The Optimal Design of A High-Temperature PCB-Embedded Transformer GaN-Based Gate-Drive Power Supply with A Wide-Input Range
The power electronics systems such as dc-dc converters and inverters for automotive applications must perform without fail- ures in the face of harsh conditions, where the power supplies are prone to undesired transients including crank and jump-start. The circuits must operate reliably over a wide input range. In additon, automotive converters work in high ambient temperatures up to 125° C, where they cannot be cooled down by force convection. This means high efficiency is mandatory to ensure safe operation. As an effective way for miniaturization, the printed-circuit- board-embedded (PCB-embedded) technique is adopted to build the transformer. Shown in Fig. 1, the magnetic core is embedded into the multi-layer PCB and serves as the substrate carrying the rest of converter circuitry. By saving the footprint of the transformer and removing the extra space between the transformer and other com- ponents, a more compact design can be achieved. Active-clamp fly- back (ACF) topology is employed, for it shows a good trade-off between efficiency and power density. There are two operating modes in ACF. One is critical conduction mode (CRM), and the other one is continuous conduction mode (CCM). CRM allows the secondary diodes to be turned off under zero-current switching (ZCS) at the expense of higher conduction loss and voltage stress of primary devices, while CCM is opposite to CRM. A tradeoff exists between the devices conduction loss and the diodes reverse- recovery loss when selecting the operating mode.
Since working conditions change with input voltage, the circuit design and the transformer design become difficult. At low voltag- es, the conduction loss of the primary devices dominates due to the higher current. As the input voltage increases, the reverse-recovery loss of the rectifier diodes plays the leading role. Apart from the operating mode selection, the design freedom of the transformer is limited. Described in Fig. 2, the transformer design is restricted by the controller duty cycle range and zero-voltage switching (ZVS) requirement, which are represented by the red and blue curve, respec- tively. Failing to meet these requirements leads to either insufficient voltage gain or low efficiency.
In this paper, a GDPS with two isolated outputs of 24 V is designed. Its input voltage ranges from 8.5 V to 28 V, and the power rating of each output is 5 W. The size is 21 mm × 20 mm × 7.35 mm, representing a power density of 53.2 W/in3. To design the targeted GDPS, this paper first presents the design approach and optimized results, including the circuit design and transformer optimization, after which the current and voltage information can be estimated and used for selecting the conponents. Layout design, hardware as- sembly, and the evaluaton of the proposed GDPS are then provided. The permeablity degradation issue of the PCB-embedded transformer caused by mechanical stress during fabrication is revealed in the end of this paper.