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Input current on switching regulator

Trojan_EE
Prodigy 10 points

Hi,

I am currently designing a PCI Express Low- Profile board. The PCI-E spec states that I must not exceed 2.1A on the +12V rail or 3.3A on the +3.3V rail. In addition, the current slew from the card must not be in excess of 0.1A/us.

How is this possible using switchers? I have done some simulations, and a typical buck converter performing a +12V  to +5V conversion, with a 0.9A load on the +5V, will draw a peak current of about 2.0A from the +12V rail, putting us damn near violating the spec, and clearly over if we have any other loads on the +12V rail. Obviously the average current is only a measly 0.5A, but the PCI-E spec seems to refer to instantaneous current draw.

Also, since most switchers operate at 500kHz- 1MHz, I don't see how one can possibly meet the 0.1A/us slew rate requirement. Any opinions/ anyone have similar issues?

Thanks!

  • 0
    TI__Mastermind 36110 points

    With proper filtering and input capacitors, the instantaneous input current from the 12V bus into the power supply will be the average input current.  High current spikes and high slew rate currents will be supplied by the capacitors.  TI has a good selection tool to pick a part for this function.  Go to www.ti.com, click on Power Management on the lower left side of the page.  The selection tool will provide many potential ICs.  I have used this often and it works well.