UCC28631: The strange behaviour of the UCC28631

Simon,

I will review the schematic and see if I can help.

Can you include waveforms showing Vout, VDD, DRV?

Is the Flyback stage operating from a PFC front-end (not shown in the schematic)? If so, what is the PFC bus voltage? What is the minimum PFC bus voltage, i.e. can the PFC stage be disabled at low line with the Flyback stage enabled?

What is the required output voltage and output load current min/max?


Can you include your design calculations (did you use the Excel design calculator)?

Can you include your transformer design, Lmag, Npri, Nsec, Nbias (very important)?


When you say that you took the EVM board transformer, do you mean from EVM572? (PMP9208 reference design uses the same transformer). Was this just for initial test and debug? or did you have problems with your own transformer?


If you can give more information, then we can offer more help.

Thanks,
Bernard

the circuit is very sensitive to transformer construction and timing. load on the bias or the primary (I was trying to steal a little power for another circuit) causes the sense circuitry to go haywire: it senses the primary voltage when the switch is turned on and the secondary voltage 1.7uS after the primary current stops flowing.

put a scope on the SD pin: the error code will likely say that either the output or the bias are going over voltage. if you look at the bias voltage it is likely flying up past the 15V or so over voltage safety level and resetting.

from an old TI msg:
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The 30 us pulse is already described in the text above - it's a preamable pulse that can be used for triggering. There is also a deliberate 10-us low interval at the end of the pulse train to signify that it has ended. It's the short 1-us positive pulses in between that give the code.

As you noted, in this case it's 26 - this is external over-temperature, or SD pin fault, i.e. the SD pin is externally pulled below the trip threshold.

At startup, and for recovery from a prior over-temperature event, the pin must pull-up to > 3.8 V worst case, 3.5 V nominal. This hysteresis to the falling trip threshold of 2.0 V ensures some off-time for the temperature to drop after a fault occurs. With the internal pull-up of 210 uA (or 185 uA min worst case), this means the external impedance must be > 20.5 k-ohm to allow startup or restart.

Above you note that the SD pin has 8.87 k-ohm in series with 10-k NTC, nominally this should just about get over the start threshold, but not worst case. I think it may be very marginal.

I would suggest opening the NTC temporarily to see if the problem goes away. If so, then I would recommend using an NTC part with higher impedance, and then drop the series resistance as required to tune the trip temperature. The SD pin will nominally trip for overtemperature when the pin drops to 2 V, which is a total net resistance of ~10 k.

The fault codes are there for internal dedug and testing, but here is the list of useful codes that may be relevant for system level debug:

4 - VDD overvoltage
5 - VDD undervoltage
12 - overlaod timer time-out - only for UCC28630
14 - AC (i.e. bulk cap level) under-voltage detection while running
15 - overtemperature internal (die temp)
18 - SD pin fault
20 - output overvoltage
21 - VSENSE pin fault
22 - DRV pin fault
23 - CS pin fault
24 - X-cap discharge event triggered - only for UCC28630/3
25 - AC (i.e. bulk cap level) under-voltage detection before startup or restart
26 - external overtemperature fault (SD pin pulled low)

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Dan

Simon,

Here are some comments on your schematic:

- You seem to be using both RCD snubber (R17/R42/C13/D4) as well as zener clamp (Z1/Z2/D4), although I see that C13 is shown as "short"
- You should just use one or other snubber, not both together
- If using zener clamp, D4 should be a fast recovery type 1N5408 is not suitable.
- If using RCD snubber, slow diode for D4 is ok
- Right now with C13 = short, there is 9k across the zeners, this will be consuming a lot of power and messign up the waveforms, a lot of magnetising energy will be diverting into these resistors instead of the secondary
- the 6.8 nF C14 across the zeners is also probably excessive.
- R39 = 3.7 k is the wrong value, this will affect the accuracy of the bulk cap voltage measurement. Per the datasheet recommendation, it should be 3.9 k.
- The main FET F3 seems to be the wrong tyoe of part - when I look up up 2SK3078, this is a low voltage high freq FET for RF amps in a SOT89 package?

I need to know youur spec (output voltage & power, input voltage range) and your transformer spec/details before I can comment any further.


Thanks,
Bernard

Dear Bernard,

       Thanks your help! I will capture the waveforms later and show to you, here is my belows reply:

1. Yes, this design include PFC, but for easily investigate and design, I separate 2 parts to design in this stage, so the schematic  is already mention all parts.

2. The actual we need output voltage is 30V and 6A in this circuit, but for first try, I just take the EVM transformer to verify the circuit (since I also try construct the transformer by myself, but maybe the transformer have some problem, the IC become fault after plug the 220V).

3. I have no use excel calculator design

4.The transformer is using this document :

5. Yes, just for debugging, and verify the layout and value is correct or not.

Thanks.

Simon

Simon,

The transformer magnetising inductance, transformer turns ratio and Rcs values are the 3 most critical choices to get the power stage and IC working together properly. If a transformer is used with the incorrect turns ratio and inductance, many things can go wrong and cause the power supply to shutdown or misbehave.

The transformer from the EVM is designed for 19.5 V output, so it will not work well for 30 V output, the turns ratios will be wrong.

Did you already try your own transformer and it caused the IC to get damaged? If so, what was the inductance, turns ratio and construction was used?


I would recommend starting with the datasheet step by step design procedure on pages 53-62 to choose the correct values for your power stage to meet your spec – in particular the mag inductance, turns ratios, and Rcs value.

Alternatively, you could use the Excel design calculator which is available on the TI website:
www.ti.com/.../sluc537


Thanks,
Bernard