IsolatedPowerSuppliesforTelecomApplications

1、Isolated Power Supplies for Telecom Applicati ons To en sure complia nee with all intern ati onal sta ndards, the desig ner of an isolated power supply for telecom applicati ons n eeds to take certa in facts into acco unt: The in put voltage is n ever 110V or 220V from the mai ns, but the lower volt

2、age of a lead-acid battery (-12V, -24V, or -48V) that is charged by means of the mains through rectification (Figure 1). (Batteries back up the telecom system in case of failure in the mains supply.) To combat corrosi on due to ion flow, the in put voltage is always n egative, with the positive term

3、 inal conn ected to ground. Figure 1. In this block diagram of the backup architecture used in telecom systems, a battery block is kept charged by the mains rectifier. I n the eve nt of a mains failure, it supplies the system to preve nt tran smissi on in terrupti ons. ISDN and other systems powered

4、 by a remote source often operate with a higher input voltage: up to 120VDC. (Losses due to line resistance are proportional to the square of supply curre nt, so the lower supply curre nt associated with higher in put voltage allows a Ion ger tran smissi on lin e.) By poweri ng the telecom pho ne th

5、rough the cable as well as the local mains, such systems eliminate the need for a backup battery at the cables user end. Whe n required, the cable therefore carries both tran smissi on sig nals and supply curre nt (Figure 2). CENTRAL OFF GE HTl +00(： NT. 2 TINTERWCElUu INTERFACE BASIC RAHAPPLIMTION

6、砂仏LLPINE闕 NON-ISDN SHALL OmCE EflU|PWfHTTEHTns|hTEfiFACE mIC ftTTEAPPUCATION (SM4LL NOME) CUSTOMEP PREMIA 丁 4 INTERFACE CENWL OFF 1屛则當 订 h INTERUCE CE HQUSE Figure 2. In an ISDN system for small-office or home applications, the phone lines no rmally carry forward tran smissi ons. During emerge ncies

7、, they carry power from the cen tral office to the remote un its n etwork term in ati on (NT) and termi nal equipme nt (TE). The select ion of power-c on versi on topology depe nds on the level of output power required: forward or flyback for low power, push-pull for medium power, and half or full b

8、ridge for high power. The simplest configuration (flyback) stores energy in the tran sformer duri ng the ON period of the primary PWM (whe n the power MOSFET is ON), and it releases it to the load during the OFF period (Figure 3). Because the sec on dary diode (D1) is forward-biased at that time, en

9、 ergy stored in the tran sformer goes to the load and charges the output capacitor. Energy stored in the output capacitor is delivered to the load during the next ON period. For this configuration, the primary-c on troller IC can have a fixed-freque ncy PWM con figurati on such as the MAX668 or a va

10、riable-freque ncy PFM con figuration such as the MAX1771. A con troller like the MAX5003 can be powered directly from the high in put voltage. MJOC66 FBPGND REF FREC GND V|H =tw Figure 3. A flyback conv erter stores en ergy duri ng every cycle: in the tran sformer when the power MOSFET is ON and for

11、ward to the load whe n the power MOSFET is OFF. In the forward topology (Figure 4), en ergy is forwarded to the sec on dary whe n the primarys power MOSFET turns ON. Energy is not stored in the transformer as in a flyback circuit, so the forward con figuratio n allows a smaller tran sformer and more

12、 power output. An output in ductor on the sec on dary side stores en ergy and also reduces ripple current in the output capacitor. During an ON period, the load receives en ergy directly from the tran sformer trough diode D1 and L1; however, duri ng an OFF period, in ductor L1 forces diode D2 to con

13、 duct. Thus, L1 and C1 together maintain a continu ous delivery of en ergy to the load. Mlu IRF7693 D D1 L1 NIBROJOOL 47|lH mm. Cl 21 0：10V MBRC74CL 4即 RETURN SMDN mi MjDOM 飞 MAX56 otherwise, the creepage dista nee is reduced to less tha n 2mm. r INULATINQ FfiFF IMSULAUNCi WBF NS Ul ATI NG OF PRIMMT

