ST AN1606 Application note handbook
Contents
1. used that opens the circuit during the off time For the L4981 controller the off time is guaranteed not to be less than 5 of the period Q1 can be a small signal transistor because its switched current is low due to the fact that the transformer secondary will have a large number of turns To realize the current sensing transformer a high permeability toroidal core ur gt 5000 has been used The secondary has 50 turns as a compromise to reduce secondary current yet not require a large number of turns al 5 18 AN1606 APPLICATION NOTE Fig 6b M2_D2 Chopping Phase M1_D1 Chopping Phase Other control circuits Input voltage sensing in the standard boost topology the rectified input voltage waveform is sensed using a re sistor that by one internal circuit delivers the mirrored signal to one of the multiplier s inputs lac pin4 For the bridgeless configuration see the circuit shown in fig 7 al 6 18 AN1606 APPLICATION NOTE Figure 7 F Coupled Inductor CURRENT MIRROR mm Input voltage sensing A and equivalent circuit B It is based on the following consideration the frequency of the signal of interest tens of Hz is much lower than the switching frequency tens of kHz The boost inductor for the low frequency behaves like a short circuit Since the Powermos s drains are in turns close to ground via the body diode the resulting equiva2. sensing resistor in the return of the current to the bridge as shown in Figure5a TA 3 18 AN1606 APPLICATION NOTE The L4981A B current loop is designed to handle this negative signal This type of resistor current sense can easily be achieve in medium power applications For high power PFC circuits it is necessary to use a magnetic current transformer for improved efficiency as shown in Figure 5b In the bridgeless PFC configuration since an input rectifier bridge is not used the current is continuously chang ing its direction and the complexity of current sensing with a simple resistor can increase Also in high power applications resistor sensing may dissipate too much power In these cases current sensing with a current transformer is the preferred approach A current sense transformer core is typically high permeability ferrite toroidal or a small core set The primary of the transformer is a single turn of wire through the core The secondary typically consists of 50 to 100 turns Figure 5 AA Inductor eee Standard Sensing IL gt Fig 5b Magnetic sensing for high power AA Inductor Typical sense transformers This type of sense transformer cannot operate at low frequency and for this reason it must be connected where the current is switched at high frequency The magnetic core must be allowed reset This is normally accomplished by using a diode In order to reproduce the induc
3. SCHEMATHIC DIAGRAM D D D D DETAIL for L2 Lon gt gt L 2c la 9 B O M for 600W version Bridgeless Evaluation Circuit Mome Vme Veme Lae Nome Tae Name Vaive Wome Tae Cr fee e oe w e w oaf e fere e ee e ee ens e e ee f e fae Cenn 0e fennas me eaaa e o e HS E E C fre OOM eT ee ee L2 1 50450 sense transformer L3 1 50 Sense transformer Ferrites Hy_perm Diam 20mm D123 4Dg911 1213 1N4148 Dz1 1N4746 Qs5 BS170 TA 15 18 AN1606 APPLICATION NOTE L1 440 uH gt E55 28 21 21 21 turns 2 5mm gapped 14 wires m 0 4 mm each D5 6 STTH8ROG6FP Q2 3 STW26NM50F Note For the evaluation circuit and external coupled inductor EMC where utilized The filter has been achieved as follows Coupled inductor 30 30 turns wire diameter 0 8mm on a toroidal 40x16x8 5 mm Magnetizing inductance each half inductor Lm 8mH and a leakage inductance Ld 50uH Conclusion The innovative bridgeless PFC configuration as described in this application note has been successfully tested Details have been presented how to implement the technology which should prove interesting to designers Figure 12 shows the test results of efficiency and power dissipation for the application s 800W prototype Figure 12 EFFICIENCY B Less Standard PFC SI 16 18 AN1606 APPLICATION NOTE Evaluation results for the 800W version Vin 110Vac Nominal po
4. d a pair of application s size For evaluation porpoise it has been realized a printed circuit Let us beginnes with and 800W P F C application 800W Target 1 Wide range input voltage variation 110Vrms to 220Vrms 2 Output power 800W 2 Output voltage 400Vdc A switching frequency of 50 kHz has been chosen as a good compromise between the coil size and the pow erMOS switching losses Boost Inductor design To design the boost inductor the parameters under consideration are the percentage current ripple as low as possible and the cost of the bobbin This portion of the design is the same as for the standard topology In this application in place of a single inductor connected to one of the phases it has been chosen to split the inductor into two sections two windings on the same core as shown in the connection diagram at fig 9 al 9 18 AN1606 APPLICATION NOTE Figure 9 Connection Diagram for the Coupled Inductor Inductor Equivalent circuit Realizing the inductor in this manner improves common mode rejection and avoids the effect of the difference between drain capacitance of the PowerMOSFETs In order to simplify the model assume a near unity coupling factor and the equivalent circuit is shown in Figure 9b The inductance is proportional to the square of the number of turns For the two windings it will be f N 5 and x ZN N N total 5 5 The required number of turns for a given inductance on t
