A guide to simplifying switch-mode power supply design

In secondary-side regulated flyback converter designs, feedback from the secondary side needs to be passed to the primary side, while allowing both sides to remain electrically isolated from each other for safety reasons. While optocouplers perform this task well, they are expensive, slow, and their life is limited to around 10 years because of the LEDs they incorporate. If the feedback circuitry could somehow be integrated into the switcher IC while maintaining isolation, the circuit would use fewer components and be smaller, simpler and cheaper overall.

This is exactly what Power Integrations has done in its InnoSwitch range. The InnoSwitch family offers a high voltage MOSFET, primary side controller, secondary side controller and secondary feedback circuits on a single chip which provides isolation without the need for an optocoupler. It does this using an interesting patented mechanism called FluxLink, whereby the secondary side is magnetically coupled to the primary side inside the IC for communication purposes. Because FluxLink uses the metal features already inside the IC, it can be integrated at virtually no extra manufacturing cost, while meeting worldwide isolation and safety standards (it complies not only with UL and TUV global isolation standards but also with the more stringent CQC 5000 meter Chinese safety standard).

Another significant feature of InnoSwitch is that it also integrates a synchronous rectification controller on the secondary side. Synchronous rectification (SR) is a very efficient mode of operation as it does away with lossy diodes, replacing them with a low-loss MOSFET. However, it can be difficult to achieve effective synchronous rectification if load conditions are transient, or during fault conditions. This method is also traditionally prone to timing difficulties if a separate SR controller isn’t used, which can reduce efficiency or even damage the primary transistor. FluxLink communication allows InnoSwitch to easily optimise the switching timing as digital cycle by cycle feedback can be passed between the primary and secondary sides within one IC. The designer no longer has to do this manually by adding an external SR controller to the circuit. In transient load conditions, FluxLink communication is so fast that the primary side receives the feedback within one switching cycle, virtually eliminating output voltage undershoot.

The result of all this is that InnoSwitch-based mobile device chargers up to 5A can have a total component count as low as primary side regulation designs, with accurate CV and CC control (±3% and ±5% respectively) and low voltage ripple. With high operating efficiency and less than 10mW no-load consumption, worldwide efficiency standards are also not a problem.

The InnoSwitch range currently comprises four different families. The CH family is for USB chargers, the EP family is for appliances and industrial equipment, the CP family is for USB-PD and USB3.0 designs, and the CE family is for consumer electronics applications. 

InnoSwitch-based power supply circuit

To complement Power Integrations’ range of switching ICs, including InnoSwitch, Avnet Abacus has launched a new transformer and choke design service, in partnership with Premier Magnetics, an American supplier of custom designed magnetics for power supplies and other applications. Premier Magnetics has been designing magnetics for more than 25 years and already works closely with Power Integrations on reference designs.

The Avnet Abacus transformer and choke design service works as follows. Use Power Integrations’ Expert or XLS software to design your circuit as per your application’s particular requirements. This software will allow you compare different PI switching ICs, and optimise the circuit design by trying different transformers, input filters and output choke designs. When you’re done, you can order samples (or bigger quantities) of both the PI switching IC and the custom magnetics required from Premier Magnetics through Avnet Abacus by simply sending us the Expert or XLS file (under NDA, if required). Magnetics samples are free of charge, and can typically be turned around in a few days or less. It really couldn’t be easier.

If you have any questions about our transformer and choke design service, or you would like to discuss your circuit design with one of our Product Specialists, click the Ask an Expert button the the right of this post to get in touch.


Philip Lechner

Philip Lechner studied Electronics & Telecommunications in Amsterdam. His career spans 33 years, including 13 years as a Field Sales Engineer covering Power, Semi-Conductors and IP&E, and 20 years in Product Management of Power Supplies and DC/DC converters for Avnet Abacus.


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