14、 second nr Figure 8. The in sulati on in this cross secti on of a wound tran sformer con sists of sleeving and insulating tape. Because sleeves are defined as insulation, this approach lets you reduce the safety dista nces by a factor of two. Additi onal in sulat ing material is required betwee n th

15、e primary and the sec on dary windin gs. If rein forced or double in sulati on is n eeded, there must be at least two layers of insulation. Because it allows good coupling between windings, this product ion tech no logy mini mizes leakage in ducta nee and is useful in con struct ing tran sformers wi

16、th multiple sec on dary outputs. It also allows a good compromise on delivered watts per unit volume of tran sformer. Because protect ion sleeves cannot be placed automatically, the manual work required makes this tech nique quite expe nsive. 2. Ano ther method en tails providi ng in sulati on by pl

17、ac ing the primary windings close to one side of the coil former and the sec on dary wi ndi ngs on the other side (Figure 9). Dista nee is kept at 4mm, but coupli ng betwee n windings is very poor. This approach is almost impossible for small tran sformers. Figure 9. This is a cross section of a wou

18、nd transformer, in which the insulation is provided by insulating tape (enameled copper is not defined as insulation). Safety regulati ons man date the minimum dista nces betwee n windin gs. 3. The use of special wire for the sec on dary winding, covered by two layers of insulating material (recogni

19、zed by UL as double insulation), can be a solution for small-geometry tran sformers. This approach can be automated up to 100%. On the other hand, the high cost of the wire required makes it less attractive. 4. A coil former con sisti ng of two concen tric half sect ions in which all the primaries a

20、re contained in one sect ion and all the sec on dary wi ndings in the other allows efficie nt assembly of the tran sformer. The two concen tric parts are first wound e y) separately (primary windings in one, sec on dary windings in the other) and the n joined in a sec ond operatio n. This process ca

21、n be automated completely. It reduces manual re-work almost to zero and yields the least expe nsive tran sformer of its type availab today. Coupli ng betwee n primary and sec on dary is poor, however, and (unfortun ate the split-primary tran sformer un der exam in ati on cannot be built this way. Th

22、is approach is of great in terest for low-power tran sformers in flyback topologies that operate from the 120VAC mains (a cellular-pho ne battery charger, for example). Mini mizi ng mecha ni cal dime nsions is a com mon requireme nt for telecom conv erters in PC-board applicati ons. Because safety s

23、pecificatio ns defi ne a mini mal creepage distance related to the pollution degree of the external environment, you can improve the pollution degree and reduce creepage distance by placing the transformer in a box filled with res in, un der vacuum. By modifyi ng the tran sformers in ternal en vir o

24、nment this way, a pollution degree of 1 is easily achievable (refer to Table 4). Transient-Voltage Protection Virtually all electrical equipme nt is subject to excessive voltage pulses duri ng no rmal operati on. Such pulses can be gen erated by light ning or by n earby electrical equipme nt such as

25、 large electrical motors. I nternatio nal specificati ons EN61000-4-5 and EN41003 define the pulse types that certified equipment must be able to withsta nd. In telecom power supplies operati ng from rectified voltage, a 1.5kW tran sie nt-voltage suppressor (TVS) is gen erally sufficie nt to protect

26、 the supply and meet all the intern ati onal specificati ons n ecessary for CE approval. More complex is the protectio n of an RS232 or RS485 in terface used for maintenance and com muni cati ons in side the end equipme nt. It can be expe nsive to place a TVS on each line, especially if the lines mu

27、st maintain low parasitic capacitance to support high transmission data rates. A wide choice of RS-232 and RS-485 in terface ICs for this purpose are available from Maxim. All include ESD protection tested per IEC1000 specifications at levels up to 5kV. That spec en sures an in terface complia nt wi

28、th the CE requireme nts, without n eed for further tests in product ion. Output Overcurrent and Overvoltage Protection Output protect ion preve nts damage to the power supply as a con seque nce of load currents ranging from zero to that of a short-circuited output, as well as damage to the load as a

29、 result of irregular supply voltage caused by such faults. The following is an exam in ati on of two circuits: one to protect the power supply from unu sual load curre nts and the other to protect the load from unu sual output voltages. Telecom power supplies require a con sta nt output-curre nt cha

