1、 Novel Capacitor-Isolated Power Converter Jingpeng Zhu，Ming Xu，Julu Sun, Chuanyun Wang FSP-Powerland Technology Inc., Nanjing, P. R. China, 210042 jingpengzhufsp- Abstract - This paper presented a novel power conversion method of realizing the galvanic isolation by dual safety capacitors (Y-cap) ins
2、tead of conventional transformer. With limited capacitance of the Y capacitor, series resonant is proposed to achieve the power transfer. The basic concept is to control the power path impedance, which blocks the dominant low-frequency part of touch current and let the high-frequency power flow free
3、ly. Conceptual analysis, simulation and design considerations are mentioned in this paper. An 85W AC/AC prototype is designed and verified to substitute the isolation transformer of a CCFL LCD TV backlight system. Compared with the conventional transformer isolation, the new method is proved to meet
4、 the function and safety requirements of its specification while has higher efficiency and smaller size. Index Terms- Touch current, leakage current, isolation, safety capacitor, resonance. I. INTRODUCTION The Touch Current (TC) is the electric current through human body while one touches the access
5、ible conductive parts of a piece of equipment1. To guarantee the safety, IEC 60990 has standardized the measuring methods of TC, one of which is shown in Fig.1a. In this method, the user is approximated by a measuring network (Fig.1b for e.g.). If the voltage (U2, namely VTC_test) detected on the me
6、asuring network exceeds the corresponding limit, the EUT will fail the TC test. Such limit values can be found in standards like IEC60950 etc. Fig.1a. A Test configuration for TC/PCC 2. Fig.1b. A Measuring Network for TC test 3.To limit TC and meet the primary to secondary clearance requirements, is
7、olation is needed. Currently, most of the primary-secondary isolation is realized by using transformers as shown in Fig.2. Since the stray capacitance C is small and the AC mains frequency is low, ICis small enough to meet TC limitation. However, the conventional isolation transformer is always larg
8、e in size and height, and has poor light load efficiency. This has become a bottleneck for the further improvement of the isolated converters, such as the power supplies for LCD TV backlight. One alternative is the piezoelectric transformer. However, its application is greatly limited by the drawbac
9、ks like complicated in control, sensitive to temperature and so on. Fig.2. Illustrative diagram of electrical shock 4.Fig.3 Y-cap isolation method. In this paper, a novel method using Y-cap to realize the primary-secondary isolation is proposed. (A “Y-Cap” is a capacitor of Class Y which is “of a ty
10、pe suitable for use in situations where failure of the capacitor could lead to danger of electric shock”. Its commonly used to bridge basic or supplementary insulation 5 .) Conventionally, Y-caps of several nF are adopted between primary side and secondary side to improve the EMI performance. The ca
11、pacitance value is mainly limited by TC safety standard. Similarly, if TC standards can be met, the Y-caps can also be used to transfer power from the primary side to the secondary side while achieving demanded isolation. As shown in Fig.3, an ac-input, two-stage power conversion system is symbolize
12、d: the first stage is a high-frequency inverter with an AC-DC rectifier input; the isolated secondary stage can be with a high-frequency rectifier or a voltage regulator. Both stages normally run at frequency finvof tens of kHz to several MHz. In Fig.3, Cs1and Cs2are Y-caps of several nF, while indu
13、ctors 978-1-4244-5287-3/10/$26.00 2010 IEEE 1824Ls1and Ls2are used to achieve series resonance with Cs1and Cs2respectively at the frequency of finv. Analysis shows that the main part of TC is driven by low frequency mains. Consequently, these two resonance branches can provide large impedance at lin
14、e frequency to block the TC, while very small impedance at finvto transfer power. As a result, isolation to limit TC can be realized while maintaining high efficiency. Comparing with conventional transformer isolation, this method can be much slimmer while optimizing the power transfer efficiency. I
15、t can be a new option for isolation, especially in ultra compact and high efficiency applications. In contrast to existing capacitor galvanic isolation patents 6or applications 7, this novel method adopts resonance circuit hence can transfer relatively larger power. II. TECHNICAL WORK PREPARATION A.
