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Impact of efficiency standards on AC/DC power supply

31 Jul 2015  | Scott Brown

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The USB BC 1.2 specification for battery chargers using a USB connector specifies both a DC voltage for constant voltage output and an AC output voltage range to ensure proper operation of any smartphone using a USB BC 1.2 compatible charger. The specification also calls out a recovery time under which the output can drop from the nominal DC voltage to the minimum AC voltage and then recover. The output needs to be back within the DC tolerance in the time specified.

The table shows the specifications required for compatibility to the USB BC 1.2 specification. The response time and voltage seem easy to achieve, especially when compared to DC/DC converters, but AC/DC power supplies have to comply with the DoE specifications, making this quite a challenge.

Table: USB Battery Charging Specification, Rev 1.2Vage Tolerances.

The iW1760 from Dialog's Power Conversion Business Group offers compatibility to the USB BC 1.2 specification while meeting both the DoE's newest efficiency standard effective February 2014 and the EU's strictest efficiency standards, the Code of Conduct Version 5, Tier 2. Figure 4 shows the response time of the iW1760 in a 10W USB charging application where the output responds to a 2A load step within 6ms and keeps the output voltage within the required USB BC 1.2 AC specification with some margin.

Figure 4: iW1760—Load Transient Response Graph.

A faster responding part can offer better response time with less capacitance while meeting the energy standards. The iW1786 is an example of a digital controller that works with a secondary-side component (iW671), a companion IC that detects changes to the output voltage and sends immediate feedback to the primary side, allowing the output to be enabled quicker than compared to stand-alone primary-side feedback. Quicker response time and less voltage droop add a significant amount of design margin to the adapter design. Alternatively, the designer can reduce the amount of bulk capacitance used to hold up the output during these transitions.

An initial review of this idea of adding a secondary IC might seem like a neutral size and cost trade-off, but the iW671 incorporates a synchronous rectification circuit for the secondary side, which removes two Schottky diodes while improving efficiency. The improved response time permits lower output capacitance on the output, improved efficiency reduces the necessary heat sinking requirements and removing secondary-side components enables an overall compact solution.

Figure 5 shows the transient response of the iW1786+iW671 (companion IC), with a noticeable improvement over the iW1760 original response. The dynamic load response improves significantly, providing ample margin to meet the USB BC 1.2 charging specification(1).

The minimum AC voltage on the output of figure 5 is 4.8V, giving a total 200mV droop vs the 700mV droop shown in figure 4. The response time measured in figure 5 is approximately 3ms, about half of the response time in figure 4. This improved speed accounts for the droop of less than half of the original droop shown in figure 4.

Figure 5: iW1786+iW671 Load Transient Response Graph.

The iW1786 uses a complex and proprietary digital core that has multiple control loops. Modern digital control loops such as the technology integrated in the iW1786 controller, enable a combination of very fast response time with small external components while maintaining stability over multiple control loops with no external compensation components. Analogue circuits are more than capable of achieving the same type of circuit, but the end result is larger, more costly and much more difficult to compensate.

Digital technology is ushering in a new era of power supply design offering flexible solutions and ease of use, even for the non-power savvy design engineer. Fast response time is now possible thanks to advancements in digital power management technology, permitting consumer electronics power supplies to meet international efficiency regulations without sacrificing performance.

(1) The circuit used to test the two different devices was identical, using the same magnetics and passive components. The only difference was the device under test (DUT) used to generate the two waveforms in figures 4 and 5.

About the author
Scott Brown joined Dialogue Semiconductor when it acquired iWatt in July 2013. He joined iWatt in October 2011 with over 20 years of experience in the analogue semiconductor industry. Scott has broad experience in all forms of semiconductor marketing from hands-on tactical to high-level strategy and many years of experience in semiconductor business and functional management. Scott has extensive global experience and a deep knowledge of the Power Management market. Prior to iWatt Scott held marketing and management positions at National Semiconductor, Micrel, ON Semiconductor, Catalyst Semiconductor and Semtech. He holds a BSc in Electrical and Electronic Engineering from Brunel University in the UK.

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