A TUTORIAL ON PRINTED CIRCUIT BOARD (PCB) LAYOUT
THE GUIDELINES FOR SWITCHING POWER SUPPLIES (SMPS) DESIGN
A careful PCB layout is critical for proper operation of power electronics devices. While routing of control circuits can be done by an auto routing pcb software, critical power circuits should be placed by hand. This tutorial provides you with some guidelines and tips.
|1. High frequency circuit design requires careful grounding. The "ground" in a circuit is supposed to be at one potential, but in reality it is not. When ground currents flow through traces which have non-zero impedance, voltage differences will occur at different points along the ground path. To minimize these voltages use ground plane for control circuit. Try to make most of signal ground connections through vias to this ground plane rather than through PC traces.
2. For each power supply stage, keep power ground and control ground separately. Tie them together [If they are electrically connected] in one point near DC output return of the given stage.
4. Minimize areas and lengths of the loops which contain high frequency switching currents.
5. Place capacitors that bypass bias supply voltages and reference pins (if any) of all ICs physically close to the corresponding pins. For driver chips use a combination of a large capacitor (10 µF - 100 µF) and a small ceramic capacitor (0.1 µF – 1.0 µF).
6. Place filter capacitors so that their leads physically go right into the printed circuit board traces that carry mainstream of the current to be filtered.
7. If you parallel power semiconductors, when doing PCB layout try to use symmetrical routing with equal impedances for each of the paralleled devices.
8. Choose the width of circuit board traces based on acceptable temperature rise at the rated current per IPC2152 as well as acceptable DC and AC impedances. Also, make sure that the PC trace will not fuse at any abnormal current (such as short circuit current) that could develop in the circuit before a electronic protection activates or a fuse clears.
9. PWB distances between various circuits should be determined according to the requirements of applicable standards. For example, for the product covered by UL 60950-1 the creepage and clearance from primary circuits to secondary circuits and safety ground should be determined from the Tables 2K through 2N of this standard. In a typical commercial application with 120/250 VAC input, creepage between primary and low-voltage secondary circuitry per UL/IEC 60950 should be 6.4 mm minimum. For more details see our guide to PCB trace spacing.
10. For circuit spacing in non-UL applications you can generally use the recommendations of Table 6-1 of IPC-2221A. IPC-2221 is a generic standard for PCB design which replaced old IPC D-275. The recommended spacing for power conversion circuits is given by IPC-9592. In my view, the above IPC guidelines are too conservative.
Quality and inspection issues for Rigid Printed Boards are covered by Qualification and Performance Specification IPC-6012B. Note that all IPC standard are voluntary rather then mandatory.
11. Schematic design and PCB layout are often done by different engineers. A PCB designer usually does not know the details of the circuit operation and criticality of components location. In this case, the electrical designer should provide this information to the circuit board designer, help set design rules and closely supervise the routing process. Professional schematic capture software allows you to set various constraints for specific nets or groups of components. Particularly, you can specify minimum line width, net spacing type, and even maximum and relative signal propagation delays.
For more circuit board layout guidelines, free software and design aids see our PCB design tutorial and trace width calculation tool.
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