC converter in Partition B will accept 24V DC input either from the AC/DC converter in Partition A or from an external source.
The external AC and DC inputs and I/O circuitry provide noise-coupling paths (for conducted /radiated emissions) from the converters. Additional noise paths exist between the two converters themselves, as well as between the converters and the rest of the circuitry in Partition B.
Switching Class D power converters contain switching waveforms that produce harmonic noise and ringing that causes broadband high-frequency emissions. The implementation of EMC design controls and PCB layout will affect the EMC performance of the PCB assembly and associated cabling.
The converter stage employs a filtering block, full-wave rectifier, controller, and a transformer which provides isolation between the two partitions.
24V DC input to the converter comes either from the AC/DC converter or from an external linear power supply input. The control IC contains the switching transistor and the feedback signal detection.
The I/O circuitry contains a microprocessor powered by 3.3V DC as regulated by the DC/DC converter. A real-time clock is provided so that analog values from the thermocouple can be recorded in memory. An unshielded multi-conductor cable with a length of 1 meter will be connected between Partition B and a thermocouple. This cable is likely to carry some of the common-mode emissions from the converters and the microcontroller.
Figure 6 shows the grounding scheme for Case 1.1, where the ground is routed exclusively as traces on the top of the board.
Figure 7 shows the grounding scheme for Case 1.2, where ground floods are introduced on the top of the board.
Case 1.2 is similar to Case 1.1, but with fewer space constraints in its application. Here the designer has more opportunities to improve grounding and reference areas. Adding additional ground and/or reference areas improves RF return paths and can reduce RF emissions. The additional copper areas will likely help with thermal power dissipation, as well.
Figure 8 shows the grounding scheme for Case 2.1, where the bottom layer is a mostly solid reference plane with some slots accounting for the need to route signals on the secondary layer.
Figure 9 shows the grounding scheme for Case 2.2, where the bottom layer is a complete ground flood with via stitching to the top-layer ground areas.