A
Allan Herriman
- Jan 1, 1970
- 0
On Mon, 4 Feb 2013 18:32:25 -0600, "Tim Williams"
message Some switchers current limit and some "burp", namely shut down
quickly on overload and try again later, a few times per second
maybe. The latter don't pull up loads very well.
I've had that problem with USB 5-volt supplies, especially when
driving electronic loads that have switchers - negative impedance -
loads inside.
I find they work better. A [continuous] foldback limit is guaranteed
to get stuck forever, each and every time the load is the same.
That's boring, and dangerous besides.
Switchers don't need to fold back, and I haven't seen one that does.
Some switchers do need to fold back.
Here's why: (Assuming a steady output current) at low output voltages
the duty cycle will be low. This means the synchronous rectifier fet is
carrying a much larger RMS current than it normally would at full load.
Adding foldback to reduce the output current when the output voltage is
low can save money on thermal management for the lower fet in certain
designs.
But foldback will further reduce output voltage (not current) putting
you right in that box.
In some situations, that may be true. In others, untrue.
For the OP's load with negative resistance, foldback is a bad idea.
For the general case, it may be possible to design the load to draw low
current at low voltage, e.g. by means of a UVLO. In that case foldback
current limiting doesn't cause system problems at all if the UVLO
threshold is higher than the voltage at which the foldback current limit
kicks in.
Regards,
Allan