Mechanical Design
Physical dimensions and mechanical considerations for PCB layout and enclosure design.
Component Heights
Physical height reference for PCB layout and enclosure design.
Power Components
| Component | Designator | Package | Height | Notes |
|---|---|---|---|---|
| LM2596S-ADJ | U2, U3, U4 | TO-263-5 | 4.5mm | DC-DC converters |
| L7812CD2T-TR | U6 | TO-263-2 | 4.5mm | +12V linear regulator |
| L7805ABD2T-TR | U7 | TO-263-2 | 4.5mm | +5V linear regulator |
| CJ7912 | U8 | TO-252-3 (DPAK) | 2.3mm | -12V linear regulator |
| CYA1265-100UH | L1, L2, L3 | SMD 13.8x12.8mm | ~6-7mm | Power inductors |
| 1217754-1 | J6, J7, J8, J9 | FASTON 250 THT | 8.89mm | Power terminals |
| 470uF 25V | C3, C11, C20-C21, C24-C25 | D10xL10.2mm | 10.2mm | Electrolytic (tallest) |
| 470uF 16V | C4 | D6.3xL7.7mm | 7.7mm | Electrolytic |
| 470uF 10V | C22, C23 | D6.3xL7.7mm | 7.7mm | Electrolytic |
| 100uF 25V/50V | C5, C7, C9 | D6.3xL7.7mm | 7.7mm | Electrolytic |
Height Profile
PCB Design Implications
- Tallest components: 470uF 25V electrolytic caps (10.2mm)
- Second tallest: FASTON terminals J6-J9 (8.89mm)
- Third tallest: Smaller electrolytics & inductors (7.7mm / ~6-7mm)
- Total board height: ~12mm including PCB thickness (1.6mm)
- Clearance: Keep space around TO-263 packages for thermal dissipation
- CJ7912 advantage: Lower profile (2.3mm) allows flexible placement
- FASTON placement: Position at board edge for cable access
- Capacitor placement: Consider grouping tall caps for enclosure fit
PCB Dimensions
To be determined based on component placement.
Heatsink Design
Aluminum PCB Heatsink Concept
A cost-effective heatsink solution using JLCPCB's aluminum PCB service. The aluminum PCB acts as a custom-fit heatsink that sits on top of the voltage regulators (U6, U7, U8) and DC-DC converters (U2, U3, U4).
Design Approach
Cross-section view:
Aluminum PCB (heatsink)
┌─────────────────────────────────────────────────────┐
│░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░│
└──┬──────────┬─────────────────┬─────────────────┬───┘
│ Thermal │ │ │
│ Pad │ (cutout) │ (cutout) │
┌──┴──┐ ┌──┴──┐ ┌──┴──┐ ┌──┴──┐
│ IC │ │ IC │ │Cap │ │Cap │
│ U6 │ │ U7 │ │ │ │ │
└─────┘ └─────┘ └─────┘ └─────┘
════════════════════════════════════════════════════════
Main PCB
Key features:
- Same outline as main PCB - Easy alignment and mounting
- Cutouts for tall components - Electrolytics (10.2mm), inductors (6-7mm), FASTON terminals
- Solid areas over ICs - TO-263 (4.5mm) and TO-252 (2.3mm) packages for heat transfer
- Thermal gap pads - Fill the gap between IC top surface and aluminum PCB
Target Components for Cooling
| Component | Designator | Package | Height | Heat Dissipation |
|---|---|---|---|---|
| L7812CD2T-TR | U6 | TO-263-2 | 4.5mm | High (1.5V × 1.2A = 1.8W) |
| L7805ABD2T-TR | U7 | TO-263-2 | 4.5mm | High (2.5V × 0.5A = 1.25W) |
| CJ7912 | U8 | TO-252-3 | 2.3mm | High (1.5V × 0.8A = 1.2W) |
| LM2596S-ADJ ×3 | U2-U4 | TO-263-5 | 4.5mm | Medium (switching losses) |
Gap Calculation for Thermal Pads
The gap between IC package top and aluminum PCB bottom depends on:
- Aluminum PCB thickness: ~1.6mm
- Standoff/spacer height: TBD
- IC package height: 2.3mm (TO-252) to 4.5mm (TO-263)
Recommended thermal pad thickness: 1mm - 2mm (get multi-thickness pack to test)
Thermal Gap Pad Selection
Thermal gap pads (サーマルパッド) provide thermal interface between ICs and the aluminum heatsink.
Required Specifications
| Spec | Requirement |
|---|---|
| Thermal Conductivity | 12+ W/m·K (higher = better) |
| Thickness | 0.5mm - 2mm (measure gap) |
| Size | 100×100mm (cut to fit) |
| Electrical | Non-conductive |
| Material | Silicone-based |
Recommended Products
AliExpress (Budget, 2-4 week shipping):
| Product | W/m·K | Size | Price | Link |
|---|---|---|---|---|
| 24W/mK Thermal Pad | 24 | 100×100mm | ~$3 | AliExpress |
| Upsiren 24W/mK | 24 | 100×100mm | ~$3 | AliExpress |
| Rgeek 12.8W/mK | 12.8 | 100×100mm | ~$5 | AliExpress |
Amazon Japan (Fast delivery, 1-2 days):
| Product | W/m·K | Size | Price | Link |
|---|---|---|---|---|
| OwlTree 3-pack | 12.8 | 100×100mm (0.5/1/1.5mm) | ~¥1,500 | Amazon.co.jp |
| Thermalright | 12.8 | 120×120mm | ~¥1,000 | Amazon.co.jp |
| 17.3W Non-conductive | 17.3 | 100×100mm | ~¥1,500 | Amazon.co.jp |
Application Tips
- Clean surfaces - Remove dust and oil from IC and aluminum PCB
- Cut to size - Match IC package footprint (TO-263: ~10×9mm, TO-252: ~6.5×6mm)
- Slight compression - Pads should be 10-20% compressed for optimal contact
- No air gaps - Ensure pad fully covers IC thermal surface
Ordering Aluminum PCB from JLCPCB
- Create edge cuts in KiCad matching main PCB outline
- Add cutouts for tall components (capacitors, inductors, FASTON terminals)
- Export Gerber files with edge cuts only (no circuit needed)
- Order as aluminum PCB from JLCPCB
- Select thickness based on thermal mass requirements
Detailed KiCad procedure to be added after prototype testing.
Enclosure Considerations
To be added.