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Adv. Radio Sci., 9, 317–321, 2011 www.adv-radio-sci.net/9/317/2011/ doi:10.5194/ars-9-317-2011 © Author(s) 2011. CC Attribution 3.0 License. Advances in Radio Science Reduction of heat sink common-mode currents in switching mode power supply circuits J. Kulanayagam, J. H. Hagmann, K. F. Hoffmann, and S. Dickmann Helmut-Schmidt-University/University of the Federal Armed Forces Hamburg, Germany Abstract. In this paper, a new filter design for a heat sink is presented. The parasitic couplings between electric power devices and the heat sink are responsible for common-mode currents. The main focus is on the reduction of these currents to reduce the heat sink radiation. For this purpose a new filter design is proposed. In addition, experimental results are shown to validate the proposed filter. 1 at different frequencies depending on the heat sink geometry (Brench, 1994), (Das and Roy, 1998). In particular, the heat sink can become an efficient antenna when resonant phenomena occur (Damiano et al., 2004). This paper investigates the origin of common-mode currents generated by SMPS and shows a filter design to reduce common-mode currents of ungrounded heat sinks and an experimental setup for validating the input impedance and the heat sink radiation. Introduction In general, power-switching semiconductors (MOSFET’s/IGBT’s) are mounted on heat sinks to keep the power semiconductors within a given temperature range. A parasitic capacitance can be formed between the case of the semiconductors and the heat sink. The insulating thermal compound (Damiano et al., 2004) is the dielectric of this capacitance. It is shown as isolator 1 in Fig. 1. Common-mode EMI is usually the result of parasitic effects (Tihanyi, 1995). Due to these capacitances commonmode currents are generated in Switching Mode Power Supply (SMPS) circuits. In consequence, these currents could violate the EMC standards by their radiation. Therefore, the heat sink should be well designed. The drain-source voltage variation increases by decreasing the commutation time of the transistor and by increasing the insulation voltage level. Consequently, a high number of harmonic components are generated which makes the heat sink an EMI generator (Sinclair et al., 1993). The shape and the dimension of the heat sinks play a major role in the power electronic system design. In general, as the size of the heat sink increases, the radiation efficiency also increases. However, the maximum radiation will take place Correspondence to: J. Kulanayagam ([email protected]) 2 EMI generator and heat sink radiation A parasitic capacitance Cdh (see Fig. 2) exists between the metal case of the semiconductors and the heat sink. The insulating thermal compound acts as a dielectric for this capacitance. Due to this capacitance and the time varying drainsource voltage a noise current inoise can be calculated according to the following equation, as a first approach for low frequencies and Cdh Chp Chg inoise (t) = Cdh · duds , dt (1) where duds /dt is the time derivative of the drain-source voltage. Cdh can reach approximately 100 pF (Tihanyi, 1995). This noise current inoise is split into the conducted EMI current ic and the radiated EMI current ih inoise (t) = ih (t) + ic (t)· (2) The noise current paths through the heat sink in AC/DC converter are shown in Fig. 2 as an example. In order to reduce the radiated emissions of the heat sink, it may be grounded. Consequently, the conducted EMI current ic raises in the power supply (Tihanyi, 1995). The results of the previous works (Li et al., 1993; Radhakrishnan et al., 2000; Archambeault et al., 2001), have proved that grounding a heat sink can reduce radiated emissions. Published by Copernicus Publications on behalf of the URSI Landesausschuss in der Bundesrepublik Deutschland e.V. heat sink current. If the heat sink is not grounded, the conducted EMI current ic can be reduced but the radiated EMI J. Kulanayagam et al.: Reduction of heat sink common-mode current ih to the SMPS raises (Felic and Evans,currents 2001). nd S. Dickmann Armed Forces 318Hamburg sink ower mode rents new esults Heat sink 2 Cdh Heat sink J.Kulanayagam et al.: Reduction of Heat Sink Common-Mode Currents in Switching Mode Power Supply Circuits Isolator 1 udsthe capacitance can be minimized. This screen is apdevice cording to the following equation, as a first approach for low plied as aCurrents shield forinthe heat sink. Furthermore, the sides and Cdh >>etCal.: Chg hp >> 2 frequencies J.Kulanayagam Reduction of Heat Sink Common-Mode Switching Mode Power C hp Supply Circuits of the screen are insulated with a well thermally conductdu MOSFET/IGBT inoise (t) = Cdh · ds · ing dielectric material. This screening design is illustrated in (1) dt approach for low device capacitance can be minimized. cording to the following equation, as a first Fig. 4. the In this work the radiated EMI currentThis loop screen (see theis applied as a shield for the heat sink. Furthermore, the sides frequencies and Cdh >>ofCthe Chg hp >> The time derivative drain-source voltage is duds /dt. PCB of the screen are insulated with a well thermally conductCdh