UCC28700

Karsan

It's not so easy to test or evaluate the IC stand-alone, since it has so many built-in protection features and built-in closed-loop control.

If you can post your schematic with component values, we can try to help. Also include details of your test setup (input volateg, output load setting, etc).


Firstly, is the VDD pin charging through the startup resistor to the start level (~21 V)? If VDD does not reach this level initially, then the IC cannot start.

When the VDD pin has reached the start level (~21 V), the IC should deliver at least 1 pulse initially at the DRV pin. If not, then maybe there is a shorted pin?

If only a single DRV pulse is being produced, then it could indicate a short on the CS pin, or else a combination of transformer magnetising inductance and current Rcs value that are limiting the CS signal to a very low level.

If the DRV pin shows just 3 successive pulses, followed by a decay and recharge of the VDD level, then it could be that the input voltage is too low, or the RS1 resistor value is not tuned correctly for your transformer turns ratio.

Are you sure there are no DRV pulses at all? Maybe you are only getting a single or 3-pulse set, but they would be so infrequent that they may be missed?

I would also suggest that you verify your design values against the UCC28700 datasheet equations 7 to 30.

Thanks,
Bernard

Hello Bernard 

 i have attached bom and schematic layout .let me know what i have to change in schematic.

Thanks,

Karsan8546.Schematic Circuit.pdfBOM.pdf

Bernard

 I am  getting ramp waveform in the VDD  pin ,and there is no Pulse in DRV  pin .i have changed capacitor value (C6  ) and tested after that removed the capacitor (C6) and tested , still i am not getting any pulse in the DRV pin .

Thanks

Karsan

Hello Karsan,

Bernard is on holiday. So I will look into this for you.  This looks like the device starting up into a fault.  I don't believe it has anything to do with the VDD capacitor.

When the UCC28700 is powered up it will give three sample pulses to the gate drive to figure what the output voltage is. If there are any faults the device will shutoff off and discharge the VDD capacitor to VDD turn off.  The trickle charge resistor will recharge the capacitor to Vdd turn on and initiate sampling again.  If there are no faults present gate drive switching will continue. 

The faults for this device are over current, over voltage, input under voltage and over temperature.

If you reevaluate the VDD waveform with the gate drive, and the auxiliary winding voltage, you should be able to see which fault is being triggered.

Regards,

Mike

Hi,
We are giving Input voltage upto 230 V. VDD pin generating Ramp waveform only. If we increase the voltage the ramp waveform charging time only reduced. we are not getting pulses in gate DRV pin it's always zero. There is no voltages on auxiliary winding, VS and CS pin.

Just a suggestion but check the actual winding polarity on your transformer. I always do a sanity check on an LC meter just in case (I have gotten one or two wrong before).

hi,

 Mosfet got  switched but cs pin current sense resistor(Rcs) got burnt ,while switching .to short out the problem whether we have to increase wattage of resistor ,if so ,let us know

Thanks

Karsan

The EVM uses a 1206 thick film for the current sense, which is rated up to 0.5 W. What size are you using? Is it a low-inductance surface mount? It looks like you have the same schematic as the EVM; do you have an EVM so that you can compare waveforms and layout?