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UCC2895: UCC2895

Stephan Reuscher
Prodigy 60 points
Part Number: UCC2895
Other Parts Discussed in Thread: UCC3895, , , LM5046

Hi,

two questions to the UCC2895:

What's the minimum switching frequency of the UCC2895 and UCC3895?

Is this IC a good choice for a complete new design or is there any other IC part which is recommended?

Please forward this message to David Wilson.

Regards Stephan

  • 0
    TI__Mastermind 25570 points
    Hi Stephan

    The minimum switching frequency of the UCC2895 device is about 90kHz. Based on eq 2 in the DS and CT = 880pF and RT = 120kOhms (100pF < CT < 880pF and 40k < RT < 120k - bottom of page 11) .

    This device is widely used and is often the best choice. It is mainly intended for primary side regulation although if better transient response is required it can be used for secondary side regulation. It does not include SR drive signals.

    A newer and alternative device is the UCC28950 (or UCC28951-Q1 for automotive applications). This device has an expanded feature set over the UCC2895. For example it includes SR drive signals with adaptive delays on both the primary switching signals (OUTA, OUTB, OUTC, OUTD) and on the SR drives (OUTE and OUTF). It is also more geared towards secondary side regulation applications although it can be used in primary side regulation too. The UCC28950 also has better burst mode control and a feature to disable the SR drives at light loads.
    It will also work down to 50kHz versus the 100kHz of the UCC2895.

    If the input voltage is <105V then the LM5046 may be worth considering too.

    Please let me know if you want any further information

    Regards
    Colin
  • 0 in reply to Colin Gillmor
    Prodigy 60 points
    Hi Colin,

    thanks for the quick and detailed answer. It resolved my issue.

    One more question:
    Can the switching frequency be lower than 90 kHz by using an external clock connected to the SYNC pin?

    Regards
    Stephan
  • 0 in reply to Stephan Reuscher
    TI__Mastermind 25570 points
    Hi Stephan

    That's a good question and I don't have any direct experience of trying this on hardware.

    I built myself a PSIM model of the oscillator circuit shown in Figure 3 of the datasheet and I can pull the switching frequency down (70kHz from 90kHz nominal). I cannot however increase the switching frequency at all.

    Basic reason for this is that if you try to synchronise to a higher frequency the CT ramp does not reach the 2.5V threshold and so the RS flip flop does not change state. If you synchronise to a lower frequency the CT ramp reaches 2.5V and the flip flop changes state. When this happens CT begins to charge again towards 2.5V but it gets reset before it reaches that point by the SYNC pulse.

    As I said, I have not tried this in hardware so there may be some unexpected behaviour. I can send you my PSIM model if you wish.

    Overall, and getting back to your original question, I would think that the UCC28950 would be a better choice - it's oscillator can be set to 50kHz. It too can be synchronised to an external source if desired.

    Do you have a particular switching frequency in mind ?

    Regards
    Colin
  • 0 in reply to Colin Gillmor
    Prodigy 60 points
    Hi Colin,

    Thanks again. I thougt about a frequency of approximately 35 kHz in order to use IGBT technology.
    I've investigated a system which is working with 35 kHz. Therefor I'm a little confused.
    Apparently it's possible to decrease the switching frequency down to 35 kHz. But this value is out of the specification.

    Do you have any idea?

    Regards
    Stephan
  • 0 in reply to Stephan Reuscher
    TI__Mastermind 25570 points
    Hi Stephan

    I've passed this question on to a colleague who may be able to help. I'll also look into pulling the UCC28950 down to 35kHz or so. Please give me a few days to run the tests and see if my colleague has any further information on the UCC2895.

    Regards
    Colin
  • 0 in reply to Colin Gillmor
    Prodigy 60 points
    I forgot to say that the system I mentioned is running with the UCC3895DW.

    Regards
    Stephan
  • 0 in reply to Stephan Reuscher
    TI__Mastermind 25570 points
    And, of course - I forgot to ask !

    Thanks for the information - I'll pass it on.

    Regards
    Colin
  • 0 in reply to Colin Gillmor
    TI__Mastermind 25570 points
    Hello Stephan

    My colleagues haven't tried this. I have an open loop test bench running on my bench now. I have it set with the reccomended max RT and CT values so I should get an oscillator frequency of about 90kHz which I do. The individual OUTx signals are running at half this so this is all correct.

    I have not been able to get the oscillator to pull down enough to get 35kHz at the OUTx pins.

    Can you check the RT and CT values that are being used in the system you have to hand - maybe you have a schematic ?
    Could you also send me a plot of the SYNC and CT pin waveforms please?

    Thanks
    Colin
  • 0 in reply to Colin Gillmor
    Prodigy 60 points

    Hello Colin,

    here are Values:

    RT=470kOhm & CT=820pF

    I'm working on the waveforms.

    Regards

    Stephan

  • 0 in reply to Stephan Reuscher
    TI__Mastermind 25570 points

    Hello Stephan

    Thanks for the component values. I put them into my test board and the free running oscillator frequency dropped to 25kHz (12.5kHz on the OUTx signals).

    I was then able to synchronise the oscillator up to 70kHz (35kHz on the OUTx signals) as you can see in the image below.

    I am waiting for some more information from some colleagues but I don't need the waveforms from you any more.

    Regards

    Colin

  • 0 in reply to Colin Gillmor
    Prodigy 60 points

    Hello Colin,

    these are good news. Thanks. 

    How did you synchronise the oscillator frequency up to 70 kHz?

    Did you change the RT and CT values or did you use the SYNC pin?

    Can the UCC28950 set to this frequency as well?

    Regards

    Stephan

  • 0 in reply to Stephan Reuscher
    TI__Mastermind 25570 points

    Hello Stephan

    UCC2895: I used the CT and RT values you sent me - 470k and 820pF. If you leave the SYNC pin open circuit then the controller runs at its nominal frequency which was about 25kHz (12.5kHz at the OUTx pins).

    I then connected a signal generator -0 to 5V directly at the SYNC pin - no other changes. The signal generator output was 70kHz and the oscillator synchronised up to this frequency so the OUTx pins ran at half this, 35kHz.

    The UCC28950 oscillator will pull up to the SYNC signal but it will also pull down a little (the oscillator on the UCC2895 pulls up only). I will check the SYNC range of the UCC28950 - probably on Monday.

    Regards

    Colin