Packing Power: New Family of Flyback Switcher ICs Integrates USB PD Controller

Power Integrations has recently introduced a new family of USB-C PD+PPS power adapter ICs called InnoSwitch3-PD that integrates a flyback converter controller with a USB-PD controller into a single package. This can reduce the number of the required components and simplify the design process. Let’s get more familiar with this technology.

The InnoSwitch3-PD is said to promote charger power density. Image used courtesy of Power Integrations

USB-C With and Without Power Delivery

The USB-C is a connectivity standard designed to provide high-speed data transfer up to 10Gb/s and offer a power flow capability of up to 100 W. The USB-C supports a single-wire protocol called USB power delivery (PD) that lets devices negotiate and choose an appropriate level of power flow through the interface. 

The USB-C power exchange can be performed with and without PD. Without PD, a constant voltage of 5 V with a maximum current of 3 A is delivered at the power pin of the interface (VBUS), leading to a maximum USB-C non-PD power level of 15 W.

On the other hand, with PD, the VBUS voltage can increase up to 20 V at a maximum current of 5 A, delivering a maximum power of 100 W. With the USB-C PD, a single converter can be used to deliver the optimum power level for various devices. For example, a converter with the input from the grid can provide a 5-V DC output voltage for a cellphone and a 19-V DC output voltage for a laptop computer load.

The following figure shows a simplified block diagram of a USB-C-based power system for smartphone applications.

Block diagram of a USB-C-based power system for smartphones. Image used courtesy of Wei-Hsu Chang

The InnoSwitch3-PD devices incorporate the USB PD to meet the growing needs of today’s power-hungry applications.

USB-C Power Exchange Requires a Controller

In order to exchange power through a USB-C interface, a controller unit is required to manage the communications between the source and sink devices and make the required adjustments to the system. For example, a USB-C PD controller, such as the STUSB4761 from STMicroelectronics, can:

• Determine the cable attachment
• Check cable power capabilities
• Autonomously negotiate a power delivery contract with a sink device
• Regulate the output voltage and current according to the PD contract
• Monitor VBUS, manage transitions, and handle protections to ensure user and device safety  

In addition to the above basic functions, a USB-C PD controller might include other features to further simplify the design process.

The following simplified block diagram shows how a PD controller is used in a USB charger.

Schematic of USB type-C PD charger for type-C receptacles. Image used courtesy of Texas Instruments

The PD controller communicates with the sink device through the channel configuration (CC) pins of the USB-C interface to determine the power level suitable for the application. It then pulls the CTL1 and/or CTL2 pins low to adjust the output voltage of the flyback converter and get a VBUS voltage higher than 5 V as needed by the sink device.  

Flyback Converters in USB PD Applications

The flyback converter is widely used in adapter applications below 100 W because of its simplicity and low cost. The basic structure of a synchronous rectifier (SR) flyback regulator is shown below.

Structure of a typical flyback regulator. Image (modified) used courtesy of Analog Devices

Control circuits are required to switch the transistors on/off at the appropriate moments to optimize the efficiency. 

Putting It All Together: a Flyback Converter with USB-C PD Capability

The simplified block diagram below shows how we can add USB-C PD capability to a synchronous rectifier flyback converter.

Block diagram of ST's EVLSTCH03-45WPD evaluation board. Image used courtesy of STMicroelectronics

The bridge rectifier converts the AC line voltage to a DC voltage and delivers a full-wave rectified voltage at the input of the flyback converter. The PD controller monitors the VBUS voltage and transfers the output voltage information via an optocoupler back to the primary controller (the STCH03) to get the output voltage requested by the sink device at the VBUS pin.

InnoSwitch3-PD Integrates a PPS Power Adapter

Conventional USB-C PD power adapters such as the one discussed above commonly require a relatively large number of components.

Power Integrations has recently introduced a new family of USB-C PD+PPS power adapter ICs called InnoSwitch3-PD that, according to the company, can offer a higher level of integration compared to existing solutions. InnoSwitch3-PD integrates the primary switch and controller, isolated feedback, and the drive circuit for the SR switch of a flyback converter with a USB-PD controller into a single package.

The diagram below shows a typical application schematic along with some details about the circuitry integrated into the new device.

Application schematic of the InnoSwitch3-PD. Image used courtesy of Power Integrations

In addition to the circuit components discussed above, the InnoSwitch3-PD also includes the drive for the load switch, the discharge path for VBUS, and a temperature sense input.

However, the key feature of the new device is the integration of the USB-PD controller that allows the InnoSwitch3-PD to directly communicate with the channel configuration (CC) pins of the USB-C interface. This can reduce the bill of material compared to existing solutions and eliminate a lot of software development required for a USB-C PD+PPS power adapter.

According to the following table from the Power Integrations, the new device can reduce the number of the required components by about 50 percent compared to conventional solutions. 

How InnoSwitch3-PD sizes up to competitors. Image used courtesy of Power Integrations

Reducing the number of components also simplifies the design and increases the manufacturability. The company mentions a 65 W power adapter reference design that offers a power density of 1.36 W/cm3 and needs 53 components.