A quick and practical view of USB Power Delivery (USB-PD) design

USB Power Delivery (USB-PD) now offers faster, more efficient, and more versatile power handling solutions. As we can all see, it’s an exciting advancement that significantly enhances the capabilities of USB connections.

This mechanism uses the USB configuration channel (CC) to allow a device to request a specific voltage. While this might seem complex at first, it’s pretty easy to utilize in practice.

Figure 1 The module has several jumpers to set the DC output voltage at multiple levels. Source: Author

What makes it easy nowadays is that we can buy compact USB-PD Trigger/Decoy modules that do the complicated background tasks for us (Figure 1). You can see such a module has a number of jumpers to set the DC output voltage to 5 V, 9V, 12 V, 15 V or 20 V.

This module acts as a trigger or decoy to request specific power profiles from USB-PD power sources such as USB-C chargers, power banks, and adapters. So, with this module, you can trigger USB-PD protocols and thus, for example, charge your laptop via a PD-capable USB-C power supply.

Note at this point that a USB-PD Trigger, sometimes called a USB-PD Decoy, is a small but clever circuitry that handles the USB-PD negotiation and simply outputs a predefined DC voltage.

Some USB-PD Trigger/Decoy modules are adjustable with a selector switch, or cycle among voltages with a pushbutton press, while others deliver a fixed voltage, or will have solder jumpers (or solder pads to install a fixed resistor) to select an output voltage. The output connection points on these modules are typically just two bare solder pads, or small screw terminals in certain cases (Figure 2).

Figure 2 The output connection points are shown on the modules. Source: Author

For just a few bucks each, these smaller and slenderer USB-PD Trigger/Decoy modules are useful to have in your tool chest, both for individual projects and for use in a pinch. In my view, for most applications, the fixed voltage type power provider is preferable, as this prevents accidental slips that could destruct the power consumer.

I recently bought a set of these fixed voltage modules. As you can see, the core part of these single-chip modules is the IP2721 USB Type-C physical layer protocol IC for USB Type-C input interfaces.

Figure 3 IP2721 is a USB Type-C PD protocol IC for USB input port that supports USB Type-C/PD2.0/PD3.0 protocols. Source: Author

The USB Type-C device plug-in and plug-out process is automatically detected based on CC1/CC2 pins. The chip has an integrated power delivery protocol analyzer to get the voltage capabilities and request the matched voltage.

Figure 4 The schematics shows a design use case built around the USB Type-C PD protocol IC. Source: Injoinic Technology

Surprisingly, the newly arrived module—designed for a single, fixed-voltage output—features the IP2721 controller in a bare minimum configuration without the power-pass element.

Figure 5 The module features the IP2721 controller in a bare minimum configuration. Source: Author

Hence, the output voltage will be whatever V_BUS is, and this could be 5 V during initial enumeration or stay at this voltage in case negotiations failed. Luckily, for many applications, this will not be much of an issue. But on paper, to comply with the USB power delivery specifications, the device is supposed to have a high-side power MOSFET as the power-pass element to disconnect the load until a suitable power contract has been negotiated.

For this writing, I needed to test the output of my module. So, below you can see a little snap taken during the first test of my IP2721 USB-PD trigger 9-V module; nothing but the process of testing the module with a compatible power source and a DC voltmeter.

Figure 6 DV voltmeter shows the output of the IP2721-based USB-PD module. Source: Author

Here are some final notes on the power delivery.

USB-PD is a convenient way of replacing power supply modules in many electronics projects and systems. Although USB-PD demands specialized controller chips to be utilized properly, easily available single-purpose USB-PD Trigger/Decoy modules can be used in standalone systems to provide USB-PD functionality.
Interestingly, legacy USB can only provide a 5-V power supply, but USB-PD defines prescriptive voltages such as 9 V, 15 V, and 20 V in addition to 5 V.
Until recently, the USB-PD specification allowed for up to 100 W (5 A@20 V) of power, called Standard Power Range (SPR), to flow in both directions. The latest USB-PD specification increases the power range to 240 W (5 A@48 V), called Extended Power Range (EPR), through a USB-C cable. So, if a device supports EPR expansion commands, it can use 28 V, 36 V, and 48 V.
Since the most recent USB-PD specification allows to realize up to 240 W power delivery through a single cable, it’s possible to provide ample power over USB to multiple circuit segments or devices simultaneously.
Electronic marking is needed in a Type-C cable when V_BUS current of more than 3 A is required. An electronically marked (E-Marked) cable assembly (EMCA) is a USB Type-C cable that uses a marker chip to provide the cable’s characteristics to the Downstream Facing Port (DFP). It’s accomplished by embedding a USB PD controller chip into the plug at one or both ends of the cable.
The USB-PD Programmable Power Supply (PPS) was implemented with USB PD3.0. With PPS, devices can gradually adjust the current (50-mA steps) and voltage (20-mV steps) in the range from 5 V to 20 V. PPS can directly charge a battery, bypassing the battery charger in a connected device.
Adjustable Voltage Supply (AVS) was implemented with USB PD3.1 and extended with PD3.2, allowing it to work within SPR below 100 W, down to a minimum of 9 V. AVS is similar to PPS in terms of function, but the difference is that it does not support current-limit operation, and the output voltage is adjusted in 100-mV steps in the range from 9 V to 48 V.

Note that USB-PD, which is combined with USB-C, takes full advantage of the power supply and multi-protocol functions over USB-C. Implementing USB-C for portable battery-powered devices enables them to both charge from the USB-C port as well as supply power to a connected device using the same port.

So, devices using a single or multicell battery charger can now be paired with a USB-C or USB PD controller, which enables the applications to source and sink power from the USB-C port. Below is an application circuit based on MP2722, a USB Type-C 1.3 compliant, highly integrated, 5-A, switch-mode battery management device for a single cell Li-ion or Li-polymer battery.

Figure 7 The application circuit is built around a 5 A, single-cell buck charger with integrated USB Type-C detection. Source: Monolithic Power Systems (MPS)

In the final analysis, it’s important to recall that the USB-PD is not just about the power delivery-related negotiations. Feel free to comment if you can help add to this post or point out issues and solutions you have found.

T. K. Hareendran is a technical author, hardware beta tester, and product reviewer.

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