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Off-line switching power supply eliminates optocouplers

Posted: 19 Nov 2014 ?? ?Print Version ?Bookmark and Share

Keywords:Power Integrations? power supply? optocoupler? charger? STB?

Power Integrations (PI) has released several generations of off-line switching power supply ICs that derive feedback information to control output regulation direct from the output, and feeds that data back to the primary switch controller via an air-core magnetic coupler. The InnoSwitch family is geared for smart mobile device chargers and adapters for various applications that include STBs, networking equipment and computer peripherals, noted the company.

Calling it FluxLink, PI said its safety-isolated communication technology combines primary- and secondary-switcher circuitry to reduce component count, eliminate slow and unreliable optocouplers, outperform primary-side controllers and reduce manufacturing costs.

A single package with all necessary creepage and clearance spacing includes primary switching controller with integrated power FET on one die, secondary synchronous rectifier and associated circuitry on a second die, and the secondary-to-primary feedback path. This is implemented using the copper lead frame of the part. Part of the lead frame and chip bond wires form the 'primary' of a single-turn, air-core transformer (the 'primary' of the transformer is on the secondary or output side of the power supply layout, as it is passing output voltage level information back to the input switching controller). This loop couples to a further loop in the lead frame, connected to the controller IC. The physical layout is configured to cancel stray magnetic field effects. Clearances between the transformer loops are sufficient to give safety and regulatory isolation levels. On the PCB, the single package spans the gap between input and output sections.


According to the company, the InnoSwitch family combines primary, secondary and feedback circuits into a single, worldwide safety-rated, surface-mount package that exceeds all global regulatory standards for efficiency and no-load consumption, while minimising component count and providing highly accurate constant voltage and constant current, up to 25W.

Inside the device, highly accurate secondary-side direct voltage and current measurements are communicated across the safety isolation barrier using the high-speed digital FluxLink technology. This proprietary new feedback technique permits precise control without the need for a bulky optocoupler, while avoiding the performance compromises inherent in primary-side regulation (PSR), such as limited accuracy and efficiency and poor transient response versus no-load consumption. Unlike primary-side regulated switchers, InnoSwitch-based secondary-side regulated (SSR) designs are inherently less sensitive to the tolerance of external components such as transformers, diodes, resistors and capacitors. This increases manufacturing yield and reduces total power supply cost. Now, mobile device chargers up to 5A can have a total component count as low as PSR designs, with accurate CV and CC control (±3 per cent and ±5 per cent respectively) and low voltage ripple. With high operating efficiency and under 10mW no-load consumption, the ICs easily comply with efficiency standards such as the California Energy Commission, European Union Code of Conduct (CoC) Version 5, Tier 2 and the upcoming US Department of Energy standards (DoE 6), which will become mandatory in February, 2016.

InnoSwitch power-supply ICs include a high-voltage power MOSFET, primary-side controller, FluxLink feedback technology and a secondary-side controller with synchronous rectification (SR). By combining the SR function with the secondary-side master controller and by exploiting the speed of the FluxLink communication channel, the SR switch timing is optimised for maximum efficiency. The fast communication link also ensures highly reliable SR operation, eliminating shoot-through in either discontinuous conduction mode (DCM) or continuous conduction mode (CCM), even during transient loads and fault conditions. Effective SR operation in both DCM and CCM modes is especially beneficial in adaptive-voltage charger applications.

InnoSwitch ICs start up using bias current drawn from a high-voltage current source connected to the DRAIN pin, eliminating the need for external start-up components. An external bias winding reduces no-load and increases system efficiency during normal operation. The ICs also include comprehensive system-level features such as output over-voltage protection, overload power limiting, hysteretic thermal protection and frequency jitter to reduce EMI.

InnoSwitch-CH ICs are the first released devices in the family and samples are available. The devices are priced at $0.59 to $0.78 (10,000).

Reference design RDR-420 describes a 5V, 2A USB charger design and is available for download on the Power Integrations website at

- Graham Prophet
??EE Times Europe

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