On 3/2/22 6:00 AM, paul swed wrote:
As Ed stated Switchers feeding switchers can be problematic. What I found I had to do was put a relay in series with a slight time delay. This allows the primary switcher to come up and then gets slammed into the secondary SPS. Could be a fet/transistor switch. So linear is preferable. Regards Paul
Sequencing is important - You can also use the "shutdown" input on the cascaded switcher with an RC time delay (maybe), or an explicit timer (555 or counter or 8 pin FPGA programmed appropriately or...)
The challenge comes when your first stage switcher needs a minimum load to work properly - Shutting off the secondary switcher removes the load. A resistor might work, if you're not total power consumption constrained.
But yes, since switchers are "constant power" devices, they draw a lot of current when the input is low voltage, so you can get into a motorboating problem with the first one in the chain's overcurrent protection (or just regulation).
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