I'm designing a bunch of power supplies using your switchers and modules. With using Webench, I'm not sure how to design for minimum and maximum current draws. For example, if my normal current draw in nominally 40mA, but needs to run at 600mA for a short period of time, how do I spec that in Webbench? If I use 40mA as my Iout, how do I simulate with the same comps to see if it hits 600mA? And, if 600mA is my calculated known load, how much more do I want to ocer spec for - 20%, (720mA)? If I do that, my 40mA efficiency drops to under 30%.
I have 3 supplies that all need this kind of design done. Some in the 3A range. How do you pick the best switcher for these applications, and then simulate them to see if they keep efficiency at light loads (without sputtering as this is for a sensitive analog circuit). Also, one of the supplies is negative 2.5V, so I can't use a linear.
Thanks,
Wally
Wally,
In WEBENCH(R) Power Designer, you should put the maximum current you will draw from the supply as the design 'Iout'; in your case, that is 600mA. This will ensure that the components selected can support the most demanding operating conditions, even if you only operate at these conditions for a fraction of the time. The component selections made by WEBENCH(R) Power Designer take into account safe margins behind the scenes so it is not necessary to add an extra 20% buffer when specifying the requirements.
Since the nominal load current for these supplies is a fraction of their maximum intended output current, I recommend you use the Light Load filter so that the Visualizer will only show devices which have pulse frequency modulation (PFM) or other pulse-skipping modulation (PSM) schemes to achieve high efficiency at low output currents. See a snapshot below of where this filter is enabled in the Visualizer:
Devices in the resulting list will have efficiency curves like shown below, where the efficiency remains near peak values over more than a decade range of output current:
We often see requirements such as yours, where the nominal output current is much lower than the peak currents, and are looking into ways to make it easier to specify and design such power supplies in WEBENCH(R) Power Designer.
I hope this helps. Please let us know if you have more questions, and thanks for using WEBENCH(R) Designer.
Kris
Wanted to mention a few more things:
1) Once you create a design for, say Iout = 0.6A, you can see the operating values of the design at a lower output current by clicking on the Op Vals button in the top navigation bar, editing the Iout, then pressing Recalculate. The BOM components will not change, just the output current used for the calculations.
2) For online simulation, you can edit the output current from the max (design) value of 0.6A by clicking on the current source load, editing the textbox value, and clicking 'Save Changes':
