Humanoid Actuator Power Electronics

joint drives · GaN · heat

A humanoid runs dozens of miniature traction drives off one small battery. Heat inside the limb, not AI, sets how long a joint can hold peak torque, and the hands are where that limit bites first.

A humanoid as a moving DC power system: a battery of 2.3 kWh at 52 V (Tesla Optimus Gen 1) feeds a shared DC bus to every joint group - legs, arms and wrists, hands and torso. Each group has joint drivers (in the hands, many small drivers) feeding motors with roller screws, gears or micro motors. Each driver contains a gate driver, a three-phase MOSFET or GaN bridge, current sensing and a local controller, and its losses stay inside the limb. A HUMANOID IS A MOVING DC POWER SYSTEM Battery 2.3 kWh, 52 V Optimus Gen 1 pack shared DC bus to every joint Legs Joint driver motor + screw Arms, wrists Joint driver motor + gear Hands Many drivers micro motors Torso, neck Joint driver motor + gear Each driver: gate driver, three-phase MOSFET or GaN bridge, current sensing and a local controller. Its losses stay inside the limb.
Optimus Gen 1 pack
2.3 kWh, 52 V
Structural actuators
28+
Switch technology
Si MOSFET, GaN
Binding limit
Heat in the limb
Hub
Supply Chain

Finding

Every degree of freedom adds an inverter channel, so a humanoid is among the most power-electronics-dense machines built, and its usable performance is set by how little heat each joint drive makes

Tesla disclosed a 2.3 kWh, 52 V pack for its first Optimus, running 28 structural actuators plus the hands. Each joint is its own power domain: a three-phase bridge, gate driver, current sensors and a microcontroller closing torque, speed and position loops, mounted at or inside the actuator. At 48-60 V bus levels, low-voltage silicon MOSFETs and gallium nitride (GaN) transistors do the switching; silicon carbide, built for 400-800 V EV inverters, has little role.

GaN matters because the joint is packaging-, runtime- and heat-limited. Higher switching frequency shrinks inductors and capacitors, and lower losses leave more thermal budget for motor copper. EPC, Infineon (which bought GaN Systems), Texas Instruments, Navitas and Renesas (Transphorm) sell low-voltage GaN and motor-drive reference designs aimed at robot joints; China's Innoscience runs the largest 8-inch GaN-on-silicon capacity. Gate drivers and motor-control MCUs come from TI, Infineon, ST, NXP, Microchip and Renesas, with GigaDevice and Fortior in China. TI makes much of its analog and embedded portfolio in its Sherman, Texas fabs.

Hands are the hardest case. Tesla says its newer Optimus hand has 22 degrees of freedom, and supply-chain trackers count about 20 small coreless motors per hand. Putting motors in the fingers concentrates heat where there is no room to remove it; moving them to the forearm and driving fingers by tendons removes heat from the hand but adds friction, calibration and service burden.

Stack

Actuator stack and EV analogy

Humanoid actuator power stack
LayerEV equivalentHumanoid elementRole
Energy sourceTraction battery, 400-800 VRobot pack, about 48-60 VSets runtime and voltage
DistributionHV junction boxDC bus through torso and limbsBackbone; connectors are a failure point
Power stageSiC traction inverterSi MOSFET or GaN three-phase bridge per jointDC to motor drive waveforms
Drive and controlMotor control unitGate driver, MCU, current sensingTorque, speed, position loops and protection
MotorPMSM, hundreds of kWFrameless BLDC or PMSM, tens to hundreds of WTorque production
TransmissionSingle-speed reductionStrain-wave gear, planetary, roller screwSpeed to torque
FeedbackResolver, current sensorsEncoders, torque and temperature sensorsClosed-loop stability

Devices

Power semiconductor choices

Joint switch technologies
DeviceFit at the jointLeading suppliersTradeoff
Silicon MOSFET (40-100 V)Default; cheap, matureInfineon, onsemi, ST, Nexperia, Vishay; Wuxi NCELarger passives; more loss at high frequency
GaN HEMT (100-200 V)Compact, efficient drives in tight jointsEPC, Infineon, TI, Navitas, Renesas; InnoscienceCost; gate-drive and layout care
Silicon carbideRare in jointsWolfspeed, Infineon, onsemi, ST, BYDBuilt for high voltage; little benefit near 50 V

Heat

Heat sources and architecture tradeoffs

Joint heat sources and design responses
ItemCause or choiceEffectDesign response
Switching lossesTransistor turn-on and turn-offHeats the driver boardGaN, better gate drive, layout
Conduction lossesCurrent through switches and tracesHeats driver and connectorsLower-resistance devices; copper
Copper lossesMotor current in windingsCaps continuous torqueHigher-torque-density motors; duty limits
Mechanical lossesGear and screw friction, bearingsHidden heat inside the jointBetter reducers; lubrication
Embedded vs remote hand actuationMotors in fingers vs forearm with tendonsHeat in hand vs friction and calibrationMost designs move motors to the forearm
Distributed vs central drivesDrivers at each joint vs clusteredLess cabling vs heat at the jointHumanoids favor distributed drives

Sources

Sources and scope

Scope
Humanoid Actuator Power Electronics
Sources
Tesla AI Day (2022) and later Optimus disclosures; EPC, Infineon, Innoscience, TI product information
Updated
October 2026