large voltage spike over shunt resistor even at no load

Question

I'm trying to build a stepper-motor driver. Each windig is driven as shown:

at the moment, im feeding this with PWNM signals of constant duty cycle, just to see how everything response. (PWMs are complementary to drive current in only one direcion) So i'm not using the OPAMP and MCU to measure any currents. What i see is that when a Mosfet gets turned on or off i see a spike over the shunt resistor, even with no load attached.

CH1 = voltage over shunt resistor CH2 = voltage at switching node

output voltage looks ok, same under load (testet with up to 3A). But im not shure where these spikes are comming from. I used a Ground-clip to measure this.

Any idea how i can get rid of this? Or is there anythin obvios wrong with my schematic. My current prototype uses a 4-Layer pcb with an (almost) solid GND-plane and seperate supply-planes under each H-bridge.

Update:

Dead-time of PWM signal is 460ns. I changed the gate resistors to 10R to allow for faster rise/fall of each gate voltage. but no improvment on the shunt signal

Output signal of the gate driver: Gate voltages with RG = 22R Gate voltages with RG = 10R Better Measurement of gate voltages, might this peak be the problem?

Update 2:

i've been testing and probing around to see if i can narrow it down. Wiring C49 and C54 directly to GND (as mentioned by peufeu) improved the signal over all, but it did not affect the spikes. But the signal is much cleaner right befor the spike, during dead time.

I found out that this Spike/swing is also present on the supply voltage. I changed C47 to 1000uF and still the supply voltage swings about +/-5V and more. Under no load, and there is no current drawn form the power supply. Wouldn't i take quite some current to cause such fast voltage change over an Elko? The swing on the supply voltage looks basically like the one over the shunt, but inverted.

Answer 1

My first guess is that the newly activated leg of the H-bridge is starting to conduct before the leg being deactivated is fully off. Do you have any mechanism on the source signals to prevent overlap at the edges?

Answer 2

I think it might be worth considering the gate-source capacitance of your MOSFETs: -

They are nominally about 5 nF and I've added them on a portion of your circuit for you to consider. If you remove T8 and T10 you should be able to prove this. With T8 and T10 removed, if you still get the spike then it's gate capacitance passing the gate drive voltage through to the sense resistor.

It’s quite a common problem in some switch mode controllers and those that suffer from it use “blanking” techniques to avoid false over-current detection.

Answer 3

I found out that this Spike/swing is also present on the supply voltage... The swing on the supply voltage looks basically like the one over the shunt, but inverted.

This is proof of current going through both upper and lower FETs rather than from lower FET Gate drive, but it is not 'shoot-through'. It is caused by Drain-Source capacitance, it's perfectly normal, and there's not much you can do to prevent it.

Your FETs have a Drain-Source capacitance of over 5 nF at 0V reducing to ~2.5nF at 15V, so a simplified equivalent circuit looks like this:-

^{simulate this circuit – Schematic created using CircuitLab}

SW1 and SW2 are the upper and lower FETs, C1 and C2 are their Drain-Source capacitances, and L1 is the wiring inductance. Imagine that SW1 has just turned off so C1 is discharged. When SW2 turns on it causes C1 to charge via L1 and R1. This is effectively a series tuned circuit with (relatively) high Q, so instead of just charging C1 it rings at its resonant frequency.

The correct way to deal with this is hold off reading the shunt resistor voltage until after the switching surge is over. However a simple low pass filter will work if you don't need great accuracy.