1.3.2 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%.
  • 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.

Figure 1-7 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 becomes inactive, while the output of 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 the power devices QC and QD (see Figure 1-3) for the duration of ‘TST’. 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, a high duty cycle is required for an application. Two possible solutions for eliminating the shoot-through current are:

  1. Reduce the CWG duty cycle for one period before changing directions.
  2. Use switch drivers that can drive the switches off faster than they can drive them on.
Figure 1-7. Example of PWM Direction Change at Near 100% Duty Cycle
Note:
  1. TON is the turn-on delay of the circuit’s power switch QC and its driver.
  2. TOFF is the turn-off delay of the circuit’s power switch QD and its driver.
  3. TST is the circuit’s potential shoot-through current time due to both switches, QC and QD (Figure 1-3), conducting simultaneously.