New Reference Design Bridges the Gap Between Primary and Secondary Windings for Low-Voltage Applications

January 12, 2021 by Antonio Anzaldua Jr.

Combining a new AC-DC controller and patented "Inde-Flux" transformer technology, Microchip and Würth Elektronik eiSos aim to help designers create reliable bi-directional communication between the primary and secondary elements in isolated applications.

Last week, Microchip released a 15 W reference design that allows designers to evaluate a microcontroller (MCU) located on the secondary windings of a transformer, which can control and start-up an application on the primary side. The IC can also be used to monitor the battery levels of all home devices.


The new AC-DC controller along with the Inde-Flux transformer

The new AC-DC controller along with the Inde-Flux transformer. Image used courtesy of Microchip Technology

In this article, we'll assess the new reference design and discuss Microchip's specialized technique for isolated feedback called Inde-Flux transformer technology.


The Purpose of Conventional Secondary Controllers

Typical high-voltage controllers are located on the primary side with no easy path of communication with any secondary controllers. Secondary controllers are used to regulate the secondary-side output voltage and current. 

To control devices on the primary side via any secondary MCUs, there are a few hurdles a device must achieve. For instance, the device must provide a power supply start-up for 500 V input, drive the primary MOSFETs, and protect against secondary-side failures.

One of the hardest challenges to overcome is finding a primary, high-voltage device that can act as a regulator, self sustain gate drivers, accept incoming PWM commands, and provide protection to the system.


Typical application circuit using a primary-side start-up IC

Typical application circuit using a primary-side start-up IC (in this case, the MCP1012) for isolated converters. Image used courtesy of Microchip


Microchip recently presented a combined hardware and software solution for designers seeking to improve interfacing capabilities between low-voltage smart home devices with power systems. 


Microchip’s 15 W Reference Design

Microchip has an extensive portfolio of design references for various products that assist engineers in system development. Near the end of 2019, Microchip developed a 1 W, MCP1012 evaluation board, which was its first go-around toward having a secondary MCU control an offline primary device.

One year later, Microchip launches the design reference, EV37F82A, which is the "next-generation reference design" intended for the MCP1012. This product is said to provide designers with options for more complex, robust systems by running through a 500 V continuous primary supply at 15 W output. 


Block diagram of the MCP1012

Block diagram of the MCP1012. Image used courtesy of Microchip

The MCP1012 15 W reference design demonstrates the ability of the MCP1012 to jumpstart a 15 W AC-DC flyback converter as it is commanded via a secondary microcontroller. 


Microchip’s Patented Inde-Flux Technology

The reference design uses a patented isolation technique for isolated feedback known as Inde-Flux transformer technology. Inde-Flux transformers are being licensed with the help of German manufacturer, Würth Elektronik eiSos (WE).  They are known throughout Europe for custom magnetics, PCBs, and wireless intelligent power systems. Microchip and WE have brought Inde-Flux technology into fruition by developing the first transformer that is a part of Microchip’s 15 W MCP1012 offline reference design. 


IC is able to transmit PWM commands from the secondary to the primary side

The integrated magnetic replaces the need for a separate isolator. Through the flux paths, the IC is able to transmit PWM commands from the secondary to the primary side. Image used courtesy of Microchip

Microchip says the Inde-Flux transformer combines the signal power and signal communication into one device, eliminating the need for optical feedback or an independent signal transformer. The secondary-side control is then enabled through a combination of the transformer, the MCP1012 primary-side auxiliary controller, and an Arm Cortex-based SAM D20 series 32-bit MCU.

Microchip’s SAM D Cortex-based MCU is said to provide 256 KB of flash memory and up to 32 KB of SRAM. Rich Simoncic, Microchip's senior VP of the analog, power, and interface business unit, stated, "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."

When used in systems with a secondary MCU, Simoncic also asserts that "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."

The new evaluation board along with the reference design may give engineers design flexibility while eliminating external components and monitoring power dissipation.    


Benefits of Secondary Control

Why would designers pursue secondary control? The main benefit is the system can anticipate analog, digital, or hybrid signals entering through the primary inputs without an auxiliary power supply. Secondary MCUs are also adaptive and allow users to directly regulate converters’ outputs. 


The MCP1012 reference design

The MCP1012 reference design. Image used courtesy of Microchip

At first glance, designers may see this as a limited solution since this reference design is focused specifically on powering a flyback converter. However, many developers will reach for a flyback converter to monitor various smart home devices since they can regulate multiple low-voltage devices. 

A unique feature of Microchip’s MCP1012 reference design is the availability of fault-mode protection. These modes include overvoltage lockout, under-voltage lockout, and overcurrent protections that ensure input voltages and currents stay at comfortable operational units.


A Strong Approach to New IoT Devices

By providing users with a secondary-side control between the system and the load, Microchip intends to help reduce the cost and footprint for flyback converters, a solid approach for new smart home devices. Designers will need to run Microchip’s MPLab IDE to run the design reference’s simulations and testing capabilities