What kind of dual power supply is this that can't produce only -Vs?

In the following generic power supply from China which I just got, it says "Note: Positive and negative voltage are used at the same time, otherwise negative voltage is not output"

Why? How does it differ to power supply that can produce a negative voltage only without the positive also being used?

In the cut portion, the positive output is the same DC +3~30 V as in the middle picture.

• if you want only negative voltage you can use one of the buck regulator modules (eg LM2956) just connect supply negative to the positive output. The allowed regulator input voltage must be more the the sum difference between input and output voltage Oct 25, 2022 at 4:17

In the following generic power supply from China which I just got, it says "Note: Positive and negative voltage are used at the same time, otherwise negative voltage is not output"

Why?

The converter you have uses the feedback from the positive output for regulation. What happens to the positive supply thus affects what happens to the negative supply, but not vice-versa.

If the current out of the positive supply is too small, the converter enters a state where the output of the negative supply is less than it's nominal value, perhaps close to zero.

Typically, the type of state that the converter enters is what is known as Discontinuous Conduction Mode (DCM). In the DCM mode of operation, there is insufficient current through the inductor (in this case positive supply inductor) during its "charging" phase of a cycle to maintain current through the entire "discharging" phase of the cycle. Very often, and most probably in your case, the efficiency of the converter drops dramatically when the converter is "deep" in DCM.

Although it means wasting power, you could connect a resistor across the output of the positive supply if you want to use only the negative supply. This will draw current through the positive supply and allow the converter to operate in a region where it provides a regulated negative output as well.

• you may as well connect a LED in series with the resistor it might fool some people that the resistor current is not wasted. Oct 25, 2022 at 4:19
• Instead, why not just connect the positive voltage to the circuit's ground, and the PSU's ground to the circuit's input? the negative input becomes even more negative, but can be left floating Oct 25, 2022 at 5:53
• I bought it to power the legendary AD620 -Vs to increase the negative swing. Can I connect the dual supply only to the chip, that is, the positive right to the AD620 +Vs and the negative to the -Vs. If yes, should I connect its ground to the circuit ground? Wont it mess it up since the AD620 has input resistor to pin 2 (-IN) with a 2 volts (from separate battery supply) in series below it (this is to respect the minimum input voltage)? Thanks.
– Jtl
Oct 25, 2022 at 20:35
• @jtl. I would need to see a schematic to tell you whether the arrangement you are thinking of will work. Post another question with your proposed circuit. (Don't add this question to your current question). Oct 25, 2022 at 21:52
• this is the exact circuit electronics.stackexchange.com/questions/639661/… i need to add -5V to make it inside the diamond box. Tnx.
– Jtl
Oct 25, 2022 at 22:43