TI - Delivering Ultra-High Power Density for 100W USB Power Delivery Adapters

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TI - Delivering Ultra-High Power Density for 100W USB Power Delivery Adapters [Copy link]

The USB Power Delivery (PD) standard supports up to 100W of power delivery over USB, which is enough to drive high-power devices such as laptops, monitors, and DLP? movie projectors.
　　USB PD adapters usually use inductor-inductor-capacitor (LLC) or quasi-resonant (QR) flyback for isolated DC/DC conversion. The advantage of LLC is that it supports power switches with soft switching, but it is not suitable for a wide output voltage range due to its narrow gain limit. Moreover, QR flyback operates in discontinuous current conduction mode and cannot achieve full soft switching in all output voltage ranges. In addition, the loss of transformer leakage inductance limits the practical maximum switching frequency. When designers want to achieve high power density and small adapter size, the above shortcomings impose great limitations on the implementation of functions.
　　Active clamp flyback (ACF) is a new resonant topology that overcomes these limitations. Unlike QR flyback, ACF can achieve soft switching and leakage inductance energy recovery. Critical conduction mode ACF uses a small negative magnetizing inductor to achieve zero voltage switching (ZVS) over the entire load range. In addition, the output rectifier can achieve zero current switching (ZCS) during shutdown, which will reduce the reverse recovery loss of the rectifier and improve the electromagnetic interference performance. With these advantages, high-frequency and high-efficiency adapters can be achieved.
　　The circuit using the Texas Instruments 30 W/in3, 93.4% efficiency, 100 W AC/DC adapter reference design includes a front-end conversion mode power factor correction (PFC) circuit based on UCC28056, followed by an ACF for isolated DC/DC conversion based on UCC28780. Figure 1 is a detailed system block diagram of the reference design.

　　EMI filter and rectifier
　　TM-PFG power stage
　　ACF power stage
　　PD controller
　　SR driver
　　UCC24612
　　isolator
　　IS07710F
　　ACF controller
　　UCC28780
　　PFC controller
　　UCC28056
　　amplifier
　　TLV171
　　output port
　　MOSFET
　　CSD17575Q3
　　Figure 1: AC/DC adapter reference design system block diagram
　　The most important feature for designers is the ultra-high power density of 30 W/in3 without a housing. First, the critical conduction mode ACF can achieve high switching frequency, allowing the RM8 core to be used in the transformer. Moreover, due to the ultra-high converter efficiency, no heat sink is required, which saves space and improves power density. Figure 2 shows the AC/DC adapter reference design circuit board.

　　Figure 2: AC/DC adapter reference design board
　　Another important feature that designers focused on was the high efficiency of the design. The UCC28780 achieves ZVS for the main switch and ZCS for the secondary rectifier. The secondary resonant configuration shapes the primary current waveform, which reduces the primary root mean square (RMS) current and significantly improves the synchronous rectifier operation. GaN FETs are used for the ACF primary side switches to improve efficiency because they have smaller parasitics. Efficiency can be further improved by shutting down the PFC stage at 5 VOUT and 9 VOUT. Figure 3 shows the average efficiency curve for the reference design.

　　Vout(V)
　　Efficiency(%)
　　Figure 3: AC/DC adapter reference design average efficiency curve
　　In summary, the 100 W AC/DC adapter reference design provides a complete solution for USB PD adapters, suitable for any 100 W application. High efficiency and high power density make the adapter more powerful and portable.