UCC27714: Low-side MOSFETs disabled but getting hot, with 5% high-side PWM.

Hi Richard,

Many thanks for the help.

I have since tried another MOSFET type with a much faster body diode recovery time of 50nS (CSD18537NKCS) but unfortunately the heating problem has persisted. This is with the gate and source connected as before. I have taken scope readings of the gate/source and drain signals and can see that in our design the drain signal overshoots on the rising edge to about 50V and undershoots on the falling edge to about -4 or -5V when using a power supply of 30V. It looks as though it is the undershoot that is causing the heating problem.

The motor winding current is about 150mA measured using a current clamp. It does not appear that the motor current goes negative.

Regards

FarmerJo

Correction, the average motor current is approximately 150mA as shown on the external PSU and the peak motor current is about 1.5A indicated by the current clamp.

Regards

FarmerJo

Hi Richard,

Thanks for the help and advice. This is what I have found so far.

The problem appears to be solved when instead of connecting the phase output (Phase A) through the load and to GND, it is instead connected to a different phase where the low-side is enabled (and the high-side is disabled). The hardware has three channels in total. Don't understand why this would be but the low-side devices no longer suffer with the heat problem! Any ideas on this?

There is another, possibly related issue, where we are destroying UCC27714 devices on a regular basis.The failure mode is that the HO output just stops working and no longer drives the high-side MOSFETs. Under this failure the device EN is high (enabled), LI is low and HI has a 10% PWM signal. I am wondering about the requirements for the 15V supply that we are using to power the UCC27714 and how stable it needs to be. Currently the 15V is being generated using a step-down regulator (MP9488). Is it possible that a problem with this supply could damage the UCC27714 devices?

Will also get some scope traces this weekend as suggested and will add them shorty.

Regards

FarmerJo

Thanks FarmerJo,

Richard will be back on Monday and I agree let's start with scope shorts. you could trigger during start up and other scenarios. Richard mentioned switch node current into load will be good one to see any abnormal conditions.

Good luck and look forward to your waveform.

Wei

Hi,

Appologies for the slow reply.

I have now fixed the stability issues with our 15V supply used by the UCC27714 device.

I have attached a number of scope traces of phase W bridge output in addition to some VDS measurements across the MOSFT that is disabled but still get hot.

The first two waveforms are of the phase W output. We are using a 30Khz PWM frequencey with an on time of about 10%.

How is it that the upper voltage driven from the bridge is higher than that of our supply voltage of 40V?

Also I notice that waveform 2 shows that the low-level driven voltage is about -1V where I would have expected it to be 0V!

The remaining 4 waveforms were taken using a x1 probe and show the voltage between the source (GND) and the drain of Q12 (see previous schematic).

From these it appears that the drain pin of Q12 is driven negatively for a very short period in each PWM cycle.

Is there any explaination why we are seeing a -ve drain voltage under these circumstances?

Since attaching the original schematic I have changed all the 2R7 gate drive resistors with 100R values.

I have not included any gate drive waveforms because previously I connected Q12 source and gate together and still the device gets hot. If it would help I can include these.

Regards

FarmerJo

Hi,

Appologies, I am having difficulties in attaching the last waveforms, will try again shorly.

Regards

FarmerJo

Hello,

You can post the additional information when you have the plots available

Regards,

Hi Richard, I cannot seem to add the remaining 3 waveforms. I am getting reports that my new posts are duplicates, although I am not able to see the new plots on the forum. Can you see them? Can they be attached in another way? I have been taking screen grabs so far and dropping then into the post. Worked for the first 3 but not the rest.

Will try again soon.

In the meantime, another thought has come to mind.

If due to some timing issue both the high-side PWM signal was enabled and the low-side (non-PWM) was enabled at the same time for a very short time, would it be possible to cause damage to the UCC27714 devices. Another possibility of incorrect timing would be where two high-side (PWM) signals were driven into the remaining third enabled low-side driver. I can see that this would not do the MOSFETs a lot of good but is ther a mechanism by which the UCC27714 could be destroyed?

Regards

FarmerJo

Attached?

Hi Richard,

Since we last spoke I have made two major changes to our drive as follows:

There was a problem with our 15V supply to the UCC27714 where it was actually operating at about 12V with poor regulation. This has since been resolved.

I have rewritten the driver firmware using C replacing the original FlowCode code. With the FlowCode code there was occasional bridge timing issues where at high speed there was an occasional judder in the motor output which was incorrect drive for two consecutive sensor transitions. Perhaps there was a bridge short here. Using the new firmware there is no judder at all across the entire speed range (up to about 12K RPM). Also there has been no damage to the UCC27714 devices either.

Is there a failure mode that when the bridge high-side and low-side drive is enabled at the same time, even for short time periods in the mS region, that it could somehow cause the UCC27714 devices to become damaged?

There is also another oddity in our 15V supply, in that when we power-down the controller (i.e. switch off the external voltage supply) and it falls below about 10V, it looks as though it tries to restart a number of times before the supply voltage finally dies. It swings between about 10V to GND at a frequencey of a couple Hz. On the first GND the processor should have reset and all the controll signals to the UCC27714 should be disabled. During this time the bridge will be powered at about 10V and the 15V input will be swinging between 10V and GND a couple of times.

Does it seem feasable that this could cause damage to the UCC27714 devices?

Regards

FarmerJo

Hi,

Having looked further into the problem of damaging the UCC27714 devices it appears to happen under two circumstances that both cause high bridge current and the external PSU to current limit.

(*) Driving the motor hard.

(*) Setting the external PSU current limit too low.

I have since used slower ramp up times and have set the external current limit to 15A both of which prevent the sudden loss of the bridge supply voltage.

My questions are now as follows.

Would the sudden loss of the bridge supply voltage and corresponding loss of the UCC27715 15V supply cause damage to the UCC27714 devices if the bridge was driving motor phases at the time?

Is it possible that somehow during very high bridge currents (probably higher than the MOSFET rating of 50A for a short while) that an incompatible voltage could be back driven into the UCC727714 high-side output pin destroying the device?

Is there a method of protecting the UCC27714 devices under these supply voltage conditions? Is it quite possible that the external power supply could fail when motor is in operation, for example.

Any help would be appreciated.

Regards

FarmerJo