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DCR022405 Smoking

Daniel McClure
Prodigy 20 points
Other Parts Discussed in Thread: DCR022405, DCR012405

Hello,

I was planning to use the DCR022405 in a design, and made a 2 layer prototype board using the circuit in figure 4 of the datasheet (with only one converter). I input 28V, and regulated it down to 24V using an LM7824. There was no load. When I applied power, it smoked.

The only differences I can see between the datasheet recommendations and this prototype are that there is no ground plane for heatsinking, and that the caps used were not low ESR. Has anyone else had a similar experience?

  • 0
    TI__Mastermind 38230 points

    The DCR022405 is not recommended for  any application when the  input voltage is absolute maximum limit is exceeded  29V  or transient  for any duration. The input power  source during ramp-likely had a transient overshoot which exceeded the maximum voltage. The input capacitor must be a 2.2uF ceramic low ESR  or the converter will oscillate  and not function . Either condition will force the converter to be thermally   overstressed. The in-rush current can be > 500mA from the LM7824. The input voltage wire if longer with add impedance and increased transient voltage  If the LM7824 is hiccuping during power -up transition, this can also increase the  power dissipation with thermal overstress and subsequent  package failure.

    Tom

    tguerin@ti.com 

  • 0 in reply to Tom Guerin
    Prodigy 20 points

    Tom,

     

    Thanks for the quick reply. So you're saying that what most likely happened (if it wasn't strictly the fact that the capacitor was not low ESR) is that the regulator's output transient exceeded 29V for some period? If this is the case, how can I prevent it from happening in the future? Is it just a matter of increasing the capacitance on the output or adding a TVS, or should I just not use a linear regulator altogether?

  • 0 in reply to Daniel McClure
    TI__Mastermind 38230 points

    Daniel:

    Thanks for the e-mail. There are several concerns or reasons for this type of failure.

    1. The IR losses associated with long wires or small power wires plus   input power sources driven into current limit and oscillating  can overstress the converter. This power source isolation  oscillation can droop the 28V to lower voltages in the  18-13V range.

    2. The  28V input power supply must decreased to an average of 24V   to prevent voltage overstress on the  converter. The absolute maximum voltage on the 24V converter is 29V. Use the input ceramic capacitors , as recommended

    3. Add a soft start circuit  on the input sync. pin , pin 18, which is also a on-off control pin.

    Add a  soft start control circuit which will delay the start-up of the DCR022405 for 7-10 msec or until the  24V bus is actively high.This delay will prevent the activation of the 24V bus current limit  . Additional input capacitance 10-100uf  may be required to hold-up the  24V source bus during the DCR012405 active switching transition .

    See the attachement for delay and control of the on-off pin.

    DCR01_02 24 Vin 05Vout USVSUVLO.pdf