buckemcfccpower supplyswitch-mode-power-supply
I need to design switching mode power supply with input voltage range of 25V-45V and output voltage 5V@2A.
Power supply must be ON while the system is in sleep mode(~1mA consumption). In that particular case switcher will be highly inefficient and will be working in dicontinuous mode. Since I need to pass both FCC and CE will that be a potential unresolvable problem? I have previous experience in designing BUCK converter but none of them had to go through certification.
TI's got a tool, named WEBENCH which can make a lot of charts and calcualtions for you. Here is its output with your parameters in pdf.
Let me highlight the one about the efficiency. The simulations shows that this IC has a better efficiency when Vin is 20V, but this difference is not that much.
It is not just the Vin that matters, if you change the supplied current from 200mA to 3A a different efficiency chart will be shown. In this case the Vin = 30V is the better choice.
Usually, there are similar charts in the datasheets if tools like this are not available.
If you only need 200mA, you should choose converter which is capable of, let's say 300mA maximum current rather than 3A, efficiency is better near the max current. Another converter, which can drive max 300mA, LMR14203's efficiency chart:
It is again the worst at 30V, but it is around 88% while with the LM2596 it is 79% which is a significant difference. On 20V it is above 90% which is pretty good.
12V AC reaches a peak voltage of about 17 volts, then via the rectifier it might produce about 15V DC across a suitable capacitor (rated at 25 volts). Maybe you had the capacitors wired back to front or maybe you misapplied the bridge rectifier. Maybe the bridge was broken in a previous experiment.
I'd still look for a buck regulator like this: -
Found here on ebay costing $2.86. It's a switching buck regulator that has an adjuster to set the output to be anywhere from 3V to 40V: -
Type / name:LM2596HVS DC-DC step-down module
Input Voltage:4.5V ~ 53V
Output Voltage:3V ~ 40V
Output Current:3A (max)
Conversion efficiency:92% (the highest)
Output Ripple:<30mV
Switching frequency:150KHz
Operating Temperature:-45 ℃ ~ +85 ℃
Size:43mm * 21mm * 14mm (L * W * H)
Best Answer
DCM is not necessarily less efficient --it depends on the controller.
If it is possible to raise the 5 V a little, you could add a linear regulator, limited to 1 mA in parallel with the SMPS. If the linear regulator regulates at (say) 5.2 V, the SMPS will see that VOUT is OK, and will fully switch off -- no switching cycles. When the load exceeds the 1 mA capability of the linear regulator, the SMPS will take over.
in addition, the inductor node will ring in DCM when the driver turns off -- put ~ 1kohm in parallel with the inductor to dampen this ringing. It will have a slight effect on efficiency though.