can reach approximately 100 pF (Tihanyi, 1995). This MOSIsolator 2 duds sink This screening design is illustrated in noise currentPCB inoise is(t) split EMI current(1) ic ing dielectric Heat material. inoise =into Cdhthe · conducted · p the dt Fig. 3. Radiated EMI current via the heat sink. and the radiated EMI current ih Fig. 4. In EMI this work the radiated ange. Fig. 3. Radiated current via the EMI heat current sink loop (see the case ating ectric g. 1. ffects mode upply could e, the The time derivative of the drain-source voltage is duds /dt. inoise (t) = ih (t) + ic (t)· (2) Isolator 2 Cdh can reach approximately 100 pF (Tihanyi, 1995). This Heat sink The current paths through heat sink in AC/DC conefficient method Fig. 1.noise of parasitic capacitance noise current inoise isrepresentaion split into the conducted EMI current iAn c Screen to reduce radiated emissions of a heat Fig. 1.Schematic Schematic representaion of the parasitic capacitance. verter are shown Fig. 2 as and the radiated EMIincurrent ih an example. In order to reduce sink was introduced Isolatorin 1 a previous study (Nagel, 1999). By in- Heat sink serting a copper screen the heat sink and the inoise (t) = ih (t) + icon (t)·the (2) Cdhheat sink geometry at different frequencies Isolator between 2 Rectifier depending L inoise 1994), (Dasthrough and Roy, The(Brench, noise current paths the1998). heat sink in AC/DC conMOSFET/IGBTScreen C C x In particular, the heat sink can become efficient antenna y s verteruare shown in Fig. 2 as an example.udsInan order to reduce Isolator 1 when resonant phenomena occur (Damiano etChpal., 2004). Isolator 3 ih Heat sink This paper investigates the origin of common-mode curPCB Cdh Chg L shows a filter Cy Rectifier PCB design to reduce rents generated by SMPS and i asing noise ic MOSFET/IGBT common-mode currents of ungrounded heat sinks and an exg the us Cx Cy uinput ds perimental setup for validating the impedance and the Ground Fig. 4. Reduction of the common-mode current by screening Chp f harIsolator 3 heat sink radiation. ih sink PCB major as the also place Fig. Fig. 2. 2. Noise Noise current currentpaths pathsthrough throughthe theheat heatsink sinkininananAC/DC AC/DCconconChg Cy PCB verter verter. 2 EMI generator and heatic sink radiation the radiated emissions of the heat sink, it may be grounded. the conducted EMI current ic 2. raises in thethe Ground The noise current loops shown in2)Fig. The current AConsequently, parasitic capacitance Cdhare(see Fig. exists between power supply (Tihanyi, 1995). The results of the previous path i via C and ground represents the common-mode curc hg metal case of the semiconductors and the heat sink. The inFig. 2.works Noise current paths through the C heat sink et in al., an AC/DC con(Li et al., 1993), (Radhakrishnan 2000), (Arrent and the current path i via and the PCB is the heat h hp sulating thermal compound acts as a dielectric for this capacverterchambeault et al., haveisproved that grounding a heat sink current. the2001), heat sink not conducted itance. Due toIfthis capacitance andgrounded, the time the varying drainsink can reduce radiated emissions. EMI current i can be reduced but the radiated EMI current c source voltage a noise current i can be calculated acnoise the radiated emissions ofloops the heat sink, itagain may beFig. grounded. current are shown in 3. The to thenoise SMPS raises (Felic ih The and Evans, 2001). Consequently, conducted EMI current ic raises in athe current paththe ic method via Chg to and ground represents the commonAn efficient reduce radiated emissions of heat mode current and the current path i via C and the power supply (Tihanyi, 1995). The results of the previous h hp 1999). PCB sink was introduced in a previous study (Nagel, By inthe heat sink current. The common-mode ic (Arproworksis (Li eta al., 1993), (Radhakrishnan et sink al.,current 2000), serting copper screen between the heat and the power duces conducted EMI, while i generates radiated EMI h chambeault et al., 2001), have proved that grounding a heat device the capacitance can be minimized. This screen isvia apthe heat sink as an antenna. If the heat sink is not grounded, sink can radiated pliedreduce as a shield for emissions. the heat sink. Furthermore, the sides the conducted EMI current can bea reduced the 3. radiated The current areicshown againthermally inbutFig. The of noise the screen areloops insulated with well conductEMI current i to the SMPS raises (Felic and Evans, 2001). in h ingpath dielectric screening designthe is illustrated current ic viamaterial. Chg andThis ground represents common4. and theHeat modeFig. current current path ih via Chp and Heat the sink PCB sink Cdh Cdh In this work the radiated EMI current via current the heat isink (see is the heat sink current. The common-mode c pro3) is considered. ducesFig. conducted EMI, while ih generates radiated EMI via the heatudssink as an antenna. If the heatudssink is not grounded, Chg Chp the conducted EMI ih current ic can be reduced buticthe radiated EMI current ih to the SMPS raises (Felic and Evans, 2001). PCB Cdh Ground Heat sink Cdh Heat sink Fig. 3. Radiated EMI current (left) and conducted EMI current (right) loop Sci., 9, 317–321, 2011 Adv. Radio uds uds An efficient method a hgheat Chp to reduce radiated emissions of C ic 1999). By insink was iintroduced in a previous study (Nagel, h left hand side of Fig. 3) between the heat sink and PCB is considered. Fig.4.4.Reduction Reductionof ofthe the common-mode common-mode current Fig. currentby byscreening. screening 3 Filter design left hand of Fig. the heat sink and PCB is Due to the side extensive use3) of between digital information and commu3 Filter design considered. nication equipment, EMC problems have increased. To mitigate this ferriteuse cores usually attached and on the caDue toproblem, the extensive of are digital information commuble/ wire (see Fig. 5) to suppress electromagnetic noise emisnication equipment, EMC problems have increased. To mitisions from digital information andonFuji3 gate Filter design this problem, ferrite coresequipment are usually(Samir attached the cawara, 1999). This idea was integrated in the heat sink filter ble/ wire (see Fig. 5) to suppress electromagnetic noise emisdesign attenuate the heat radiated emissions. and The heat Due totofrom the extensive use sink of digital information commuFujisions digital information equipment (Samir and nication equipment, EMC problems have increased. To mitiwara, 1999). This idea was Filter integrated in the heat sink filter gate thisto problem, cores usuallyemissions. attached on the cadesign attenuateferrite the heat sinkare radiated ble/ The wireheat (seesink Fig.is5)extended to suppress electromagnetic emiswith a metallic ring noise bar which EMI Source EMI Receiver sions from digital information equipment (Samir and Fujihas the same material characteristics like the heat sink. Furwara, 1999). idea waspart integrated in sink the heat sinkcore filter thermore, on This the extended of the heat a ferrite is attached. The ferrite core is radiated fixed between the heat design to attenuate the heat sink emissions. Thesink heat Fig. 5. the Block diagram of theThis filterisdesign and power device. illustrated in Fig. 6. Filteris attenuated due to the ferrite The high frequency current sink is extended with a metallic ring barby which has the same core. This current can be calculated the following equamaterial characteristics like the heat sink. Furthermore, on tion : Source EMI EMI Receiver the extended part of the heat sink a ferrite core is attached. The ferrite core is fixed between the heat sink and the power device. This is illustrated in Fig. 6. The high frequency current due to ferrite core. This current can be Fig. is 5. attenuated Block diagram of the thewww.adv-radio-sci.net/9/317/2011/ filter design calculated by the following equation : sink is extended with a metallic U ring bar which has the same I = ds · (3) power . The mmone PCB c proMI via unded, diated 001). ble/ wire (see Fig. 5) to suppress electromagnetic noise emissions from digital information equipment (Samir and Fujiwara, 1999). This idea was integrated in the heat sink filter J. Kulanayagam et al.: Reduction of heat sink common-mode currents design to attenuateetthe sink radiated The heat Currents in Switching Mode Power Supply Circuits J.Kulanayagam al.:heat Reduction of Heatemissions. Sink Common-Mode J.Kulanayagam et al.: Reduction of Heat Sink Common-Mode Currents in Switching Mode Power Supply Circuits Heat sink Heat sink 319 3 3 Filter Heat sink Heat sink EMI Source EMI Receiver sink Ferrite core Ferrite core Fig. 5. Block diagram of the filter design. Fig. 5. Block diagram Isolator 1 of the filter design J.Kulanayagam Power Supply Circuits Isolator 1 Isolator et 1 al.: Reduction of Heat Sink Common-Mode Currents in Switching Mode Chg current a heat By inpower Isolator 1 sink is extended with a metallic ring bar which has the same MOSFET Heat sink MOSFET material characteristics like the heat sink. Furthermore, on the extended part of the heat sink a ferrite core is attached. The ferrite core is fixed between the heat sink and the power Isolator 2 device. This is illustrated in Fig. 6. The high frequency Isolator cur2 Ferrite core PCB PCB due to the ferrite core. This current can be rent is attenuated calculated by the Isolator 1 following equation : MOSFET Cdh Cdh Uds IhZ=h · Zh Z Heat sink MOSFET MOSFET S11 S11 Network Analyzer Network Analyzer Isolator 2 Isolator 2 Isolator 1 PCB PCB (3) Zf Zf Ih Ih Fig. 7. 7. Measurement Measurement setup setup for for the the input input impedance Fig. impedance of of the the reference reference Fig. 7. Measurement setup for the input impedance of the reference heat sink sink design. design heat MOSFET heat sink design Chp ChpIsolator 2 Uds Uds PCB Heat sink Heat sink S11 Network Analyzer Ferrite core Ferrite core Fig. 6. Schematic representation of filter design and equivalent cirFig. 