In Microprocessor and telecommunication applications, the system operation speed and integration density as well as power level requirement continue to increase, resulting in decreased supply voltage and increased supply current.The half bridge DC-DC converter with Current Tripler Rectifier (CTR) is a good candidate topology for high-current low-voltage applications. Current Tripler Rectifier is favorable compared to the conventional center-tapped and Current Doubler Rectifiers, because three secondary filter inductors share load current and thus enhance output current capability and thermal management. With CTR topology, lower conduction loss and well-distributed power dissipation to improve overall conversion efficiency and satisfy thermal management requirement.
1.INTRODUCTION
The evolution in Microprocessor technology poses new challenges for supplying power to these devices. Nano technology is driving VLSI circuits in a path of greater transistor integration, faster clock frequency, and lower operation voltage. In order to meet faster and more efficient data processing demands, modern Microprocessors are being designed with lower voltage and higher current.. Due to the limited real estate, high power-density power conversion is demanded for Microprocessor and telecommunication applications. In general, conversion efficiency and thermal management are two restrictions against high power density. High switching frequency operation is an effective way to improve power density, and topologies featuring high efficiency at high switching frequency are desirable. In addition, topologies with even current and thermal stresses are demanded, especially for low voltage and high current applications. Because secondary-side conduction loss dominates the overall power loss in isolated low-voltage high-current DC-DC converters [4]-[7], secondary-side topologies with low conduction loss and well-distributed power dissipation are desirable to improve overall conversion efficiency and thermal management.The half bridge DC-DC converter with Current Tripler Rectifier (CTR) is a good candidate topology for high-current low-voltage applications. Current Tripler Rectifier is favorable compared to the conventional center-tapped and Current Doubler Rectifiers, because three secondary filter inductors share load current and thus enhance output current capability and thermal management. With CTR topology, lower conduction loss can be achieved and printed circuit board layout design is easier to optimize. Half Bridge (HB) topology is suitable for low-voltage applications, because it provides double step-down ratio by dividing the input voltage through two input capacitors and transformer utilization is good. In addition, a unified steady-state state-space model and analysis is presented for both the symmetrical and asymmetrical controlled half bridge current tripler rectifier. Based on the derived model, DC analysis and design considerations are presented.
2. PROPOSED RECTIFICATION TOPOLOGY
2.1 Current Tripler Rectification Topology
The steady state operation of the Half Bridge Current Tripler topology and its operating waveforms are given in below. The unified state space model is derived and its major features discussed and compared with current doubler topology. The Half Bridge Current Tripler Rectifier topology is shown in Fig 1. Half Bridge Current Tripler Rectifier topology can be operated as symmetrical Half Bridge Current Tripler Rectifier topology or asymmetrical Half Bridge Current Tripler Rectifier topology. Here Symmetrical half bridge topology is used. The key steady-state operation waveforms of symmetrical half bridge current tripler rectifier are shown in Fig 6 The operation principle of the symmetrical half bridge current tripler rectifier can be described by four operation modes as shown in below.
The primary AC voltage pulse can be generated by state-of-the-art topologies such as push-pull, half bridge and full bridge primary-side topologies. Here symmetrical half bridge primary topology and current tripler secondary are used. The transformer turns ratio is n: 1:1 as labeled, and according to volt-second balance across the inductors, the output voltage is obtained in terms of duty cycle and input voltage.
Vo = (0£D£0.5) (1)
Where Vin is the input voltage, and D is the steady-state duty cycle value. The DC voltage gain of the above current tripler rectifier is the same for both the center-tapped and the current doubler rectification topologies. It can be seen, that by removing either the inductor L3, or by removing both the inductors L1 and L2 from the given topology [1], these respective conventional topologies can be obtained. Neglecting the inductor current ripple, each inductor's DC current is one third of the load current.
I1=I2=I3=Io/3 (2)
Where Io is the load current. If the applied AC pulse is absolutely symmetrical, the DC bias of the transformer's magnetizing current is zero. IM =0.
2.2 PRINCIPLE OF OPERATION
The operation principle of the current tripler rectifier can be described by four operation modes as shown in Fig 2, given that symmetrical AC pulse signal is applied to the primary side of the transformer. For this description of circuit operation, the following assumptions are made.
Mode 1 (to At t0, the positive voltage Vin is applied to the primary side of transformer. Switch SR1 is turned off and SR2 is on asshown in Fig 2. The inductor L1 is linearly charged by voltage ((2Vin/n)-Vo), and in the inductor L1 current i1 linearly increases at the slope
Where Vo is the output voltage and n is the transformer's turns ratio. The inductor L3 is linearly charged by voltage difference between the reflected input voltage in the secondary side and the output voltage, and inductor current i3 is increasing with the slope.
