Power Integrations — Offline Flyback Regulator Increases Efficiency, Accuracy and Protection in Consumer Electronic Devices’ Embedded Power Supplies

The InnoSwitch™3-CE family of power regulator ICs enables designers to create highly efficient, compact power supplies for consumer products such as set-top boxes, networking adaptors, wireless speakers and gaming terminals.

The highly integrated InnoSwitch3‐CE parts are comprised of a primaryside Quasi‐Resonant (QR) flyback controller which operates in continuous and discontinuous conduction modes, and a 650V primary-side power MOSFET. They perform secondary-side regulation without the use of optocouplers, taking advantage instead of a FluxLink™ isolated feedback link which also facilitates precise control of secondary Synchronous Rectification (SR), as shown in Figure 1.

Fig. 1: Key features of the InnoSwitch3-CE

The conversion efficiency of the InnoSwitch3-CE is up to 94%, and is almost flat across the load range. Often eliminating the need for a heat-sink, the InnoSwitch3-CE offers excellent noise immunity and very low output ripple, and this allows small power supplies to be embedded in a wide range of consumer products which previously would have used an external power adapter.

High switching efficiency is complemented by low power consumption in stand-by mode of <15mW with no load enabling OEMs to meet the requirements of both existing and proposed global energy efficiency regulations. A comprehensive array of protection and safety features has also been integrated into the InnoSwitch3‐CE family. These include:

• Short-circuit protection
• Line input over‐voltage and under‐voltage protection
• Open-loop and pin‐short protection
• Latching or hysteretic over‐temperature protection
• Output over‐current and over‐voltage protection

InnoSwitch3‐CE ICs incorporate Power Integrations’ proprietary FluxLink inductive-coupling communications link. Using this technology, they accurately control the operation of the primary-side switch with secondary-side SR to maximize efficiency. Accurate control prevents potentially damaging crossconduction, or shoot-through, even as load changes force the power supply to repeatedly switch between discontinuous and continuous conduction modes. In addition, FluxLink is a safe barrier‐crossing technology which is approved worldwide, providing galvanic isolation between the primary side’s high voltage and the low-voltage secondary side. This removes the need for unreliable discrete isolation components such as optocouplers in the feedback circuit.

The primary-side QR flyback controller employs a combination of on‐off control, variable-frequency and variable-current control techniques to achieve flat efficiency across the entire load range. This approach allows the InnoSwitch3-CE family to provide almost noiseless power solutions producing an output of up to 65W, well beyond the power capability of traditional on‐off control schemes.

The secondary side consists of a controller, a transmitter circuit magnetically coupled to the primary-side receiver via FluxLink, Constant Voltage (CV) and Constant Current (CC) control circuits for output voltage and current regulation and a synchronous rectifier driver. Numerous safety features can be selected to provide either an auto‐restart or latching shutdown fault response.

Excellent output-voltage regulation of better than ±3% and output-current regulation of better than ±5% across line and load is achieved by secondary sensing. The secondary-side controller directs the operation of the primary side, initiating a primary-side switching cycle only when it has finished its own switching cycle.

Fig. 2: Typical design for a 36W flyback converter operating from a 90V to 265V AC supply and producing a well‐regulated 3A/12V output

QR or valley switching force the integrated primary-side MOSFET to turn on at the minimum MOSFET drain‐source voltage. This significantly reduces switching losses and EMI. Excellent noise immunity enables designs using the InnoSwitch3‐CE to achieve Class A performance levels in accordance with the suite of relevant EN 61000‐4 standard specifications, making it easy for integrated power-supply designs to comply with EMC regulations.

The InnoSwitch3‐CE is available in a compact new InSOP 24D package which offers reinforced isolation up to 4kV and carries UL1577 and TÜV (EN 60950) safety approvals. An online selection tool, Build Your Own InnoSwitch, guides the designer to the InnoSwitch device that best meets the needs of their application. Power Integrations also supplies the comprehensive PI Expert™ design software, and reference designs, including the 36W circuit, featured in Figure 2.