In a half-bridge stage (Fig 2), if both transistors are driven simultaneously, even for a few nanoseconds, a shoot-through condition might occur (Fig 1). This occurence depends on multiple factors, like transistors sensitivity, gate voltage threshold, driver’s speed and many others, but the most important factor is the overlapping of the driving commands. The consequences of the shoot-through starts from loss of efficiency, power supply stress due to overcurrent and going up to MOSFETs destruction. The phenomena is also called cross-conduction.
What to do to avoid the shoot-through?
Simply, insert a guard time between the high-side transistor duty-cycle and the low-side one. This guard time is called a dead time (Fig 3).
The presented project is an automatic verifier of the overlapping and near-overlapping of the two signals: the high-side drive and the low-side drive.
A simplified block diagram of the device is below (Fig 4):
Basically, the detector generates a pulse of at least 1 us every time the two input signals have a gap shorter than 100ns. The output pulse might be longer if the input signals are overlapped.
How to use the shoot-through checker?
Connect the wires to the output of the digital driver, a pair for each driver.
I.e. “driver_high” to I0.a and “driver_low” to I0.b. Or viceversa, it doesn’t matter. The detected shoot-through event is available for visualization with oscilloscope or logic analyzer on pin O0. Also, the LD0 will turn on for an indefinite anount of time to signal that a shoot-through occured. It can be cleared by pressing button BTN0.
The signalling LEDs are LD0, LD1, LD2, LD3 and the unique “clear” button is BTN0 which clears all four LED events.
One detection block is composed out of three monostable blocks and an AND gate (Fig 5):
The code for the detector is quite simple and is based on the monostable