25.2.3.2 Dead-Band Delay in Full-Bridge Mode
Dead-band delay is important when either of the following conditions is true:
- The direction of the CWG output changes when the duty cycle of the data input is at or near 100%, or
- The turn-off time of the power switch, including the power device and driver circuit, is greater than the turn-on time.
The dead-band delay is inserted only when changing directions, and only the modulated output is affected. The statically-configured outputs (CWGxA and CWGxC) are not afforded dead band, and switch essentially simultaneously.
The following figure shows an example of the CWG outputs changing directions from forward to reverse, at near 100% duty cycle. In this example, at time t1, the output of CWGxA and CWGxD become inactive, while output CWGxC becomes active. Since the turn-off time of the power devices is longer than the turn-on time, a shoot-through current will flow through power devices QC and QD for the duration of ‘t’. The same phenomenon will occur to power devices QA and QB for the CWG direction change from reverse to forward.
When changing the CWG direction at high duty cycle is required for an application, two possible solutions for eliminating the shoot-through current are:
- Reduce the CWG duty cycle for one period before changing directions.
- Use switch drivers that can drive the switches off faster than they can drive them on.
