Charging Handshake: How PD and QC “Talk” to Your Phone

Introduction: The Silent Conversation You Never Knew Existed

Every time you plug your smartphone, tablet, or laptop into a charger, a complex digital negotiation takes place in milliseconds. This is called the “handshake.”

Without this handshake, your expensive 100W GaN charger is nothing more than a glorified 5W brick. The handshake determines how much voltage and current your device receives, dictating whether you wait 30 minutes for a full charge or three hours.

In the world of fast charging, there are two dominant “languages” for this handshake: USB Power Delivery (PD) and Qualcomm Quick Charge (QC) .

While both achieve the same goal—faster charging—their handshake processes are entirely different. One uses physical voltage pulses on data wires; the other uses sophisticated digital data packets on a dedicated pin.

This article dissects the anatomy of both handshakes, explains why they fail, and reveals how the new PPS standard is rewriting the rules of communication.

What Exactly is a Charging “Handshake”?

In electrical engineering terms, a handshake is the initial negotiation protocol between a power source (charger) and a power sink (device).

Its primary purposes are:

1. Safety: Prevent overvoltage or overcurrent that could fry the battery or start a fire.
2. Compatibility: Identify what fast-charging standards the device supports.
3. Optimization: Request the specific voltage and amperage that matches the battery’s current state of charge.

If the handshake succeeds, the charger ramps up from the default 5 Volts to 9V, 12V, 15V, or even 20V. If it fails, the charger remains locked at 5V—the universal “safe mode” used for basic USB charging.

Part 1: The QC (Quick Charge) Handshake – “Morse Code” Over Data Lines

Qualcomm Quick Charge is the older, more established standard. It was born during the Micro-USB era, long before USB-C existed. Because Micro-USB had no dedicated pins for power negotiation, Qualcomm got creative: they hijacked the D+ and D- pins.

These pins were originally designed for data transfer (syncing photos or music). In the QC handshake, they are repurposed to carry voltage signals—essentially electrical pulses that act like Morse Code.

Step-by-Step: The QC Handshake Process

Step 1: The Default State (5V)
When you first plug your QC-compatible phone into a QC charger, the charger outputs a safe, conservative 5 Volts. This is the “idle” state that every USB port uses.

Step 2: The “Ping” (Phone to Charger)
The phone’s power management IC (Integrated Circuit) pulls the voltage on the D+ pin down to 0.6 Volts. This specific drop is a signal that says: “Hello, I am a device that supports Quick Charge.”

Step 3: The “Acknowledgment” (Charger to Phone)
The charger detects this 0.6V signal on the D+ pin. If it is a QC-capable charger, it responds by pulling the D- pin down to 0.6 Volts.
This back-and-forth confirms that both sides “speak” QC.

Step 4: The Voltage Request
Once the acknowledgment is sent, the phone sends a new signal via the D+ and D- pins requesting a higher voltage.

• For QC 2.0, this is a simple jump (e.g., “Give me 9V” or “Give me 12V”).
• For QC 3.0, this is a continuous negotiation. The phone sends pulses asking for 200mV (millivolt) incremental steps, moving from 3.6V up to 20V in tiny increments.

Step 5: The Continuous Tweak
Unlike a one-time handshake, QC 3.0 and above never stop negotiating. As the battery fills up and internal resistance increases, the phone continuously sends voltage requests down the D+ pin to dial the power down, reducing heat generation.

Why the QC Handshake Fails

• Broken Data Wires: If your USB cable has broken D+ or D- pins (common in “charge-only” cables), the handshake fails, and you get 5V slow charging.
• Incompatible Charger: A standard non-QC charger simply ignores the 0.6V pulse on the D+ pin because it doesn’t know what it means.

Part 2: The PD (Power Delivery) Handshake – “Digital Email” Over the CC Pin

USB Power Delivery is the modern, universal standard mandated by the USB-IF (Implementers Forum). It was designed exclusively for the USB-C connector, which features an entirely new pin called the CC (Configuration Channel) pin.

The PD handshake does not use voltage pulses. Instead, it uses binary data packets—exactly like sending a text message or an email between the charger and the device.

Step-by-Step: The PD Handshake Process

Step 1: The Default State (5V)
Just like QC, a PD charger initially outputs a safe 5 Volts when idle.

Step 2: The “Source Capabilities” Menu (Charger to Phone)
The moment you plug in the USB-C cable, the charger sends a digital data packet down the CC pin. This packet is essentially a menu:

“Hello. I am a 65W charger. My available power profiles are:
– 5V at 3A (15W)
– 9V at 3A (27W)
– 15V at 3A (45W)
– 20V at 3.25A (65W).
What would you like?”

Step 3: The “Request” Packet (Phone to Charger)
The phone reads this “menu,” checks its own battery voltage and temperature, and sends a digital request packet back down the CC pin:

“Thank you. I am currently at 50% battery. I request 9 Volts at 3 Amperes.”

