For designing a PCB, many people use a brushless DC motor with end application mandating that the motor rotates in both directions, i.e., clockwise and anticlockwise. Some people argue to use FETs, diodes, and I/O ports on the chosen microcontroller so that the direction of rotation of the DC motor could be manually controlled but they forget to make use of the TI’s DRV8801PWPR motor driver. The DRV8801PWPR is an H-bridge motor driver which offer various features. These features include slow decay current mode, fast decay current mode, and a low-power sleep mode (which is a major requirement for many battery-operated projects).
Just like the DRV8801, the MAX14870 from Maxim also contains the FETs which help to make up the H-bridge. The MAX14870 is used as a motor controller in many C-BISCUIT designs and many other motor drivers which allow engineers to use external FETs.
The most common example of such gate-driver ICs is the DGD0506 from Diodes Incorporated. Before discussing the modes of decay, let’s have a quick analysis of how H-bridge works.
Image: DGD0506 from Diodes
An H-bridge is a simple circuit which contains four independently controlled FETs(BHTs were used in the past but FETs are more common in use now a days). These FETs serve as the switching elements and can be used for channelling current flow through the load. The load is usually an inductive load, such as a motor. The following image illustrates the H in the H-bridge.
Image: Circuit diagram of an H-bridge where the two current paths resemble the letter “H”
The free-wheeling or fly backdiodes, also called as the external diodes, are not always included in an H-bridge circuit because the FET’s body diodes can easily substitute them.
Fast Decay Mode
Many a times engineers come at a point that the installed DC motor, when disabled, would stop quickly at a particular position. The requirement certainly requires employing fast decay mode as is reasonable to assume that fast decay corresponds to fast deceleration. This is in fact wrong. The terms ‘fast decay’ and ‘slow decay’ are associated with the current flow through the inductor and they are not related to the working of the DC motor in any way.
The term ‘fastdecay’ actually means that the motor’s coils current will decay to zero within seconds. The following image illustrates the current flow which started from Vba, then travelled through Q1, the winding and then Q4 leading to the ground node. This is the condition where the motor is energized and working properly.
Image: The motor is energized and turning normally
As we all know that the current through an inductor cannot vary rapidly; therefore, if we disable Q1 and Q4 current will flow back through fly back diodes or FET body diodes. This current will then gradually decrease to zero. So, fast decay mode is the technique which uses FETs instead of diodes for decaying inductive current. The following image shows Q2 and Q3 enabled while Q1 and Q4 are both disabled.
Image: Q2 and Q3 enabled (fast decay mode)
In short, depending on your use, the fast decay mode may or may not be required, conversely you can use fly back diodes or body diodes. However, the starting time of these diodes cannot be known. According to TI’s Community Support page, “Generally, fast decay usually is needed for high inductance motor[s], high running speed, [or] high degrees [of] micro stepping which all need the current [to] change quickly.”
To summarize fast decay mode: Do not think that fast decay will stop the motor quickly because, in reality, it’s the opposite.
Slow Decay Mode
For a better explanation go to image above when the motor is running normally. Now, instead of switching Q2 and Q3 on and turning off Q1 and Q4, as was done for fast decay mode (image 3), we will now disable Q1 and enable Q2 (see Image 5 below).It must be noted here that both low-side FETs or both high-side FETs can be used for slow decay mode.
Image: Q2 and Q4 enabled (slow decay mode)
The current of the inductor changes to zero as it passes through Q2 and Q4 (recirculating-type fashion). In this case, no applied voltage is present to rapidly discharge; rather the current dissipates in the form of heat as friction increases in the inductor and also between the two FETs. Despite the slower current decay, this mode provides a faster reduction in motor speed. So, when a DC motor is rotating, it generates back EMF, which can be manifested by the rapid current decay. The motor will coast toward zero angular velocity as the stored energy is gradually dissipated.
In the image above, when the Q2 and Q4 are enabled, a low-impedance path between the two motor terminals is created which essentially shorts out the back EMF, thus allowing the motor’s stored energy to be dissipated much more quickly. The result is rapid deceleration, to the point that the term “brake” is associated with slow decay mode.
It must be remembered that the names “slow” and “fast” are directly associated with the rate of decay of the current through the inductive load (such as a motor winding). This is not associated with the reduction in the angular velocity of the motor. Fast decay mode causes a rapid reduction in inductive current and allows the motor to coast toward zero velocity. Slow decay mode leads to a slower reduction in inductive current but produces rapid deceleration. The side image summarizes the current pathways of these two decay modes.