30W mains chip improves cross-regulation without opto-coupler

Power Integrations is aiming at domestic appliances the next iteration of its FluxLink in-package inductive feedback technique for secondary-side regulation without opto-isolators.

The chips, to be known as the InnoSwitch-EP family, have three die within a single package: a 725V primary-side switching mosfet, a primary-side control IC, and a secondary-side control IC – see diagram. The secondary-side synchronous rectifier mosfet is an external component.

Within the package, primary and secondary components are separated by a safety-rated dielectric barrier.

Feedback from secondary-side to primary-side IC across the barrier is through an inductive coupler constructed from bond-wires and the two lead-frames. Exactly the scheme works is not something the firm likes to talk about, except to say it gives good output regulation (+/-3% voltage, +/-5%), and that it is highly resistant magnetic interference from outside.

It also uses less static power than an opto-isolator – no-load power for a single output version can be under 10mW.

Multiple output power supplies with good cross-regulation are possible without resorting to separate primary components (compare ‘before’ and ‘after’ output diagrams).

This is due to synchronous rectification and sensing both outputs.

With diodes rectifiers and single output sensing, the un-sensed output not only has variation it its own diode’s voltage drop to contend with as its load varies, but its winding voltage will also vary as the current though diode on the regulated output varies from min to max, varing its Vdiode by 200-300mV.

With synchronous rectification, the mosfets drop only tens of mV, reducing interaction between the outputs, and the two-input potentiometer weights feedback to reduce it further.

Reduced voltage drops through synchronous rectification also increase efficiency.

“For example, InnoSwitch-EP ICs enable 20W power supplies to achieve approximately 90% efficiency in a multi-output design, while minimising no-load consumption to less than 30mW,” said the firm. “They enable designers to meet new ENERGY STAR 7 efficiency standards for monitors.”

Particularly for countries where mains line voltage is poorly controlled, input voltage is sensed and switching paused if the input exceeds 320Vac. Switching re-commences when it drops to a safe level for over 150ms. According to Power Integrations, line over-voltage sensing is +/-5%.

Applications are expected in microwaves, washers and dryers, as well as in stand-by power supplies for PCs, servers, air conditioning, displays, TVs, and lighting.

Samples are available now, as is a reference design (RDK-469).

steve bush

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