Step 4: The “Accept” Packet (Charger to Phone)
The charger verifies the request, sends an “Accept” data packet, and swiftly switches its internal circuitry to output exactly 9V/3A.

Step 5: PPS (Programmable Power Supply) – The Dynamic Handshake
With the introduction of PD 3.0 PPS, the handshake becomes dynamic. Instead of sticking to fixed steps (9V, 15V, etc.), the phone sends request packets every few seconds asking for granular changes (e.g., 7.2V, then 7.1V, then 6.8V) to match the battery’s precise charging curve. This is similar to QC’s 200mV steps, but done via digital packets rather than voltage pulses.

Why the PD Handshake Fails

• Lack of a CC Pin: If you use a USB-A to USB-C cable (which lacks a CC wire), there is no physical pathway for the data packets to travel. The PD charger cannot send its “menu,” so it stays at 5V.
• Wrong Cable Wattage: If you use a cable rated for 3A but the charger requests 5A, the E-Marker chip inside the cable interrupts the handshake and forces a fallback to 3A (or 5V).

Part 3: The Visual Analogy – QC vs. PD

To summarize the difference in a single mental image:

QC is like walking into a bar and shouting: “Beer!” The bartender sees you speak English and gives you the standard beer.

PD is like sitting at a fancy restaurant: The waiter hands you a printed menu (Source Capabilities), you point to the dish you want (Request), and the waiter nods (Accept) and brings it to you.

Part 4: The Great Misconception – Can QC and PD Handshakes Merge?

Here is the most critical question: If I plug a QC phone into a PD charger, will the handshake work?

The strict answer is: No.

• The QC phone sends its “0.6V pulse” down the D+ pin.
• The PD charger is physically ignoring the D+ pins and only “listening” to the CC pin for digital packets.
• The charger never responds to the QC pulse. The handshake times out, and the charger remains stuck at 5V.

The only exception: If the PD charger is a multi-protocol chip (common in modern GaN chargers), it contains separate hardware that also listens to the D+ pins. In that case, the charger speaks both languages. However, this is not “PD compatible with QC”—it is simply a charger with two translation chips.

Part 5: The PPS Bridge – The Handshake That Unites Them

With USB PD 3.0 PPS (Programmable Power Supply), the USB-IF introduced a handshake that mimics QC’s flexibility but uses PD’s digital packet system.

Qualcomm integrated PPS into Quick Charge 4.0 and 4+.

• When a QC 4.0 phone connects to a PD 3.0 PPS charger, the charger sends its “Source Capabilities” packet over the CC pin.
• The QC 4.0 phone, which is programmed to understand PD packets, reads the menu and sends a request for “7.2V” (PPS mode).
• The handshake succeeds, and fast charging begins.

However: This only works for new devices (Snapdragon 845 and above). If you have an older QC 2.0 or 3.0 phone, it only listens to the D+ pins and cannot read the PD packets, so the handshake remains broken.

Part 6: Troubleshooting – What to Do When the Handshake Fails

If your phone is charging painfully slow (5V/1A), the handshake has failed. Here is the ultimate checklist to fix it:

1. Check the Cable: Ensure it is a data-capable cable (not just a charge-only cable). If it lacks D+ and D- wires, QC fails. If it lacks a CC wire (USB-A to USB-C cables often lack this), PD fails.
2. Check the Port: For PD, you must use a Type-C to Type-C cable. PD does not work over USB-A ports.
3. Check the Power Brick: Look at the fine print on the charger. Does it list “QC 3.0” or “PD 3.0” on the label? If it only lists one, it won’t handshake with the other.
4. Try the Original Cable: OEM cables (the one that came in the box) are guaranteed to have all the necessary pins for that specific handshake.

Conclusion: The Future of the Handshake

As the industry consolidates around USB-C, the fragmented handshake era is ending.

• PD 3.1 now supports up to 240W, capable of charging gaming laptops.
• QC 5.0 is essentially a software layer built on top of PD PPS.

Within the next 2-3 years, the “QC vs. PD” debate will become obsolete for smartphones. Nearly every new device will use the PD+PPS digital handshake over the CC pin.

For now, remember the golden rule: If your charger and device don’t complete the handshake, you are leaving 80% of your charging speed on the table. Always check the pin configurations, buy high-quality 5A cables, and look for chargers that explicitly state support for both PD and QC if you own legacy devices.

Key Takeaways (Quick Reference):

• QC Handshake: Voltage pulses on D+/D- pins (Morse Code).
• PD Handshake: Digital packets on CC pin (Email Exchange).
• Fail State: 5V slow charging.
• Cable Rule: PD requires USB-C to USB-C (with CC pin). QC requires functional D+/D- pins.
• Future: PPS is the universal handshake that bridges the gap.