233 lines
6.4 KiB
Markdown
233 lines
6.4 KiB
Markdown
# Trace Width Guide: Power Distribution
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**Date:** 2025-12-28
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**Based on:** ESP32-WROOM-32E, AMS1117-3.3, TXB0104PWR
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---
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## Current Requirements Summary
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### 3.3V Power Rail
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- **ESP32 peak current:** 500mA (WiFi transmission)
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- **LEDs (2×):** ~12mA
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- **TXB0104 VCCA:** 5µA (negligible)
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- **Pull-up resistors:** <0.1mA (negligible)
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- **Total peak current:** **~512mA**
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### 5V Power Rail
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- **TXB0104 VCCB:** 5µA (negligible)
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- **AC Connector:** Minimal (if just signaling)
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- **Total current:** **~10mA** (very low)
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---
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## Trace Width Calculations
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### IPC-2221 Standard Reference
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**Current Carrying Capacity (10°C temperature rise):**
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| Copper Weight | Layer Type | Capacity |
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|---------------|------------|----------|
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| 1oz (35µm) | External (F.Cu/B.Cu) | ~1.0A per 1mm width |
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| 1oz (35µm) | Internal | ~0.5A per 1mm width |
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| 2oz (70µm) | External | ~2.0A per 1mm width |
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| 2oz (70µm) | Internal | ~1.0A per 1mm width |
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---
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## Recommended Trace Widths
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### For Standard 1oz Copper PCB (Most Common)
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#### 3.3V Power Traces
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**Minimum Requirements:**
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- **External layer:** 0.51mm (for 512mA)
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- **Internal layer:** 1.02mm (for 512mA)
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**Recommended (with safety margin):**
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- **External layer:** **1.0mm** (50% margin, easy to route)
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- **Internal layer:** **1.5mm** (50% margin)
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**Best Practice:**
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- **Main power rail:** Use **1.0mm to 1.5mm** wide traces
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- **Branch traces:** Can be narrower (0.5mm) for short runs to components
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- **Copper zones/pours:** Even better for power distribution
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#### 5V Power Traces
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**Minimum Requirements:**
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- **Any layer:** 0.01mm (for 10mA) - theoretical minimum
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**Recommended:**
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- **Standard signal width:** **0.3mm** (sufficient and standard)
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- **Main rail:** **0.5mm** (if using dedicated trace)
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**Note:** 5V current is very low, so trace width is not critical. Use standard signal trace width (0.2-0.3mm) or slightly wider (0.5mm) for main rail.
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---
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## Detailed Recommendations
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### 3.3V Power Distribution
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#### Option 1: Wide Traces (Recommended)
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```
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Main 3.3V rail: 1.0mm - 1.5mm width
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Branch to ESP32: 0.8mm - 1.0mm width
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Branch to LEDs: 0.3mm - 0.5mm width
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Branch to TXB0104: 0.3mm - 0.5mm width
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```
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#### Option 2: Copper Zones/Pours (Best Practice)
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- Create a **3.3V copper zone** covering the board area
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- Provides lowest impedance
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- Best for power distribution
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- Use **1.0mm clearance** from other nets
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#### Option 3: Hybrid Approach
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- Main rail: **1.5mm wide trace** from regulator to ESP32 area
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- Copper zone: **3.3V zone** around ESP32 and other components
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- Branch traces: **0.5mm** for short connections
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### 5V Power Distribution
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```
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Main 5V rail: 0.3mm - 0.5mm width (sufficient)
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Branch to TXB0104: 0.3mm width (standard)
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Branch to AC Conn: 0.3mm width (standard)
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```
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**Note:** 5V current is so low that trace width is not a concern. Use standard signal trace widths.