6. Schematic representation of filter design and equivalent circuit cuit Cdh Zh Zf Ih I is the heat sink current and Uds is the drain-source voltage Ihh is the heat sink current and Uds is the drain-source voltage of the transistor and Z is the impedance of the network. Chp Udstransistor and Z is the impedance of the network. of the The initial heat sink setup of Fig. 1 changes to the filter The initial heat sink setup of Fig. 1 changes to the filter design of Fig. 6. An additional impedance Z has to be added design of Fig. 6. An additional impedance Zff has to be added in series to the resonant circuit. Z is the impedance of the in series to the resonant circuit. Zff is the impedance of the ferrite core. ferrite core. representation Fig. 6.Schematic Schematic representation of and equivalent cirFig. 6.This of filter filter design and equivalent cirnew design has a higher heatdesign transfer resistance than This new design has a higher heat transfer resistance than cuit. cuit the original setup, but it has a lesser heat transfer resistance the original setup, but it has a lesser heat transfer resistance than the screening design. than the screening design. Ih is the heat sink current and Uds is the drain-source voltage U ds ofI 4the transistor and setup Z is the impedance of the network. (3) · h = Experimental 4 Experimental setup Z The initial heat sink setup of Fig. 1 changes to the filter design of Fig. sink 6. An additional Zf has to be added Input impedance of the heat sink Ih4.1 is the heat current and Uimpedance is the drain-source voltage Input impedance of the ds heat sink inof4.1 series to the resonant circuit. Z is the impedance f the transistor and Z is the impedance of the network. of the ferrite core. The initial heat sink setup ofwere Fig.necessary 1 changestotoanalyze the filter Three different configurations the Threeof different configurations were necessary toto analyze the design Fig. 6. An additional impedance Zf has be added input new impedance ofhas theaheat sink designs. This design higher heat transfer resistance than input impedance of the heat sink designs. in original series to setup, the resonant circuit. Zf issink the design impedance of the the but has a lesser heat transfer The first setup is anit original heat forresistance measurThecore. first setup is an original heat sink design for measurferrite ingthe thescreening input impedance than design. as a reference. It was placed to the ing thenew input impedance as a reference. It was placed tothan the This design has a to higher heatthe transfer resistance vector network analyzer measure scattering parameters vector network analyzer to measure the scattering parameters the it hasand a lesser heat transfer setup resistance S11original (see Fig.setup, 7). Abut second third measurement were S11 (seescreening Fig. 7). Adesign. second and third measurement setup were than the made for the extended heat sink design without ferrite core for the extended heat sink design without ferrite core 4 made Experimental setup and with ferrite core, respectively. The scattering parameters and with ferrite core, respectively. The scattering parameters S11 of these setups were measured by a vector network anof these setups were by a vector network an11Input 4.1S of measured the heat sink alyzer (seeimpedance Fig. 8). From these scattering parameters of all alyzer (see Fig. 8). From these scattering parameters of all three configurations, the input impedances were calculated three configurations, the input impedances were calculated Three different configurations were necessary to analyze the input impedance of the heat sink designs. www.adv-radio-sci.net/9/317/2011/ The first setup is an original heat sink design for measuring the input impedance as a reference. It was placed to the vector network analyzer to measure the scattering parameters 3 PCB Isolator 1 Isolator 1 Isolator 2 Fig. 7. Measurement setup for the input impedance of the reference MOSFET MOSFET heat sink design Network Analyzer Network Analyzer Heat sink S11 S11 PCB core Ferrite PCB Isolator 2 Isolator 2 Fig. 8. Measurement setup for the input impedance of the ferrite Isolator Fig. 8. 8. Measurement setup setup for for the the input1 impedance Fig. input impedance of of the the ferrite ferrite heat sinkMeasurement design heat sink sink design. design heat MOSFET by using (Diepenbrock et al., 2001) 4by using Input(Diepenbrock impedance ofetthe al.,heat 2001)sink 1 + S11 S110 · 1 + S11 Zsetup , (4) input = Z 4.1 Experimental Network Analyzer Zinput = Z0 · 1 − S11 , (4) 1 − S11 Three were necessary to analyze the wheredifferent Zinput isconfigurations the input impedance to the network, the used Isolator 2 whereimpedance Zinput is the input impedance toPCB theThe network, the used input of the heat sink designs. first setup is an normalization impedance Z0 is 50 Ohms, and the measured normalization impedance Z is 50 Ohms, and the measured 0 original heat sink design the input impedance scattering parameter datafor is Smeasuring 11 . In this work, the frequency Fig. 8. Measurement setup the input impedance of the ferrite scattering parameter data is Sfor In this work, the frequency 11 .the as a reference. wasup placed to vector network analyzer range from 300ItkHz to 1 GHz was considered. heat from sink design range 300 kHz up to 1 GHz was considered. to measure the scattering parameters S11 (see Fig. 7). A sec4.2 and Radiation of the heatsetup sinkwere made for the extended ond third measurement 4.2 Radiation of the heat sink heat sink design without ferrite core and with ferrite core, using