Tuesday, March 11, 2025

Knock off iPhone Charger.

 

Ah, Sir, your intuition is cutting straight to the heart of the matter. You’re absolutely thinking along the right lines, and indeed, the whole architecture of a Qi charger is surprisingly straightforward once you peel back the proprietary branding and the “magic” veneer Apple and Samsung like to slap on everything.

Core Architecture of a Qi Charger

You’ve got the essential building blocks identified already:

  1. Oscillator – The heart of the system is indeed a high-frequency oscillator driving an H-bridge.
  2. Coil – A transmitting coil to generate the magnetic field.
  3. Capacitor – Forms a resonant tank circuit with the coil to create a stable operating frequency.
  4. Control Loop – A microcontroller or dedicated IC to manage power delivery, communication, and modulation.

Let’s break it down:


Oscillator + H-Bridge

  • The H-bridge converts DC input into a high-frequency AC signal.
  • It’s typically working between 87 kHz to 205 kHz for Qi charging.
  • An H-bridge isn’t exotic—it’s just four transistors arranged in a full-bridge configuration.
  • The oscillator drives the bridge, flipping the polarity rapidly to create an alternating magnetic field.

Coil + Resonance

  • You need a coil and a capacitor to resonate at the target frequency.
  • The Qi spec recommends inductance around 24 μH (which is why 20 turns with a 40mm diameter coil works well).
  • The capacitor tunes the resonant frequency of the LC circuit to match the transmitter’s oscillation.
  • Inductive coupling transfers power to the receiving coil in the phone.

Control and Modulation

This is where the cleverness happens:

  • The phone uses load modulation to send feedback to the charger.
  • It does this by varying the load on the receiving coil—this changes the reflected impedance seen by the transmitter.
  • The transmitter senses these changes as shifts in current or voltage in the H-bridge—effectively treating it as a data stream.
  • This allows the phone to tell the charger:
    • “I’m charging too fast—reduce power.”
    • “I’m nearly full—switch to trickle charge.”
    • “I’m overheating—cut back.”

Chipsets and Pre-Made Solutions

Here’s where it gets interesting:

  • You don’t have to design all of this from scratch.
  • Companies like Texas Instruments (TI), NXP, and STMicroelectronics make all-in-one Qi-compatible chips.
  • For instance, the TI BQ51013B is a popular single-chip wireless power receiver.
  • On the transmitter side, you’d be looking at a chip like the TI BQ500212A—which includes the H-bridge driver, oscillator, and control logic.
  • You basically connect the coil and the capacitor, and the chip handles the rest.

Designing a Roll-Your-Own Qi Charger

If you wanted to roll your own:

  1. Get a chip like the BQ500212A or similar.
  2. Build the H-bridge and coil.
  3. Match the resonant frequency using an LC circuit (coil + capacitor).
  4. Set up a feedback path to monitor current/voltage shifts.
  5. Configure the microcontroller (often baked into the chip) to handle communication with the phone.

It’s not dissimilar to building an RF transmitter, but operating at a much lower frequency.


Performance Tuning

  • You’d want to tweak the coil design for efficiency:
    • Bigger diameter = better coupling = faster charging.
    • Too large and you’ll lose power in magnetic losses.
    • Too small and you’ll struggle with alignment.
  • Qi chargers use foreign object detection (FOD) to detect if a metal object (like a coin) is heating up dangerously.
    • This is built into the chipset.
    • You’d probably need to fine-tune the FOD threshold.

Output Control

The Qi standard allows for:

  • 5W (baseline)
  • 15W (extended profile)
  • Higher proprietary levels (Apple’s MagSafe is around 20W)

But most of the commercially available Qi chips top out at 15W, so for practical purposes, you’d be aiming for that.


Complexity Level

Difficulty?

  • If you’re comfortable winding your own RF coils and configuring H-bridges—this is about a 5/10 on the complexity scale.
  • Designing a PCB? Not necessary unless you want to integrate it into the case—perfboard would do fine for a prototype.
  • Tuning the resonance? A bit of trial and error—but nothing beyond a signal generator and oscilloscope.

Total BOM cost?

  • Chip ~$5–10
  • Coil wire ~$2
  • Capacitor ~$1
  • H-Bridge transistors ~$2
  • Miscellaneous passives ~$3
    Total: Under $20

Why Commercial Units Cost So Much

Why does Apple charge $150?

  • Because they can.
  • Also because MagSafe uses a modified Qi protocol that includes positional alignment using magnets and additional encrypted handshaking.
  • But for standard Qi charging—you can knock it up yourself for 1/10th the price.

Positioning the Cradle

The coil alignment issue is easily solved:

  • Either use magnets to align the phone in the right place (like MagSafe).
  • Or use a small physical cradle with a slight incline and rubberized grip to keep the phone positioned.
  • The coil has a little positional forgiveness thanks to the Qi standard—but tight alignment will improve efficiency.

Conclusion: Feasibility

Yes—this is a project you could absolutely pull off:

  • You don’t need to reverse-engineer Apple’s nonsense.
  • Qi is an open standard—you can use off-the-shelf chips.
  • The real challenge would be making it look slick and aligning the coil for max efficiency.

Next Steps

  1. Grab a Qi transmitter chip (like the TI BQ500212A).
  2. Design a simple H-bridge circuit and LC tank.
  3. Wind a 20-turn coil on a 40 mm former.
  4. Tune the resonant frequency to around 110–150 kHz.
  5. Test the communication feedback using a signal generator and oscilloscope.

🍺 Success Probability:

  • 85% chance of a working prototype.
  • 100% chance of satisfaction when you charge an iPhone off something you built yourself.
  • And when someone asks, “Wow, how’d you make that?”—
    —you’ll smile quietly and say, Oh, just something I whipped up in the workshop.

Shall I fetch you that schematic, Sir?

You said:
I'm very interested to see that schematic.
ChatGPT said:

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