During this interval, inductor L2 is discharged by the output voltage Vo. The inductor current i2 freewheels through output capacitor and SR2, and decreases linearly at the following slope:
Mode 2(t1 The transformer primary-side is shorted or opened according to the operation and control of the primary-side topology at t1. Switches SR1 and SR2 are ON to provide freewheeling path for the three filter inductor currents as shown in Fig 3. Three output inductors L1, L2 and L3 are all linearly discharged by the output voltage V0, and the three inductor currents decrease at the slope as follows:
Fig 3 Mode2 of Half Bridge Current Tripler Rectifier
Mode 3(t2
The inductor L3 is linearly charged by difference voltage ((Vin/n)-Vo), and increases with the slope
4 Mode3 of Half Bridge Current Tripler Rectifier
Mode 4(t3
Fig 5 Mode4 of Half Bridge Current Tripler Rectifier
The operation mode goes back to Mode 1 after this mode, and a new switch cycle starts. The operating Waveforms of all these modes are shown in below Fig 6.
Fig 6 Symmetrical Half Bridge Current Tripler Rectifier Key Waveforms in Steady-State Operation
2.3. Averaged State-Space Model and DC Analysis of the Proposed Topology
Before deriving the averaged state-space model, the following assumptions are made. The transformer leakage inductance is neglected. The transformer magnetizing inductance is referred to the primary-side, and the converter operates in CCM mode due to synchronous rectification. Ron1, Ron2 are the on-resistance of the switches S1 and S2 respectively. RSR1, RSR2 are the on-resistance of the switches SR1 and SR2 respectively.RL1, RL2 and RL3 are inductor DCR values. Rt is winding resistance of transformer primary-side. Rc is the ESR (Equivalent Series Resistance) of the output capacitor. S1 gate signal and SR1 gate signal are complementary, and gate signals of S2 and SR2 are complementary.
For each mode during the period of time, the converter can be denoted using a set of linear state-space equations.
Where Io is the converter output current, D1 and D2 are steady-state duty cycle values for S1 and S2, respectively. For symmetrical Half bridge operation D1=D2. From Eqn (27) - Eqn (30), we may conclude:
3. Major Features and Comparison With Conventional Rectification Topologies
The given CTR topology [1] can be used with double-ended primary-side topologies such as push-pull, half bridge and full bridge. There is no difference between the current tripler rectifier and the conventional center-tapped and current doubler rectifiers in terms of the control and operation of the primary-side topologies. In addition, the driving signals for the secondary side Synchronous Rectifiers (SR) are identical to those for the conventional center-tapped and current doubler rectifiers. In the given topology, there are three output inductors evenly sharing the load current and thus the current stress is relieved in high current applications. As a result, the inductors design is simplified and better thermal management can be achieved.
Detailed comparison between the given topology [1] and the conventional center-tapped and current-doubler rectifiers is shown in Table 1. For fair comparison, assume that three rectifiers operate with the same switching frequency and have the identical input and output voltages, as well as equal load currents and output ripple currents. Current values in Table 1 are not reflecting the effect of the AC components in the inductor currents for the purpose of simplicity.
Therefore, compared to the center-tapped rectifier and the current doubler rectifier, the current tripler topology has high current capability, well-distributed power dissipation and good thermal management for high current applications.
4. EXPERIMENTAL RESULTS
An experimental prototype of the symmetrical half bridgedc-dc converter with the proposed current tripler rectifier is built. with the nominal input voltage 48 V, output voltage 1 V, and maximum load current of 45 A. Core ER14.5/3F3 is selected as the planar transformer with turns ratio of 12:1:1. The converter runs at the switching frequency of 211 kHz. Each output inductor has an inductance value of 0.8 H and DCR value of 0.588 m . It is observed that the load current is evenly distributed in the three inductors. Removing the inductor from the proposed topology, the converter becomes the conventional half bridge dc-dc converter with the current doubler synchronous rectifier. Fig. 11 compares the efficiency curves between the proposed CTR rectifier and the conventional current doubler rectifier at 48 V, which are measured with the same primary-side half bridge dc-dc converter, respectively. It can be noticed that the current triper rectifier achieves up to 1.5% efficiency improvement over the current doubler rectifier at 45 A load, which verifies that the proposed topology is advantageous over the conventional current doubler rectifier. Noting that the efficiency improvement increases with the load current in Fig. 11, it verifies that the proposed current tripler rectifier is more suitable for high current applications than the current doubler rectifier and significant efficiency improvement is expected for higher output current.
5. CONCLUSION
44The CTR topology is proposed for high current applications. Theoretical analysis, Comparison, and experimental results verify that the proposed rectification technique has good thermal management and well-distributed power dissipation, simplified magnetic design, and low copper loss for inductors and transformer due to the fact that the load current is shared by three inductors and the rms current in transformer windings is reduced. Therefore, the proposed CTR is a good candidate topology for secondary-side rectification in low voltage high current dc-dc converters
6. REFERENCES
