DRV8825: About the output current waveform.

Kengo,

What decay and Toff setting do they use? If the winding current cannot be driven lower enough during to Toff period with slow decay mode, the winding current could be driven up even with minimum high side FET turn-on time.

Please set the longest Toff and mixed decay or fast decay mode to see any change on the winding current regulation.

Hi Wang.

Thank you for your reply.

They use a Slow Decay mode.

And this phenomenon is only Slow Decay mode.

So, I think problem is that the motor current flow is insufficient too.

However, I think there is an toff like bellow the waveform.

Could you give me some your advice?

1. Why doesn't our waveform fall ?

2. How to set the toff on DRV8825 ?

3. Why is there no current limit ?

Sincerely.

Kengo.

Kengo,

1. The current ripple could be caused by the input voltage ripple. Please measure the input voltage to confirm it and add more input capacitor to filter it.

2. Sorry, DRV8825 cannot set different Toff because it has a 30kHz fixed PWM frequency.

3. "Why is there no current limit?" What voltage is on the input  terminal? What is the winding resistance? What the winding full scale current do you set?

Hi Wang.

I got a below waveform as you said.

(They added 0.01uF capacitor at close to VM pin.)

This is their use motor spec.

The winding resistance : 0.55ohm / Phase.

https://catalog.orientalmotor.com/item/all-categories/42mm-pkp-series-2-phase-bipolar-stepper-motors/pkp243d15a2-sg18

 FullScale：100% 1.15A

Their stepping setting : Full step (2-phase excitation) with 71% current)  → 0.86A

Sincerely.

Kengo.

Wang.

This phenomenon is only slow decay mode.

Changing the mode returns to a normal waveform.

Sincerely.

Kengo.

Kengo,

Yes. If you want to keep that waveform, you can keep slow decay.

I would recommend to use mixed decay or fast decay mode. Do you have further question?

Wang.

I know that using mixed decay or fast decay mode can solved.

Actually, we also confirmed that the phenomenon did not occur.

and more, if we could change toff time, it be longer I think this phenomenon is not occur.

So, I have a question that is not clear.

My customer's waveform is not up and down.

I think it should be like this.

So, we confirmed whether the inductance motor saturates using low current excitation.(0.5A)

However the result was the same.

Therefore, we would like to ask again follows;

 <Question1>

If we expand the scope of the measurement waveform, is it possible to confirm up and down fluctuating?

<Question2>

Does this phenomenon depend on the inductance value of the motor?

Sincerely.

Kengo.

Kengo,

When you saw the abnormal output voltage, please help me monitor the input voltage also. That is why I want to add more input capacitors to see any output change. Right now, I don't get the waveform with mixed decay, more input capacitor and monitor the input and output voltage. So, I don't have any clue.

Wang.

This is with VM(bule).

It does not seem to fluctuate.

Do you think about any other factors?

If you need any more information, please let me know.

Sincerely.

Kengo.

Kengo,

If you don't follow my test condition, I cannot move further and give any comment. I kind of agree with you, this phenomenon may depend on the inductance value of the motor. You can follow your thought to try different motors.

BTW, I am asking a waveform with mixed decay, more input capacitor and monitor the input and output voltage. I only saw you did the last two conditions: monitor the input and output voltage. How about mixed decay mode setting and more input capacitors?