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I have been working on a SMPS design using the NCP11187. It should output 24V DC nominal with a maximum ripple of no more than 100mV Pk-PK and be able to output up to 50 Watts. I have generally followed the example design from the datasheet of the NCP11187. After designing and building the PCB, my testing showed that my design can not successfully start regulating with a load attached. It does automatically try to restart the switches but fails everytime with a load connected. Based on the datasheet this makes me think that it could be tripping the feedback open−loop protection. with no load initially connected it does successfully start regulating and then I am able to pull current from the power supply however not the full 50 watts it was designed for. I can pull about 25 watts from the power supply before the voltage sags too low and it fails to regulate. ultimately here are my questions:

  1. what can I measure to be certain of which protection is being triggered by the ncp11187 when attempting to start up with a load connected?
  2. which part of my design is limiting the amount of current I can pull to ~ 1A
  3. Is there anything wrong with my PCB layout?

Additional Information:

Circuit Schematic

PCB Layoutenter image description here

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  • \$\begingroup\$ Much more information is required. Input voltage, transformer turns ratio and primary inductance, waveforms of DRAIN and CS... as-is it will be purely speculation as to why the converter isn't running properly. \$\endgroup\$ Commented May 12 at 20:58
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    \$\begingroup\$ Please show your design calculations, particularly regarding the output capacitor values. Have you followed the datasheet? Appnotes? Other design tools if applicable? Please include calculations or design parameters in any case. (I notice a datasheet is not linked, nor other references; providing at least a datasheet helps us answer your question quickly.) \$\endgroup\$ Commented May 12 at 21:10
  • \$\begingroup\$ I have provided more information. I will also provide waveforms as requested by Adam asap. \$\endgroup\$
    – Folgar
    Commented May 12 at 23:26
  • \$\begingroup\$ Do you have a means to isolate your AC supply? In other words, are you able to SAFELY connect scope GND to Vdd? NOTE: DO NOT ATTEMPT THIS UNLESS YOU ARE SUITABLY EXPERIENCED AND QUALIFIED. ONLY IF you can do this SAFELY, then please supply the following waveforms: Vcc, Vdd, Vin, Cs (U1, pin 1), under 2 conditions: 1. Load not connected, and 2. with the load connected that causes unit to NOT start. \$\endgroup\$ Commented May 13 at 1:47
  • \$\begingroup\$ Could you provide a reference to where you obtained the "uF per W capacity" rule of thumb? Was this the extent of your design calculations, is there anything more--? \$\endgroup\$ Commented May 13 at 2:51

2 Answers 2

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I suspect you will need a small capacitor placed at U1 pin 1 to Vdd. The current sense may have a spike at the leading edge that needs to be filtered.

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Is there anything wrong with my PCB layout?

I don't see a major thing except creepage and clearances. But they don't seem to be causing the problems you have.

what can I measure to be certain of which protection is being triggered by the ncp11187 when attempting to start up with a load connected?

You can probe the CS pin of the IC and see if it hits the IC's trigger level, which appears to be 0.7~0.8V. BE CAREFUL whilst doing this because you'll be dealing with live voltages.

which part of my design is limiting the amount of current I can pull to ~ 1A

Your primary current sense resistance seems a bit high for the power level, switching frequency, and the primary inductance.

  • Use the higher frequency variant of the IC (100k or 130k), or
  • Use a transformer with higher primary inductance (*), or
  • Decrease the primary current sense resistor (this must be properly calculated)

You may end up with doing all of the above.


(*) I can see from the WE product page that the transformer is designed for 65 kHz but the inductance is a bit low since it's designed for the universal input.

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