et parameters al., 2001) Inby order to (Diepenbrock validate the attenuation of the heat sink respectively. The scattering ofsink theseradiated setups In order to validate the attenuation of theS11 heat radiated emissions of the proposed heat sink design an experimental 8). were measured by a vector network analyzer (see Fig. emissions of the proposed heat sink 1design an experimental + S11 test was done inside anZinput anechoic chamber to ,avoid the pres= Z · From these scattering parameters of all three configu0 test was done inside an anechoic chamber to avoid the pres- (4) 1EMI − S11 ence of environmental EMI noises in measurements. A rations, the input impedances calculated by using ence of environmental EMI noiseswere in EMI measurements. A simplified schematic of the experimental setup is shown in (Diepenbrock et is al.,the 2001) where Zinput impedance tosetup the network, simplified schematic ofinput the experimental is shownthe in used Fig. 9. The RF signal generator was connected via a coaxFig. 9. The RF signal generator was viathe a coaxnormalization impedance Z0 is 50connected Ohms, and measured scattering parameter data is S11 . In this work, the frequency Sci., considered. 9, 317–321, 2011 range from 300 kHz upAdv. to 1Radio GHz was 4.2 Radiation of the heat sink 4 J.Kulanayagam et al.: Reduction of Heat Sink Common-Mode Currents in Switching Mode Power Supply Circuits 100 320 J. Kulanayagam et al.: Reduction ofthird heat configuration sink common-mode currents The capacitances the heat Currents J.Kulanayagam et of al.:in Reduction ofMode Heat Power Sink (extended Common-Mode J.Kulanayagam et al.: Reduction of Heat Sink Common-Mode Currents Switching Supply Circuits reference J.Kulanayagam et al.: Reduction of Heat Sink Common-Mode in Switching ModeasPower Supply Circuits sink withCurrents ferrite core) are the same first and second configferrite core1 80 100 uration. But the inductances of the heatChamber sink are increased by Anechoic ferrite core2 The capacitances of the third configuration (extended heat Das, S 100 the ferrite cores. This is because all two ferrite core heat consist Thesink capacitances the third configuration (extended without ferrite core reference Antenna hea with ferriteofcore) are the same as first and second configreference 60 80 ferrite core1 sink with ferrite core) are thetherefore same as first and second configof different materials and different magnetic charneti Heat sink uration. But the inductances of the heat sink are increased by ferrite core1 ferrite core2 80 uration. ButThe the inductances of of thethe heatsystem sink are increased by the Diepen acteristics. quality factor is reduced by ferrite core2 the ferrite cores. This is because all two ferrite core consist without ferrite core the Signal ferrite This is because all two ferrite core consist without ferrite core gro ferrite core.cores. Furthermore, it depends the ferrite materials 40 60 of different materials and thereforeon different magnetic charGenerator 60 of different materials and therefore different magnetic charEle which is used. acteristics. The quality factor of the system is reduced by the 996 acteristics. The quality factor of heat the system isincreased reduced by the The input impedance of the sink is because 20 40 ferrite core. Furthermore, it depends on the ferrite materials Felic, Ground Plane core. Furthermore, it depends on the ferrite 40 Spectrum aferrite ferrite core hasVa very large impedance due to itsmaterials permeabilwhich is used. circ Analyzer which is used. ity. This means that the heat sink currents will be minimized The input impedance of the heat sink is increased because 0 20 Com The input impedance of the heat sink is increased because 20 by aadding the ferrite core large between the heat sink and power Li, K. ferrite core has a very impedance due to its permeabilaFig. ferrite core has a very large due to its permeabilFig. 11.Experimental Experimental setup emission 12. setup forimpedance emission measurements. measurements device and the heat sink radiation is currents attenuated by filter. of F This means the heat will bethis minimized -20 5 00 ity.ity. This means thatthat the heat sinksink currents will be minimized 6 7 8 9 VL 10 10 10 10 10 by adding the ferrite core between the heat sink and power by adding the ferrite core between the heat sink and power 30 Frequency/ Hz mag Antenna vertical and Heat Sink by frontal device and heat attenuated this filter. without ferrite core andsink Vw/radiation isisthe received RFfilter. signal device and the the heat sink radiation is attenuated by this Ferrite -20 5 -20 Antenna vertical and Heat Sink sideways Nagel, 6 7 8 9 6 7 8 9 105 10 of the sink with ferrite core. The measurement setups 10 10 1010 1010 10 10 25 heat tron 30 Frequency/ Hz enhanced heat sink Fig. 9. Input impedance of the Frequency/ reference, Hzthe andSink Heat frontal were30configured for frontal Antenna andAntenna lateral radiation. InSink this work, verticalvertical and Heat frontal Wis with and without ferrite core andSink Heat Sink sideways Antenna and Heat sideways the frequency range from 20 upAntenna tovertical 300vertical MHz was considered. 