 Current-polarity dead-time compensation

Current-polarity dead-time compensation adjusts a switching command to oppose the dead-time output-voltage error predicted from the direction of load electric current. During bridge switching dead time, an inductive current can flow through a MOSFET body diode; the resulting rail clamp makes the cycle-average error depend on current direction. The manufacturer’s bridge-state diagrams establish that current-path mechanism.^[ref]

Consider an original ideal half-bridge model with rail separation $V_d$, switching period $T$, duty fraction $D$, and effective dead-time fraction $\delta =t_d/T$. Assume current has one nonzero sign throughout the affected interval and that device drops are negligible. Let positive current leave the midpoint, so that the adopted clamping model givesFor a desired normalized average $D_{\ast }$, chooseSubstitution cancels the average error in this model. The correction changes the commanded pulse duration; it does not remove the nonoverlap needed to avoid H-bridge shoot-through. Different full-bridge switching patterns require applying the appropriate event model to each leg before subtracting the terminal averages.

For $V_d=48\mathrm{V}$, $T=50\mu \mathrm{s}$ and $t_d=0.5\mu \mathrm{s}$, $\delta =0.01$ and the uncompensated error magnitude is $0.48\mathrm{V}$. At $D_{\ast }=0.40$, use $D_{\mathrm{cmd}}=0.41$ for positive current and $0.39$ for negative current. Each gives the intended $19.2\mathrm{V}$ average under the stated assumptions.

Command limits matter. Suppose the allowed duty range ends at $0.98$ because a minimum off-time is required. With positive current and $D_{\ast }=0.975$, the formula asks for $0.985$. Clipping to $0.98$ leaves an actual normalized average of $0.97$, an error of $-0.24\mathrm{V}$ at this bus potential difference. Thus a compensation law can be correct locally while the requested average lies outside its achievable range. Minimum pulse durations impose further limits.

Current-sign uncertainty in dead-time compensation is particularly important near a zero crossing. If an estimated sign $\hat{\sigma }$ is used while the actual sign is $\sigma$, the residual error, before command clipping, isA wrong opposite sign doubles the uncompensated error magnitude: $0.96\mathrm{V}$ in the example. Current-sensor delay, noise and a sign reversal within the switching interval can all invalidate a sign held constant for the cycle. Using a zero correction in an uncertain interval avoids that particular wrong-sign doubling, but does not cancel the original error.

A proposed bench comparison records gate commands, midpoint electric potential difference and load electric current together, then integrates each switching cycle with and without compensation. Compare positive-current, negative-current and crossing intervals separately, and repeat near pulse limits. This separates the average-error cancellation being tested from preservation of nonoverlap, device voltage drops and imperfect knowledge of switching delay.