开云体育

Hi All,

Just wondering if there are any SMPS guru’s out there that can help me with a problem. I have a PS 1690 power supply that will cycle when it is cold. I can here it ticking, a 12 volt test lamp attached to the output will cycle along with the ticking.

After a few minutes, the PSU will fire off and stabilize and all is well. All of the voltages look good, the primary and secondary caps all test good (ESR and capacitance).

Once it finally lights off, the voltages all seem normal the output ripple looks fine. I’ve tried different load combinations and it doesn’t seem to matter much.

Heating the board ?seems to reduce the time the issue occurs.

Looking at the primary with another scope, all seems to look fine. The start up voltage on the hybrid IC is fine and the output to the gate of the FET is cycling on/off when the problem is occurring.

Any change the Opto-Isolator could be causing the is concern or is it more likely that a resistor value has changed value? It so hard to figure out without a schematic.!

Thanks for your feedback!

Jim

Sent from Windows Mail

#104576

Usually I would say the primary or secondary side capacitors are bad but you
checked them.

There is a bootstrap circuit on the primary side which powers the regulator
control circuits when it is operating.  If it is developing low power, then the
regulator will cycle on and off as the bootstrap supply filter capacitor charges
and discharges.

Show quoted text

On Fri, 28 Feb 2014 03:35:20 +0000, you wrote:

Hi All,

Just wondering if there are any SMPS guru’s out there that can help me with a problem. I have a PS 1690 power supply that will cycle when it is cold. I can here it ticking, a 12 volt test lamp attached to the output will cycle along with the ticking.

After a few minutes, the PSU will fire off and stabilize and all is well. All of the voltages look good, the primary and secondary caps all test good (ESR and capacitance).

Once it finally lights off, the voltages all seem normal the output ripple looks fine. I’ve tried different load combinations and it doesn’t seem to matter much.

Heating the board  seems to reduce the time the issue occurs.

Looking at the primary with another scope, all seems to look fine. The start up voltage on the hybrid IC is fine and the output to the gate of the FET is cycling on/off when the problem is occurring.

Any change the Opto-Isolator could be causing the is concern or is it more likely that a resistor value has changed value? It so hard to figure out without a schematic.!

Thanks for your feedback!

Jim

#104580

开云体育

Hello Jim,

?

Double check the capacitor that supplies the control IC / Hybrid as this would be the likely culprit based on your observations (high ESR)

?

Often heating helps solve this problem as ESR is proportional to temperature (Low temp = higher ESR) - hence heating improves start up reliability

?

Also, check the biasing components, probably a resistor connected to the high voltage rail (may have gone high in value but not O/C)

?

I have also encountered the diode connected to the feedback (bootstrap)?winding of the main transformer increase its dynamic resistance which can cause these symptoms

?

Once the unit initially "kick starts" via its biasing network, it doesn't get enough feedback current via the winding to sustain operation and the "bias" supply drops below the operating voltage of the IC, and the cycle repeats itself

?

Often the diode will appear to measure OK on?a normal?digital multimeter

?

This is an?uncommon?fault (and difficult to find as you would normally only arrive at this diagnoses when everything else is ruled out)

?

Let me know how you go

?

Hope this helps

?

Regards,

Brenton

?

Show quoted text

-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...] On Behalf Of Jim Stadtfeld
Sent: Friday, 28 February 2014 2:35 pm
To: tekscopes@...
Subject: [TekScopes] TDS-420 SMPS

?

Hi All,

Just wondering if there are any SMPS guru’s out there that can help me with a problem. I have a PS 1690 power supply that will cycle when it is cold. I can here it ticking, a 12 volt test lamp attached to the output will cycle along with the ticking.

After a few minutes, the PSU will fire off and stabilize and all is well. All of the voltages look good, the primary and secondary caps all test good (ESR and capacitance).

Once it finally lights off, the voltages all seem normal the output ripple looks fine. I’ve tried different load combinations and it doesn’t seem to matter much.

Heating the board ?seems to reduce the time the issue occurs.

Looking at the primary with another scope, all seems to look fine. The start up voltage on the hybrid IC is fine and the output to the gate of the FET is cycling on/off when the problem is occurring.

Any change the Opto-Isolator could be causing the is concern or is it more likely that a resistor value has changed value? It so hard to figure out without a schematic.!

Thanks for your feedback!

Jim

Sent from Windows Mail

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#104585

These 'scopes use OEM (non-Tek made) PSU's. I have repaired the PSU in a TDS420A using the service manual of the LeCroy 9354, which can be found on the web. It turned out to use an identical circuit, only replacing one or two positive secondary voltages with negative ones, but?same layout.

The TDS420's PSU is not the same (at least it wasn't with my samples), though they are interchangeable.

Raymond

#111819

Sorry to resume an old thread, hoping to help.. I had the same startup problem on my TDS420 PSU; after a bit the psu would turn on correctly and continue to work until next day.
 Next day the psu wasn't starting again; protection was kicking in every 1-2 seconds.
 The main problem was caused from the overcurrent protection; even without load, the protection would kick in.
 I've disabled the protection (risky, i know) and now it works correctly.
 Let me know if you need more details.