AN1031 FAIRCHILD | Alldatasheet
Document overview
- Manufacturer or author: Provided By ALLDATASHEET.COM(FREE DATASHEET DOWNLOAD SITE)
- PDF pages: 15
Technical content
Considerations in Designing the Printed Circuit Boards of Embedded Switching Power Supplies The importance of a good printed circuit board (PCB) layout in switching power supplies cannot be overstated. Developing the schematic and debugging the breadboard is a good start, but the final, critical challenge is to layout the PCB. Fortunately, understanding the phenomenon behind the operation of the typical switching power supply makes the effort much easier. 1. Introduction 2. Discussion The primary rule for the designer is to be involved with every aspect of the design of the switching power supply, including the PCB. He or she is the only person who best understands the functional requirements of the power supply within the final product. In doing this, the power supply designer should never allow a PCB designer to use the auto-routing routines within the PCB layout program. The autorouter routine only strives to connect nodes that utilize the same signal as stated in the netlist. It disregards the length of the traces needed to accomplish this. The autorouter also considers all grounds the same signal and connects them together without consideration of the actual types of signals running through certain traces. For the power supply designer and the PCB designer to execute a good PCB layout, knowing the signals that flow between components is very important. Appreciating the subtle “black magic” aspects to the PCB layout is essential to the success of the product. These layout factors can affect the performance of the switching power supply and can also affect the product’s ability to be released into the market. The aspects of the product’s opera- tion that affect the printed circuit board design are: radiated electromagnetic interference (radiated EMI), conducted EMI, power supply stability, efficiency and operational longevity. The two forms of EMI are tested by regulatory approval bodies such as UL, IEC, and numerous other regulatory bodies throughout the world. The product must pass these stringent EMI tests before it can be sold into its respective market. The remaining factors affect the product's basic operation and customer satisfaction. April 30, 1999 Marty Brown Switching power supplies have large current pulses with very sharp edges flowing within the power supply circuit. These large current pulses have the greatest effect on the creation of EMI, and should be the primary focus of the PCB designer. These currents flow in definable “loops” and the circuits carrying these currents should be laid-out first. The low-level control circuitry is then subsequently coupled into specific spots in the layout. These loops are diagrammed in figure 1 for the three major basic topologies of switching power supplies. All of the other topologies are varia- tions of these three. 3. Current Loops AN1031 Rev A, July 1999
- The power switch high current loop.
- The rectifier high current loop.
length of a conductor, is radiated into the product’s environment. trapezoidal current waveforms with high peak currents and very sharp edges (di/dt). Figure 1. The Major Current Loops within Switching Power Supply Topologies
Figure 2. The Modes of Operation of Switching Power Supplies
very small circumference and is composed of traces that are short in length and wide in width. example of a layout for the power switch and rectifier loops in a buck converter is shown in figure 3. Notice the very short distances between all members of the two main AC loops. Figure 3. Example layout for a Buck Converter Power Stage
Figure 4. Example Layout for a Transformer-isolated Output paralleled capacitors can be seen in figure 5.
Figure 5. Laying out Parallel Capacitors switching power supply topologies are shown in figure 6.
Figure 6. Ground Arrangements for various Topologies b. Non-isolated, Transformer-based Topologies. way, noise voltages from the high current loops are not summed into the low-level sense signals. well as drive the gate or base of a power switch.
If there are separate analog and power ground pins on the controller IC, they should be routed separately to the ground side of the current sensing resistor. If the IC does not have separate ground pins, then the trace between the IC and the ground end of the current sense resistor should be short and wide. Another good practice to reduce radiated EMI is to place large areas of ground plane on the opposite side of the PCB and around these high current traces. The ground planes act as electro- static shields for some of the RF energy already radiated. These large conductor areas trap radiated EMI and dissipate them within eddy currents created by the RF energy. One last and very important factor in designing PCB layouts for switching power supplies is the capacitive coupling of the AC node voltages into their heatsinks or into nearby ground planes. The problem is very severe in through-hole designs, but can also be a problem in surface-mount applications. The problem is created by the high AC voltages that appear on specific nodes within the switching power supply. Examples of these nodes are the drain connection of the power switch, the AC node connected to an output rectifier, and any snubber or clamp networks connected to these nodes. In through-hole applications, the power switch is typically a power package with a tab which is bolted to a heatsink with a 5 mil (0.005 inch, 0.13 mm) insulator between them. The drain tab of the power switch has AC peak-to-peak voltages of either one or two times the input voltage. In many supplies, the heatsink is earth grounded which provides a path for the capacitively coupled noise energy to exit the enclosure. Insulator makers have pads that have embedded foil which cuts the capacitance in half. The problem is less significant in surface-mount applications since the capacitance formed by 0.062 inch (1.6 mm) thick F4 material is much smaller. Additionally, it is rare that earth ground is brought onto the PCB, but the noise could couple into other sensitive signals. The goal is to reduce this parasitic capacitance by creating PCB structures that exhibit low capacitance, such as locating susceptable signals one the same side instead of underneath the noisy node or cross- hatching any ground planes beneath the noisy node. 7. The EMI Filter Layout Now is the appropriate time to consider the layout of the EMI filter. An EMI filter is needed anytime a power lead or leads are allowed to exit the product’s enclosure, which hopefully also provides some RF shielding. These filters are intended to reduce, but cannot completely eliminate, the high frequency currents conducted within the DC input and/or output wiring. Conducted EMI is tested by the regulatory bodies by placing a special cur-rent transformer (a line impedance stabi- lization network (LISN)) in series with the input and/or output power lines. The tester then plots the spectrum of the emerging current waveform from DC to over 1 GHz. The product under test must emit a current spectrum lower than the specified limits at all frequencies. The filters are designed to not pass the high frequency noise created by the PWM switching power supply. If the parasitic factors of the filter components themselves are not well known and the components are not laid-out properly, some switching energy can couple around the filter compo- nents to traces on the other side of the filter. This allows some of the high frequency energy to escape into the environment or into the rest of the system. Once in the external wiring, this conducted RF energy will then radiate into the surrounding environment as radiated EMI. Some good guidelines are to place the EMI filter as close as possible to the point where its signal exits the enclosure. The layout of the actual EMI circuitry should also be as close to “in-line” as possible. “Zig-zaging” the layout can cause input and output traces to be in close proximity to each other, thus promoting inductive coupling.
