 H-bridge shoot-through

H-bridge shoot-through is unintended conduction directly between the supply rails through devices in the same bridge leg. It differs from the intended load-current path through opposite legs of an H-bridge. Overlapping device conduction during switching can create that rail-to-rail path even when the intended logical states never command it. The manufacturer application note explicitly identifies cross-conduction as shorting the supply and introduces nonoverlap timing to prevent it.^[ref]

An original lumped fault-loop model shows why a short overlap can matter. Let a previously unenergized parasitic loop have electrical resistance $R$, inductance $L$ and constant supply electric potential difference $V_d$. During overlap,soFor $t\ll \tau$, the initial ramp is $i\simeq V_{d}t/L$; the steady resistive value $V_d/R$ is not reached instantaneously.

Take stipulated $V_d=24\mathrm{V}$, $R=0.10\Omega$ and $L=100\mathrm{nH}$, giving $\tau =1\mu \mathrm{s}$. An overlap lasting $100\mathrm{ns}$ reaches about $22.84\mathrm{A}$. The supply deliversSome of this energy remains in the loop's magnetic field at turn-off; it should not all be labeled heat dissipated in one switch. A $500\mathrm{ns}$ overlap in the same ideal model reaches about $94.43\mathrm{A}$, illustrating the nonlinear dependence on duration. Device and supply behavior can invalidate these constant-parameter assumptions.

Bridge switching dead time addresses timing overlap by allowing one conduction path to cease before the complementary one starts. An inductive load still needs a recirculation path while the controlled devices are off. TI's primary bridge lesson illustrates current paths during that interval.^[ref]

A proposed diagnostic compares actual device control waveforms and supply-current transients with load current during transitions, using a limited-energy test arrangement. A large load current by itself does not identify shoot-through; the timing and path establish the mechanism. The calculated fault-loop pulse is an original model, not an asserted device rating.