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An H Bridge allows the control of current on both directions through an inductive load such as a motor. Figure 1-1 shows how, by choosing which FET is enabled, current is made to flow in one direction or the other.
Due to the physical properties of inductive loads, once current is flowing in one direction, said direction must be maintained. This is also true when the H Bridge is disabled or when the opposing voltage polarity is applied (e.g. when the Direction command is switched).
Not giving a safe path for this current to flow, while it decays down to zero or switches to the new direction current, will result in damage to the H Bridge’s power switches.
A proper path for this current decay is often offered as free wheeling diodes in parallel with the FET switch, which will start conducting as soon as the FET switches are disabled. A more efficient way to handle this current is to enable/disable FET switches in a sequence that carries the decaying current, but without causing shoot through.
These alternatives to control current flow until it decays are referred to as current recirculation methods. On this application note we will detail each style of current recirculation and decay modes.
When diodes are used to accept the current flow while it decays, this is referred to as asynchronous decay. It is asynchronous to the controller turning the FET switches ON and OFF. The timing of when the diodes will start conducting is not known, but it is highly recommended for this turn ON time to be as short as possible in order to avoid possible damage to the FET switches. Schottky diodes are often used for this purpose.
Although FET switches often have a body diode associated with them, it is often much more efficient to utilize the FET ON resistance as a safe path for current decay. When the controller coordinates the turning ON and OFF of FET switches as a means to offer a safe path to current during decay, this is referred to as synchronous decay. The time of when the FETs are brought online to carry the current is known and fixed.
It is impossible to instantaneously offer a safe path for decaying current by turning opposing FETs, since this would cause shoot through. As a result, every controller employing a synchronous decay mechanism will, for a very small period of time, employ a form of asynchronous decay through the FET switches’ body diodes.
On the upcoming definitions, the words “fast” and “slow” are meant to correlate to how fast the current decays down towards zero. They do not imply any form of speed of actuation on the inductive load.
During a fast decay recirculation mode, current is said to decay towards zero as fast as possible. This is attained by disabling the energizing FET switches and then enabling the opposing FET switches (synchronous decay), or letting current flow through free wheeling diodes (asynchronous decay).
Current decays the fastest possible because a voltage of greater magnitude but opposing polarity is applied to the inductive load.
On synchronous decay, the proper technique to achieve fast decay mode is to employ a break before make mechanism in order to avoid shoot through. If the opposing FETs are enabled as soon as the energizing FET switches are disabled, there will be a short period of time in which all four FET switches will conduct. This is extremely hazardous to the device.
The solution is to add a period of time in which all FET switches are off, called dead time. During this time, energizing FET switches are allowed to switch to their OFF state and inductive load existing current is carried by either body diodes or external Schotky diodes.
H Bridges employing asynchronous decay will let the diodes conduct the current while it decays.
H Bridges employing synchronous decay will turn the opposing FET switches until current decays down to zero, or a fixed time off elapses.
Note that the voltage applied to the inductive load is that of the source plus two diode forward voltage drops, or current multiplied by respective switch RDSon.