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:

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✅ 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.

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✅ 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.

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✅ 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.”

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✅ 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.

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✅ 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.

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✅ 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.

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✅ 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.

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✅ 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

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✅ 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.

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✅ 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.

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✅ 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.

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✅ 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.

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🍺 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: