Getting lesser load current in LNK306G based design.

Hi, 

I agree with you that this behavior is little bit confusing, as it works well under low/high voltage, but not in the middle level voltage.  

Just to get some ideas, would you please try these:

1. Use full rectify bridge in stead of half bridge.

2. Remove the capacitor on C26, is anything changes. 

If you can send me the drain voltage and current just before the power supply enters into restart mode, it will be also helpful to find the reason.  

HI Terry,

                We did following trials and found no improvements.

1. Used full rectifier but found same issue in same 150VAC to 207VAC range. Please find attached drain voltage waveform with full rectifier. We don't have current probe so unable capture current waveform.

2. Removed capacitor C26 in full wave configuration but problem worsened and extended down upto 100VAC. Actually this is soft start capacitor. Attached drain waveform with full wave rectifier and no capacitor.  

3. Pl find attached half wave waveform with capacitor. 

In fact, I can't see any difference between any scenario as mentioned above. At present above waveforms blink on scope display meaning SMPS gets shutdowna and again starts. One thing I have noticed that restart pattern through 150VAC to 207VAC range is similar to overload pattern. We are doing overload test throughout 90VAC to 305VAC at 500mA. Don't know whether SMPS is getting loaded in this particular range or what?

Awaiting your support. 

Jagdish Karamchandani 

Hi, 

Thanks for the waveforms. Were these waveforms taken with 90Vac input?

From the drain voltage, it is clear that the duty cycle is over the device maximum duty cycle, and the power supply is on the edge of over load. The inductor with higher inductor should be used. Would you please do a measurement of the inductance value, it may be out of the spec. 

If you could measure the drain current, the inductor value can be calculated with the waveform if the input voltage is fixed.  

Hi Terry,

       Thanks for your support.

1. Today tried measuring current  waveforms by inserting very low resistance 0.15E in series with inductor but SMPS gone into cyclic restart mode when we attach scope probe. It seems we may need current probe.

2. Regarding previous drain waveforms, we did measurement at 170VAC where SMPS fall into cyclic restart mode. This is to know what happens during that time. Actually waveforms were blinking due to restart nature but we captured stable waveform by pressing stop button of scope. Pl let me know for any more query.

3. Inductance shown by inductance meter is 1.7mH for inductor rated for 1.5mH. Specs of inductor are +/-10% tolerance. P/N: RFB1010-152L from coilcraft. Actually this is the P/N suggested in application note for 360mA current output. I feel that KRP ratio might be decreasing during 150VAC to 205VAC which might require higher value inductor. Pl let us know what you think.

4. We shall order higher value inductors with same current rating and let you know performance. 

Jagdish 

Hi,

Thanks for the information. If you can record the waveforms just before the power supply enter in to auto restart, it will be hlepful too. Maybe with input voltage equal to 149V. Like you said, the lowest KRP ratio should be with the lowest input voltage, not the mid range, so it is not so clear what really happen during the mid range input voltage (150V to 207V.).

Please keep me update about the high value of inductors. You may try to increase the bus capacitor value to 40uF, see if the auto restart goes away. 

Hi Terry,

We measured drain waveforms near 150VAC and it remains same as posted you earlier. We also did following exercise with higher inductance,

1. With 2700uH, we can increase current capacity to 345mA from 320mA at 1500uH. So even with 2700uH we can't increase to 360mA. Moreover we observed humming with coilcraft series 2700uH (RFS1317-275KL) and hence we did trial with 2200uH (RFB1010-222L). With 2200uH we can increase current capacity to 330mA and no humming. Finally, we have decided to settle for 2200uH since there seems no scope to further increase capacity.

2. It seems increasing inductance doesn't add appreciably to current and we are not able to get specified load current with said design in mid range.

3. Don't know if LNK306 is thoroughly checked by power integration for 360mA output load across whole input range or not. 

If you have anything else to get it tried out, please advice or else we shall finalize design capacity to 330mA. 

Jagdish 

Hi,

Thanks for the update.  

Looks like with the 2200uH inductor, you are okay with the design, which is good. I do not really understand why it does not work with 2700uH, did you check the waveforms? Every time when we release the products, the maximum output power is checked and verified under some conditions, which stated in the data sheet.

Hi Terry,

   Thanks for your reply. Even 2700uH increase only about 10mA w.r.t 2200uH inductor. We did measured waveforms and were almost similar as posted to you. I checked LNK306 datasheet and found 360mA current to be achievable in certain conditions which I have attached for your reference. Out of 7 test conditions I am not sure about conditions 3 and 5.

1. Condition 3 is KRP ratio which I suspect might be dropping in that range and hence needing higher inductance.

2. Condition 5 is of efficiency. PI have tested with 75% efficiency but they have not mentioned effect of lesser or higher efficiency. Don't know whether this can impact current capacity or not. You can better comment on this.

3. After doing lot of exercise it seems 2200uH inductor fit well w.r.t others. However, we have increased only 10mA capacity w.r.t 1500uH from same RFB1010 series.

4. One other questions that I can think is of mode switching in this range. Don't know whether some transition is going on from CCM to MDCM mode or what which overall gives lesser current capacity. 

Awaiting your comments. 

 Jagdish 

Hi,

The Krp should be increase when the input voltage increase, as the di/dt for the inductor will be increase. This is the point that I do not know why the power supply goes to auto restart only at some mid range of input voltage.

Did you tried to use high DC bus capacitor, what was the result, does the issue go away?

The waveforms you attached in previouse post, is under the auto restart period, would you please record the drain voltage and current when the power supply is working okay? For example record the waveforms with 90Vac, 120Vac and 230Vac conditons. thanks

Hi Terry,

    We did increased input bus capacitor by 9.4uF (2X4.7uF) but issue remained unsolved at load current of 350mA.

Please find attached waveforms at 90VAC, 120VAC, 230VAC and 300VAC.  Your further advice awaited.

Jagdish 

Thanks for the waveforms.

If I understand well, these waveforms are the drain voltage during startup. Is the situation changed now, is the power supply have problem on 90Vac now, which was not originally?

Hi Terry,

    All these waveforms are of drain node. Power supply is having no issue at 90VAC, 120VAC, 230VAC and 300VAC. As informed earlier supply only has issue from 150VAC to 205VAC range and that too at 350mA. Issue doesn't occur even at 150VAC to 205VAC if we reduce load current to 320mA.

Jagdish 

Hi,

Got it. Did you hear anything back from the field application about your design?

To really figure out what caused the failure, you could recore the drain current, and drain voltage at the point when the input voltage increase slowly from 140Vac to 150Vac, which the issue just start to appear. You can set the scope time scale to be long, and zoom in the waveforms to see what is the change when the issue coming in. The same record can be done from increasing the voltage slightly from 200Vac to 210Vac, right at the point when the issue dispears. To do this record, you could use a DC input supply instead of AC input to avoid the line variation.  

Hi Terry,

     I shall arrange for some setup to measure current waveforms and update you later on this. 

Jagdish