16、 Analysis on Capacitor Isolation System The basic requirements are as follows: 1. Basic System Functions: such as output voltage, power, etc.; 2. TC Safety: meet the spec, such as U2 0, so its shorted by PE, load current will mainly flows through Iebranch instead. This is not acceptable. Another cas
17、e is to put LSin the bottom branch, as Ls. Still, the Cs2-LS-D4-Vac(N)loop impedance |Z| = Ls-1/Cs2 0 (Ls = 1/Cs1 + 1/ /Cs2). As a result, its necessary to select Ls1 = Ls2 = Ls/2, Cs1 = Cs2 = 2Cs, 0Ls = 1/0/Cs, 0 = inv. So that the impedance of both top and bottom resonance branches as well as the
18、total loop are small. When under TC test, the impedance of the Ieloop is the impedance of the measuring network ZTCas shown in Fig.1b, which is thousands of ohms, this symmetrical structure can greatly help to reduce the TC. However, only series resonance circuit is not enough in most practical case
19、s. The first reason is the PCC is still unacceptable large. PCC is formed when there is no TC measuring network. The resonance branch is shorted by Iebranch, so Ie(PCC) is always too large. This causes lower efficiency and EMI problems, or even cant maintain system functions. Furthermore, even TC ma
20、y be too large for some other topologies. For example, when the half bridge inverter is substituted with a full bridge circuit (Fig.4b), the analyzed operation mode becomes much worse, even if two LSare used in the series resonance loop - the TC Iemay be unacceptably large for certain light load RL
21、ZTC.Considering VA and VBas two square wave voltage source with inverse phase, for VBpart (VA = 0), the Cs1-Ls1-RL-PE loop will be bypassed by ZTCbranch, so the TC will become much larger than that in the half bridge case. Similarly, the PCC will be much larger than that of half bridge system. Fig.4
22、a half bridge system circuit with one LS.Fig.4b Full bridge system circuit with only resonance branches. For these reasons, a CM choke Lcmis proposed to attenuate the high frequency earth current Ie, which is introduced by common mode voltage. The circuit is shown in Fig.5. Lcmcan be placed at the D
23、C-bus or LS-CSbranch where the function has no significant difference. Fig.5 half bridge system circuit with Lcm.B. Calculation on Capacitor Isolation System Take the system shown in Fig.5 for example. When the PFC works properly, D1System function: Output Power: 85 W; Voltage gain: 110%; Output fre
24、quency: 65 kHz; U2 meet safety spec: UL60950, 0.35MIU (equally 1.75 Vrms); Profile: H 5mH, Lcmhas little impacts on VTC_test, and the calculation is more precise. Fig.9 shows that, for certain Csand Ls, IPCCincreases dramatically as Lcmdecreases; when Lcm 4mH, Csand Lshave little impacts on IPCC. Fi
25、g.8 VTCcurves (calculated ICs_rms = 0.51 A, 65 kHz; VCs_bias = 165 Vmax, VCs_peak = 1.2 kVmax (264 Vacin). Cshas to be realized as Y1 cap or the like. This is needed by double/reinforced insulation between primary-secondary sides, which is demanded in IEC60065. However, usually Y-cap is used between
26、 L/N to PE for EMI suppression, so its permissible peak voltage and AC current is relatively small, and operation frequency is 50/60 Hz. Consequently, nearly all 1826the Y caps are produced with class II ceramic material of which the frequency and temperature characteristics are not so good. The iss
27、ue is that the Y-caps are used to transfer power in this solution. This leads to relatively high voltage and current stress and high DC bias as well. So Y-caps which are made using film or class I ceramic material is preferred. In this design, two film Y2 caps of 10 nF and 15 nF in series is selecte
28、d for Cs. 2. Flat inductor LsLs= 1 mH; ILs_rms = 0.51 A. The key of Lsdesign is the limitation of height and power loss optimization. To limit the VTC_test and PCC current, the larger Ls(smaller Cs) is the better. So the design of Lsgreatly influences the selection of the resonance parameters. Flat
29、core EFD20 of 10mm height is selected in this design. Core material is 3C96 of Ferroxcube. The winding is designed as 150 Turns using AWG27 wire. 3. CM choke LcmLcm = 4 mH(self and mutual); ILcm_rms = 0.51 A; ILcm_pk 15 mA. Unlike conventional EMI filter application, Lcmis used under relatively larg
30、er CM current. Commonly, lower permeability material leads to lower core loss at certain B. So a material of i = 7000 was selected. The core is T14 (14 OD * 8 H / mm) with a winding of 30 Turns. In addition, in case that the Lcmvalue is too large as tens of mH, the nanocrystalline material can be us
31、ed, which features higher permeability and low loss at high flux density. So larger Lcmor Iecan still be acceptable. D. Experimental Results Fig.10 shows that the VTC_testwaveform is the same as simulation one. The results meet the spec. with an 85 W output power, and a voltage gain of 0.97. When Va
32、c = 264 V, VTC_test= U2 = 0.367 Vrms, which is much less than 1.75 Vrms. Fig.11 shows TC test results under a condition of two faults when L-N polarity is opposite and the neutral line is fused. Result shows that the peak value of VTC_test does not increase. Its RMS value is still less than the 1.75