to be added to make the designs practical. Figure 7. Schematic of the Buck (Step-down) Converter
Parts List for the Buck Converter in Figure 7: Designation Mfr Part No. Value Rating Package C1 Sprague 593D226X0035E2W 22uF 35V E-case C2 Sprague 593D226X0035E2W 22uF 35V E-case C3 Mepco C105M1206ZFLB 0.1uF 50V 1206 C4 Mepco C102M0805ZFLB 1nF 50V 805 C5 Sprague 293D155X0010A2T 1.5uF 10V A-case C6 Mepco C102M0805ZFLB 1nF 50V 805 C7 Sprague 293D475X0010B2T 4.7uF 10V B-case C8 Sprague 594D227X0010D2T 220uF 10V D-case C9 Sprague 594D227X0010D2T 220uF 10V D-case L1 Coilcraft DO3316-103 10uH 3.8A Custom Q1 Fairchild FDS4410 - 10A,30V SO8 Q2 Fairchild FDS4410 - 10A,30V SO8 Q3 Fairchild MMBD914 - 0.1 A,100V SOT23 Q4 Fairchild MBRS130L - 1A, 30V SMC R1 Dale WSL-2010-R020F 20m ý 1% 2010 R2 Dale CRCW08052211FB02 2.11K 1% 805 R3 Dale CRCW08051101FB02 1.10K 1% 805 U1 MAXIM MAX1636 - SO20-W
Figure 8. PCB Layout for the Buck Converter any associated analog functions or interface circuits. Once again, the design can be scaled.
flyback converter can replace two or more buck or boost supplies within a system. Figure 11. A Flyback Converter
Figure 12. PCB Layout for a flyback converter
- Brown, Marty, Power Supply Cookbook, Butterworth-Heinemann, 1994, pp 89-94, 224,
- Brown, Marty, Practical Switching Power Suppy Design, Academic Press, Inc., 1990,
- Linear Technology, Linear View v2.0, CD, Linear Technology Corp. 1997.
ACEx™ CoolFET™ CROSSVOLT™ E 2CMOS TM FACT™ FACT Quiet Series™ FAST FASTr™ GTO™ HiSeC™ The following are registered and unregistered trademarks Fairchild Semiconductor owns or is authorized to use and is not intended to be an exhaustive list of all such trademarks. LIFE SUPPORT POLICY FAIRCHILD’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF FAIRCHILD SEMICONDUCTOR CORPORATION. As used herein: ISOPLANAR™ MICROWIRE™ POP™ PowerTrench™ QS™ Quiet Series™ SuperSOT™-3 SuperSOT™-6 SuperSOT™-8 TinyLogic™ 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, or (c) whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in significant injury to the user. 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. PRODUCT STATUS DEFINITIONS Definition of Terms Datasheet Identification Product Status Definition Advance Information Preliminary No Identification Needed Obsolete This datasheet contains the design specifications for product development. Specifications may change in any manner without notice. This datasheet contains preliminary data, and supplementary data will be published at a later date. Fairchild Semiconductor reserves the right to make changes at any time without notice in order to improve design. This datasheet contains final specifications. Fairchild Semiconductor reserves the right to make changes at any time without notice in order to improve design. This datasheet contains specifications on a product that has been discontinued by Fairchild semiconductor. The datasheet is printed for reference information only. Formative or In Design First Production Full Production Not In Production DISCLAIMER FAIRCHILD SEMICONDUCTOR RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE TO ANY PRODUCTS HEREIN TO IMPROVE RELIABILITY, FUNCTION OR DESIGN. FAIRCHILD DOES NOT ASSUME ANY LIABILITY ARISING OUT OF THE APPLICATION OR USE OF ANY PRODUCT OR CIRCUIT DESCRIBED HEREIN; NEITHER DOES IT CONVEY ANY LICENSE UNDER ITS PATENT RIGHTS, NOR THE RIGHTS OF OTHERS. UHC™ VCX™