Motor Control and Timing

Sensorless BLDC Motors and Switching Dead Time

Understand back-EMF sensing and why opposing switches need separation in time.

By MySunDirection
Published 13 October 2026
Updated 14 October 2026
13 min read

One-minute answer

You need to understand the internal functions in “Sensorless BLDC Motors and Switching Dead Time” before selecting, connecting or troubleshooting the real equipment.

Learn how a sensorless six-step BLDC drive observes the floating phase, why startup needs another strategy, and how half-bridge dead time prevents cross-conduction.

Select or check first: The 30-degree delay is nominal, not a universal timer constant. Half a sector is 30 electrical degrees in the ideal six-step picture. The equivalent time changes with speed, and practical drives adjust for detection delay, winding current lag, load and desired phase advance.
Controlling assumption: Sensors, controller logic, actuator limits, timing, fault response and environmental conditions are defined together.
Top risk: Do not tune dead time on an unprotected full-energy bus. Start with the power-stage manufacturer's recommended design process, current limiting and fault shutdown. High-side measurements can exceed ordinary oscilloscope probe ratings, and an inverter can regenerate into its DC link.
Proof: Check the exact manufacturer data, units and limits, then verify the result by calculation, measurement or a controlled acceptance test.

Start with the basics Optional foundation before “Sensorless BLDC Motors and Switching Dead Time”

Automation-and-control basics

An automatic system measures a condition, decides what it means and commands an actuator while checking whether the expected result actually happened.

Sensor: A sensor converts a physical condition into a signal. Its range, uncertainty, response time, environment and failure behaviour matter as much as its nominal output.
Controller: A controller reads inputs, applies logic or a control law and issues outputs. Software cannot correct missing measurements, inadequate hardware authority or unsafe architecture.
Actuator: An actuator turns a command into physical action, such as motor torque, valve position or switching. Its real limits depend on load, supply, drive and duty.
Feedback: Feedback compares measured behaviour with the desired state so error can be corrected. Open-loop command assumes the action occurred; closed-loop control checks it.
Fault and safe state: A fault is an abnormal condition that may invalidate control. The safe response must be defined from the hazard and may require independent hardware, not only software.

How to use this article: Trace one complete chain—physical condition, sensor, signal conditioning, controller, drive, actuator and feedback—then ask how each link can fail.

On this page

The undriven phase carries rotor-timing information
Sensorless startup is a controlled handover
Dead time separates opposing half-bridge commands
Commission rotor timing and switch timing separately

A sensorless six-step BLDC controller removes Hall-position wiring by extracting timing from motor voltage. That does not make the drive unaware of position: it replaces a physical position sensor with an electrical observation and an estimator. The same controller must also switch each inverter leg safely. These are separate timing problems—deciding when the rotor needs the next commutation state, and ensuring two opposing power switches never conduct together.

The undriven phase carries rotor-timing information

Sensorless BLDC motor and controller with qualitative floating-phase back-EMF crossing neutral halfway through a 60-degree electrical sector. — In a six-step sector, two phases conduct and the floating phase can be observed. A zero crossing is a timing reference, not the commutation command itself.

Rotor magnets moving past a stator winding induce back EMF. During 120-degree six-step control, the undriven phase is available for observation. Its voltage is compared with a neutral reference—sometimes the motor neutral, sometimes a synthesized midpoint, and sometimes a processed bus-related threshold. In the idealized trapezoidal model, the relevant crossing occurs halfway through the 60-electrical-degree sector. The controller then schedules the next commutation after an appropriate interval.

Back EMF grows with speed

E = k_e ω

This simplified proportional relation explains the startup limitation: at zero speed there is no useful rotational back EMF. Units for E, the motor constant and angular speed must be compatible, and a measured terminal waveform also contains switching and circuit effects.

E: induced back EMF
k_e: motor voltage constant in compatible units
ω: rotor angular speed

The crossing detector needs context
Condition	Why the raw signal is difficult	Design response
Standstill and very low speed	Back EMF is absent or too small relative to offsets and noise	Align and start open-loop, use Hall sensors, or use another estimator
Immediately after commutation	Inductive ringing and switching transients contaminate the floating terminal	Apply a justified blanking interval before accepting samples
PWM operation	Switch-node edges and common-mode motion enter the measurement	Synchronize sampling or filter without excessive phase delay
Rapid acceleration	The sector period changes while the next event is predicted	Update timing from recent crossings and bound acceleration
Lost crossing	No trustworthy rotor event is available	Timeout, reduce drive or enter a defined restart/fault sequence

Sensorless startup is a controlled handover

Typical conceptual sequence

Establish a known or acceptable initial rotor orientation with bounded current.
Advance commutation open-loop slowly enough that the rotor can follow.
Monitor the floating-phase signal while speed and signal-to-noise ratio increase.
Require several plausible crossings with correct order and timing before declaring lock.
Blend or switch to crossing-based commutation under explicit acceptance criteria.
If timing becomes implausible, leave closed-loop operation safely rather than continuing a guessed sequence.

