Hoping you’ll be forgiving if my response crosses into “editorializing” in some way, I’d say the problem is, at least partially, your approach.
You can’t create a document at 150ppi in Illustrator. This is where the flaw takes root. The Illustrator artboard is fundamentally resolution-independent, with the only connection to resolution being the “Document Raster Effects” setting, which, if you don’t apply any raster-based effects, is meaningless.
To be blunt, Illustrator’s raster export stinks. As Illustrator’s primary function is vector graphics creation and output, it simply doesn’t offer enough control over raster output.
It can be an over-simplification, but the way I see it, a workflow has two ends; input and output. When devising the methods that will steer workflow, it is best to consider output first. If raster output is the objective, raster input is most logical. So in that sense, one might ask you, why don’t you just do this work in Photoshop? Of course a totally valid answer would be that the graphics you’re creating are best composed in vectors, since as we all know, many on-screen-destined elements do rightly originate that way.
So how to avoid Illustrator’s pitfall-laced raster export?
Bingo. Your image size/resolution objectives are not an Illustrator strength, so leave that to Photoshop.
- Set up your canvas of the target size and resolution in Photoshop
- Design your graphic in Illustrator with no concern whatsoever for size and resolution
- Copy/paste to Photoshop as a Smart Object, and
- Enjoy full control over the size and positioning prior to committing it to pixels
- Benefit from Photoshop’s much wider array of options for PNG output, from Save As at any resolution, to legacy Save for Web, and the newer Quick Export and Export As functions