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Why Chargers Change Their Voltage: USB PD Negotiation, PPS/AVS, and What "40W Dynamic" and "PPS" Actually Mean
Charger boxes say “PPS,” “variable voltage,” or “40W Dynamic (60W max),” and it’s not obvious what any of it changes. The short version: a USB-C charger isn’t a device that pushes a fixed voltage; it negotiates with your phone and picks one. And a charger that can adjust its voltage finely wastes less power inside the phone as heat, so the speed holds up. This article follows that negotiation in diagrams, then turns it into a decision: what to look for on a charger for iPhone 18 Pro / 17, Android, and laptops.
The short answer: three questions the mechanism settles
| Question | What the mechanism says | So when you choose |
|---|---|---|
| Why does a charger “negotiate”? | Under USB PD the charger lists the voltages it can supply and the device picks one. Power the device doesn’t request never flows | A big charger won’t harm a small device. Wattage is a ceiling |
| What’s good about “variable voltage”? | With fixed steps (5/9/15/20V) the phone has to step the voltage down internally, and that drop becomes heat. Variable voltage (PPS/AVS) delivers close to what the battery needs, so less heat and sustained speed | iPhone 18 Pro: “60W or higher variable-voltage adapter.” Android: look for “PPS” |
| What is “40W Dynamic (60W max)”? | Apple’s adapter: 40W normally, up to 60W dynamically when the device asks. iPhone 17 wants 40W+, iPhone 18 Pro wants 60W+ for the advertised speed | iPhone 17 → 40W+. iPhone 18 Pro → 60W+ and variable voltage |
Mechanism 1: USB PD negotiation—the charger lists what it can do, the device chooses
Plug in a USB-C cable and, before real power flows, the charger and device have a short exchange. The charger presents a list of voltage steps (“5V at 3A, 9V at 3A, 15V at 3A, up to 20V at 5A”), and the device requests the combination it needs (“9V at 2A, please”). That exchange is USB Power Delivery (USB PD) negotiation, and only after it settles does charging start in earnest.
Two things follow. Power the device doesn’t request never flows, so plugging earbuds into a 100W charger won’t hurt them (wattage is a ceiling). And if the charger’s list and the device’s needs don’t line up, they settle on a lower step, which is why the labels (PD / PPS / variable voltage) decide real-world speed.
Mechanism 2: with fixed steps, the voltage the phone has to “drop” becomes heat
The classic USB PD list is fixed steps of 5V / 9V / 15V / 20V (28 / 36 / 48V in the extended range above 100W). But a phone battery sits around 3.7–4.4V. Power arriving at 9V has to be stepped down by circuitry inside the phone before it reaches the cells. That step loses some energy as heat, and the bigger the drop, the more heat.
Decision: charging that runs hot doesn’t stay fast. A phone lowers its charging power as temperature rises (how fast charging works), so read “choose a fast charger” as “choose a charger that can deliver power without making heat.”
Mechanism 3: variable voltage (PPS / AVS) delivers close to what’s needed
To trim the waste of fixed steps, USB PD includes modes that adjust voltage finely.
- PPS (Programmable Power Supply): added in USB PD 3.0. The device can request voltage in 20mV steps and set a current limit. Delivering close to battery voltage means a smaller internal drop and less heat. Most Android fast charging—Samsung’s Super Fast Charging, for one—assumes it (“PPS” on the box is this).
- AVS (Adjustable Voltage Supply): introduced with the USB PD 3.1 extended range (above 100W, up to 240W), and extended to the 100W-and-below range in PD 3.2. Adjusts in 100mV steps and suits steady supply to laptops.
Both let the charger change voltage to the device’s request; they differ in step size and intended use. They only work when both charger and device support them. With one side missing, you’re back to fixed steps.
Decision: for Android, pick a charger labeled PPS. Without it, the same wattage won’t reach the advertised speed (charging standards explained).
Mechanism 4: reading Apple’s “40W Dynamic (60W max)” and “variable-voltage adapter”
Apple’s official fast-charge conditions differ by model.
- iPhone 17 / 17 Pro / 17 Pro Max: “up to 50% in around 20 minutes with a 40W or higher adapter” (Apple Support)
- iPhone 18 Pro / Pro Max: “up to 50% in 15 minutes with a 60W or higher variable-voltage adapter” (Apple specs)
- iPhone Air / 17e: “up to 50% in 30 minutes with 20W or higher”
Apple’s 40W Dynamic Power Adapter with 60W Max supplies 40W normally and up to 60W dynamically when the device asks. Apple doesn’t name a mode (PPS or AVS), but writing “variable voltage” into the spec means a charger that can deliver finely adjusted voltage, not fixed steps, is the condition—which is exactly Mechanisms 2 and 3.
Decision:
- iPhone 18 Pro at advertised speed → 60W or higher plus variable voltage. In doubt, Apple’s 40W Dynamic (60W max). For third parties, pick products that state “iPhone 18 Pro” and “variable voltage” (diagnosing slow iPhone 18 Pro charging).
- iPhone 17 → 40W or higher. The practical minimum is the 45W class (45W chargers compared).
- Other iPhones → 20–30W is plenty. A bigger charger won’t make them faster.
Mechanism 5: laptops—single-port wattage, not the total, plus the extended range
Laptops sit on the high 20V step (28V and above beyond 100W) for long stretches. Multi-port chargers advertise a big total wattage, but what one port can deliver is a separate figure, and it’s shared when several devices connect. Above 100W you need both a USB PD 3.1 (EPR) charger and a 240W-class cable (do you need a 240W cable?).
Decision: decide the device’s required wattage first, then choose a charger whose single-port rating ≥ that number (what wattage charger to buy).
Products that follow the mechanism
Apple’s own, meeting the iPhone 18 Pro / 17 condition: 40W normally, 60W max, variable voltage.
Apple 40W Dynamic Power Adapter with 60W Max
A compact 45W that clears the iPhone 17 “40W or higher” bar: folding plug.
Anker Nano Charger (45W) with USB-C cable
For Android’s PPS assumption: 45W, two ports, PPS.
UGREEN Nexode Mini 45W (2× USB-C, PPS)
Single-port 100W for a laptop: judged by the single-port figure, not the total.
UGREEN Nexode 100W Charger (4× USB-C, PD 3.1)
FAQ
Q. Is a high-wattage charger risky for a small device? A. No. USB PD delivers only what the device requests. Wattage is a ceiling; the device sets the actual power.
Q. What’s the difference between “PD” and “PPS”? A. PD is the basic negotiation (fixed steps). PPS is an addition on top that lets the voltage move finely. Android’s advertised speeds usually assume PPS, so a PD-only charger may fall short.
Q. Does the cable matter? A. Yes. Above 3A (60W) and above 100W, a chip in the cable (eMarker) declares its capability; without it, you’re dropped to a lower step (how to choose a USB-C cable).
Sources (primary)
- USB-IF “USB Charger (USB PD)” / “USB Power Delivery specification” (definitions of PPS, AVS, EPR)
- Apple Support “Fast charge your iPhone” (iPhone 17 series: 40W+ for ~20 min to 50%; Air / 17e: 20W+)
- Apple “iPhone 18 Pro Tech Specs” (60W+ variable-voltage adapter, 15 min to 50%)
- The PD 3.2 AVS extension is confirmed via a secondary source (Power Electronics News, “Understanding USB Power Delivery 3.2”)
- Related mechanisms: how fast charging works / volts, amps, watts / why GaN chargers are small
Summary
- A USB-C charger negotiates voltage with the device. Unrequested power never flows, so wattage is a ceiling.
- Fixed steps force the phone to drop voltage internally, and that drop is heat. Variable voltage (PPS / AVS) trims the waste and keeps speed up.
- iPhone 18 Pro: 60W+ and variable voltage. iPhone 17: 40W+. Android: PPS. Laptops: single-port wattage.
Related: Apple genuine vs third-party chargers / what wattage charger to buy / iPhone 18 Pro charging
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