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opethmc

DMC60 Causing Power Supply Fault

Question

Close full duty cycle (93% in attached images) the DMC60 pulses through zero to -12V and upon trying to recover causes an over voltage failure on the power supply. This happens unloaded and with a small load of 4Amps. Anyone else have this issue?

 

Cheers,

matt

image_6483441.JPG

image_6483441 (1).JPG

 

The onset of this behavior can be seen as you increase the duty cycle you get an occasional incorrect extra pulse as shown in the following figure

image_6483441 (2).JPG

Edited by opethmc

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Hi @opethmc,

Can you attach a picture of the setup that you are using so that we can better understand your configuration? I presume also that you are keeping PWM pulse width within the parameters described in this section of the Reference Manual here?

Thank you,
JColvin

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Hi @JColvin I will attach a photo of my setup as soon as I get to the office tomorrow. I have a +/- 12V supply connected to the DMC60, and the output of the DMC60 directly connected to a 12V 360W brushed DC motor. For the PWM input to the DMC60 I am using a 4ms period and keeping the duty cycle between 1ms and 2ms as described in the manual. Everything looks fine until around a 90% duty cycle you start seeing incorrect pulsing as I show in the above figure. Once I get close to 2ms (or 1ms) on the DMC60 , eg full positive or full negative, it trips the supply as shown above.

Appreciate any help!

Cheers,

matt

 

@JColvin attached image here of my setup.

 

image_123923953.JPG

Edited by opethmc
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Hi @opethmc,

I heard back from the other engineer. They let me know that the following:

Quote

We use N-Channel mosfets and the bridge driver IC has an integrated charge pump that allows the highside mosfets to be turned on. The charge pump charges the bootstrap capacitor during the time that the complementary low side mosfet is turned on. The bridge driver monitors the voltage across the bootstrap capacitor, and if it gets too low then it will forcefully turn off the high side fet and briefly turn on the lowside fet to recharge the capacitor. If you were to scope VGS of the highside fet this would look like a glitch in the 64us period. This should never happen at lower duty cycles. However, when you get close to 100% the low side fet is barely on, if at all, and that's when the bridge driver will need to forcefully turn off the high side fet and turn on the low side fet to recharge the boot capacitor so that it’s at a high enough voltage to drive the highside gate to a voltage that will result in the fet being turned all the way on. Therefor the DMC60 doesn't really do a true 100% duty cycle. We redesigned the DMC60C to use a different bridge driver, which uses a different scheme for charging its bootstrap capacitor, and is able to do a true 100% duty cycle.

 

I read through the post and noted the following statement: “I have a +/- 12V supply connected to the DMC60, and the output of the DMC60 directly connected to a 12V 360W brushed DC motor.” If the red wire is connected to +12V and the black wire is connected to -12V then during the on time current will flow from +12V through the highside mosfet, then through M+ (assumes forward direction), then through the motor, then through M-, then through the lowside mosfet, and then through the black wire to the -12V supply. In theory this should be the same as connecting a +24V supply to the red wire and a 0V ground to the black wire, and will effectively apply 24V across the motor. We always assumed that the black wire would serve as the common, or ground in the system, and that the PWM input signal would be referenced to the same voltage as the black wire. However, this shouldn’t matter since the PWM input signal is isolated through a CAN transceiver. In any case I don’t think that’s causing this issue. I think what is actually happening is that there is a large transient when the bridge driver abruptly turns off the highside mosfet to force the lowside on to recharge the bootstrap capacitor. We don’t really notice an impact from that during our use because we are typically battery powered or we use a lead acid battery in parallel with a bench supply, which effectively serves as a large capacitor to sources the additional charge required to startup the motor from a stall or stores any extra energy when the motor abruptly stops spinning. I don’t really have any advice for powering the DMC60 from a +/- supply since that isn’t something that we’ve done.

Let me know if you have any questions.

Thanks,
JColvin

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Hi @JColvin thanks for the insight. I have just tried this on a DMC60c and I see the same issue. It does not seem to happen with a regular frequency, sort of random. I should have noted 12V supply rather than +/- 12V. I confirmed that I am supplying 12V with reference to 0V. Does the engineer think that powering from a battery will fix this issue?

 

Cheers,

matt

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Hi @opethmc,

Powering it from a battery should definitely help. They mentioned to me that when they use a bench power supply, they use a 12V lead acid battery in parallel with it and set the bench supply to output 13.7V which effectively float charges the battery. This way the battery acts like a very large capacitor unless the continuous current draw is more than the continuous output current that the bench supply can provide.

Let me know if you have any questions.

Thanks,
JColvin

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