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Charging glossary

Plain-English definitions of the terms used across our charging pages. Each entry says what the term means and why it matters when you choose or use a charger.

By ERONIX · Last substantive update:

A–C

AC (alternating current)
The electricity in your wall socket, which reverses direction many times a second. In India it is about 230 volts at 50 cycles per second (50Hz). A charger's first job is to turn it into the low, steady direct current a phone uses.
AC coupling
An oscilloscope setting that blocks the steady DC part of a signal so only the small variations, such as ripple, are shown. Used when measuring charger noise 1.
Active-clamp flyback (ACF)
A refinement of the flyback converter that recycles energy otherwise lost at each switching cycle, allowing higher efficiency and frequency. More common in chargers of about 30W and above 2.
Adjustable Voltage Supply (AVS)
A USB Power Delivery 3.2 mode in which the phone sets the voltage in 100-millivolt steps between 9V and 20V. It applies to adapters rated 27W to 100W. iPhone 17 and iPhone 18 Pro use it for their fastest charge; a 20W adapter cannot offer it 34.
Ambient temperature
The temperature of the air around the charger during a test or in use. Charger surface temperatures are only comparable when the ambient temperature is stated.
Ampere (A)
The unit of electric current, the rate at which charge flows. A 20W USB-C charger supplies up to 3A at 5V or 2.22A at 9V.
APDO (Augmented Power Data Object)
A USB Power Delivery offer that lets the device adjust voltage within a range rather than choose a fixed level. PPS and AVS are both APDOs 5.
Average active efficiency
The average of a charger's efficiency at 25%, 50%, 75% and 100% of its rated load, as used in US and EU regulations. The minimum at a 20W nameplate is about 85.5% 67.
Bandwidth limit (20MHz)
An oscilloscope filter that removes very high-frequency pickup so ripple readings are repeatable. Standard practice for charger ripple measurement 18.
BIS (Bureau of Indian Standards)
India's national standards body. It runs the Compulsory Registration Scheme under which power adapters for IT equipment must be tested and registered before sale 9.
BIS CARE app
The official BIS mobile app. Its "Verify R-No under CRS" function lets anyone check a registration number and the models it covers 10.
Bridge rectifier
Four diodes that turn alternating current into pulsing direct current at the input of a charger.
Bulk capacitor
The large high-voltage capacitor near a charger's input that smooths the rectified mains. Apple's 20W uses two 400V 15µF electrolytic capacitors 11.
Charge Limit
An iPhone 15 and later setting that stops charging at a chosen level between 80% and 100%, in 5% steps 12.
Charge pump
A circuit that converts voltage using capacitors rather than inductors, often very efficiently. iPhone 17 uses one together with AVS for fast charging 13.
Clearance
The shortest distance through air between two conductive parts, such as the mains side and the USB side of a charger. It prevents electricity from arcing across 14.
Common-mode noise
Electrical noise that appears equally on both output wires relative to earth. Excessive common-mode noise from a poorly designed charger can disturb touchscreens.
Compulsory Registration Scheme (CRS)
The BIS scheme, set up under a MeitY order, that requires listed electronics, including power adapters, to be tested by a recognised laboratory and registered before sale in India 9.
Constant current, constant voltage (CC/CV)
The two-stage way lithium-ion batteries charge: first at a high steady current, then at a steady voltage while current tapers off. Explains why charging slows near full 15.
Creepage
The shortest distance along a surface between two conductive parts. Safety standards set minimum creepage between mains and the low-voltage output 14.