30、racteristic. Whe n load curre nt exceeds a certa in level, gen erally 20% to 40% above the nominal value (Ino m), the conv erter output becomes a con sta nt-curre nt gen erator. For that con diti on, reduc ing load resista nce only lowers the output voltage. In terms of power man ageme nt, the worst

31、-case con diti on for such telecom supplies is the worst-case maximum load curre nt (Inom plus 40%), which occurs whe n the output voltage is still regulated. Delivered power is 40% above nominal and can persist indefinitely. The entire power supply must be capable of withstanding this level of outp

32、ut power. Because the n ecessary over specificati on can affect mecha ni cal dime nsions and price of the heats ink, a system can ben efit by restrict ing the allowed tolera nce on output curre nt. Maxim offers a wide choice of high-side precisi on curre nt mon itors for this purpose, ra nging from

33、the simple MAX4173 in a SOT23 package (Figure 10) to the MAX471, which can also detect the curre nt flow direct ion. Figure 10. In high-side current measurements, the MAX4173 provides a voltage referred to ground and proportional to current flowing in the sense resistor. These ICs employ a resistor

34、in series with the load current that drops only a few millivolts, thanks to an internal current mirror that produces, in a resistor refereneed to ground, an analog voltage proportional to the high-side load current. For the MAX471, this measurement has an accuracy of 2%. A simple op amp such as the

35、MAX4040 can act on this sig nal to gen erate curre nt-protectio n feedback that acts directly in the converters voltage feedback loop. It affects the feedback loop only whe n output-curre nt protect ion is inv oked. Although power-supply stability for that condition must also account for the current

36、-protection circuits response time, stability is seldom compromised because the required resp onse time is not fast. Multiple outputs (ma in plus auxiliary voltages) can be protected by devices such as the MAX869L and the MAX890L. In addition to curre nt-measureme nt circuitry, these ICs include a p

37、ower MOSFET in series with the measured load current, which can disc onn ect the load in resp onse to overcurre nt con diti ons. All these curre nt measureme nts are performed on the positive rail, i.e., on the output. The alter native (measuri ng on the n egative rail, or the com mon of the power s

38、upply) in cludes all curre nt returned by the system. For a three-output system, for example, this means that every output must be desig ned to withsta nd all the curre nt. Such n egative-path loops in a complex system can actually produce wrong curre nt measureme nts. Because any failure withi n th

39、e conv erter can cause an out-of-spec output voltage with a catastrophic effect on the load, most systems include a simple circuit that monitors the output voltage continuously and stops the converter in case of an over- or un dervoltage con diti on. Maxim offers a wide choice of such voltage mon it

40、ors, such as the MAX6806. When the mon itor reports a voltage failure via a protect ion loop, it must first stop the primary IC con troller. If the failure persists, it must also (through a driver circuit) turn on an SCR connected in parallel with the converters output. Once the SCR turns on, it can

41、 exit from the ON latching condition only when its current flow falls to zero. Not all load circuits like to endure the dv/dt of a converter output as it rises from zero to nominal duri ng tur n-on. Most prefer to rema in in a reset con diti on un til the output voltage stabilizes. Maxim is the worl

42、dwide leader in generating such resets during startup. Simple ICs like the MAX809 or the MAX6351, for example, available in SOT23 packages and able to monitor 1 or more voltages, provide a Power OK signal with delay (typically 140mS). They also advise the system, with considerable precision, to save

43、 all necessary data before a declining supply voltage disappears completely. Operating Temperature, Flammability, and Thermal Protection UL 94VO, V1 or V2, defi nes limits for the maximum temperature in side an electrical system. Tests performed by qualified pers onnel in a certified laboratory must

44、 verify compliance before a UL stamp on the system can be approved. Thus, the thermal-ma nageme nt aspect of power-supply desig n should en sure, for the sake of system reliability, that the temperature of any hot spot caused by worst-case conditions remain within specified limits. The more critical

45、 comp onents in a power conv erter are gen erally the primary power switches, the sec on dary rectifier diodes, and the isolati ng tran sformer. Good thermal coupli ng betwee n the active comp onents and a heats ink is desirable, uni ess the in creased parasitic capacita nce betwee n the switch elem

