Current limiting for ADUM3070

Hi there,

I checked out the behavior of current limiting on ADUM3070 Evaluation board.

But I noticed the performance is a bit different from what I expected.

The datasheet says it works with cycle by cycle during the short circuit(2msec), but I could not observed it.

And I just wonder why 600mA is following?

I need to know  how the current limiting works correctly to choose right components for the robust system.

Please look at the attached file.

Thanks

 

Best regards,

Tarzan

ADUM3070_currentlimit.pdf
  • 0
    •  Analog Employees 
    on Oct 3, 2014 2:21 AM

    Tarzan:

    From the Soft Start Mode and Current Limit Protection section of the datasheet:

    If a short circuit occurs, the push-pull converter shuts down for about 2 ms and then enters soft start mode. In soft start mode, to limit the peak current during VISO power-up, the primary converter gradually increases the width of the PWM signal.

    The 2 ms shut down occurs on your scope photos when the output short circuit occurs. After the 2 ms shutdown, the ADuM3070 enters the softstart mode as described in the datasheet and as seen on your waveforms.

    More from the datasheet to help answer your question about the 600mA output current during the short:

    The ADuM3070 has a pulse-by-pulse current limit, which is active at startup and during normal operation, that protects the primary switches, X1 and X2, from exceeding approximately 1.3 A peak, protecting the transformer windings.

    The peak currents are limited on the primary side switches to 1.3A peak, which due to ripple from the inductance of the primary windings, limits the average primary current to a lower value.  The effect in your circuit is to limit the output current to about 550mA.

    Can you describe your intended application and the requirements needed for output short circuit?

    Regards, Brian

  • Thanks for your reply.

    I thought 2msec shout down take places over the short circuit.

    Is my understanding incorrect?

    And one more, how can I estimate the limit output current? you said 550mA, but I don't know where this come from.

    I'm planning to use this for Vin=5V, Vout=12V, Iout=0.01A .

    Since output current is very low, so I want to selcect smaller and reasonable external compontens.


    Best regards,

    Tarzan

  • 0
    •  Analog Employees 
    on Oct 7, 2014 10:01 PM

    Tarzan: I am asking the designer to look at your question & I will get back to you soon.

    Regards, Brian

  • 0
    •  Analog Employees 
    on Oct 10, 2014 7:15 AM

    Tarzan: I asked the designer your 2 questions and he had a thoughtful response to the L and C selection and an attachment to explain the frequency and transformer design:

    Your Question 1 is from thread 46249 and repeated here::

    1. I understand what you mean, for ADUM3070 trasformer inductance should be 60 to 100uH.

    But I just wondered how the switching frequency affects the selection of transformer.

    Because this device has flexible Fsw setting (200kHz-1MHz).

    And for the selection of the LC components on the secondary side,

    Don't you have any calculating formula?

    I don't know why the datasheet says Cout=47uF,and L1=47uH.

    I'd like to use smaller capacitor and inductor to adjust Fsw since the load current will be less than 10mA in our system.

    Do you have an application note about ADUM3070, which explains more about how to select external components at the different Fsw?

    1) Answer to question 1, It is a little bit complex topic.  Small output current doesn’t mean to choose small inductance.  Small inductance tends to make DC to DC converter work in DCM mode at low load current; this will make the transient response bad; if they don’t care load transient response they can try some inductances with small values. Attached is one document regarding magnetizing inductance and filter inductance for your information; both of them will affect the work mode (CCM or DCM), that is the transient response.  The switching frequency affects the work mode, also while choosing transformer we need to pay attention to its saturation characteristic.

    Typically the filter inductance decides the work mode and the bypass capacitance decides the ripple (V= I*DT/C).  Also the L and C relates to the loop compensation design, if they change the values, they can change the small signal response of the converter; but this is tricky, they can change the L and C, however try to maintain the stability of converter.

    Regarding question 2), yes, the converter will be shut down for 2mS and the soft start process is about 8mS.  The current limit threshold is designed inside the chip, it is fixed.

    Regards, Brian

    ADuM3070load_transient_sim_results.pdf
  • Hi Brian,

    Thanks for your detaied explanation.

    Let me confirm the fllowing matter.

    You said that 550mA load during  the short cirucit is fixed due to the chip-design.

    Do you mean that 550mA is the max current in any condition?

    I just worried that if external component's selectons and Vin/Vout  condition  affects this.

    Best regards,

    Tarzan