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Original question:

LM74670-SQEVM: replace diode in bridge rectifier

LM74670-SQEVM: Three phase bridge rectifier design contains large spikes on output

Desmond Good
Prodigy 10 points
Part Number: LM74670-SQEVM

Hello,

I'm testing out a three phase bridge rectifier design to rectify three phase from a brushless DC motor driven with this ideal diode controller and it's not working as I expect. The circuit is similar to the following, except the diodes are replaced with an N-channel MOSFET and the LM74670. The load resistance I've tested with is 1k and 100 ohms.

if a standard three-phase 400V AC connection is rectified what DC voltage comes out of it? - Electrical Engineering Stack Exchange

The problem is as the generator starts to spin, the rectified DC waveform contains large spikes (up to 30V) that occur when the gate drive switches. I've also noticed that the gate drive does not hold steady and switches a lot during a period where it should be steady.

Questions:

- Do I need some kind of snubber network to reduce these spikes? But I'm not sure how that will prevent the gate drive from switching as often as it is.
- Can I add some hysteresis to the switching of the gate drive somehow?

Can provide oscilloscope images if needed.

Thank you!

Desmond
ECE Student

  • 0
    TI__Guru 57737 points

    Hi,

    Thanks for your query. I am looking into the query and will get back to you within a couple of days. 

  • 0
    TI__Guru 57737 points

    Hi Desmond,

    I think you already understand that the Gate of  LM74670 is turned OFF for some time to charge the internal charge pump. When the Gate is pulled low the load current will transfer to the body diode of the FET from its channel. 

    It looks like the inductance of the DC motor is causing the spikes. Can you please share waveforms for load Current, Gate, Source and Drain when you see spikes.