46、e nt and the heats ink exacerbates EMI. High-temperature wire and insulating material per UL specifications ensures a high operati ng temperature for the isolati on tran sformer. All materials must have a self-extinguish rating per UL94V0, 94V1; the use of such materials simplifies final UL recognit

47、ion for the system. Even if the thermal operating temperature of a telecom system is defi ned by specificati on, an over- or un dertemperature con diti on duri ng no rmal operati on (ma inly over-) can have a catastrophic effect on the power supply. Barring immediate failure, the temperature fault c

48、an reduce equipment life by loweri ng the MTBF of comp onents stressed by the hot temperature. To avoid irreversible damage, thermal protecti on should in clude a double threshold: a logic alarm to advise that temperature is going out of specificati on and a sec ond alarm (with a higher threshold) t

49、hat shuts dow n the con verter. Thermal protectio n must be placed close to the most critical component; that component is identified easily in a thermal map produced with an in frared camera. Maxims MAX6501/6502 thermal sen sors mon itor the ambie nt temperature with high accuracy and provide a log

50、ic sig nal whe n the threshold is reached. Whe n compared with the traditi onal thermal switch, their high precisi on, semic on ductor reliability, small mecha ni cal dime nsions (SOT23 packages), and low prices make them very attractive. MAX6503/6504 sen sors for n egative temperatures can advise w

51、he n the temperature is lower than a specified minimum value; this is a condition that can affect the converters ability to start up. An undertemperature logic signal can turn on a heat gen erator. Feedback The regulated output of a telecom power supply is gen erally tight (5%), but in some cases gr

52、eater accuracy is required. In flue nces that must be opposed in mai ntai ning tight output accuracy in clude the effects of load variatio n (load regulatio n), li ne-voltage variati on (li ne regulati on), and temperature. Thus, the con troller on the primary side must receive feedback from the reg

53、ulated output voltage on the sec on dary side. To main tai n the in here nt isolati on betwee n primary and sec on dary windin gs, this feedback sig nal should be isolated as well. Three circuits are useful for this fun cti on: Feedback via auxiliary tran sformer windings Feedback via optocouplers F

54、eedback via extra mag netic parts, which allow com muni cati on betwee n the primary and the sec on dary circuits Feedback based on an auxiliary tran sformer wi ndi ng is very simple. The auxiliary-winding voltage, which usually supplies the primary side of the converter, is mon itored by the primar

55、y con troller. Coupli ng betwee n sec on dary and auxiliary windings cannot be very tight, because safety dema nds creepage and cleara nee, and double or more insulation between them. As a result, the typical tolerance on an auxiliary-winding voltage is only 10%. Some loads can tolerate this loose t

56、olerance; others employ post-regulati on in the form of a synchronous step-dow n conv erter such as the MAX1623 or the MAX1714. Major adva ntages for the auxiliary-wi ndi ng approach in clude small size and a low-cost con trol circuit. To obta in optocoupler feedback, an op amp on the sec on dary si

57、de (such as the MAX4122) continu ously compares the output voltage with that gen erated by a voltage refere nce (such as the MAX6002). The op-amp output the n drives the optocoupler diode, produci ng a feedback sig nal from the optocoupler proporti onal to the error differe nce betwee n output and r

58、efere nce voltages. That is, a curre nt tran sfer betwee n the diode and the tran sistor (isolated from each other) produces a curre nt output on the primary side. Flow ing through a resistor, this curre nt produces the signal voltage read by the primary-side controller (Figure 3). By offeri ng a wi

59、de choice of optocouplers already approved for this function by the safety age ncy, todays market aids safety qualificatio n of the power supply. To further simplify procureme nt, all comp onents in this circuit are sta ndard parts. For feedback using extra magn etic parts (which allow com muni cati

60、 on betwee n the primary and the sec on dary circuits), the prin ciple is the same as for an optocoupler. The sec on dary circuit is somewhat complex in this case, because the magn etic parts must be drive n by a sig nal of a certa in freque ncy rather tha n a DC curre nt. The result ing extra expe