20 Radha Input Inputimpedance/ impedance/dB dB Input impedance/ dB 4 4 RRh h LhLh Uds U Uds ds Fig. 10. Equivalent circuit for original heat sink design 5.2 15Results and discussions 20 20 The attenuation of the heat sink radiation is calculated for 15 10 frontal 15 and lateral measurement of the heat sink is illustrated in Fig. 11. The maximum attenuation value and the average 10 5 10 C Ihhp Ih attenuation value of the frontal measurement of the heat sink are 25 dB and 15 dB, respectively. Furthermore, the maxiChp 5 Chp mum05attenuation value and the average attenuation value of 0 the lateral measurement of the heat sink are 10 dB and 5 dB, -5 0 0 0.5 1 1.5 2 2.5 3 respectively. 8 Frequency/ Hz Ih Fig. Fig.10. 10.Equivalent Equivalentcircuit circuitfor fororiginal originalheat heatsink sinkdesign design. Fig. 10. Equivalent circuit for original heat sink design impedance of the reference heat sink design can be expressed as: impedance of the reference heat sink design can be expressed 1 1 impedance of the reference heat sink design can be expressed as: Zinput · (5) 1 += S11jωCdh1 + Zh + jωChp 1 as: Zinput = Z0 · Zinput = , (4) + Z + · (5) h 1 1 −input S11 =jωC1dh + Z jωChp · (5) h+ The impedance of Z the heat sink is given by jωCdh jωChp The impedance heatimpedance sink is given by network, the used where Zinput isof thethe input to the The impedance of the heat sink is given Z = R + jωL · (6) h h h normalization impedance Z0 is 50 Ohms,byand the measured Zh = Rh + jωLh · (6) scattering parameter data is S11 . In this work, the frequency Zthe Rh + jωLheat h = original h · sink design can (6) The resonant frequency of range from 300 kHz up to 1 GHz was considered. The resonant be expressed as: frequency of the original heat sink design can The resonantas: frequency of the original heat sink design can be expressed 4.2 Results and discussions be expressed as: 1 1 fres1 = p · (7) ploop · Cloop =configurations · (7)9. 2π L The results of allfres1 three are illustrated in Fig. 1 2π p Lloop · Cloop f = · (7) res1 The curves of the first (reference heat sink design) and sec2π L, loop · Cloopimpedance beUp toUp the resonant frequency f the input res1 ond heat sink without core) configuration to (extended the resonant frequency fres1 ,ferrite the input impedance behaveshaves like athat capacitance and above the resonant frequency show, heat sink structures behave like series resonant athe capacitance and above resonant frequency Up to like the resonant frequency fres1 ,the the input impedance befres1fhaves the impedance behaves likelike anan inductance. The resocircuits. the impedance behaves inductance. The resores1 like a capacitance and above the resonant frequency nant nant fres2 of second configuration The result of thethe configuration can bewas explained by frequency fres2 offirst the second configuration wasshifted shifted ffrequency behaves like an inductance. The resores1 the impedance into into the lower MHz frequency range because the extended the following equivalent circuit (see Fig. 10.) lower MHz rangeconfiguration because the was extended nantthe frequency fres2frequency of the second shifted The input impedance of the reference heat sink design can heat heat sink design has a larger current loop than the reference sink design has a larger current loop than the reference into the lower MHz frequency range because the extended be expressed as: design. In design other words, second configuration has larger design. other words, second configuration has heatInsink hasthe a the larger current loop than thea alarger reference inductance than the original heat sink setup. In consequence, inductance than the original heat sink setup. In consequence, design. In other 1 words, the1 second configuration has a larger the resonant frequency is ·smaller than resonantfrefre+ Zfhoriginal +fres2 (5) Zinput =frequency the resonant is smaller than thetheresonant res2 inductance than the heat sink setup. In consequence, jωC jωCfrequencies dh hp quency f . The resonant of the first and secres1 quency f . The resonant frequencies of the first and secres1 the resonant frequency fres2 is smaller than the resonant freond configuration are 300 MHz 180 MHz,respectively. respectively. ond configuration MHz andand 180 MHz, quency fres1 . are The300 resonant frequencies of the first and secAdv. Radio Sci., 9, 317–321, 2011 ond configuration are 300 MHz and 180 MHz, respectively. Attenuation/ dB CCdhdh 25 25 Attenuation/ dB Attenuation/ dB Fig.9. Impedance of the of reference, the enhanced heat sink with and Fig. thethe enhanced heat sink Fig. 9.9. Input Inputimpedance impedance ofthe thereference, reference, enhanced heat sink without ferrite core. Cdh ferrite Lh with with and andwithout without ferritecore coreRh -5 0 -5 0 x 10 3 2.5 8 3 x 10 8 12.Attenuation Attenuation of of the the heat frequency x 10 6Fig.11. Conclusions Fig. heat sink sinkferrite ferritedesign designininthethe frequency 0.5 0.5 1 1.5 2 1 1.5 Frequency/ Hz Frequency/ Hz 2 2.5 rangefor forfrontal frontaland and lateral lateral measurements. range measurements Fig. 11. Attenuation of the heat sink ferrite design in the frequency In Fig. this 11. paper a new filter design introduced. can