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UCC28950: Ucc28950 gate signal noise at the gate of the mofet

Prodigy 230 points

Replies: 9

Views: 91

Part Number: UCC28950

Hi,

Hope you are doing good.

We are using ucc28950 as a controller for our 700 W design

Reference post link:e2e.ti.com/.../969039

We are testing the controller alone for the gate drive signals. ( We scoped out D  signal of the controller)

( Test condition : we did not power up the DC-DC board, we connected the bias supply to the controller and measured the signal)

We measured the gate drive signals at the following stage :

stage 1: 

output of the controller and input of the MOSFET driver IC

( division: y axis : 5V/ div, x axis : 1us/div)

Stage 2:

The output of the gate driver IC

( division: y axis : 5V/ div, x axis : 1us/div)

Stage 3;

Gate signal at the mosfet 

initially, we observed the signal for few seconds

and then see that the signal is dropping down. 

( division: y axis : 5V/ div, x axis : 1us/div)

During all the processes, we observed that the MOSFET driver IC is getting very hot in the span of few seconds. 

We are using ucc27324 Mosfet driver as suggested in the 600 W design application. 

could you show us the waveform on how the gate signal reads at the MOSFET gate? Is our waveform expected one?

We need help solving this issue. 

Thank-you 

Warm Regards 

Harini Krishna 

  • hello Harini Krishna,

    I am looping in the UCC28950 application engineer to support you, you will receive the feedback ASAP.

  • Hi ,

    we did a few more testing as we were suspecting the gate drive transformers. 

    We probed the signals at pin no.3 and 4 of tgate1 ( figure in the previous post)

    ( division: y axis : 5V/ div, x axis : 1us/div)

    We changed the gate resistance: r16, r17,r18,r19 from 3.01 ohm to 2.34kohm and observed that the MOSFET driver is not getting unusually hot ( as mentioned in the previous post). 

    However, we measured the gate signals at the ( gate A+, gate A-), (gate b+, gate b-)( from the schematic )and found that 

    gate A+, gate A- measures the following

    ( division: y axis : 5V/ div, x axis : 1us/div)

    gate B+, gate B- measures the following

    ( division: y axis : 5V/ div, x axis : 1us/div)

    we see that the signals are not complimentary. 

    are the scoped waveforms expected? what can be the reason that both the waveforms are not identical and not complementary?

    Is it okay to change the gate resistance form 3.01 ogm to 2.67kohm? what is the recommended range?

    Thank-you

    Warm Regards

    Harini krishna 

  • In reply to harini bukkapatnam:

    Hello,

    The following link will bring you to the user's guide for the evaluation module.  It has waveforms for the gate driver that you can evaluate.  It might also be worth while for you to order the EVM as well to evaluate.

    There is also a Webench design tool that you can use that will generate a design for you; as well as, recommended FETs and FET drivers.

    If your FET drivers are getting hot the gate capacitance may be higher than the FETs are capable of driving.  The following link will bring you to an application not that discusses how to calculate FET switching and driving losses.  You might want to study this to see how much power the FET driver is dissipating by driving your FET.  You just might need to select a FET driver with higher current capability for your FET.

    In regards to what size gate resistor that you should should.  I generally set it = to > (Vdrive max)/(Idrive Max Current)  Generally this resistor values for most applications will be between 2.2 and 100 ohms.

    There is an application note that was written on how to design with the UCC28950 that you can find at the following link.  You might find this helpful as well.

    Regards,

  • In reply to Mike O':

    Hi Mike,

    Thank you for the resources. 

    While testing the board we observed that gate A and Gate B are showing the same kind of PWM (they are supposed to be complimentary). 

    We are giving an input of Dc source ( Variac connected to rectifier) for the PSFB and a DC of 12 V as bias supply (controller). When we slowly increase the input voltage from 0 to 230V AC we observe that at 40V AC the fuse goes hot and RED. and found out that the controller is giving the same signal (not complimentary) for both A and B ( in the same leg) the same signal which is creating a short on the board. 

    all the testing is been done on no-load condition. 

    For the same circuit, we simulated the design on Tina TI for verification before going to the prototype stage. 

    What could be the possible reasons for such behavior of the UCC28950 IC??

    Would you suggest where to start debugging this issue?

    X axis : 5v/ div; Y axis : 0.1us/Div

    Thanks

    Warm Regards

    Harini Krishna 

  • In reply to harini bukkapatnam:

    Hello,

    Could you double to see if you have one of your channels inverted?

    The device cannot convert power if phase A and B, and C and D are in phase.

    Are these the signals off the damaged design?

    You might want to get a hold of one of the EVMs and compare it to your design to see what is different.

    Regards,

  • In reply to Mike O':

    Hi Mike, 

    Thank you for the reply

    We double-checked the signals. The measured signals are at the controller not after the driver( the channels on the scope were also not inverted on the scope during measurement). We are using ucc27324 and for our MOSFET driver and from the datasheet these are non-inverting channels. 

    The measured signals are from the new design. Not the old damaged one.

    Where can we suspect the area of the issue here on the controller?

    Thanks

    Warm Regards

    Harini Krishna

  • In reply to harini bukkapatnam:

    Hi Mike

    Hope you are doing good. 

    Could you please reply to the previous post?

    thank-you

    Warm Regards

    Harini Krishna

  • In reply to harini bukkapatnam:

    Hello Hirani,

    The devices are 100% tested and the A and B and C and D phase operate at 180 degrees out of phase.

    Could you double check to make sure the frequency being used is in the recommended range, just to make sure that is not causing and issue.

    Regards,

  • In reply to Mike O':

    HI Mile,

    The programmed frequency for the device is 200kHz.

    Thank-you

    Warm Regards

    Harini Krishna