How to fast demagnetize solenoid (High frequency solenoid switching)

Thanks, is there any method to find out zero crossing of back emf? since it's a marking machine i need to know when to give next pulse.
I think i will be having more questions :)

Hi Suyog,

To determine the zero crossing, a current sense amplifier can be used to measure the current across the sense resistor.

Another option is to use a current probe to determine how long to drive the fast decay and add margin. This assumes you can afford the timing margin.

1) First option looks fine to be implement in final product version. I can give next pulse as soon as current go zero (+3.75uS blanking time of FET)
2) Second option looks more like for R&D and testing once blanking margin is decide and then hard code it.

in first option using external amplifier or solution like INA219 or AC712 will they affect working of Rsens i.e. DRV8840?

Thanks
Following is our strategy first test current with oscilloscope on EVM of DRV8840. Get an idea of blanking time required. In final product if required we will go with current monitor.

hi

we did purchased DRV8840EV

following is the configuration still i am not able to switch on  my solenoid. Any mistake i am doing in configuration?

Hi Suyog,

We will investigate and get back to you soon. While you are waiting, would you please check the VREF testpoint (TP7) for 2.5V? If 2.5V is not observed, please check the JP2 jumper setting to verify it it connected between pins 1 and 2 (GVREF to VREF).

Hi
I am getting 2.5v no problem. And I am not getting any nFault output!

Hi Suyog,

This helps a lot. It confirms you have communication with the mcu and the mcu is accepting the commands.

A few more questions if you don't mind.

What is the VM voltage?
What are the parameters/model number of the solenoid?
Can you provide scope captures of the OUT1/OUT2 pins when driving the solenoid?

Thanks.

Hi
1) VM is 24v and supply is rated for 6A
2) It's a custom made solenoid and apart from its voltage ratings and coiled resistance I don't have any details. Resistance is 0.99 ohms. If you want any more parameters let me know I will try to measure them
3) I don't have scope :( but I can say solenoid sometimes get some power. I recall I was playing with it and EVM was enabled and I manually pulled solenoid it stayed in same position and moment I untick enable it slowly went back in its original position and second time I untick phase it went back little faster but not as expected. This is the only two time EVM worked :(
If you want scope output I will arrange for it.

Thanks
Suyog Vyawahare.

Hi Suyog,

Please provide scope captures. I am not sure I will be able to duplicate what you are observing without it.

Thanks.

Hi
Kindly give me couple of days for arrange for scope. Till then keep this thread open. I also also try to provide current graph too.

Hi

Following are attached Images of Waveforms. Channel one is Out1 wrt to ground and Channel two is Out2 wrt to ground. And one thing i realised that solenoid takes upto to 6A if it is switch on for more than 6ms (this is happening with existing control circuit. I will post its waveforms on saturday) 

Hello Rick,
Can you have a look at my scope captures?

Hi Suyog,

No I have not looked at them yet. I was expecting a second set of scope captures based on your message.

Are you still intending to send the second set?

Channel one is the demagnatising pulse and channel 2 is driving pulse signals are inverted when solenoid is not working both terminals of are pulled up.

Sorry I forgot to uplod this on time

Hi Rick,
Should we drop DRV8840 and go with discrete mosfets?

Hi Suyog,

Switching to discrete MOSFETs is a decision that you will have to make. I am sorry I was not able to help your debug.

The DRV8840 does have additional protection features over discrete FETs, such as over current and short circuit protection, undervoltage lockout, and overtemperature.

Any other chip which can source higher current also which is pin compatable with DRV8840? so i can reuse evaluation board? If not then going with mosfets

Hi Suyog,

Unfortunately no. The DRV8829 and DRV8842 are pin compatible, but have the same current.