View FDS3570_1719354.PDF datasheet online --- IC-ON-LINE

Datasheet File OCR Text:

1 june 2001 rev. a, june 2001 ?001 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 speci?ation 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 designers 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 speci?ations. 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 ?al 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 ?back 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 ?back designs. the rms currents in output recti?r(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 ef?iency 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 ef?iency. while non-dissipative active clamp schemes have been developed, they are achieved at the expense of signi?ant added complexity. application note 7004 power converter topology and mosfet selection for 48-v telecom applications craig varga june 2001
2 rev. a, june 2001 ?001 fairchild semiconductor corporation figure 1. simplified flyback regulator schematic. for 36- to 72-v input designs, the fds3670, 100v mosfet is the lowest voltage part that should be used. even then, the drain voltage will have to be carefully controlled with some form of primary side clamp circuit. if the input supply can be expected to experience transient voltages on the order of 100v, the 150v fds2570 should be considered. it is important to note, two devices would have to be connected in parallel to maintain the same power level that can be achieved with a single fds3670. the off-state voltage is determined by the input voltage, output voltage, transformer turns ratio, transformer leakage inductance, and snubber/clamp circuit. ignoring the effects of leakage inductance, the off voltage is equal to the input voltage plus the output voltage multiplied by the transformer turns ratio. as an example, assuming a 1:1 turns ratio and a 5v output, the off- state drain voltage is equal to vin plus 5v. the effects of leakage inductance complicate the matter considerably. the energy stored in the leakage inductance is equal to li 2 /2, where l is the leakage inductance and i is the peak primary current. this energy appears as a voltage impressed on the various stray capacitances around the circuit. the energy is equal to cv 2 /2 and, therefore, v pk =i pk (l/c) 1/2 . the problem is complicated by the fact that most of the stray capacitances are nonlinear and vary greatly as a function of the instantaneous voltage applied. as such, an empirical approach to designing the snubbers and clamps is generally used. figure 2 shows typical waveforms for the ?back regulator. generally in a 48v application, the maximum useful power level for a ?back design will be around 20 to 30 watts. above that, consider a single transistor forward converter as shown in figure 3. gate q1 rtn c2 d1 t1 + c1 + u1 pwm gnd v out v in
3 rev. a, june 2001 ?2001 fairchild semiconductor corporation figure 2. typical waveforms for the flyback regulator. single transistor forward converters the forward converter achieves much better transformer utilization than the yback design since in theory, the transformer stores no energy. there is an extra inductor in the output power path and an additional diode as well. single transistor forward converters are useful in 48v system designs, up to 150 watts or so. a similar problem exists in the forward converter as in the yback design because the drain voltage is inherently unconstrained. leakage nductance in the power transformer will produce large voltage spikes that must be snubbed or clamped. v gs -v spike -v in + v fb -v in v ds i d(q1) i d1 v sw v in i pk 0 time sw on vsat v flbk i pri 0 time i sec 0 time pwm'ed edge
4 rev. a, june 2001 ?2001 fairchild semiconductor corporation figure 3. a single transistor forward converter. figure 4 shows a design for a typical rcd clamp circuit. the leakage inductance energy is stored in the capacitor, and subsequently burned up in the resistor. the yback voltage during the primary switch's off state is not forced by the turns ratio as in the yback design. during the off state the forward output recti er (d1) essentially disconnects the transformer from the secondary circuit. the resonant effects of the transformer s primary inductance, and the stray capacitance at the switch s drain node control the primary switch drain voltage. this capaci- tance includes the fet s output capacitance and miller capacitance, as well as transformer winding capacitances, and re ected secondary recti er capacitances. again, the severe nonlinearity of many of these elements leads one to a largely empirical approach to circuit optimization. in general, it is safe to assume that the maximum drain-voltage observed in the off state will be 2.5- to 3-times the input voltage. therefore, the 200-v fds2670 is the best choice for these designs. figure 5 shows the important current and voltage waveforms. figure 4. a typical rcd clamp circuit. gate q1 mosfet n d1 d2 t1 c1 + u1 pwm gnd v in rtn c2 l1 + v out r1 to fet drain to ground of v in d1 c1
