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Microchip’s Reference Design Simplifies Secondary MCU Control of Primary Power

February 24, 2021 by Gary Elinoff

An innovative transformer developed with Würth Elektronik eiSos and Microchip’s MCP1012 auxiliary AC/DC controller are key to the new design.

Microchips MCP1012 enables the secondary-side MCU to control an AC to DC converter’s primary duty cycle with no need for an expensive, bulky, independent bias power supply.

The EV37F82A kit enables implementation of the reference design. Image courtesy of Microchip
The EV37F82A kit enables implementation of the reference design. Image courtesy of Microchip


The reference design itself is described in Microchip’s product announcement. Before digging in, it will be useful to discuss the MCP1012 and the Inde-Flux transformer.


The MCP1012

In this reference design, the MCP1012 serves as a primary-side start-up IC for starting an offline auxiliary switching supply working in conjunction with a secondary-side controller. A primary-side auxiliary controller, its function involves providing for the startup of the flyback converter, using an internal open-loop, peak current-mode current regulator. 

Communication from secondary to primary is achieved through accepting PWM commands from the secondary side via an isolation medium, in this case the Würth Elektronik eiSos pulse transformer.

No independent bias supply is needed.

The IC enables direct measurement and active regulation of voltage and/or current, and simplifies communication for load-referenced systems.


The Inde-Flux Transformer

The Inde-Flux Transformer is the second part of the story. It is actually made by Würth Elektronik eiSos, employing Microchip’s Inde-Flux  IP. This transformer, as per Microchip’s announcement “combines the signal power and signal communication into one device, eliminating the need for optical feedback or an independent signal transformer.” 


The Reference Design

Microchip’s 15W MCP1012 offline reference design demonstrates a 15 Watt offline power design and incorporates the necessary functionality to allow for the elimination of primary side the auxiliary power supply.  Of interest to OEM designers, Inde-Flux transformer technology can be scaled to enable transformer designs for different voltages and power levels. 

As per Rich Simoncic, senior vice president of Microchip’s analog, power and interface business unit, “The combination of our Inde-Flux technology in a Würth Elektronik eiSos transformer, our MCP1012 AC-DC controller and our SAM D20 series 32-bit MCU creates a unique solution to offline power management.” He goes on to state that “These devices enable a simpler, more reliable implementation of complex bi-directional communication between the primary and secondary elements used in many isolated applications utilizing offline power. Where this solution is used in systems with a secondary MCU, customers can realize up to 60% savings of the bias supply area and reduce the bias supply bill of materials costs by $3 or more.” 


Some Specifications for the MCP1012

The device is described in great detail in its Datasheet. The minimum input voltage is 16 volts, typical, and 500 volts maximum. The unit can tolerate 700 volts on a transient basis. Minimum and maximum oscillation frequencies are 37 and 63 kHz, respectively.


Employing the Reference Design

The 15W MCP1012 offline reference design is pictured at the front of this article. In addition to the board, populated with all necessary parts. The item includes all the design files as well as a user’s guide.


Microchip also makes available the DT100118 1W reference design. It offers a 1 Watt reference design that also employs the MCP1012.

Microchip’s Reference Design Simplifies Secondary MCU Control Of Primary Power

This package includes a fully populated 1-watt evaluation board, also with a useful jumper pin.