USB-PD vs Traditional USB: Why Power Hubs Don't Exist
This page explains the fundamental differences between traditional USB power distribution and USB Power Delivery (USB-PD), and why you can't simply use a "USB-C hub" to split USB-PD power.
Traditional USB (2.0/3.0) - Simple Power Distribution
Traditional USB hubs were simple because of the fixed voltage system.
AC Adapter (5V)
│
▼
┌─────────────┐
│ USB Hub │ ← Just splits 5V to all ports
└─┬───┬───┬───┘
│ │ │
5V 5V 5V ← Same voltage everywhere, no negotiation
Key characteristics:
- Fixed 5V - no voltage negotiation needed
- Simple current limits - 500mA (USB 2.0) or 900mA (USB 3.0) per port
- Passive distribution - hub just connects 5V rail to all ports
- Devices draw what they need (up to the limit)
The hub is essentially just a power splitter with some current limiting. Very simple, very cheap.
USB-PD - Complex Negotiation Required
USB Power Delivery is fundamentally different. Each device negotiates its own voltage and current requirements.
Charger (supports 5V/9V/15V/20V)
│
▼
┌─────────────┐
│ USB Hub │ ← Must negotiate with EACH device separately
└─┬───┬───┬───┘
│ │ │
?V ?V ?V ← Each device wants different voltage!
Device A wants 20V/3A (laptop)
Device B wants 9V/2A (tablet)
Device C wants 15V/3A (zudo-PD)
Why it's complicated:
- Voltage negotiation per port - Each device negotiates via CC pins
- Dynamic power budget - Hub must track total power and reallocate
- Voltage conversion - If charger provides 20V but device wants 9V, hub needs DC-DC converter
- Each port needs PD controller IC - Adds cost and complexity
- Re-negotiation - When devices plug/unplug, everything must re-negotiate
Comparison Table
| Feature | Traditional USB | USB-PD |
|---|---|---|
| Voltage | Fixed 5V | 5V/9V/12V/15V/20V (negotiated) |
| Negotiation | None | Required per device |
| Hub complexity | Passive splitter | Active controller per port |
| Power sharing | Simple current limit | Complex budget management |
| Cost to distribute | Very low | High (needs ICs, DC-DC per port) |
How Multi-Port USB-PD Chargers Actually Work
Multi-port USB-PD chargers are not simple hubs. They have sophisticated internal architecture:
┌─────────────────────────────────────────────────────────────┐
│ Multi-Port GaN Charger │
│ │
│ ┌─────────────┐ │
│ │ AC → DC │ Single main power stage │
│ │ (GaN) │ Converts AC to internal DC bus │
│ └──────┬──────┘ (e.g., 24V or 48V internal) │
│ │ │
│ ▼ │
│ ┌─────────────┐ │
│ │ Central │ "Brain" - manages total power budget │
│ │ MCU │ Decides how much each port can have │
│ └──────┬──────┘ │
│ │ │
│ ┌────┴────┬────────┬────────┐ │
│ ▼ ▼ ▼ ▼ │
│ ┌──────┐ ┌──────┐ ┌──────┐ ┌──────┐ │
│ │DC-DC │ │DC-DC │ │DC-DC │ │DC-DC │ Per-port converters │
│ │ + PD │ │ + PD │ │ + PD │ │ + PD │ (voltage conversion │
│ │ IC │ │ IC │ │ IC │ │ IC │ + PD negotiation) │
│ └──┬───┘ └──┬───┘ └──┬───┘ └──┬───┘ │
│ │ │ │ │ │
└────┼────────┼────────┼────────┼─────────────────────────────┘
▼ ▼ ▼ ▼
USB-C USB-C USB-C USB-C
Port1 Port2 Port3 Port4
Key Components
| Component | Role |
|---|---|
| AC-DC Stage | Single conversion from AC to internal DC bus (GaN for efficiency) |
| Central MCU | Power budget manager - decides allocation per port |
| DC-DC per port | Converts internal bus to negotiated voltage (5V/9V/15V/20V) |
| PD Controller per port | Handles CC negotiation with each device |
Dynamic Power Allocation Example
Example: 200W charger, 4 ports
Device plugs into Port 1, requests 100W
→ MCU: "OK, 100W available for Port 1"
Device plugs into Port 2, requests 65W
→ MCU: "OK, 65W for Port 2, total 165W used"
Device plugs into Port 3, requests 100W
→ MCU: "Only 35W left! Re-negotiate..."
→ Tells Port 1 & 2: "reduce power"
→ Redistributes: 65W + 65W + 65W = 195W
This is why good chargers (Anker, UGREEN, etc.) publish power distribution tables - the central MCU follows specific rules for allocation.
Key Terms
GaN (Gallium Nitride)
A semiconductor material - alternative to traditional silicon.
| Property | Silicon (old) | GaN (new) |
|---|---|---|
| Switching speed | Slower | Much faster |
| Heat generation | More | Less |
| Size | Larger | Smaller |
| Efficiency | ~85% | ~95% |
Result: GaN chargers are smaller, cooler, and more efficient.
Same 65W output:
┌─────────────┐ ┌───────┐
│ Silicon │ vs │ GaN │
│ Charger │ │ │
│ │ └───────┘
└─────────────┘
Large Compact
MCU (Micro Controller Unit)
A tiny computer chip - the "brain" inside devices.
┌─────────────────────────┐
│ MCU │
│ ┌─────┐ ┌─────┐ ┌───┐ │
│ │ CPU │ │ RAM │ │I/O│ │ All in one tiny chip
│ └─────┘ └─────┘ └───┘ │
└─────────────────────────┘
- Runs simple programs
- Reads sensors, controls outputs
- Very low power, very cheap
- Found in almost everything: chargers, appliances, toys, cars...
In USB-PD charger: MCU monitors all ports, calculates power budget, tells each port how much power it can provide.
Why You Can't Use a Regular USB-C Hub for Power
A regular USB-C hub (for data) cannot distribute USB-PD power because:
- No PD controllers - Data hubs don't have PD negotiation ICs
- No DC-DC converters - Can't provide different voltages per port
- No power management - No MCU to manage power budget
- Fixed 5V only - Most hubs only pass through 5V for charging
Bottom line: Multi-port USB-PD chargers exist (each port has its own PD controller and DC-DC converter), but USB-PD hubs that split power from a single upstream source are rare, expensive, and complex.
Implications for Modular Synth Power
For powering multiple zudo-PD units, use a multi-port USB-PD charger (not a hub):
- Each port independently negotiates 15V
- Shared ground eliminates ground loops between cases
- Central MCU manages power allocation
See USB-PD AC Adapter for recommended multi-port chargers.