42007 FAIRCHILD | Alldatasheet
Document overview
- Manufacturer or author: Provided By ALLDATASHEET.COM(FREE DATASHEET DOWNLOAD SITE)
- PDF pages: 6
Technical content
Figure 3. Second Order Filter levels below 200W is shown in figure 3. series, but this would increase cost, weight and space.
- DC isolation
- Circuit grounding
- Susceptibility to audio frequency and RF noise conducted to the supply on power lines
- Interference generated in the supply and conducted to the other parts of the system on power lines
- Radiated interference and susceptibility
- Turn-on, turn-off transients All of these problems can be solved by placing a component in the right place such as to avoid interference. This makes the layout of the power supply one of the most crucial steps when developing a switcher. Here are some hints on what to keep in mind when developing and laying out a power converter: Care should be taken in the generation of fast rise current spikes resulting from the sudden reverse biasing of diodes used in the transformer rectifier circuitry. Ringing in the transformer can be clamped out by placing snubbers across the windings. Snubbers are discrete circuits which reduce the switching transients dissipatively as heat, or non-dissipative, by returning the energy back to the input. A small ferrite bead on the leads of the main MOSFET switching devices will suppress the generation of high frequency ringing at turn-on and turn-off periods. A faraday shield, returned to DC ground, and placed between the primary and the secondary windings will prevent capacitive coupling of voltage transients into the power supply and output leads. In order to minimize the lower and higher frequency harmonics generated in the transformer, a high permeability material must be chosen. This results in low exciting volt amperes and low core losses, which means better efficiency, smaller size and reduced weight. Also, decoupling RF noise at the source by using good high frequency capacitors with low inductance and low ESR is helpful. Lead lengths needs to be kept to a minimum to avoid RF energy radiating into free space. Decoupling outgoing leads with small capacitors will localize conducted energy before spreading to the rest of the circuitry. PCB LAYOUT CONSIDERATIONS It is always possible to contain or fix EMI and RFI to a certain degree, but the real trick is at the layout stage of the PC board. Consider switching waveforms as the ones generated from a DC to DC converter and the traces which carry these signals as the antennas. These traces are related in a complex way with the radiated fields from the converter. The prediction of radiation from a circuit requires the identification of the frequency content and their absolute magnitudes of the time domain waveform. Of particular importance is that there is no energy contained below the fundamental frequency, and 99% of the energy is contained below 1/(π tr) (where tr is the rise time of the waveform). Note that the edge rate is a key driver (or limiter) for the frequencies of concern. Power supply and DC to DC converter designers try to achieve fast rise and fall times to minimize the switching losses, but this high di/dt conditions will generate ground and trace bounce voltages which can excite components and cables to radiate in the common mode manner. Multi- layered boards which use good ground and power planes provide the low impedance power distribution necessary for good power supply decoupling. The following are some of the techniques used to minimize EMI and RFI generation: Enclosing signals between power and ground planes achieves a locally shielded enclosure that reduces radiation by 30 to 40dB and also reduces radiated susceptibility and ESD susceptibility. Think of traces as transmission lines. A good rule is to keep signals with 1nsec rise or fall times shorter than 9cm (3.5in). Traces which carry high frequency currents should be surrounded by a coplanar ground trace in order to reduce both radiation and crosstalk to other traces. REV. 1.0 10/25/2000