5. e heat in limited surface area The dissipated power is important from an efficiency point of view Figure 2 Inductor controller The bridgeless configuration topology presented in this paper avoids the need for the rectifier input bridge yet maintains the classic boost topology This is easily done by making use of the intrinsic body diode connected between drain and source of PowerMOS switches A simplified schematic of the bridgeless PFC configuration is shown in Figure 3 Figure 3 Inductor Controller SI 2 18 AN1606 APPLICATION NOTE The circuit shown from a functional point of view is similar to the common boost converter In the traditional to pology current flows through two of the bridge diodes in series In the bridgeless PFC configuration current flows through only one diode with the PowerMOS providing the return path To analyze the circuit operation it is necessary to separate it into two sections The first section operates as the boost stage and the second section operates as the return path for the AC input signal Referring to Figure 4 the left side Figure 4a shows current flow during the positive half cycle and the right side Figure 4b shows current flow during the negative half cycle Figure 4 Positive half cycle Negative half cycle return uc controller controller Positive HALF Cycle When the AC i
6. he same core is the same as it is for one winding or two windings The only difference is that the two windings are separated into two sections For simplicity we can design the coupled inductor using the same criteria as for a standard inductor core size number of turns and size of copper wire For the core the preferred design is a gapped ferrite core set The size of the core can be chosen considering the maximum current Ipk that for the 800W target s parameters can exceed 14A placing lpk 15A 2 3 9 Voore Veore min K L I peck mm Where K 1 4 104 core Igap For the 800W application the nominal current ripple has been chosen around 25 This fixes the boost induc tance value L 450uUH 10 18 G AN1606 APPLICATION NOTE The coil requirements can be met using a gapped core set type E66 33 27 characterized with the following key parameters Ae 550mm 3 Icore 146mm m Core gt 1600 Vcore 80 4 10 3mm 3 The air gap needed to avoid saturation and optimize the coil size is equal to lap 3mm Using the parameters in the formula g Vcore gt 67 5mm 3 This result confirms the core is well above the minimum size The used formula for the number of turnes needed to design the total required inductance L is h N L _ l core 1gap core A 2 Ho A t9ap The resulting N 38 in our solution has been realized with 19 turns 19 turns In order to minimize the high frequency l
7. icroelectronics All Rights Reserved STMicroelectronics GROUP OF COMPANIES Australia Brazil Canada China Finland France Germany Hong Kong India Israel Italy Japan Malaysia Malta Morocco Singapore Spain Sweden Switzerland United Kingdom United States http www st com 18 18 G
8. lent circuit is shown in fig7b The relation between the voltage from the inductor and the current that flows in to lac pin is 1 1 a Y 9 paa Trst Where Req R1 2 R2 and t m c1 1 2 m t The net introduces one pole at fp The pole must be located at a frequency high enough not to distort the input waveform and at the same time low enough to filter the switching frequency In this application the equivalent resistance has been choosen Req 324 k that fits well with the current amplifier design The resulting R1 is 300k and R2 is 12kQ The pole has been placed a decade before the switching frequency Fp 5kHz that gives b Ci 1 2 87nF 2 n fp Bure In practice in our test a standard value of 2 7 nF as ben used ky 718 AN1606 APPLICATION NOTE Voltage feed forward Voltage feed forward a useful function in wide range applications requires a DC voltage proportional to the rms value of the input mains For the L4981 this value must be between 1 5V to 5 5V so that it can mirror over a wide range Since the rectified mains frequency is 100 120Hz we need a large rejection for this frequency and because the feed forward reaction time is proportional to the bandwidth we introduce a second order filter that allows good compromise between attenuation of the fundamental frequency and response time The circuit Fig 8 is similar to the fig 7 described earlier Figure 8 Coupled Induc
