Comparison of Current Doubler Rectifier and Center Tapped Rectifier for Low Voltage Applications P Alou J A Oliver O Garc a R Prieto J A Cobos Universidad Polit cnica de Madrid UPM Divisi n de Ingenier a Electr nica DIE Jos Guti rrez Abascal 2 28006 Madrid SPAIN Tel 34 91 336 31 92 Fax 34 91 564 59 66 email pedro alou upm es Abstract It is well known that the Current Doubler Rectifier is very suitable for low voltage 1V 2V and very high current 50A 100A dc dc converters However at lower current applications 30A 20A 10A is not clear which is the more appropriate rectifier the Current Doubler or the Center Tapped The goal of this paper is to identify the application area of these rectifiers The output current together the output current ripple output current ratio I IO are important parameters to select the more suitable rectifier I INTRODUCTION Current Doubler Rectifier Fig 1 right 1 2 is very appropriate for low voltage and very high current applications because it presents two main advantages 1 Simpler transformer with only one secondary winding Besides the corresponding secondary current is half the output current These two features make possible the reduction of conduction losses in very high current applications 2 High current connections can be minimized since the secondary winding and the inductors are in series However there exist some misunderstandings related to the Current Doubler rectifier and the ripple cancellation of its output inductors inductors are operating the output current each 1 Both output in parallel handling half inductor Analyzing in detail the design of the inductors this feature can be not a real advantage as this paper shows 2 Although the inductor currents are cancelled as in the 2 phase Buck converter the Current Doubler rectifier does not present the good dynamic performance under load steps than the 2 phase Buck does In the other hand although the Center Tapped rectifier Fig 1 left presents only one output inductor it operates at double the switching frequency of the semiconductors becoming an interesting feature at low and medium current applications This paper analyzes and compares these two rectifiers trying to identify the corresponding application area The higher current ripple of the Current Doubler inductors becomes an important disadvantage at low and medium current applications Applicability of each rectifier stage is analyzed over five different specifications and two prototypes based on the Half Bridge topology are presented II RECTIFIERS COMPARISON DESIGN OF DIFFERENT SPECIFICATIONS To perform the analysis Half Bridge topology is selected since it is a suitable topology for these applications However main conclusions obtained in this paper do not depend on the selected topology and only depend on the rectifier stage in the Center Output to input voltage ratio is V O V IN n CT d Tapped rectifier and in the Current Doubler V O V IN n CD 2 d rectifier being nCT 1 1 the turns ratio of the Center Tapped transformer and nCD 1 the turns ratio of the Current Doubler transformer If both converters are designed to operate with a centered duty cycle d 25 2 n n CT DC VSEC SR2 S IRS2 SEC 2 SEC 1 IRS1 G2 G1 SR1 VO IO L IL VSEC SEC S G2 G1 ISR2 SR2 SR1 ISR1 IL1 IO VO IL2 IL L1 L2 a Center Tapped Rectifier b Current Doubler Rectifier Figure 1 Center Tapped and Current Doubler rectifiers 0 7803 9547 6 06 20 00 2006 IEEE 744 VSEC IL ISR1 ISR2 T 1 fSW T 2 VIN nCT I IO IO IO 2 ton dT toff ton toff VSEC T 1 fSW T 2 VIN 2nCD IL1 IL2 IL1 IL2 ISR1 ISR2 IL I IO 2 IO 2 IO IO IO 2 a Center Tapped Rectifier b Current Doubler Rectifier Figure 2 Main waveforms of Center Tapped and Current Doubler rectifiers ton dT toff ton toff Figure 2 shows the main waveforms of these rectification stages Output inductors of the Current Doubler operate with a 180 shifting providing a ripple cancellation that depends on the duty cycle Due to the ripple cancellation I I L d 1 d 21 effect the current ripple of each inductor IL is higher than the output current ripple I This feature penalizes the performance of the Current Doubler rectifier under some conditions To compare both rectifiers and determine their applicability area five specifications are considered Two parameters are very important to determine the proper rectifier 1 the maximum output current IO and 2 the output current ripple output current ratio I IO Table I shows the five different specs considered Input voltage is 48V and output voltage is 1 5V in all the cases The comparison between the Center Tapped and the Current Doubler rectifier is carried out under the following conditions 1 same input voltage range and same output voltage 2 primary MOSFETs operate at the same switching frequency in both rectification stages 3 turns ratio is selected to operate with the same duty cycle 25 to response symmetrically under up and down load steps and 4 both rectifiers are designed with the same output current ripple I through the output capacitors SPECIFICATIONS SELECTED TO COMPARE BOTH RECTIFIERS TABLE I Specification 5A 10A Specification 5A 25A Specification 5A 50A Specification 1A 10A Specification 1A 25A I 5A 5A 5A 1A 1A IO 10A 25A 50A 10A 25A I IO 1 2 1 5 1 10 1 10 1 25 A Size and Losses of Output Inductors The Center Tapped inductor handles all the output current however it operates at double the frequency of the Current Doubler inductors Otherwise the Current Doubler inductors operate at the same frequency than primary switches and handles half the output current each inductor In spite of the current cancellation effect the inductance value of the Current Doubler Table II doubles the inductance of the Center Tapped rectifier to meet the same output current ripple Besides the current ripple through the Current Doubler inductors is higher Table III and Table IV increasing the high frequency effects like fringing flux and skin effects in the conductors 3 Hence the only advantage of the Current Doubler inductors is that the current is half the output current 745 Center Tapped Rectifier Current Doubler Rectifier Inductance IL Core and material Losses Inductance IL ECD ECT Core and material Losses COMPARISON BETWEEN CENTER TAPPED AND CURRENT DOUBLER INDUCTORS Ner of Current ripple at each Inductance RMS inductor current inductors 1 2 V O 2 V O value 21 f SW 21 f I SW d I d TABLE II inductor I I d 1 d 21 TABLE III 2 IO I 2 12 IO 2 2 I 2 12 d 1 d 21 2 INDUCTOR DESIGN OF THE CENTER TAPPED RECTIFIER Specifications I IO 5A 10A