D–I

DC (direct current)
Electricity that flows in one direction. A charger outputs DC at a set voltage, such as 5V or 9V.
DCP (Dedicated Charging Port)
A basic USB charging signal that tells a device it may draw current at 5V without a data connection. ChargerLAB detected it on Apple's 20W alongside USB PD 11.
E-marker
A chip inside some USB-C cables that tells the charger how much current the cable can carry. Required above 3A; not needed for 20W charging 16.
Efficiency
The share of power taken from the wall that reaches the device: output power divided by input power, as a percentage. The rest becomes heat inside the charger.
Electrolytic capacitor
A capacitor type with high capacity in a small size, used for smoothing. Its life shortens with heat, so temperature ratings matter.
EMI (electromagnetic interference)
Unwanted electrical noise a charger can emit through its cable or into the air. Chargers use filters to keep EMI within legal limits.
EMC (electromagnetic compatibility)
A product's ability to work without causing or suffering from interference. Tested to regional standards before sale.
EPR (Extended Producer Responsibility)
The obligation on producers under India's E-Waste (Management) Rules 2022 to arrange collection and recycling of listed products. Whether standalone chargers are covered needs a written determination 17.
Fixed PDO
A USB Power Delivery offer of one specific voltage with a maximum current, such as 9V at 2.22A. iPhones charge on fixed PDOs on 20W adapters 518.
Flyback converter
The circuit used in most small chargers. It stores energy in a transformer during each switching cycle and releases it to the output, providing isolation from mains 19.
Gallium nitride (GaN)
A wide-bandgap semiconductor used in some charger transistors. It switches with lower losses than silicon, which helps make high-wattage chargers smaller. At 20W the advantage is smaller 2011.
GoodCRC
The short acknowledgement message a USB Power Delivery device sends after receiving each message, confirming it arrived intact 21.
Hard Reset
A USB Power Delivery recovery step that returns both devices to a safe 5V state when communication fails 5.
IEC 62368-1
The international hazard-based safety standard for audio, video, information and communication technology equipment, including chargers. It replaced IEC 60950-1 and IEC 60065 22.
Interaction to Next Paint (INP)
A web performance measure of how quickly a page responds to taps and clicks. Included here because our pages publish their performance targets.
IS 13252 (Part 1)
The Indian safety standard for IT equipment, including power adapters, currently used for BIS registration. It is being replaced by IS/IEC 62368-1:2023, with both accepted until 1 November 2028 923.
IS/IEC 62368-1:2023
The Indian adoption of IEC 62368-1, which becomes the only accepted standard for these products after 1 November 2028 2324.
Isolation
The barrier that keeps the dangerous mains side of a charger separated from the low-voltage output you touch. Provided by the transformer and insulation distances.

L–P

Lightning
Apple's proprietary connector used on iPhone 5 through iPhone 14. Fast charging these models needs a USB-C to Lightning cable and a USB Power Delivery adapter 4.
Line regulation
How well a charger keeps its output voltage steady when the mains voltage changes.
Load regulation
How well a charger keeps its output voltage steady as the current drawn changes.
Load step / transient response
How a charger's output voltage dips or overshoots, and how quickly it recovers, when the current drawn changes suddenly.
MFi (Made for iPhone)
Apple's licensing programme for accessories that use Apple connectors, such as Lightning cables. It does not apply to USB-C wall adapters.
Milliampere-hour (mAh)
A unit of battery charge capacity. Larger batteries take longer to charge at the same power.
mVp-p (millivolts peak to peak)
The unit for ripple: the difference between the highest and lowest points of the output's small variation, in thousandths of a volt.
No-load power
What a charger draws from the wall with nothing connected. US and EU rules cap it at 0.10W for this class; Apple's 20W measured 0.021W at 220V 718.
Optimized Battery Charging
An iPhone feature that learns your routine and delays charging past 80% until shortly before you usually unplug 12.
Optocoupler
A component that sends a feedback signal across the isolation barrier using light. Some designs replace it with magnetic coupling.
Over-current protection (OCP)
A circuit that limits or cuts output when the current drawn exceeds a safe level.
Over-temperature protection (OTP)
A circuit that reduces or cuts output when the charger gets too hot.
Over-voltage protection (OVP)
A circuit that limits or cuts output if the voltage rises above its intended level, for example after a component fault.
PDO (Power Data Object)
One entry in a USB Power Delivery charger's list of offers, describing a voltage and maximum current 5.
PMIC (power management integrated circuit)
A chip in a phone that manages charging and power distribution. Apple does not publish the iPhone's charging circuit details.
Power density
How much power a charger delivers per unit of volume, in watts per cubic centimetre. Apple's 20W measured about 0.42 W/cm³ 18.
Programmable Power Supply (PPS)
A USB Power Delivery 3.0 mode in which the phone adjusts voltage in 20-millivolt steps. Widely used by Samsung Galaxy phones; iPhones charge on fixed levels 25.
PS_RDY
The USB Power Delivery message a charger sends to confirm it has switched to the requested voltage and the phone may draw power 21.

