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:
- Oscillator – The heart of the system is indeed a high-frequency oscillator driving an H-bridge.
- Coil – A transmitting coil to generate the magnetic field.
- Capacitor – Forms a resonant tank circuit with the coil to create a stable operating frequency.
- 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:
- Get a chip like the BQ500212A or similar.
- Build the H-bridge and coil.
- Match the resonant frequency using an LC circuit (coil + capacitor).
- Set up a feedback path to monitor current/voltage shifts.
- 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
- Grab a Qi transmitter chip (like the TI BQ500212A).
- Design a simple H-bridge circuit and LC tank.
- Wind a 20-turn coil on a 40 mm former.
- Tune the resonant frequency to around 110–150 kHz.
- 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?
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