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---
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## Implementation Guidelines
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### For KiCad PCB Layout
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#### Setting Up Trace Widths
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1. **Design Rules Setup:**
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- Go to: `File → Board Setup → Design Rules → Net Classes`
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- Create net classes:
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- `Power_3V3`: Min width 0.5mm, Preferred 1.0mm, Max 2.0mm
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- `Power_5V`: Min width 0.2mm, Preferred 0.3mm, Max 1.0mm
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- `Signal`: Min width 0.2mm, Preferred 0.2mm, Max 0.5mm
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2. **Assign Net Classes:**
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- `+3.3V` net → `Power_3V3` class
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- `+5V` net → `Power_5V` class
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- All other nets → `Signal` class
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3. **Routing:**
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- Route power traces first (widest)
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- Use copper zones for power distribution where possible
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- Keep power traces short and direct
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### Trace Width by Location
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| Location | 3.3V Width | 5V Width | Notes |
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|----------|------------|----------|-------|
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| **Regulator output** | 1.0-1.5mm | - | Main power source |
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| **To ESP32 VDD** | 1.0mm | - | High current path |
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| **To TXB0104 VCCA** | 0.5mm | - | Low current |
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| **To LEDs** | 0.3-0.5mm | - | Low current |
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| **To pull-ups** | 0.2-0.3mm | - | Very low current |
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| **5V main rail** | - | 0.3-0.5mm | Low current |
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| **To TXB0104 VCCB** | - | 0.3mm | Very low current |
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---
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## Copper Zone Recommendations
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### 3.3V Copper Zone
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- **Layer:** F.Cu or B.Cu (or both)
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- **Clearance:** 0.5mm from other nets
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- **Min width:** 0.5mm (for narrow areas)
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- **Coverage:** Around ESP32, regulator, and power distribution area
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### GND Copper Zone
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- **Layer:** Both F.Cu and B.Cu (ground plane)
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- **Clearance:** 0.3mm from other nets
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- **Coverage:** Entire board (ground plane)
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- **Vias:** Connect both layers with multiple vias
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### 5V Copper Zone (Optional)
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- **Not necessary** due to very low current
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- Can use if board space allows
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- **Width:** 0.3mm minimum if used
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---
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## Thermal Considerations
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### Power Dissipation
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- **3.3V @ 512mA:** ~1.69W (ESP32 peak)
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- **Trace resistance:** Lower with wider traces
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- **Voltage drop:** Minimize with wide traces and short paths
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### Trace Heating
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With 1.0mm trace width and 512mA:
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- **Temperature rise:** ~10°C (acceptable)
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- **Voltage drop:** <50mV for typical trace lengths
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---
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## Design Checklist
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- [ ] 3.3V main rail: **1.0mm minimum** (1.5mm preferred)
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- [ ] 3.3V to ESP32: **1.0mm minimum**
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- [ ] 3.3V branches: **0.5mm minimum** for short runs
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- [ ] 5V traces: **0.3mm minimum** (standard signal width)
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- [ ] Ground plane: **Full coverage** on one or both layers
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- [ ] Power zones: Consider copper zones for 3.3V
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- [ ] Vias: Use multiple vias for layer transitions on power nets
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- [ ] Clearance: Maintain proper clearance from other nets
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---
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## Quick Reference
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### Minimum Trace Widths (1oz copper, external layer)
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| Net | Current | Minimum | Recommended |
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|-----|---------|---------|-------------|
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| **+3.3V** | 512mA | 0.51mm | **1.0mm** |
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| **+5V** | 10mA | 0.01mm | **0.3mm** |
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| **GND** | - | - | **Ground plane** |
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| **Signals** | <10mA | 0.2mm | **0.2mm** |
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### Summary
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- **3.3V traces:** Use **1.0mm** width (or copper zone)
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- **5V traces:** Use **0.3mm** width (standard signal width)
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- **GND:** Use **ground plane** (copper zone covering entire board)
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---
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## Additional Notes
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### Why 1.0mm for 3.3V?
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- Provides **50% safety margin** over minimum requirement
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- Easy to route and manufacture
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- Low voltage drop
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- Good thermal performance
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- Standard practice for power traces
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### Why 0.3mm for 5V?
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- Current is very low (10mA)
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- Standard signal trace width
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- Easy to route
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- Sufficient for the application
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### Copper Zones vs Traces
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- **Copper zones:** Best for power distribution (lowest impedance)
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- **Traces:** Good for point-to-point connections
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- **Hybrid:** Use zones for main distribution, traces for branches
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---
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*Guide created: 2025-12-28*
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