Q–Z

Quasi-resonant (QR) switching
A flyback control method that switches the transistor when voltage across it is at a low point, reducing losses and noise. Common in 20W chargers 19.
R-number
The registration number printed with the BIS Standard Mark, in the form R-XXXXXXXX. Verify it in the BIS CARE app and check the model is listed 2610.
Reinforced insulation
A single insulation system that gives protection equivalent to two independent layers, used between mains and accessible parts 14.
Request / Accept
The USB Power Delivery messages in which the phone asks for one of the charger's offers and the charger agrees 21.
Ripple
The small, fast variation left on a charger's DC output by its switching and rectification, measured in mVp-p. One indicator of design quality; not a measure of battery safety on its own 8.
Series registration
A BIS provision letting one registration cover a lead model and up to nine related models that share key construction features 27.
Silicon (Si)
The conventional semiconductor in most electronics. Apple's 20W adapter uses a silicon power IC 11.
Silicon carbide (SiC)
A wide-bandgap semiconductor best known in high-voltage, high-power systems. It appears in some 45–65W and larger chargers; none was found at 20W 28.
Sink
The device receiving power in USB Power Delivery, such as an iPhone.
Source
The device supplying power in USB Power Delivery, such as a wall charger.
Source_Capabilities
The USB Power Delivery message in which the charger lists the power levels it can supply 21.
Standard Mark
The BIS mark printed on registered products or packaging, shown with the R-number 26.
Standby power
See no-load power.
Surface temperature
The temperature measured on a charger's case. Always reported with ambient temperature, load and duration.
Synchronous rectification (SR)
Using a transistor instead of a diode to rectify a charger's output, reducing losses and heat. Standard in good 20W designs 11.
Taper
The gradual fall in charging power as a battery approaches full.
Thermal throttling
A phone reducing its charging power because the battery is warm. Apple says fast charging may not work when iPhone is too hot 4.
Transformer
The component that steps voltage down and provides isolation between mains and output in a flyback charger.
USB Implementers Forum (USB-IF)
The industry body that publishes USB specifications and runs the USB compliance and logo programme 29.
USB Power Delivery (USB PD)
The USB standard that lets a charger and device agree on voltage and current over the USB-C cable's CC wire. The current specification is Revision 3.2 30.
USB Type-C (USB-C)
The reversible USB connector used by iPhone 15 and later and by modern chargers.
vSafe5V
The default 5V level a USB-C charger supplies before any Power Delivery negotiation.
Watt (W)
The unit of power: volts multiplied by amperes. 9V × 2.22A ≈ 20W.
Wide-bandgap semiconductor
A material, such as GaN or SiC, that needs more energy to conduct than silicon, allowing higher voltages, temperatures and switching speeds 20.
Y-capacitor
A small safety-rated capacitor across the isolation barrier that reduces electrical noise. Its value is limited to keep leakage current safe.

Sources

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  4. Fast charge your iPhone (102574) Apple Support · 2026-09-18 · checked 2026-10-20 A04
  5. usbc_pd.h API docs Zephyr Project · accessed 2026-10-20 · checked 2026-10-20 B10
  6. Power Adapters Design Techniques to Meet the New DoE Level VI Efficiency Standards GlobTek · n.d. · checked 2026-10-20 B39
  7. Commission Regulation (EU) 2019/1782 (OJ adopted text) EU / legislation.gov.uk · 2019-10-01 · checked 2026-10-20 B40
  8. Understanding Switching Regulator Output Artifacts Expedites Power Supply Design Analog Devices (A. Limjoco) · 2014-08 · checked 2026-10-20 B35
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  10. BIS CARE app Bureau of Indian Standards · 2021-11-18 · checked 2026-10-20 C08
  11. Updated Power IC - Teardown of New Apple 20W Charger (A2940) ChargerLAB · 2023-09-05 · checked 2026-10-20 A08
  12. About Charge Limit and Optimized Battery Charging on iPhone (HT210512) Apple Support · 2025-12-08 · checked 2026-10-20 A05
  13. Apple iPhone 17 series debuts with 40W Dynamic Power Adapter and PD 3.2 AVS ChargerLAB · 2025-09-15 · checked 2026-10-20 B19
  14. Isolation glossary (SLLA353) Texas Instruments · n.d. · checked 2026-10-20 B59
  15. BU-409: Charging Lithium-ion Battery University · n.d. · checked 2026-10-20 B29
  16. SSZTBJ8 (USB Type-C cable e-marking) Texas Instruments · 2016-05 · checked 2026-10-20 B23
  17. E-Waste (Management) Second Amendment Rules 2023 LegitQuest · 2023-07-24 · checked 2026-10-20 C20
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  21. Build a USB-C Power Supply Circuit Cellar · n.d. · checked 2026-10-20 B09
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  23. MeitY notification amending CRO 2021 (S.O.4997(E)) TeamLease RegTech · 2025-11-07 · checked 2026-10-20 C02
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  28. 1080W Peak Power: An In-Depth Analysis of Silicon Carbide Chargers from 26 Brands ChargerLAB · 2026-06-12 · checked 2026-10-20 B50
  29. USB-IF Compliance Program USB-IF · n.d. · checked 2026-10-20 B24
  30. USB Power Delivery (document library) USB-IF · page shows 2026-09-14 · checked 2026-10-20 B01

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