5 rev. a, june 2001 ?2001 fairchild semiconductor corporation figure 5. voltage and current waveforms for a single transistor forward converter. the average switch current in a forward converter is equal p out / v in where p out is the max- imum output power, is the ef ciency, and v in is the minimum input voltage. with a maximum rating of 3 amps for the fds2670 and assuming an ef ciency of 80%, the maximum output power that can be obtained with a single primary-side fet is approximately 80w. utilizing two fds2670 devices in parallel are therefore capable of a theoretical output power of about 160 watts. note the instantaneous peak switch current is going to be 2.5 to 5 times higher than the average current, depending on the input voltage range. the wider the input voltage range, the higher the peak switch current. two transistor forward converters for still higher power levels, up to about 200 watts, the two transistor forward converter of figure 6 is a good choice. the basics of the circuit s operation are similar to the single transistor design. the difference is that the leakage inductance s energy, and therefore, the drain voltage, is no longer unconstrained. during the off state, the transformer s primary v gs (q1) -v spike -v reset -v in v ds (q1) i d (q1) i d1 i d2 i l1 i out v sw v sat 2v in i pk 0 time sw on i pri 0 time i min pwm'ed edge
6 rev. a, june 2001 ?2001 fairchild semiconductor corporation becomes a current source that forward biases diodes d1 and d2, returning the energy stored in the transformer s primary to the input capacitor. no snubbers or clamps are required, and the maximum off voltage the primary side fet will see is the input voltage, plus a couple of diode drops. therefore, the 80v FDS3570 is useful in designs that will never have the input exceed of 76v. the 100v fds3670 will provide a little more design margin if desired. all wave- forms are essentially similar to figure 5 except the fet s drain voltages, which are shown in figure 7. figure 6. two transistor forward converter. figure 7. voltage and current waveforms for two transistor forward converter. note that the primary current is the same as the single transistor design example above. however, the FDS3570 has an on-resistance of only 15% than that of the 200v fds2670 (20 m ? vs. 130 m ? ). even with two transistors in series, the net on-resistance is still much lower than the single high-voltage transistor, so a higher power level can be obtained. ef ciency will also be several percentage points higher than the single transistor design since the energy that would be dissipated in snubbers and clamps is, in this case, recovered to the input capacitor. using the fds3670 s maximum rms current rating of 9 amps, an assumed ef ciency of 83% with 36v in, and a maximum 45% duty factor, gives a maximum output power of 180 watts. again, it is permissible to parallel fets in an effort to lower the conduction losses and raise the useable output power of the converter. d1 d2 t1 d3 d4 q1 q2 isolation input gate gnd u1 pwm gate source gnd rtn c2 l1 + c1 + v out v in v gs (q 1 tq 2 ) -v in -can vary from 0 to v in v sd (q 1 tq 2 )
7 rev. a, june 2001 ?2001 fairchild semiconductor corporation double-ended topologies there are several commonly employed variants of double-ended converters. the terminology implies that the magnetic ux swing in the transformer primary is bi-directional. in other words, the transformer is being actively driven in two directions, as opposed to the forward and y- back topologies where the transformer is driven in one direction and allowed to reset its core ux naturally. the double-ended approach has the advantage of utilizing the core volume more ef ciently, and therefore allowing the use of a physically smaller transformer core. the price paid for this better utilization is more power switches and more complex transformer windings. there are two main topologies to discuss, push-pull and bridge. push-pull designs figure 8 shows a basic push-pull design. the main advantage of this design is grounded- source switches. this allows for a simple gate-drive design, requiring only two switches. the main disadvantage, like the single transistor forward and yback designs, is the drain voltage of the two switches is inherently unconstrained. therefore, there will typically be some form of snubbing and/or clamping, usually passive, on the fet drains. operation is quite simple. each fet gets turned on for some fraction of one half of an entire cycle. if each fet were to be operated for exactly 50% of a full period, the effective duty factor at the output of the second- ary recti ers would be 100%. this cannot be accomplished in practice, but a close approxima- tion can be achieved. also, the effective secondary side ripple frequency is doubled. this helps to minimize the size of the secondary inductor. figure 9 shows idealized gate drive, fet drain, and transformer secondary waveforms. it should be noted that the theoretical minimum drain voltage seen by a primary fet in this con guration is 2-times the dc input voltage. as one fet is driven on, a voltage of vin is developed across one half of the transformer primary. an equal voltage will be induced in the other half of the winding. this voltage is added to the input voltage, and impressed on the drain of the off fet. since there is also unclamped leak- age inductance associated with the transformer primary, there will be spikes substantially higher than twice the input. consequently, only the 200v fds2670 is suitable for applications where inputs as high as 72v could occur. about twice the power level of a single transistor forward converter can be obtained. figure 8. a basic push-pull design. d c out v o d t q1 control l + c in v in + + q2
8 rev. a, june 2001 ?2001 fairchild semiconductor corporation figure 9. voltage and current waveforms for the basic push-pull design. full-bridge designs full-bridge designs, sometimes called four transistor bridges, are generally used at very high power levels, mainly due to the cost of these designs. two of the four switches have oating sources, so they each require an isolated gate drive, usually transformer coupled. the power transformers primary winding design is a simple single winding. the secondary, is generally center-tapped, at least for very low output voltage designs. the main advantage of the bridge topology is that the drain voltages of all four switches are constrained to vin. energy in the transformer s leakage inductance gets returned to the input and can help force commutate the switches. the switches get turned on across a diagonal of the bridge. figure 10 shows the basic circuit design, and figure 11 shows the idealized waveforms. since the drain voltage is well clamped to vin without snubbers, there is no concern for passive energy losses. this also allows the use of the 80v, FDS3570 in all four locations. with these devices, output power v gs (q2) v gs (q1) pwm'ed edge -2v in -v in v ds (q1) v ds (q2) d1(anode) d2(anode) i l1 i 0 v sw v sat i pk 0 time sw 2 sw 1 2 vin v in i pri 0 time i min
9 rev. a, june 2001 ?2001 fairchild semiconductor corporation levels approaching 390 watts are theoretically possible. paralleling fets allows this number to be increased. the output waveforms of the bridge design look the same as the push pull. the fet drain voltages are depicted in figure 11. figure 10. the basic circuit design for a full-bridge regulator topology. figure 11. voltage and current waveforms for the full-bridge regulator. d1 d2 l1 t1 q2 q4 q1 v in gate 1 gate u1 pwm gate input 1 isolation input 2 gnd gnd source 1 gate 2 source 2 rtn c2 + c1 + v out q3 v gs (q2&q3) v gs (q1&q4) pwm'ed edge can vary from 0 to v in -v in v ds (q1&q4) v ds (q2&q3) v sw2 v sat i pk 0 2 time sw 1-4 sw 2-3 v in v in i sw2 0 time i min can vary from 0 to v in -v in
10 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 signi cantly based on whether there is input/output isolation. ef ciency can range from 70 to 85% . a summary of the rst-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 recti?ation. synchronous recti cation may be employed with all the previously mentioned topologies. this technique consists of replacing the output recti ers 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 recti cation. 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 speci?d limits. when using surface-mount power devices the primary heat sink is the pc board. the main constituents of pcbs are berglass topology max. power (watts) typical ef?iency (%) flyback 30 75 1 transistor forward 150 80 2 transistor forward 200 83 push pull 350 80 full bridge 500 85 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 ? fds3670 100v 6.3a 32 m ? fds2570 150v 4 a 80 m ? fds2670 200v 3 a 130 m ?
11 rev. a, june 2001 ?2001 fairchild semiconductor corporation and copper. for example, we pack berglass 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 ll 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, air ow 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 air ow 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"
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. trademarks 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: 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 optoplanar? pacman? pop? power247? powertrench qfet? qs? qt optoelectronics? quiet series? silent switcher smart start? fast fastr? frfet? globaloptoisolator? gto? hisec? isoplanar? littlefet? microfet? microwire? optologic? rev. h3 ? acex? bottomless? coolfet? crossvolt ? densetrench? dome? ecospark? e 2 cmos tm ensigna tm fact? fact quiet series? star*power? stealth? supersot?-3 supersot?-6 supersot?-8 syncfet? tinylogic? trutranslation? uhc? ultrafet vcx? ? ? ? star*power is used under license

▲Up To Search▲

Price & Availability of FDS3570

	To Download FDS3570 Datasheet File
If you can't view the Datasheet, Please click here to try to view without PDF Reader .