Pls. Help
SMPS output low... 6.62 volts instead of 20 volts.
No shorted components no dried up caps.
Uc2844 not shorted...

 

Pls. Help
SMPS output low... 6.62 volts instead of 20 volts.
No shorted components no dried up caps.
Uc2844 not shorted...

What is the voltage on the UC3842 feedback pin?

 

Post a schematic of your circuit design.

 

Post a schematic of your circuit design.

It's not a stand alone power supply
It's within an inverter ( Variable Frequency Drive) board.

 

Look at Uc2844 datasheet and draw PSU circuit diagram

 

Look at Uc2844 datasheet and draw PSU circuit diagram

Ok

 

Ok

Point is the 2844 readings are fine
Vi... 15.65v
Vref..... 5.01v
Vfb..... 2.5v

 

It's not a stand alone power supply
It's within an inverter ( Variable Frequency Drive) board.

Measurement findings on this ic

Vi.... 15.65v
Vref..... 5.01v
Vfb...., 2.5 v
Which seems to me quite normal

 

What is the voltage on the UC3842 feedback pin?

Vfb.... 2.5v
Vi...... 15.65v
Vref...... 5.01v

 

Where does the Vfb signal come from? Is it from a two-resistor potential divider from the output, or via an opto-isolator?

 

As Ian0 is pointing out, check the feedback loop. 2.5V is generally the 'stop' command for the chip, it needs to be below that for the converter work.

 

Where does the Vfb signal come from? Is it from a two-resistor potential divider from the output, or via an opto-isolator?

Thanks for Swift response
Problem is this smps is not stand alone. it's within an industrial 3 phase inverter all components smd. Difficult to perform whatever you recommended me.
There are no any optocouplers at all.
The output of the schottkey diode is directly tied to 269-5g regular.

 

Thanks for Swift response
Problem is this smps is not stand alone. it's within an industrial 3 phase inverter all components smd. Difficult to perform whatever you recommended me.
There are no any optocouplers at all.
The output of the schottkey diode is directly tied to 269-5g regular.

Can’t help any further.
if there is 2.5V at Vfb, then there is no fault in the UC3842.

 

What is the voltage on the UC3842 feedback pin?

Also one more thing there are 2 lm239 comps. Their Vcc reads the 6.62v.
While from their datasheet the supply is +/-18v....

 

Can’t help any further.
if there is 2.5V at Vfb, then there is no fault in the UC3842.

Thanks a lot