7004

7004 FAIRCHILD | Alldatasheet

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Rev. A, June 2001©2001 Fairchild Semiconductor Corporation Introduction With the recent proliferation of telecommunications equipment, there is more demand than ever for voltage converters that are powered by the nominal 48V telecom supply. Depending on the application and operating environment, the supply voltage range can vary widely. A typical specification can range from a low of 36V to a high of 72- with a 48-V nominal. In some designs, transients in excess of 100V need to be considered. Most of these designs will require input to output isolation of up to 1500V. Output voltages are frequently 5V and below with 3.3V probably the most common requirement, and 2.5V gaining in popularity. If a processor is on the card, voltages as low as 1.3V are not unlikely. One common approach is to regulate a distributed power bus, say the 5V rail, and then use non-isolated DC/DC converters to generate lower voltages. With the tendency away from 5V, the 3.3V rail is beginning to serve as the distributed bus, although, from the power supply designer’s perspective, this is not the most of desirable situations. Fairchild has recently introduced a family of high voltage MOSFETs ranging from 80- to 200-V drain voltage specifications. This application note will provide information helpful in the proper selection of FETs for primary side switches – available in various types of 48V power converters. Basic Topologies There are a nearly endless variety of power converter topologies that can be used for 48V conversion. A large number of considerations will enter into making the final choice. Power level is going to be the main determining factor, although output voltage and input/output isolation are factors to consider Flyback Converters Figure 1 shows a basic flyback design using the FDS3670, 100V MOSFET. The circuit illustrated has the advantages of low parts count and simplicity, making it useful for relatively low power levels. Output current is the major limiting factor in flyback designs. The RMS currents in output rectifier(s), transformer secondaries, and output capacitors tend to be large compared to the average output current (i.e., a high crest factor). As such, at high output currents, the secondary side power components tend to get physically large and efficiency suffers. The off-voltage of the primary side power switch is inherently unconstrained in this topology. As this occurs peak voltage is limited by snubbers and/or clamps that generally dissipate power and reduce efficiency. While non-dissipative active clamp schemes have been developed, they are achieved at the expense of significant added complexity. Application Note 7004 Power Converter Topology and MOSFET Selection for 48-V Telecom

Applications

Figure 1. Simplified flyback regulator schematic. can be achieved with a single FDS3670. application, the maximum useful power level for a flyback design will be around 20 to 30 watts. Above that, consider a single transistor forward converter as shown in Figure 3.

Figure 2. Typical waveforms for the flyback regulator.

0 TIME

Figure 5. Voltage and current waveforms for a single transistor forward converter. higher the peak switch current.

Figure 9. Voltage and current waveforms for the basic push-pull design.

Rev. A, June 2001©2001 Fairchild Semiconductor Corporation Table 1 shows the comparison of the topologies that have been discussed. Note that the rela- tive cost increases significantly based on whether there is input/output isolation. Efficiency can range from to 85% . A summary of the first-pass selection of key MOSFET parameters is shown in Table 2. Parameters of the MOSFETS used in Application Note are shown in Table 3. Table 1: Comparison of various switching regulator topologies. Table 2: Power MOSFET voltage and current ratings versus power supply topology. Table 3: PowerTrench® MOSFETs in SO-8 package for 48V Applications. Synchronous rectification. Synchronous rectification may be employed with all the previously mentioned topologies. This technique consists of replacing the output rectifiers with MOSFETs, striving to lower conduc- tion losses. The concept is, however, not without its share of problems. The complexity of the drive and control circuitry increases dramatically. Timing of the gate-drive to the synchronous FETs is critical to avoid cross conduction. The parasitic capacitances and body diode reverse recovery characteristics of the output FETs contribute to substantial switching losses in these devices. The technique is most applicable for 12V and lower output voltages. The lower the output voltage, the greater the advantage of synchronous rectification. Much above the 12V level, the large parasitic losses make the approach much less advantageous. Thermal Considerations One of the main limiting factors in any power supply design is the ability to keep the junction temperatures of the power devices within specified limits. When using surface-mount power devices the primary heat sink is the PC board. The main constituents of PCBs are fiberglass Topology Max. Power (Watts) Typical Efficiency (%) Flyback 30 75

1 Transistor Forward 150 80

2 Transistor Forward 200 83

Topology Maximum Drain Voltage Average Drain Current Flyback 1.5Vin(max) 751.5Pout/ Vin(max) 1 Transistor Forward 2.5 to 3.0 Vin(max) 1.3Pout/ Vin(max)0 2 Transistor Forward Vin(max) 1.2Pout/ Vin(max) Push Pull 352.5 to 3.0 Vin(max)0 0.65Pout/ Vin(max) Full Bridge Vin(max) 0.60Pout/ Vin(max) Voltage(DS) Current (ID max) ON-Resistance (@VGS=10V) FDS3570 80V 9 A 20 m Ω FDS3670 100V 6.3A 32 m Ω FDS2570 150V 4 A 80 m Ω FDS2670 200V 3 A 130 m Ω

Rev. A, June 2001©2001 Fairchild Semiconductor Corporation and copper. For example, we pack fiberglass in our walls to insulate our homes. Hardly the kind of material we would choose to conduct large amounts of heat away from our power devices. Copper is the primary means of conducting heat away from the source and into the air stream. Probably the biggest mistakes made in laying out PCBs for power devices are the removal of copper in the vicinity of the power devices, and the reliance on inner layers for inter- connecting the power devices. When possible, leave as much copper around the power devices, and interconnect devices using topside copper. This allows the generated heat to spread laterally and to be removed into the air stream passing over the board. Thicker copper is better, and with most of the manufacturing technology currently in use, thermal reliefs are not required, nor should they be used. Whenever possible, leave the plane areas under and around the power devices, and the inner layers and backside of the board intact. This helps with the heat spread and power dissipation. For optimal thermal control, add an area of large diameter vias around the power device that connect into the inner and back layers of the board. If possible, allow these vias to fill with solder to form a thermal pipe between all the layers. Avoid vias for power path interconnects, especially the commonly used micro-vias. If vias must be used to run power between layers, use a minimum of one via per amp of average current – two vias per amp are preferred. Another consideration is the use of thermal pads for conduction of heat to the chassis. Manufacturers such as Bergquist, Avid, and Thermalloy, produce thick, compliant silicone pads that are loaded with thermally conductive materials. If the power devices can be mounted on the back side of a PCB, and a thermally conductive pad sand- wiched between the PCB and a steel chassis, a great deal of heat will be very effectively removed. Surface mount heatsinks are also available from some of the previously mentioned manufacturers which drastically improve the PCB’s ability to transfer heat to the airstream. In all cases, airflow across the PCB is very important. Even a small amount of air movement will make a big difference in the device temperatures. Try to avoid putting the power devices in the airflow shadow of tall components such as a microprocessor or the power supply’s own input or output capacitors. Reference: Marty Brown, "What Everyone Should Know About Switching Power Supplies" Rudolf Severns, Gordon Bloom, "Modern DC-DC Switchmode Power Converter Circuits"

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