Attenuation of the heat sinkwas ferrite design in theItfrequency range for frontal and lateral measurements be range used for to frontal reduceand thelateral heat measurements sink common-mode currents in The impedance the heat sink iscircuits. given by The validation of Switching ModeofPower Supply 5the Radiation of the heat input impedances of thesink heat sinks have verified that the ZhRadiation = Rh + jωLofh ·the heat sink (6) 5input impedances can be increased at some frequencies up to 5 Radiation of the heat sink 5.1 setup 45 dBExperimental due to the ferrite core between the heat sink and the The resonant frequency 5.1 Experimental setupof the original heat sink design can power device. Therefore, the heat sink current can be minibe5.1 expressed as: Experimental setup In order toaddition, validate theradiated attenuation of thefrom heat the sink radiated mized. In the emissions heat sink In order toof validate the attenuation of the heatan sink radiated emissions the proposed heat sink design experimental 1 are attenuated by the filter. Again, our proposed filter design In order tothe validate the attenuation of the heat sink radiated emissions ofp proposed sinkchamber design an · heat (7) fres1 = done test was inside an anechoic to experimental avoid the the heat presreduces the heat sink noise currents anddesign attenuates 2π L · Cproposed loop loop emissions of the heat sink an the experimental test was done inside an anechoic chamber to avoid presence of environmental EMI noises in EMI measurements. A sink radiated emissions. ence ofwas environmental noises in EMI measurements. test done insideEMI an anechoic chamber to avoid theA presUp to the resonant frequency f , the input impedance besimplified schematic of the experimental setup is shown in res1 simplified of theEMI experimental setupmeasurements. is shown in A ence of schematic environmental noises in EMI haves like a RF capacitance and above theconnected resonant frequency Fig. 12. The signal generator was a coaxFig.simplified 12. The RF signal generator was connected viavia aiscoaxschematic of the experimental setup shown References fres1 thetoimpedance behaves likedesign. an inductance. The reso- ain ial cable the heat sink ferrite On the other ial Fig. cable12. to the heat sink ferrite design. Onconnected the other via sideside The RF signal generator was aacoaxthe second configuration was shifted nant frequency fres2 ofperiodic biconical/ logarithmic antenna receives the emitted biconical/ logarithmic periodic antenna receives the emitted ial cable to the heat sink ferrite design. On the other side a Archambeault, Pratapneni, S.,range Zhang, L., andtheWittwer, D.: into the lowerB., MHz frequency because extended electromagnetic fields from the heat sink. This received RF electromagnetic fields from the heat sink. This received RF Comparison of various numerical modeling tools against biconical/ logarithmic periodic antenna receives thea stanemitted heat sink design has a larger current loop than the reference signal Vproblem detectedfields byaaspectrum spectrum analyzer. signal is detected by dard sink emissions, IEEE electromagnetic from theanalyzer. heat sink.in:This design. In otherconcerning words, theheat second configuration has received a Internalarger RF From these RF signals of with and without ferrite core From these RF signals of with and without ferrite core tional Symposium on Electromagnetic Compatibility, vol. 2,onpp.on signal V than is detected by a heat spectrum analyzer. inductance the original sink setup. In consequence, the1341–1346, ungrounded heat sink, the attenuation of the heat sink the ungrounded heat sink, the attenuation of the heat sinkon 2001. From these RF signals ofsmaller with and without ferrite frecore the resonant frequency fres2 is than the resonant Brench, C.: Heatsinkisis radiation as ausing function of geometry, in: IEEE radiated emissions calculated radiated emissions calculated using the ungrounded heat sink, the attenuation thesecond heat sink quency fres1 . The resonant frequencies of the firstofand International Symposium on Electromagnetic Compatibility, pp. radiated emissions is calculated using A (dB) = V (dBµV ) − V (dBµV ), (8) configuration are 300 MHz and 180 MHz, respectively. The hs(dB) = Ferrite A105–109, Vw/o Vw/ hs w/oFerrite Ferrite (dBµV ) −w/ Ferrite (dBµV ), (8) 1994. capacitances of the third configuration (extended heat sink Damiano, A., G., Ferrite Marongiu, I., A.: A heat sink where A(dB) is Gatto, the ofofthe heat radiated emisAhs = Vattenuation (dBµV ) heat −Piroddi, Vsink (dBµV ), (8) hs w/ Ferrite where A attenuation theand sink radiated emishs is the w/o model for EMI resonance frequency determination, in: Power sions, V is the received RF signal of the heat sink w/o Ferrite sions, Vw/o theConference, receivedofRF of the heat sink Ferrite where Ahs is theis attenuation thesignal radiated Electronics Specialists vol. 1,heat pp. sink 273–277, 2004.emiswww.adv-radio-sci.net/9/317/2011/ sions, Vw/o Ferrite is the received RF signal of the heat sink gro fere 189 Samir, by Pac Sincla of E circ Con Tihany K. E J. Kulanayagam et al.: Reduction of heat sink common-mode currents with ferrite core) are the same as first and second configuration. But the inductances of the heat sink are increased by the ferrite cores. This is because all two ferrite core consist of different materials and therefore different magnetic characteristics. The quality factor of the system is reduced by the ferrite core. Furthermore, it depends on the ferrite materials which is used. The input impedance of the heat sink is increased because a ferrite core has a very large impedance due to its permeability. This means that the heat sink currents will be minimized by adding the ferrite core between the heat sink and power device and the heat sink radiation is attenuated by this filter. 