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UCC2801: Over-current limit

Hyeonseung Ju
Prodigy 40 points
Part Number: UCC2801


hi

I'm looking for external circuit to over current limit

In my case , I use auxiliary power supply that output voltage is 12V , no bias VCC

- using Isolate CT, R35 connects CS and GND

even UCC2801 have over current limit circuit that CS voltage over 1.5V

the converter that using UCC2801 maintain constant current -  Load set short current therefore output voltage is zero

UCC2801 is current mode controller  so that situation can be appear

but other device will destroy because short circuit continue during long time

so the converter need external circuit that output current (or CS voltage) is over limit point, that operate hiccup mode

I think using internal comparator is best option, how can i do?

  • 0
    TI__Guru 74115 points

    Hi,

    I think you need to design an external circuit to detect your Vout so when Vout drops to some lower say 80% of the regulation set point, your detection circuit will treat it as short so to pull down some signal pin to stop switching.

    If you want to use the ic internal comparator to pull down/disable the ic switching you can use COMP pin which is ok to pull down to GND to force switching stop (data sheet section 9.3.1.1).

  • 0 in reply to Hong Huang
    Prodigy 40 points

    thank you for help

    why don't we use CS over-current circuit that comparator with 1.5V ?

    That part is useless?

    For your help, I will make output UVP to detect OC(SC) circuit but I curious about that

  • 0 in reply to Hyeonseung Ju
    TI__Guru 74115 points

    Hi,

    In your first message, you said even UCC2801 associated part is protected by CS limit, other parts of your circuit may damage. This is not a typical situation from your design since CS limit cannot protect your other circuit. But CS limit in this part is ok in other design cases. So your case may need pull down COMP to completely stop switching. Yes, in your case, it looks CS limit does not make effective protection to your other circuit so you need to use different protection, or add margin to your other circuits to tolerate the current limit. 

  • 0 in reply to Hong Huang
    Prodigy 40 points

    Hi

    so, you mean, CS design is not well

    I will fine problem with my product

    I can't fine problem otherwise I will use COMP to protect from SC 

    thank you for your help

  • 0 in reply to Hyeonseung Ju
    TI__Guru 74115 points

    Hi,

    CS limit can terminate the pulses but your circuit need to tolerate the current limit level. If your circuit is not designed to tolerate the current limit then CS limit cannot protect your circuit. If you do not want to increase your circuit tolerance to current limit level, then you can make protection by pull-down COMP pin to avoid damage.

  • 0 in reply to Hong Huang
    Prodigy 40 points

    Hi

    the converter don't have current feedback circuit, just voltage feedback

    1st current flows CT, controller sense CT pulse

    Load become SC, the converter continue high level current & 0Vout (SC situation) not hiccup

    so.. I don't understand that happens

    in steady state the converter operate well

  • 0 in reply to Hyeonseung Ju
    TI__Guru 74115 points

    Hi,

    UCC2801 pin 3 CS can be used for current limit. When CS voltage reaches  about 1 V, the pulse will be terminated. But your circuit needs to tolerate the current corresponding to CS = 1V.

    Please provide your circuit if you want us to make additional analysis. 

  • 0 in reply to Hong Huang
    Prodigy 40 points

    - Controller circuit, VCA comes another power supply, PC2 - Feedback octocoupler

    thank you for support

    of course the converter can operate at CS=1V but in SC case , don't converter have to operate hiccup mode?

  • 0 in reply to Hyeonseung Ju
    TI__Guru 74115 points

    Hi,

    The IC is in cycle by cycle limit when CS  reaches 1 V. So the IC is not in hiccup, that means you won’t see hiccup on Vout. 

    Please show where CS pin connection. 

    Pulling down COMP pin can make hiccup if you want to have Vour in hiccup when making short circuit protection.

  • 0 in reply to Hong Huang
    Prodigy 40 points

    Hi,

    CS connect only CT that i show first script

    the point is, even the Load is short circuit, controller can't operate hiccup mode

    CS is over 1V, controller reduce duty cycle that CS continue 1V

    using UV circuit, i care about UVP doesn't operate soft-start and UV reference is too low,  it is hard to operate OC

    how can i solve that problem?

  • 0 in reply to Hyeonseung Ju
    TI__Guru 74115 points

    Hi,

    Yes, the point is that CS pin current limit is cycle-by-cycle, and not hiccup. The duty is corresponding to CS 1 V. If you need hiccup protection, you would need to add external circuit, for example, to pull down COMP pin when in short circuit. 

  • 0 in reply to Hong Huang
    Prodigy 40 points

    Hi,

    Okey, so i have to make new external circuit to protect SC

    Can you recommend circuit for me to that operation?

    it is sad for protect by SC from CS 

  • 0 in reply to Hyeonseung Ju
    TI__Guru 74115 points

    Hi, 

    You do not have to make new circuit if you can make your remaining circuit to tolerate SC - we see such design idea and work in many designs. 

    If you do not want to design margin up, and for short circuit protection, you need to a comparator to detect Vout so when Vout drops below, say 80%, that comparator output low to pull down COMP.

    Actually, I think your CS current limit set point looks too high. You may consider to lower your current limit since during SC, the current should be not greater than your current limit. I suggest you set up your current limit 120% to 150% of your rated current. Then design your circuit to tolerate 1.5x your rated current.

  • 0 in reply to Hong Huang
    TI__Guru 74115 points

    Hi,

    Actually, you may look at Section 9.3.12, this device does have hiccup protection on over current, but in order to make hiccup the CS needs to reach 1.5V. So it looks you do need to design your circuit with 1.5x rated current.

  • 0 in reply to Hong Huang
    Prodigy 40 points

    Hi,

    Actually, i design converter with 3(or more) rated current

    the converter derating starts 1.3x rated 

    but in SC, output voltage is 0V, the output current continue with 1.3x rated

    maybe i think over current comapartor in UCC2801 operate transformer saturation & Arm short, something like that situation

    the internal OCP function don't care about output section ( CT is 1st line in converter )

    the controller is current mode controller, so the converter operate CC(constant current) mode in SC

    I think if i care about output section, i design external circuit like UV circuit or other things

    thank you for your help

  • 0 in reply to Hyeonseung Ju
    TI__Guru 74115 points

    Hi,

    Ok, so in order to trigger hiccup, CS needs to increase to reach 1.5V. If your design cannot make this, then you would have to design your own protection circuit.