Another good design practice is to provide a chassis ground ring around the periphery of the board. This ring provides a formidable shield or field interceptor to prevent radiation at the circuit boundaries. This layout practice is also recommended as an interceptor for Electrostatic discharges, and thus provides a more robust design. The use of ferrites is very effective to directly suppress high frequency energy by both reactive impedance and absorptive losses. Ferrite beads for example use soft magnetic materials which require little energy to alter the magnetic flux. These ferrites become more resistive at high frequencies, which effectively reduce the bandwidth and higher frequency energy content of digital signals. Ferrite chokes are also commonly used in loose wires to cancel out common mode and differential mode currents. These ferrites come in all kinds of shapes, sizes and mounting options (including surface mount). Try and keep high frequency currents to a local area of the PCB. This practice limits the capability of those currents to excite an efficient radiator. PCB Layout and Component Selection Summary: 1. Always use a solid ground and power plane (multi layer boards are preferred). 2. Border PCB with chassis ground trace. 3. Centrally locate high frequency clock circuits. (distribute signals symmetrically) 4. Locate line drivers and receivers close to the connectors. 5. Decouple high frequency currents locally. 6. Use shielded components. 7. Use bulk capacitors near ports. 8. Use ferrites for input/output lines. 9. Use narrow and buried traces when possible. (depends on the current it needs to handle) 10. Use high frequency ceramic (surface mount when possible) capacitors and locate close to the pin. 11. Keep high di/dt lines as short as possible. 12. Keep input and output leads away from electromagnetic noise generators. 13. Minimize capacitive coupling to chassis grounds. EMI AND RFI CONSIDERATIONS FOR LOW POWER BOOST REGULATORS; ML4861 DESIGN EXAMPLE The ML4861 is a small boost converter used in 1 to 3 cell battery applications. This DC to DC converter comes in three versions which are: 3.3V, 5V, 6V or adjustable V OUT. One of the main features of this small boost converter is the fact that it only requires one inductor and two capacitors to form a complete battery boost converter operating from 1 to V OUT–0.5V and offering very good efficiency (>90%). Some applications however, require that conducted and radiated noise be maintained to a minimum within a certain window of the frequency spectrum. In such cases, the appropriate selection of components and layout is very important. For example some pager or cellular applications may require to keep EMI and RFI noise to a minimum from 1MHz to 400MHz, in which case the following considerations should be made: Use the above mentioned layout considerations when designing the PCB to minimize the radiated and conducted energy. Use a shielded inductor with the appropriate inductance, maximum current and lowest possible DC Resistance value to optimize the efficiency of the converter. Use the tables shown in the data sheet of this device, and place it as close to the IC as physically possible. Note, the higher the inductance, the better the efficiency, but the lower the maximum current available. Also, the higher the input voltage, the higher the efficiency. For a given input voltage (1, 2 or 3 cells), select the highest inductance which meets the desired maximum output current, efficiency and size limitations of the design. The recommended inductor manufacturers which offer shielded power inductors are: Coiltronics-CTX Series Tel. (305) 781-8900 Sumida-CDRH Series Tel. (708) 956-0666 Sumida-CDR63B, CDR74B and CDR105B Series Use low ESR capacitors close to the output voltage pin to minimize ripple voltage. It is also recommended to use two capacitors (one at the output of the ML4861 and one at the port) in case long VCC lines are required. The recommended capacitor manufacturers are: Matsuo-267M Series Tel. (714) 969-2491 Sprague 595D Series Tel. (603) 224-1961 To further reduce the conducted noise to the rest of the circuitry, use high frequency ceramic capacitors preceded by a small ferrite bead to shunt high frequency noise as shown in Figure 4, Option 2. Another possibility is shown as Option 3, where a small, high frequency EMI filter is connected directly to the output port. When using these small filters, it is important to have a solid ground plane connected to chassis ground to effectively shunt the high frequency energy to ground. The recommended filters are: Murata Erie Tel. (800) 831-9172 NFM40R Series (Rated current = 200mA) NFM41R Series (Rated current = 300mA) NFM61R/H Series (Rated current = 2A) REV. 1.0 10/25/2000
feedthrough at the expense of efficiency of the converter. and insertion loss characteristics. Figure 4. ML4861 EMI/RFI Circuit Suggestions
FAIRCHILD’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR CORPORATION. As used herein: 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, and (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 a significant injury of the user. 2. A critical component in 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. www.fairchildsemi.com 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.