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  • Created:
  • Updated:
  • Author:
    Takeshi Takatsudo

Project Overview

A compact power supply module that provides ±12V/+5V power for modular synthesizers using USB-C PD 15V as input.

Design Goals​

Output Specifications​

  • +12V: 1500mA max (L7812CD2T-TR)
  • -12V: 1000mA max (CJ7912)
  • +5V: 1500mA max (L7805ABD2T-TR)

Input​

  • USB-C PD 15V 3A (max 45W)

Performance Characteristics​

  • Efficiency: Approx. 75-80% (overall)
  • Ripple Noise: <1mVp-p (final output)
  • Regulation: ±1% (line & load variation)
  • Response Speed: Excellent (due to linear stage)
  • Safety Margin: 150%+ across all circuits

Architecture​

4-Stage Design​

This project adopts a 4-stage architecture to achieve efficient power conversion while minimizing noise.

USB-C 15V ──┬─→ +13.5V (DC-DC) ──→ +12V (LDO) ──→ +12V OUT
            │
            ├─→ +7.5V  (DC-DC) ──→ +5V  (LDO) ──→ +5V OUT
            │
            └─→ -15V (Inverter) ──→ -13.5V (DC-DC) ──→ -12V (LDO) ──→ -12V OUT

Stage 1: USB-PD Power Delivery​

  • STUSB4500 IC: USB-IF certified USB-C PD protocol negotiation
  • 15V Output: Obtains stable 15V through PD negotiation
  • Load Switch: AO3401A P-MOSFET enables power after successful negotiation

Stage 2: DC-DC Converter​

  • LM2596S-ADJ × 3: High-efficiency buck converter
    • +15V → +13.5V (for +12V rail)
    • +15V → +7.5V (for +5V rail)
    • -15V → -13.5V (for -12V rail)
  • LM2586SX-ADJ: Inverted SEPIC converter (+15V → -15V, 3A capable)
  • Efficiency: Approx. 85-90%

Stage 3: Linear Regulator​

  • LM7812: +13.5V → +12V (noise reduction)
  • LM7805: +7.5V → +5V (noise reduction)
  • LM7912: -13.5V → -12V (noise reduction)
  • Ripple: <1mVp-p

Stage 4: Protection Circuit (Beginner-Friendly, 2-Level Protection)​

  • PTC Resettable Fuse: Auto-recovery during overload
  • SMD Fuse: Backup protection during short circuit
  • TVS Diode: Surge voltage protection
  • LED Indicator: Status display for each voltage line

Design Features​

1. All Parts Available from JLCPCB​

  • Extensive Basic Parts Usage: No additional costs
  • Rich Inventory: From minimum 2,763 to maximum 21 million units
  • Stable Sourcing: Stable supply chain

2. High-Performance Design​

  • 2-Stage Filter: DC-DC + Linear for low noise
  • Sufficient Margin: 150%+ safety margin across all circuits
  • Modular Synth Optimized: Low noise, high stability

3. Beginner-Friendly Protection Circuit​

  • PTC Auto-Recovery: Recovers in 30 seconds even with excessive module connections
  • Staged Protection: Overload → PTC, Short circuit → Fuse
  • Visual Feedback: Instantly recognize overload by LED extinguishing
  • No Repair Needed: Users can resolve normal overloads themselves

4. Manufacturability​

  • All SMD Components: Compatible with automated assembly
  • TO-220 Package: Easy heatsink mounting
  • Separated Design: Physical separation of DC-DC and linear stages

Protection Circuit Operation​

Normal Operation​

  • PTC has zero resistance
  • LED lights brightly ✅

Overload (1.2-2A)​

  • PTC becomes high resistance
  • LED extinguishes
  • Reduce modules and wait 30 seconds
  • Auto-recovery 🔄

Short Circuit (>2A)​

  • Fuse blows
  • Repair required ❌

Next Steps​

  1. Finalize Unsearched Components:
  • PTC Resettable Fuse (1.1A, 0.75A, 0.9A @ 16V)
  • 2A SMD Fuse (+12V backup)
  1. PCB Design:
  • 4-layer PCB (power plane separation)
  • Thermal via placement
  • EMI countermeasure layout
  1. Prototype Manufacturing:
  • Use JLCPCB SMT service
  • Initial lot of approximately 10 units
  1. Performance Verification:
  • Ripple noise measurement
  • Load response characteristics
  • Thermal design verification
Last updated on by Takeshi Takatsudo