5 Gate driver and dead time effects that shape converter behaviour

Key Takeaways

• Gate driver timing is part of the power stage, so delay mismatch and gate current will shape losses, distortion, and stress before control software responds.
• Dead time should be set from worst case delay spread, then corrected with compensation for the voltage bias it introduces under load.
• Timing models that include the full path from driver to device will cut bench iteration and produce safer first hardware settings.

Dead time and gate drive timing will decide how a converter behaves before software control can clean up the waveform.

Motor systems use about 45% of global electricity, so small timing errors inside their converters deserve early design attention. A clean schematic and a stable control loop will not rescue a half bridge that turns on too early or too late. Gate driver strength, propagation delay, and dead time shape voltage error, heat, and device stress long before you start bench tuning. You will get better hardware faster when you treat those settings as part of converter design rather than a final trim step.

Gate driver timing sets converter behaviour before control loops act

Gate driver timing sets the converter’s physical switching sequence, and that sequence fixes what the power stage can do in each cycle. Turn on delay, turn off delay, and gate current define when current commutates. Control software acts later. If the hardware timing is poor, the waveform is already compromised.

A simple half bridge shows this clearly. If the high side device turns off 40 ns later than expected and the low side turns on 20 ns earlier, the overlap is enough to raise cross conduction risk even if your PWM duty cycle is mathematically correct. A motor drive will then show extra heating, noisier current, and a rougher line-to-line voltage than the controller commanded.

• Dead time shifts average phase voltage.
• Delay mismatch raises cross conduction risk.
• Weak gate drive increases switching loss.
• Extra blanking extends diode conduction.
• Timing spread breaks one fixed setting.

Those effects are often tuned late because the converter will still run at light load. That bench habit hides the fact that timing is part of the power stage itself. You’re not adjusting polish. You’re setting the physical order and duration of events that determine current path, voltage error, and thermal stress on every switching edge.

Dead time shapes output voltage across each switching interval

“Dead time inserts a small blank interval, but that blank interval changes the average phase voltage every switching period.”

Current keeps flowing while both devices are off, so it commutates through a diode or channel path that was not part of the commanded PWM state. The result is a predictable voltage error.

A low voltage inverter feeding an inductive load makes the effect easy to see. During positive current, dead time after the high side turns off forces current through the low side diode, so the phase node stays lower than the ideal PWM command. Reverse the current direction and the sign of the voltage error flips, even if duty cycle stays the same.

That is why dead time affects a converter most strongly near zero crossings and at low modulation index. The commanded duty cycle can be accurate while the applied voltage is biased every cycle. Current controllers then work harder to correct an error created in hardware, and the correction often appears as distortion, acoustic noise, or uneven torque.

Short dead time raises cross-conduction risk

Short dead time reduces voltage error, but it will raise cross-conduction risk if device turn-off is slower than expected. The unsafe case is not the nominal delay on a datasheet. The unsafe case is the slowest turn-off event across temperature, current, gate resistance, and part variation. That margin must be covered.

Consider a 400 V bridge leg where the upper device has a typical turn-off delay of 60 ns at room temperature. Add a hotter junction, a little more gate resistance from layout, and a slower lot of devices, and that event can stretch far enough that a 50 ns dead time becomes a direct overlap condition. The PWM command did nothing wrong, yet the leg still sees cross conduction current.

Cross conduction and dead time are linked through uncertainty, not just nominal timing. You’re sizing a guard band around the slowest off event and the fastest on event. Lab failures often happen after a load step or thermal soak because those conditions move both delays, and a setting that looked safe at start-up no longer covers the spread.

Long dead time extends body diode conduction

Long dead time keeps overlap away, but it also forces current through the diode path for longer than necessary. That raises conduction loss, increases reverse recovery stress in many topologies, and distorts the applied voltage. The penalty grows with current and switching frequency. Safe timing still needs to stay tight.

A traction inverter phase leg under heavy current shows the tradeoff quickly. Add a few hundred nanoseconds of extra blanking and the diode carries current longer on every edge. Junction temperature rises, the phase voltage flattens around transitions, and current ripple can climb even though the control law and bus voltage have not changed.

Long dead time also hides weak gate driver design. Some teams add blanking to stop occasional overlap, but the better fix is often cleaner layout, lower loop inductance, or a gate current profile that turns the device off with more certainty. Extra dead time is useful as insurance, yet too much of it taxes every cycle and every ampere.

Gate drive current sets switching loss during transitions

Gate drive current determines how quickly the device moves through its linear region, and that directly sets switching loss. Faster transitions cut overlap between voltage and current, but they also raise dv/dt and di/dt. Slower transitions reduce electrical stress on some nodes while heating the switch. You’re always trading one limit against another.

A bridge leg that uses a large gate resistor often looks calm on a scope, but the calm edge comes with a longer Miller plateau and more energy burned each turn on and turn off. Another design with a very small resistor switches sharply, then starts to show ringing and false triggering through common source inductance. Gate driver design basics always come back to that balance.

Wide-bandgap semiconductors can reduce power losses by up to 90% in some power conversion stages, which makes timing discipline even more important because transitions are much faster. Dead time selection, gate resistance, and layout can’t be guessed when edge speeds rise. You need settings that match the device physics and the commutation loop.

Dead time targets must cover delay spread

Dead time should cover the full spread of turn-off and turn-on delays, not just a typical number from one operating point. Temperature, bus voltage, current direction, gate resistance, and isolated driver skew all move the timing. A usable target is the smallest blanking interval that still protects the slowest switching case.

A good bench process starts with measured or modelled delay stacks. You add driver propagation delay mismatch, device turn-off spread, and layout-related variation, then keep a modest safety margin. That approach is much more stable than copying 500 ns from an old design that used a different device family and a different current range.

Dead time compensation restores commanded voltage under load

Dead time compensation corrects the average voltage error created by blanking, so the load sees something closer to the commanded waveform. It does not remove the need for safe timing. It subtracts the predictable bias after you have chosen a dead time that prevents overlap across operating conditions.

A current sign-based compensator is a common starting point. Positive phase current gets one correction polarity, negative current gets the opposite, and the magnitude reflects blanking time and bus voltage. That simple method works well in many motor drives because the voltage error changes sign with current direction and stays roughly proportional to dead time.

Compensation gets more trustworthy when you’ve already modelled the timing chain and current paths carefully. Teams using SPS SOFTWARE can inspect the gate drive sequence, the commutation path, and the resulting phase voltage in one place, then tune the correction against a transparent model rather than guessing from a noisy bench waveform. That makes dead time compensation explained in control code line up with what the switches are actually doing.

“Converters run clean when timing is designed, checked, and compensated before the first prototype forces the lesson.”

Timing simulation should include delays from driver to device

Timing simulation is only useful when it includes the whole path from PWM command to device current commutation. Driver delay, isolation skew, gate resistance, threshold spread, Miller behaviour, and parasitic inductance all matter. A simplified ideal switch model will hide the exact problems dead time is meant to control.

A useful setup starts with one bridge leg and a representative inductive load. You apply the intended PWM, sweep temperature and gate resistance, and inspect gate voltage, drain or collector current, and phase node voltage through each transition. That kind of study will show where overlap appears, where diode conduction lasts too long, and how much compensation your control law should apply.

That is why disciplined timing work belongs near the start of converter design and not at the end of bench debug. SPS SOFTWARE fits that workflow because it lets you inspect transparent switching models and set dead time from physical evidence instead of habit.