Increasing efficiency of offline flyback power conversion

-September 16, 2017

This product is featured in EDN's Hot 100 products of 2017. See all 100 here.
Designers of the next generation of Power Integrations’ InnoSwitch have somehow managed to improve the efficiency of power adapters to a level of 94%.

Let’s look at some history and put that efficiency in perspective before we can appreciate how significant this level of efficiency is. Legacy adapters have an 87% full load efficiency rating; that is the government spec right now for a 30W, 19V, rectangular adapter with 230VAC input. With a little bit of design expertise, a designer may get up to around 90% efficiency, a 3% improvement on the government spec, which is considered a ‘high efficiency’ design. In 2014, Power Integrations introduced an InnoSwitch version that could achieve 92%. Table 1 below says that the InnoSwitch from 2014 was able to double the power in the same sized legacy adapter.

Small efficiency improvements lead to significant size reductions. Surface area shrinks as the square of efficiency improvement and volume as the cube. (Image courtesy of Power Integrations)

Cell phone companies do not want a cell phone adapter to be the size of a notebook adapter; this type of adapter needs to remain small and lightweight, but with more power output being demanded by the industry.

InnoSwitch 3

With this new design, IC designers were able to achieve 94% efficiency; only 2% better than the 2014 design, but with 25% less heat than the best conventional designs in the industry right now. This 94% includes the bridge rectifier and output chokes—pretty impressive.

Image courtesy of Power Integrations

The good news is that because of this efficiency level, designers will not need a heatsink up to 65W since the InnoSwitch 3 is not burning much excess energy as heat, all in a small, slim InSOP package which has more than 11 mm of creepage/clearance.


I absolutely do not have a good feeling about optical interfaces in general from past design experience. I know that opto-isolators have improved quite a bit since my circuit design days back in the late 80s, but I still feel that there is a reliability factor in using optos, especially in power design architectures. That’s why I like digital isolation interface architectures like FluxLink, a safety-isolated digital feedback mechanism in the InnoSwitch design built into the device’s package. This also replaces primary side regulation.

FluxLink architecture and advantages over optocouplers (Image courtesy of Power Integrations)

There is a new algorithm that designers have added which gives FluxLink almost perfect step response because they have cycle-by-cycle control of the current limit and the switching frequency--amazing what you can do in power electronics with a little clever software addition. The design also has excellent 15 mW no-load draw, including input line sensing.

Creepage distance

The China market has a 5,000 meter spec and the creepage distance, in this wider package of over 11 mm, meets their needs. This is a good addition to this solution whether you are a Chinese designer or a designer who cares about the creepage at higher altitudes for your design.

Creepage and Clearance (Image courtesy of Power Integrations)

New versions of InnoSwitch 3 optimized by application

I like that designers are offered a choice between 650V and 725V internal MOSFETs in this new family trio (Table 2). The 725V version is for the high-end appliances/expensive equipment that need a bit more headroom to survive surges. This simplifies power supply design for a variety of applications.

USB PC adapter usage

A proprietary switch control allows the device to operate in CCM and quasi-resonant switching modes, and can switch modes instantly, for highest efficiency and lowest losses in the primary MOSFET (Figure 3). This is important because many adapters are now designed to USB-PD which can change the output voltage from 5V to 20V. This voltage changing capability makes life difficult for designers because the change in output voltages also changes the reflected voltages, which in turn affects the transformer winding, plus if you need to work efficiently at both 5V and 20V you can run out of design window if you try to only do quasi-resonant. The FluxLink architecture enables seamless discontinuous mode (DCM)-continuous conduction mode (CCM) transitions in the active control of the SR MOSFET as can be seen in Figure 3.


Another designer-friendly addition is an online selection tool, Build Your Own InnoSwitch, which is available to help designers customize device features for their particular design specifications.


FYI so you can judge for yourselves if this solution is for your architecture:

$1.11 (CE) and $1.15 (CP), in 10,000-piece quantities. InnoSwitch3-EP parts will be available in November 2017 at $1.18 per 10,000-pieces.

Visit Power Integrations website for more information.

Steve Taranovich is a senior technical editor at EDN with 45 years of experience in the electronics industry.

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