Electronic – Why ACS712 is abruptly dropping its voltage output

Some things to think about:

• The H-bridge was tried with Vb and micro on a lab supply, but without 5V switcher, right?

• Has the source impedance (of Vb) been compared to load impedance of motor drive and switcher combo? Source impedance needs to be much lower than load impedance. If not, supply could oscillate, especially if kicked by a motor hard start.

• Are all the PWM signals generated by timers in the micro, so that they maintain proper phasing and timing (i.e., not bit banged in code)?

• Were all the PWM inputs to MC33883 correct before giving enable? No tri-stated output,or highs when should be low.

• Are you sure the ADC readings from pot are OK? Noise here could cause trouble.

• Is motor drive frequency compatible with switcher filter, both input and output? I.e., not on top of a filter resonance.

If we look at an inverter leg

^{simulate this circuit – Schematic created using CircuitLab}

And it is such that the top switch is controlled while the bottom switch has no gate-drive (or gate-source termination). When the top switch is turn-ON/OFF the source of the upper device will be dragged up/down with respect to the load and the supply rail.

Since the top source and the bottom drain are connected, the drain of the bottom switch will also move.

Now consider the small signal characteristics of such a device

^{simulate this circuit}

The Cdg capacitance (the miller capacitance) will facilitate charge flow which can charge up the Cgs of the lower device. If this charge is enough to raise the gate voltage to Vth, then the lower FET can potentially start conducting resulting in a shoot-through.

This is why there should always be a gate termination resistor and also a low-impedance gate-drive to absorb such charge flow