33、Vrms. In addition, it has passed all the tests under cases of only one fault which is demanded by the spec. Fig.12 shows the efficiency test results versus load for the system of Fig.5. In this figure, Lsin cap scheme is 200 H larger than that of cap2 scheme. Lkis the inductance of load. XFMR curve
34、is the efficiency of the conventional double isolated transformer system. All the dashed curves mean the PE line is off, thus there is no PCC current, while the solid curve means the PE line is on. All efficiency tested with no TC measuring network inserted. It can be found that the efficiency of ca
35、pacitor isolated system is higher than that of the transformer isolated system at nearly whole load range. The light load efficiency can be improved by reducing the Lsvalue, at the cost of lower full load efficiency. Fig.13 is the image of the isolation part. The height of this part is reduced from
36、22 mm to 12 mm. Fig.10 TC test (LS= 1 mH,CS= 6 nF,Lcm= 4 mH,RL= 364 ,Vac= 110 V). Fig.11 VTC_test_pkversus input Vac. Fig.12 System efficiency comparison. Fig.13 Image of isolation part (substitution of transformer). VTC_test(U2) VPFC OUTVac1827III. EXTENSIONS OF CAP-ISOLATED POWER CONVERTERS Beside
37、s the AC/AC structure illustrated above, the application of capacitor isolation method can be extended to AC/DC or DC/DC power converters and so on. Fig.14 shows the extended structure of the capacitor isolation power converter. The cap-isolation part can either consists of safety cap, resonance ind
38、uctor, common choke as analyzed above, or only two safety caps as shown in Fig.15. When the frequency of the Vinv is high enough to ignore the impedance of Cs1amd Cs2, power can be transferred to the secondary side. Based on this cap-isolation part, both the input and output of the system can be eit
39、her AC or DC as shown in Fig.14. Taking DC/DC cap-isolated system as an example, one circuit is shown in Fig.16. Without the isolation concerns, the output voltage step-down is realized by using an auto-transformer. Similarly, other non-isolated circuits like resonant circuit can also be adopted to
40、regulate the output voltage. Fig.14 Extensions of cap-isolated power converters. Fig.15 Capacitor-isolated system using only safety caps to transfer power. Fig.16 Capacitor-isolated DC/DC converter. IV. CONCLUSION This paper presented a new method for primary-secondary isolation by using Y-caps. Fir
41、st, the conventional transformer isolation and relative touch current standard are introduced. Then the structure of the capacitor isolation circuit is proposed and analyzed. The impacts on touch current and earth current of the key parameters are calculated. Based on the analysis, an experimental s
42、etup is designed to substitute the isolation transformer in a LCD TV CCFL backlight system. Experiment results show that the new method meets the touch current requirements of the related safety standards. In addition, it can improve the efficiency while greatly reduce the thickness. Lastly, the ext
43、ended application of capacitor isolation is introduced. In this capacitor isolated system, the isolated transformer is divided to several parts with safety issues only on safety caps. This introduces great design advantages such as in bus converter cases. In addition, for DC to DC applications, by i
44、ncreasing the switching frequency of the HF inverter to MHz as well as adopting SMD Y-cap, the total size can be greatly reduced. The resonance inductor Lscan even be eliminated. This provides the new capacitor isolation method great advantage over the conventional transformer isolation. However, as
45、 a new method, the capacitor isolation system inevitably needs improvements. Its structure is more complicated than transformer in most cases. Modification is always needed for different application. The analysis of this system, especially under abnormal operation mode, is still needed. APPENDIX: EQ
46、UATIONS FOR TC TEST KTC_test_hfand KTC_test_lfrepresents the measured voltage U2 over the voltage of the measuring network; Eq.2 shows IPCCcalculation: IPCCrepresents Iein Fig.5 when there is no TC measuring network, it influences normal efficiency. In addition, when calculating PCC current, its har
47、monics led by Vhfcant be ignored. It is much larger than its fundamental component which is suppressed by the resonance network. These formulas can be used to guide the parameter selection to optimize PCC current while meeting TC demands.1828_ _ _ _ _ _ _ _ _ _ _(1) *(1 )(1 ) * *(1 )( )swlineswhf pk
48、TC hf pk i hf TC hf TC test hfLihf SRhf ihffreq fACin pkTClf pk ilf TClf TCtestlfi lf m lf TC lf SR lffreq fmTC hf SR hfihfmTC hffreq fVVK ZKKZ KRVVK ZKKZ Z ZZZ ZKZZ=+=+=+_ _,| | | |freq flineSR lfilfLSR lfTC pk TC lf pk TC hf pkZKZRVV V= + =+(1) _ 12_ 12_*,21_ _ 21_ 21_ _12_ 21_*(1 1 / )/(1 ),hhswH
49、 hhL lineswH hhf pk iPCC hf pk i i hfLcmMcm self SR hf i hff if i nACin pkPCC lf pk i lfcm M Lcm self i lf SR lfffcm self SR hfihf ilfcm MfifViKZZRZKViKZKZRZZZKKZ=+=+=+=_L linecm M Lcm self SR lfcm Mcm self SR lfffPCC pk PCC hf pk PCC lf pkZZZRZZZii i=+ + =+(2) Where: 1_/1_ _/21,11/,2 ,22222Mswsw linelineswswlinelinefhffmcmDC Shf pk SR hf lfflfffhffSflffcm Mfhfffhff cm M cm Mcm self cm selfflffflffcm selfSTC h
