I've posted previously on a similar issue which can be found here
My design is a 3 phase inverter with:
I'm using only a single high and low MOSFET for some testing.
The specifics of the setup are:
- 30VDC Bus Voltage (But the design is to be used at 310V)
- 10R Gate Resistors
- 820R Load Resistor
- Vth of the MOSFETS are 4V
- VCC to the driver is 16V
- The coloured circles match the probe points on the scope capture.
The object of the test is to check the switching response of the MOSFETS. During the testing I found that the High side MOSFET appears to switch off, in that Vgs < Vth however there is still voltage on the phase terminal (to the load). This voltage decays over some 3uS.
What I'm trying to determine:
When the lower MOSFET is switched on there is still a significant voltage at the phase connection, this is equal to DCBus and at 310V creates quite a disturbance.
Where is the stored energy coming from after the upper MOSFET is off. I don't have any additional capacitors, other than a small 100nF ceramic as the bootstrap caps and some filtering on the VCC.
Specifics of the capture:
- D0 and D1 are the PWM 1H/L digital inputs into the driver
- Yellow = Lower gate (I have to pulse this for the bootstrap caps)
- Purple is the Upper Gate
- Blue is the Phase Voltage
- White is the math Vgs (Delta between Purple and Blue)
Can anyone please describe why the phase voltage / gate voltage takes so long to decay? Obviously the load resistor plays a part in the decay, but where is the energy coming from that has to be bled by the resistor?
Is it possible that because the phase connection is linked directly to the gate driver's VS pin, that there is some magic within the gate driver preventing the phase voltage to drop? I doubt this is the cause as there is no direct connection from the gate driver to DCBus after the upper MOSFET is off.
Is the upper MOSFET actually switched off? Even though the Vgs is about zero, and by all accounts the MOSFET SHOULD be off, I'm wondering if it actually is not properly switched off..
Thanks in advance!
I've struggled to find almost anything on the web about this issue (specifically the transients that occur during switching). However I did find an Infineon application note
The image from the app note is appended below, and clearly shows the same conditions that I've encountered on page 10. It also shows the switching transients that I'm experiencing.