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[28,5V@0,6A] LNK6763E - Low efficiency (50%)

Posted by: Serras on

Hello all,

I need some help to "debug" an efficiency problem in my SMPS.

Specs: Input 86Vac - 240Vac and Output 28,5V @ 600mA. The device selected by the PIExpert was the LNK6763

Currently the SMPS is regulating near the supposed values: Output at 29Vdc and is able to deliver stable 580mA of current.

But the efficiency is around 50%. The transformer temperatures arises  70ºC @ full power.

Not only the primary measured inductance is inside limits but also I've seen on the osciloscope the "charge" current of the Primary Winding (with a series resistor...) and it seems that it is not saturating the core. (measured the efficiency without the series resistor and it is 50%)

The output diode and input snubber also doesn´t seems to heat too much (65ºC).

The output voltage on the Auxiliary Winding is also stable at around 15Vdc

Attached is the PIExpert Simulation as well the transformer specs. Also some Photos.

Data From PIExpert:

Core: E20/10/6 (EF20), NC-2H (Nicera) or Equivalent, gapped for ALG of 149nH/T2

Nominal Primary Inductance: 1197uH (+-10%)

Maximum Primary Leakage: 11,97uH

 

Measurements:

Primary Winding Inductance : 1,206mH

Secondary Winding Inductance: 96,2uH

Auxiliary Winding Inductance: 11,5uH

Maximum Primary Leakage (all other winding shorted): 8.9uH

 

Maximum Switching Frequency: ~120kHz

 

Maximum Primary Overshoot @ 328Vdc @ 240vac 

@ Open : 468Vdc

@ 100Ohm @292mA @ 29,10V : 484Vdc

@ 50Ohm @582mA @29,00V: 496Vdc

 

Efficiency (@100 ohm charge):

Input: 239,6Vac @ 77.8mA 

Output: 29,23Vdc @ 292mA 

Efficiency: 45,8%

 

Efficiency (@50 ohm charge):

Input: 239,6Vac @ 137mA 

Output: 28,9Vdc @ 580mA 

Efficiency: 51%

 

Files

Comments

Submitted by PI-Picard on 04/19/2017

Hi Serras,

Can you do the following?

1) Can you show me the connection diagram how you measured the efficiency?

2) Take measurement of the reflected voltage across the primary winding when the switch is off.

3) Did you follow the exact same wire gauge described in the PIExpert?

4) Measure the RMS voltage across R2 & R3. You may use the oscilloscope for about 20 cycles or more.

5) Is there any pre-load connected at the output?

 

Regards,

PI-Picard

 

 

 

 

Hi,

sorry for my late reply.

Its all figured out now. I was measuring the efficiency in a wrong way as you say... Seems obvious now but at that time I din't get it right away.

I also got help from the www.feryster.pl were I got the samples.

Thanks for your support and replies

Best Regards,

Serras