BLHeli 14.8 damped light and active freewheeling

Aficionados of BLHeli call out the benefits of “damped light” and “active freewheeling”, terms coined by BLHeli's author.

Since these are terms invented by BLHeli, so you might wonder whether they are truly innovative or just marketing hype for existing techniques.

Lets go to the BLHeli manual for an explanation.

Pwm damped light mode adds loss to the motor for faster retardation. Damped light mode always uses high pwm frequency. In damped light mode, two motor terminals are shorted when pwm is off

Taking the last statement first, in fact, what happens that as that during the OFF phase of the PWM drive, the high side FETs at both ends of the winding are turned ON. One FET is on for the whole phase, and the other one switches on a short time after its corresponding low side FET turns off. The short time is to allow the low side FET to cease conducting, otherwise both high and low side FETs would conduct at the same time, a current from battery +ve to -ve via the two FETs. There is a corresponding pause at the end of the PWM phase. The time delays allowed depend on the driver circuitry and FET performance, they are specified in the firmware  for a specific and don't necessarily apply to a pin compatible ESC.

This technique is known in the wider community as COMPLEMENTARY PWM, a very standard technique.

Lets look at some more insight in the manual:

Damped light mode is implemented by doing braking, and inherently active freewheeling is also implemented. Then losses due to braking are counteracted by the reduced losses of active freewheeling.

Now to the detail… until the high side FET is turned on, the back EMF in the coil set drives current through the body diode of the high side FET, and of course that current causes a slowing torque on the motor shaft. In a non complementary PWM drive, the body diode may carry current well into the OFF phase whereas with complementary PWM, the high side FET quickly takes over in third quadrant mode.

What is the difference? The FET in third quadrant mode has lower voltage drop than the body diode, and so the deceleration is greater. Further, current may flow back to the battery for a very short time during the OFF phase.

Complementary PWM does NOT add loss to the motor as such and for the most part it reduces external losses, it does permit higher degeneration current and that retards the motor more quickly, the higher current increases loss in the motor's winding resistance. Some small amount of energy is delivered to the battery under some circumstances.

It is this latter effect that is called out as “active freewheeling”, more commonly known in the wider community as regenerative braking. The regenerative current (ie back to the source) flows for only a very small part of the PWM cycle, and accounts for negligible reduction in power consumption in most cases. It is actually a greater nuisance than benefit as it causes grief with electronic regulated power supplies in bench testing, and may risk on board transients in flight if a low resistance connection to the battery is compromised. (Never disconnect the battery on an aircraft with motors running, especially with complementary PWM, as there is a risk of a voltage transient that may take out on board electronics.)

To the purist, complementary PWM would seem the best choice, however it has the following significant disadvantages:

  • regenerative braking may try to feed current back to an incompatible power supply in bench testing;
  • regenerative braking may cause supply rail transients on an aircraft with compromised battery connection (and the connections used are not high reliability);
  • the delays programmed are critical to  performance, too short and current increases, FET dissipation increases, battery endurance is decreased; and too long means most of the work is already done by the body diode and there is negligible benefit;
  • waits in the firmware to allow FETs to turn off are critically important and tuned to specific circuits and devices (ie clones of an ESC need to be evaluated separately).

The effects of complementary PWM's improved dynamic braking is most evident at lowish to mid rpm. There are two influences:

  • at max rpm, propeller torque dwarfs motor braking, but propeller torque falls as rmp^2; and
  • dynamic braking torque tends to fall with speed (zero at zero rpm).