Hi, I’m having trouble with a PCB design I’ve done that’s powered by a Power-One DC-to-DC switching power supply. I’m using the 40A model [ZY2140G but I’ll (hopefully) be switching to the popular 60A model [YV09T60-0G from the same family.
Anyways, I’m feeding it 12V input voltage and have a 71kohm trim resistor to get a ~1.2V output voltage, plus a pair of 22uF ceramic input capacitors. Unfortunately the power supply is very finicky! Sometimes it works, other times it outputs no detectable voltage at all. Sometimes it will inexplicably shut off for no reason, other times it will fail to turn on (even with very little or no load attached). When it works, it works fine; I can pull at least 15A out for an hour at least – other times it shuts off while barely being taxed at all, or simply doesn’t turn on in the morning after being left disconnected all night. It’s driving me nuts.
I’ve assembled two PCBs with two separate power modules, and they both have this behavior, so I don’t think it’s a bad soldering connection or a defective module, unless both of them are bad. I’ve tried adding 1000uF of extra input capacitance as well as dummy loads, no luck. I’ve tried both a nice lab power supply and a few ATX computer power supplies for the 12V input; same behavior. The circuit is really simple: other than the trim resistor between pins 7+8, voltage in on pin 1, output from 12+13, and positive sense (tied to Vout) on pin 14, all other pins are either grounded or unconnected.
When I probe the two ends of the resistor I see 2.0V (output from the power supply) on one end and about 0.4V on the other end (input to the power supply). Curiously, I see this even when the power supply has decided it’s in a “bad mood” and isn’t operating (producing no output voltage).
You don’t mention output capacitance. The datasheet has fairly specific capacitance / ESR requirements; it’s not unusual to get instability in switching converters when you deviate from what the manufacturer recommends.
Can you post your schematic? Without it, I am just guessing here…
What is CCA connected to? The output capacitance depends on that. Looking at the data sheet, they use 110uF of ceramic and 220uF of tantalum on the output with CCA=1, and about 50uF on the input. Look at page 5 for more info on the capacitance and ESR requirements for the output caps.
Do you have the sense pins tied to the output at the load?
When it fails, what are the status of the OK and PG pins?
n1ist:
Can you post your schematic? Without it, I am just guessing here…
Sure! Really appreciate you taking the time to look at this. Schematic for the power supply is attached below; there’s another 3x100uF electrolytic, 5x47uF ceramic and 12x4.7uF ceramic on the output not shown.
n1ist:
What is CCA connected to? The output capacitance depends on that. Looking at the data sheet, they use 110uF of ceramic and 220uF of tantalum on the output with CCA=1, and about 50uF on the input.
CCA is grounded, so I should be okay with the 3x100uF electrolytics on the output, right? BTW I’ve also tried 3x100uF polymer caps.
Do you have the sense pins tied to the output at the load?
Unfortunately I will eventually have the supply driving five separate daughtercards each with its own load, so I don’t think there’s an easy way to sense close to the load – there are five loads on separate cards. So I opted to not use the sensing and tied +SENSE directly to VOUT and -SENSE to GND.
When it fails, what are the status of the OK and PG pins?
Unfortunately, I haven’t used this particular converter. At first glance, the schematic looks OK. For output caps with CCA grounded, they suggest 220uF with a 5mohm ESR with 400uF max.
PG low implies that the input voltage has gone outside the power-good window.