Dead time separates opposing half-bridge commands

Half-bridge high-side and low-side command waveforms with both commands off briefly between complementary switching transitions. — The command gap gives one switch time to stop conducting before its opposite is allowed on. The required interval depends on the driver, devices and operating conditions.

A half bridge connects one motor terminal alternately toward the positive and negative DC rails. If its high-side and low-side MOSFETs conduct simultaneously, they create a low-impedance path across the supply. That cross-conduction, or shoot-through, can produce destructive current. Dead time delays the incoming turn-on command after the outgoing device has been commanded off.

More delay is not automatically safer overall
Timing	Immediate effect	Consequence
Too short	Outgoing device may still conduct when the opposite device turns on	Shoot-through, heating, bus disturbance or device failure
Adequate	No overlap across expected delay, gate charge, temperature and tolerance	Safe commutation with bounded diode interval
Excessively long	Load current freewheels through a diode or alternate path longer than necessary	Additional loss, voltage error, torque distortion and heating

Command waveforms are not proof of transistor conduction. Propagation delay mismatch, gate resistance, device charge, Miller coupling, bus voltage, current direction and temperature affect the actual drain and gate waveforms. Some drivers use a fixed or programmed delay; others monitor gate voltage and handshake between outputs. Read the driver and switch data sheets together and verify worst-case operation with suitable isolated measurement practices.

Commission rotor timing and switch timing separately

Two timing loops need different measurements
Question	Evidence
Does zero-crossing order match phase sequence?	Floating-phase waveforms labeled by commutation sector and direction
Is startup handover reliable?	Repeated cold, hot, loaded and low-bus startups with lock/fault logs
Does actual switch conduction overlap?	Gate-source and switch-node waveforms captured with appropriate probes plus current evidence
Is dead time excessive?	Diode-conduction interval, switching loss, temperature and torque/current distortion
Do faults fail safely?	Missing-crossing, stall, overcurrent and undervoltage tests at bounded energy

Review the phase-state and Hall-sector foundation in How a BLDC Motor Knows When to Switch. For the broader motor voltage-current relationship that produces back EMF, see Why a Brushed DC Motor Starts Differently.

Selection boundary and evidence to keep with the decision

Treat “Sensorless BLDC Motors and Switching Dead Time” as a selection problem, not only a definition. Record the required result, alternatives considered, the controlling assumption and the condition that would change the choice.

Keep evidence proportional to consequence: Check the exact manufacturer data, units and limits, then verify the result by calculation, measurement or a controlled acceptance test. Revisit the decision when the load, environment, duty, interface, regulation or maintenance plan changes.

One-minute questions

Questions to answer before you apply this lesson

What should I select or check first for Sensorless BLDC Motors and Switching Dead Time?

The 30-degree delay is nominal, not a universal timer constant. Half a sector is 30 electrical degrees in the ideal six-step picture. The equivalent time changes with speed, and practical drives adjust for detection delay, winding current lag, load and desired phase advance.

Which assumption can change the conclusion?

Sensors, controller logic, actuator limits, timing, fault response and environmental conditions are defined together.

What can go wrong if I copy the example directly?

Do not tune dead time on an unprotected full-energy bus. Start with the power-stage manufacturer's recommended design process, current limiting and fault shutdown. High-side measurements can exceed ordinary oscilloscope probe ratings, and an inverter can regenerate into its DC link.

How do I verify the result in the real system?

Check the exact manufacturer data, units and limits, then verify the result by calculation, measurement or a controlled acceptance test.

Sources and further reading

Learn Six-Step Sensorless BLDC Motor CommutationMicrochip Technology · 2026-10-13Manufacturer guidance reviewed for floating-phase back-EMF detection, midpoint crossing, nominal commutation delay and noise, ringing and filtering concerns.
AN1160: Sensorless BLDC Control with Back-EMF FilteringMicrochip Technology · 2026-10-13Primary application note reviewed for 120-degree six-step states, one floating winding per sector and practical sensorless commutation architecture.
Understanding Smart Gate DriveTexas Instruments · 2026-10-13Manufacturer white paper reviewed for half-bridge shoot-through, dead-time insertion and the tradeoff between overlap prevention and diode-conduction loss.
Configurable DeadtimeTexas Instruments · 2026-10-13Manufacturer application brief reviewed for open-loop versus gate-voltage-monitored dead time and why both too-short and too-long intervals are undesirable.