Analysis and Implementation of a Novel Asymmetrical Half-Bridge High Step-Down DC-DC Converter

A novel asymmetrical half-bridge high step-down (AHB-HSD) dc-dc converter is proposed in this paper. A buck inductor is applied to the conventional asymmetrical half-bridge step-down (AHB-SD) dc-dc converter to achieve the high step-down voltage conversion. Moreover, all switches of the proposed converter can achieve zero-voltage switching (ZVS) under turn-on transition. The operating principle of the proposed converter is presented in detail herein to prove the aforementioned results. Finally, a 400 V input voltage, 48 V output voltage, 400 W output power prototype of proposed converter is implemented. The theoretical analysis is thus verified by experimental results. The maximum power efficiency of the proposed converter is 89.8 % at output power of 250 W.


Introduction
A power supply for telecom equipments and modern computers is required to have the characteristic of high power density, which can be achieved by operating at a higher switching frequency. It is well known that the switching losses increase and the efficiency decreases with increasing switching frequency. To overcome this drawback, the soft switching techniques are taken into consideration (1)(2)(3)(4)(5)(6)(7). As a result, a conventional asymmetrical half-bridge step-down (AHB-SD) dc-dc converter as shown in Fig. 1 is usually used to obtain zero-voltage switching (ZVS) operation at turn-on transition and low voltage stress for all the switches (5)(6)(7). Its output voltage can be determined by . However, for increasing the step-down conversion ratio with high output current rating applications, one is to get extremely narrow duty cycle, the other is to have a higher turns ratio n of the transformer. Unfortunately, an extremely narrow duty cycle is difficultly controlled to regulate the output voltage. Moreover, the noise is easily affected the narrow duty cycle to cause the converter out of order. To avoid the narrow duty cycle in the high step-down conversion, a higher turns ration n is thus applied to the transformer of the conventional AHB-SD converter. However, the higher turns ratio will increase the inter-winding capacitances, winding resistances and leakage inductances. It thereby complicates the design and implementation of the transformer. The volume of transformer also becomes large and decreases the power density of the converter.
To solve this problem, many high step-down techniques are thus presented in the literature (8)(9)(10)(11). Consequently, a novel asymmetrical half-bridge high step-down (AHB-HSD) dc-dc converter depicted in Fig 2 is proposed by applying a buck inductor b L to connect with the rectifier diode 1 D . The proposed converter exhibits significant advantages of high step-down conversion ratio, ZVS operation and low voltage stress for all switches, no extremely narrow duty cycle and low turns ratio n of transformer. Fig. 1. Conventional AHB-SD converter. Fig. 2. Proposed AHB-HSD converter.

Operating Principle
The proposed asymmetrical half-bridge high step-down (AHB-HSD) converter presented in Fig. 2 contains two main switches 1 S , 2 S , which are driven complementarily with two small dead bands to realize the ZVS at turn-on operation for each main switch.
Some assumptions are made as follows before describing the operating principle of the proposed AHB-HSD converter.
1) All switches and diodes of the proposed converter are ideal. The switching time of the switches and the reverse recovery time of the diodes can be neglected.
2) Because the time intervals of the resonance, the charge and discharge time of capacitors  5) The turns ratio n of the transformer is 1.
Based on the switching of the switches and diodes, the proposed converter operating over one switching period S T can be divided into ten linear stages described as follows. The equivalent circuit of each stage is presented in Fig. 3. Before time 0 t , the switches 1 S and 2 S are on and off, respectively. The circuit operation has been achieved steady state. When the switch 1 S is turned off at time 0 t , this stage starts. In this stage, the capacitor , the diode 2 D thereby conducts and this stage ends.  In this stage, the circuit operation is similar to the 1 S turn-off state of a conventional AHB-SD converter.   In this stage, the circuit operation is similar to the 1 S turn-on state of a conventional AHB-SD converter.
The next switching period starts when 1 S is turned off again. According to the aforementioned operating principle of the proposed converter, the key waveforms over one switching period S T are schematically depicted in Fig. 4. Neglecting the dead times for brief steady-state analysis, the duty cycles of main switches 1 S and 2 S will thus be D and D  1 , respectively. In the steady state, based on the voltage-second balance principle to the magnetizing inductor m L , it shows that voltage CB V is a function as Similarly, according to the voltage-second balance principle to the output inductor o L , we thus have Solving equations (3) and (4), thus it yields is a function of variables in V , D , k and R .  Table 1.  On the other hand, it reveals from Fig. 6

Soft-switching performance
Figs. 8-9 show the experimental results of gating signals and drain-to-source voltages of all the switches. It reveals that the switches are turned on after their drain-to-source voltages drop to zero. As a result, all the switches of the proposed converter are achieved ZVS operation at turn-on transition. Moreover, we also can find that the maximum voltages across switches 1 S and 2 S are nearly input voltage V 400  in V .

Conclusions
An asymmetrical half-bridge high step-down dc-dc converter is proposed to achieve high step-down voltage conversion and ZVS operation under turn-on transition for all switches. The voltage stress across the two main switches is the input voltage V 400  in V . It means that the switches do not have the problem of high voltage stress. Furthermore, the experimental results of a prototype with 400V input and 48 V/400 W output thereby are presented to validate the theoretical analysis and ZVS performance of the proposed converter. The measured maximum power efficiency of the proposed converter is 89.8 % at output power of 250 W.