Pulsating buzz in my SMPS in standby mode

[hafrse]

Hi,

Have a SMPS with pulsating buzz sound when it is in standby mode, frequency of the pulsating buzz is about 1 hz. The buzz is comming from the secondary part near the output voltages. When I power up the instrument from standby, it is away (?) but not 100% sure, there are fans running and could be there but I can not hear it. However, the SMPS works fine. What can that be ?

Thanks

 

[schmitt trigger]

Most modern converters, to meet stringent unloaded efficiency requirements (like Energy Star), go into what is called a pulse skipping or hiccup mode.
This should be normal. Can you read the output voltage while in standby? Or better, apply an oscilloscope?

 

[hafrse]

Hello,

Thanks for the information , will try to reach the outputs and measure the voltages with a scopemeter

 

[hafrse]

Most modern converters, to meet stringent unloaded efficiency requirements (like Energy Star), go into what is called a pulse skipping or hiccup mode.
This should be normal. Can you read the output voltage while in standby? Or better, apply an oscilloscope?

Hello,

Some update:

The only output avaialble on standby is the 12V, it is all ok according to the oscilloscope , but if I measure the large 680uf 250V caps on the primary side , the voltage drops around 5 volts each buzz/whistle cycle (around 1 hz).
Also , there is a small pcb with AN8032 (power regulator active filter) with a red diode, it also blinks each buzz/whistle cycle , if I measure the voltage on one of the pins with a multimeter, the pitch of the buzz changes.
When powering up the instrument, all the buzz/whsitle goes away.
It seems to be the standby circuit (?). I have noschemtics for the power supply.
Thanks

- - - Updated - - -

Or may be I should concider to replace the primary 680uf caps (high ESR)

 

[schmitt trigger]

If the 12v output is OK, you should be OK also
It is the burst mode during standby

 

[picgak]

Hi,

Have a SMPS with pulsating buzz sound when it is in standby mode, frequency of the pulsating buzz is about 1 hz. The buzz is comming from the secondary part near the output voltages. When I power up the instrument from standby, it is away (?) but not 100% sure, there are fans running and could be there but I can not hear it. However, the SMPS works fine. What can that be ?

Thanks

In standby mode (when the smps is ON)that sound is normal
It can be rectified if you can apply a signal to the shutdown pin while on standby Or put some minimum load always

 

[hafrse]

In standby mode (when the smps is ON)that sound is normal
It can be rectified if you can apply a signal to the shutdown pin while on standby Or put some minimum load always

Hello, there is a standby line which goes directly from the front keyboard (power-up/power-down momentary push button) to the power supply, and I hav a 12V while in standby, can you please explain what signal or how to rectify the buzz?.

Thanks

 

[picgak]

just connect a 1k resistor accross 12v dc and see the change

 

[hafrse]

just connect a 1k resistor accross 12v dc and see the change

Tried with 1k and .5k , same problem, it seems that when the other voltages are activice (when the instument is on) the buzz goes away.
Thanks