9. nput voltage goes positive the gate of M1 is driven high and current flows from the input through the inductor storing energy When M1 turns off energy in the inductor is released as current flows through D1 through the load and returns through the body diode of M2 back to the input mains See Figure 4A During the off time the current throw the inductor L that during this time discharges its energy flows in to the boost diode D1 and close the circuit through the load Negative HALF Cycle During the negative half cycle circuit operation is mirrored as shown in Figure 4B M2 turns on current flows through the inductor storing energy When M2 turns off energy is released as current flows through D2 through the load and back to the mains through the body diode of M1 Note that the two PowerMOSFETs are driven synchronously It doesn t matter whether the sections are per forming as an active boost or as a path for the current to return In either case there is benefit of lower power dissipation when current flows through the PowerMOSFETs during the return phase Current Sensing The PFC function requires controlling the current drawn from the mains and shaping it like the input voltage waveform To accomplish this it is necessary to sense the current and feed its signal to the control circuit In average current conventional boost topology we sense the rectified current rather than the AC input current This can be achieved by a simple
10. osses the winding has been made using the multiple wire approach It is possible to estimate the losses for a low frequency current Imposing a maximum power value to be dissipated in the copper Pcu 5W Pe 2 wire i Pwire Hic laus max lt SW Rpc lt 2 60mQ leas max Using the formula for multiple wires N l Roo Pou are lt 60mQ d M 4 Were In practice 20 wires were used each having a diameter d 0 4 mm Output Capacitor filter For the bulk capacitor selection we consider a reasonable 100Hz voltage ripple GU 11 18 AN1606 APPLICATION NOTE Po m Co za 2t Alo Vo were f is the input frequency Imposing lt 10Vac the peak of voltage variation over 400Vo the Co value will results gt 318uF the commercial value is 330uF Power Devices The selection of the power devices is dependent upon the topology and the size of the application Operating in continuous current mode fast reverse recovery diodes are needed The TURBOSWITCH STM family in the 600V voltage range offers a very good solution for the two boost diodes the STTH8RO6FP has been chosen The insulated TO 220 package makes it easy to assemble the parts on a heat sinke Concerning the Powermos requirements a 500V blocking voltage Bvdss is needed for this application The chip selection is more complex To find the best solution it must be considered all the parameters that affect the power dissipation and to compa
11. re the results in terms of a cost to benefit ratio The devices used in the 800W application 2 2 are the type STY384NB50F The four Powermos are efficiently driven without any additional buffer thanks to the smart characteristics of the integrated driver Figure 10 800W SCHEMATHIC DIAGRAM DETAIL for L2 ty gt S K La L 9 SI 12 18 AN1606 APPLICATION NOTE B O M for 800W Bridgeless Evaluation Circuit rene ae name ete Nene ae Wane ave one var Pm fee ten om me vem r won me oe ee erm e L wm Re ee e Era L2 1 50450 sense transformer L3 1 50 sense transformer Ferrites Hy_perm Diam 20mm D123 D7 8 9 10 11 12 13 1N4148 Dz4 1N4746 Qs5 BS170 L1 450 uH gt E66 33 27 18 18 turns 3mm gapped 20 wires m 0 4 mm each Dee STTH8RO6FP Q1 2 3 4 STY34NB50F Note For the evaluation circuit and external coupled inductor EMC where utilized The filter has been achieved as follows Coupled inductor 30 30 turns wire diameter 0 8mm on a toroidal 40x17x9 mm Magnetizing inductance each half inductor Ln 8mH and a leakage inductance Ld 50uH Scaling down the application As a second step based on the same circuit a 600W P F C has been built The target specification designed for server application is 600W Target 1 Wide range input mains 110Vrms to 220Vrms 2 O
12. tor Vip t e hoo Voltage feed forward Circuit A and equivalent circuit B Defining HLp s the transfer functions between the voltage from the inductor and the voltage at the output of the filter vLp Fig 8B we have the following relation 1 R S Hip Rss Kup Ra 2Rb 2R0 The time constants cannot be expressed in simple way and so that the position of poles can be numerically cal culated The constant Kip is defined taking in to account the wide range that is V_mains is between 88V and 264V 2 2 d Vip VRMS Kip Choosing to calculate this value at the midpoint of the allowed values 2 2 88 264 _ 1545 5 e R Kesa SI 8 18 AN1606 APPLICATION NOTE To fit this it has been choosen Ra 998kQ 2 499 Rb 150kQ Re 30kQ For the capacitors we set 80 dB of attenuation on the fundamental frequency using the commercial values te en Cb 470nF The design places two poles at 3Hz and 14Hz and 80 dB of attenuation at 100Hz Practical examples The preceeding points of this note have described the topology peculiarity Remainder of the topics for PFC design are similar to standard P F C boost applications based on L4981A B see the related references and application notes Starting from now we can refer to real design examples In fact in order to verify the efficacy of the described configuration it have been checke