5 6 321 Conclusions In this paper a new filter design was introduced. It can be used to reduce the heat sink common-mode currents in Switching Mode Power Supply circuits. The validation of the input impedances of the heat sinks have verified that the input impedances can be increased at some frequencies up to 45 dB due to the ferrite core between the heat sink and the power device. Therefore, the heat sink current can be minimized. In addition, the radiated emissions from the heat sink are attenuated by the filter. Again, our proposed filter design reduces the heat sink noise currents and attenuates the heat sink radiated emissions. Radiation of the heat sink References 5.1 Experimental setup In order to validate the attenuation of the heat sink radiated emissions of the proposed heat sink design an experimental test was done inside an anechoic chamber to avoid the presence of environmental EMI noises in EMI measurements. A simplified schematic of the experimental setup is shown in Fig. 11. The RF signal generator was connected via a coaxial cable to the heat sink ferrite design. On the other side a biconical/ logarithmic periodic antenna receives the emitted electromagnetic fields from the heat sink. This received RF signal V is detected by a spectrum analyzer. From these RF signals of with and without ferrite core on the ungrounded heat sink, the attenuation of the heat sink radiated emissions is calculated using Ahs (dB) = Vw/o Ferrite (dBµV ) − Vw/ Ferrite (dBµV ), (8) where Ahs is the attenuation of the heat sink radiated emissions, Vw/o Ferrite is the received RF signal of the heat sink without ferrite core and Vw/ Ferrite is the received RF signal of the heat sink with ferrite core. The measurement setups were configured forfrontal and lateral radiation. In this work, the frequency range from 20 up to 300 MHz was considered. 5.2 Results and discussions The attenuation of the heat sink radiation is calculated for frontal and lateral measurement of the heat sink is illustrated in Fig. 12. The maximum attenuation value and the average attenuation value of the frontal measurement of the heat sink are 25 dB and 15 dB, respectively. Furthermore, the maximum attenuation value and the average attenuation value of the lateral measurement of the heat sink are 10 dB and 5 dB, respectively. www.adv-radio-sci.net/9/317/2011/ Archambeault, B., Pratapneni, S., Zhang, L., and Wittwer, D.: Comparison of various numerical modeling tools against a standard problem concerning heat sink emissions, IEEE International Symposium on Electromagnetic Compatibility, 2, 1341–1346, 2001. Brench, C.: Heatsink radiation as a function of geometry, IEEE International Symposium on Electromagnetic Compatibility, 105– 109, 1994. Damiano, A., Gatto, G., Marongiu, I., and Piroddi, A.: A heat sink model for EMI resonance frequency determination, Power Electronics Specialists Conference, 1, 273–277, 2004. Das, S. and Roy, T.: An investigation on radiated emissions from heatsinks, IEEE International Symposium on Electromagnetic Compatibility, 2, 784–789, 1998. Diepenbrock, J., Archambeault, B., and Hobgood, L.: Improved grounding method for heat sinks of high speed processors, Electronic Components and Technology Conference, 993–996, 2001. Felic, G. and Evans, R.: Study of heat sink EMI effects in SMPS circuits, IEEE International Symposium on Electromagnetic Compatibility, 1, 254–259, 2001. Li, K., Lee, C., Poh, S., Shin, R., and Kong, J.: Application of FDTD method to analysis of electromagnetic radiation from VLSI heatsink configurations, IEEE Transactions on Electromagnetic Compatibility, 35, 204–214, 1993. Nagel, A.: Leitungsgebundene Störungen in der Leistungselektronik: Entstehung, Ausbreitung und Filterung [Dissertation], Wissenschaftsverlag, Mainz, 1999. Radhakrishnan, K., Wittwer, D., and Li, Y.-L.: Study of heatsink grounding schemes for GHz microprocessors, IEEE Conference on Electrical Performance of Electronic Packaging, 189–192, 2000. Samir, A. and Fujiwara, O.: Calculation of load effect produced by ferrite core attached to wire above a ground plane, Asia Pacific Microwave Conference, 1, 1999. Sinclair, A., Ferreira, J., and van Wyk, J.: A systematic study of EMI reduction by physical converter layout and suppressive circuits, International Conference on Industrial Electronics, Control, and Instrumentation, 2, 1059–1064, 1993. Tihanyi, L.: Electromagnetic Compatibility in Power Electronics, J. K. Eckert & Company, Inc., Sarasota, Florida, 1995. Adv. Radio Sci., 9, 317–321, 2011
