Output voltage higher than expected

[JimmyShen]

When I solely test this Auxiliary power board(Just use the power supply to provide AUX_DC+, without any loads), the output voltage P_15V and D_6.5V are both a little bit higher. I got 18.8V and 10.1 respectively. Anybody know the reason?

By the way, sometimes when I give the power supply, the 'overload' will be light. But after a while, it will become normal.

 

[kellys_eye]

Can't read the schematic - too low resolution.

 

[Harald Kapp]

Please supply an enlarged image of this section:

Have you tried with some load, e.g. 100 mA on each output? Switch mode power supplies often require a minimum load for the regulation to operate correctly.

 

[kellys_eye]

Switch mode power supplies often require a minimum load for the regulation to operate correctly.

That was my first thought but since the output looks to be via shunt regulation there's no real way the voltage could exceed what the circuit is designed to deliver.

I suspect it's a case of misplaced ground probe when metering the output.

 

[danadak]

Output is classic series regulated, and its open loop, so Vout = Vz - Vbe.
Principal problem, as an aside, occurs at very high loads where R1 starts starving
the pass transistor for base current and zener drops out of regulation.

Note I cannot tell from schematic if there is fdbk from one of the outputs
due to schematic resolution/readability when magnified. But that would not
change the series regulator output behavior.




Note I did not use your part numbers, but general principles apply.

One can envision that as load drops Vbe drops due to base current drop, so
output rises to Vz (its tolerance matters) in the limit.


Regards, Dana.

 

[JimmyShen]

Please supply an enlarged image of this section:
View attachment 59934
Have you tried with some load, e.g. 100 mA on each output? Switch mode power supplies often require a minimum load for the regulation to operate correctly.

It works as a regulator and the value for these two outputs is correct. Comparing these two outputs with the remaining two, it seems that the remaining two didn't have a load.

 

[JimmyShen]

Can't read the schematic - too low resolution.

I posted part of the enlarged photos.

 

[JimmyShen]

That was my first thought but since the output looks to be via shunt regulation there's no real way the voltage could exceed what the circuit is designed to deliver.

I suspect it's a case of misplaced ground probe when metering the output.

That two outputs is correct. P_15V and D_6.5V have the problems. I am now also thinking the load. I will try it.

 

[crutschow]

the output looks to be via shunt regulation

Not exactly.
It's a common emitter-follower series regulator controlled by a shunt Zener diode reference.
It has no feedback from the output so has poor regulation.

 

[danadak]

As Ib drops so does Vbe and as previously stated Vout = Vz - Vbe, so we have Vout rising :




Regards, Dana.

 

[danadak]

The PBSS306 on your schematic, is shown as NPN, yet the 306 is PNP ? The transistor should be
NPN.....

https://assets.nexperia.com/documents/data-sheet/PBSS306PZ.pdf

Regards, Dana.

 

[kellys_eye]

Yes, I meant to type series but somehow didn't realise I'd written shunt! D'oh!

 

[crutschow]

Yes, I meant to type series but somehow didn't realise I'd written shunt! D'oh!

Welcome to the Brain Flatulence Society.
I'm a member in good standing.

 

[kellys_eye]

Welcome to the Brain Flatulence Society.
I'm a member in good standing.

I'm too close to retirement to be bothered!