13. tor s current in boost topology two of magnetic sense sections are needed and the simplified schematic is shown in figure 5b When the sense transformer solution is applied in the bridgeless topology the simple sense as in fig5b is no longer valid 4 18 GU AN1606 APPLICATION NOTE The circuitry is more complex than in the boost case because here we have two pair of PowerMOS M1 M2 and diodes D1 D2 alternating It is necessary to sense the chopping current of the PowerMOS diode section and to sum the signals to be applied to Rs The sensing of the diode s current can be simply done by placing a magnetic sensor at the common cathode L2 in fig 6 Only one of the two diodes operates each half input cycle Figure 6 For the PowerMOSFET portion of the circuit the complexity increases because during the half cycle when one of the PowerMOSFETs is chopping the other one has to handle the current flowing back to the mains Using the configuration of sensors as shown in Figure 6 it is possible to solve the problem without undue com plexity The unnecessary high frequency portion of the current signal is cancelled because of the method M1 is connected to L1A as shown in Figure 6b The problem due to the change of polarity during each half cycle is solved by using a center tapped secondary and two rectifiers Since the coupling of the two windings must not permit the demagnetization of L1 an auxiliary transistor Q1 is
14. utput power 600W 2 Output voltage 400Vdc The switching frequency has been set at 75kHz to use a reduced size and high performance PowerMOS ki 13 18 AN1606 APPLICATION NOTE Boost Inductor design The boost inductor has been design as been previously described For the 600W application the nominal current ripple has been set around 22 gives requires the inductance value L 440uH The inductor requirements can be met using the core set type E55 28 21 characterized with the following key parameters A 357mm 2 Icore 123mm m_core gt 1600 Vcore 43 7mm 3 The needed air gap is lan 2 5mm Using the relation g Vcore gt 38 8mm 3 The result confirms that the core is good enough Using the relation h the resulting N 42 For the 600W the coil has been realized with 21 turns 21 turns For minimize the high frequency losses the multiple wire solution has been used Imposing the copper losses Pcu 3 8W it has been used 14 wires having a diameter d 0 4 mm each Output Capacitor For the selection of Co the relation as been described in m The commercial value 330uF 450V used for the 800W application is still good for the 600W application Power Devices For the two boost diodes as for the 800W application the STTH8RO6FP has been used Concerning the Powermos the devices used in the 600W version application are two STW26NMB50F SI 14 18 AN1606 APPLICATION NOTE Figure 11 600W
15. wa AN1606 YZ APPLICATION NOTE A BRIDGELESS P F C CONFIGURATION BASED ON L4981 P F C CONTROLLER by Ugo Moriconi This technical document describes an innovative topology dedicated to a medium to high power PFC stage The originality of this topology is the absence of the bridge that usually is placed between the EMC filter and the PFC stage The advantages of this topology can be found in terms of increased ef ficiency and improved thermal management L4981 PFC Controller This application features the L4981 PFC controller It is a high performance device operating in average current mode with many on chip functions The driver output stage can deliver 1 5A which is very important for this type of application A detailed device description can be found in AN628 A functional block diagram is shown in Figure 1 Figure 1 Functional Diagram LFF MULT OUT ISENSE CA NUT COSC ROSC SYNC November 2002 1 18 AN1606 APPLICATION NOTE Description of Bridgeless PFC Configuration Topology The conventional boost topology is the most efficient for PFC applications It uses a dedicated diode bridge to rectify the AC input voltage to DC which is then followed by the boost section See Figure 2 This approach is good for a low to medium power range As the power level increases the diode bridge begins to become an important part of the application and it is necessary for the designer to deal with the problem of how to dissipate th
16. wer 4 Vin 220Vac Nominal power Evaluation results for the 600W version Vin 110Vac Nominal power Vin 220Vac Nominal power 395VDC 652W 624W 0 994 fp 8 96 5 References a Parsad N Enjeti R Martinez A high performance single phase AC to DC rectifier with input power factor correction IEEE APEC 93 b Alexandre Ferrari de Souza and Ivo Barbi A new ZVS Semi resonant High Power Factor Rectifier with Re duced Conduction Losses IEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS VOL 46 NO 1 FEBRUARY 1999 c STM Application Notes AN628 AN824 STMicroelectronics www st com http ccd sgp st com stonline books index htm al 17 18 AN1606 APPLICATION NOTE Information furnished is believed to be accurate and reliable However STMicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics Specifications mentioned in this publication are subject to change without notice This publication supersedes and replaces all information previously supplied STMicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics The ST logo is a registered trademark of STMicroelectronics 2002 STM
Download Pdf Manuals
Related Search
ST AN1606 Application note handbook