01-31-2018 12:26 PM
For anyone who's still trying to make this work, here's how the charger/cable actually work:
The Lenovo Yoga 3 has a unique power supply that is based off a proprietary Lenovo version of USB power delivery where the power supply uses a USB type-A receptacle that can power normal USB devices, but when the Lenovo Yoga 3’s is plugged into the adapter with a special cable the power supply will then modify its output voltage to 20V.
Additionally the power receptacle on the Lenovo Yoga 3 is also a USB type-A receptacle allowing users to plug in USB devices into this port when it is not being used to power the notebook (or charge its battery).
The complexity is that both the power adapter and the Yoga 3 are supplying 5V onto the USB V-Bus signal, when the power supply and the Yoga 3 are connected together, the power supply notifies the Yoga 3 its presence so that the Yoga 3 can then turn its USB power source (5V) to a power sink. At the same time the Yoga 3 notifies the power supply of its pretense and the power supply modifies the output voltage from 5V to 20V. When the power supply is removed the reverse happens (power supply goes back to 5V and the Yoga 3 will start supplying 5V on the USB V-bus line).
Yoga 3 EPS Architecture
A traditional USB Power Delivery systems negotiate power delivery over selected lines within the USB cable using a designated protocol; prior to the type-C this was done over the existing V-bus signal (5V). Lenovo avoids this complexity by using a method referred to as USB Sideband signaling (USB-SB). Lenovo has taken the standard USB type-A connector and added two additional fingers just outside the traditional V-Bus and GND signals (see diagram of a USB-SB plug below):
Figure 1. lenovo proprietary USB Type-A Plug with sideband signals (SB5 & SB6)
These sideband signals (SB5 and SB6) are used to communicate the voltage switches on the notebook and the power adapter.
The diagram below illustrates how we think the negotiation between the notebook and EPS (External Power Supply) works through the USB cable.
The Lenovo USB cable only has three conductors with the ends using proprietary Lenovo USB type-A plugs with the additional sideband signals (See FIGURE 1). At the Plug the sideband signals are tied together so that they only use a single conductor in the cable.
With the cable unplugged, the notebook will be asserting 5V onto the SB5/SB6 signals (through a 103 KΩ resistor) and 5V onto the V-Bus line, while the EPS has the SB5, SB6 signals at ground and asserting 5V onto the V-Bus line.
When the cable is plugged into the notebook and the EPS, both systems are driving V-Bus to 5V initially. The sideband signal is connected to ground through a 200 KΩ resistor, forming a resistor ladder which drives the sideband signal to 3.3V. Upon detecting 3.3V the Notebook will stop driving 5V onto the V-Bus line, while the EPS will start driving 20V onto the V-Bus line.
03-01-2018 12:53 AM
This was super-helpful information!
I couldn't figure out why the 3rd wire was present, nor why I was only seeing 5volts across those wires. This helped me understand what was going on.
Two things I wanted to add the this post -
1) Some people were talking about wire colors...they are different depending on what country your device is from, so just keep that in mind.
2) I did manage to "fix" my laptop. You can see my entire writeup here, but basically I just replaced proprietary plug with my own from spare parts I had laying around. Works like a champ!
03-01-2018 02:06 AM
I ended up buying one of those eBay adapters I mentioned above after all...and it works. Just stick it on the end of any universal laptop adapter & you're good to go