# Buck/Boost Power Supply: What happens if we use much bigger Bulk Capacitance (i.e. 10x)?

If we take a buck/boost converter that is known to function correctly, but increase the bulk capacitance by 10x, what can we expect to happen electrically?

Background: I took the design below off TI's WEBENCH tool. Input: 10-14 V, output 12 V @ 8 A. The total input capacitance is 68 uF (CBulk, aluminum) + 3*15 uF (Cin, ceramic) = 113 uF.

After building it out on a board, I realized there were some serious instabilities occasionally; especially at higher loads, or if the load current suddenly changed. After a lot of debugging I finally realized a larger bulk capacitor (1000uF = 1 mF) upstream was causing it. After removing it, the PSU functioned as intended. In reality, my input stage was:

simulate this circuit – Schematic created using CircuitLab

Now everything is working, but I'd like to understand better before proceeding:

1. Why did the large bulk capacitor cause the instability in the output?
2. How can we calculate the bulk capacitor size limit?
3. Is the bulk capacitor size limit a function of the output capacitors?
• Not much. What’s feeding the input though and how will that part handle the increased inrush, that’s the question. Commented Oct 30, 2018 at 20:37
• I'm kind of surprised at this, but I guess it's not entirely unexpected. A switching supply, of necessity, has a negative Thevanin equivalent resistance on its input. Just plot the current vs. voltage for anything that's consuming constant power -- it'll jump out at you. So you cannot be entirely blithe about connecting one to a load. Commented Oct 30, 2018 at 20:41
• All bets are off and strange thing may happen if your actual PCB layout doesn't follow manufacturer's board template. If you, God forbid, use some breadboard, then just forget it. Commented Oct 30, 2018 at 20:49
• Are you sure the instabilities where on your supply and not on the primary source? What is supplying Vin? 1mF would be a fairly large load for a switcher. Commented Oct 30, 2018 at 22:12
• "In reality, my input stage was:..." - are you really using an IRF9530 to switch more than 8 Amps? What exactly is the power source? Commented Oct 30, 2018 at 22:48