Low power not isolated 5V adapter

Posted by: DrSlump on 02/01/2013

Hi, i would like to create a 230V to 5V / 100mA peak not isolated adapter. The main goal is to obtain a low power device.

I'm testing a basic circuit using the lnk304 (i'll use the lnk306 for the final version) configured as buck converter:

- half wave rectifier

- PI filter with 1mH inductor and two 4.4uF capacitors (optained with two couples of 2.2uF capacitors)

- MURS160 freewheeling diode, 1mH output inductor and 120uF low esr capacitor.

- Feedback circuit is obtained with a 3.9V zener plus a tlp127 optocoupler and a 1K resistor in paralel with opto's emitter diode.

Measuring the input power consumption with a power meter it reports 0.7W, in open circuit. Is it normal? How to lower the adapter power absorption?

Regards,

Michael.

Edit:

i'm testing the circuit, and i found that inserting 1K resistive load, the power consumption lowers to 0.2W, instead of 0,7W in no load condition. Pleas can you help me understanding the problem?

Comments

You need to attach schematic -- I suspect there is something off with feedback circuit and/or associated component values

Thankyou for your answer, here you are the actual schematic.

The resisitve laod allows the converter to maintain regualtion. With no bleeder the buck regualtor cannot regulate to no load. The opto coupler input and outtput will be conducting more heavily when converter output is above regalation and hence the input power will rise.

Thankyou, but i have some doubts. I've modified the circuit as shown in the attached image file, introducing a 2.2K load and changing the zener diode with a MMS74688Y1G and using a 470uH output inductor, to obtain about 6V output voltage.

In this way the measured input power consuption is about 0.2W, wich is a tad greater than the 80mW no load power consuption declared in datasheets. I would expect about 0.1W.

Do you have any ideas how to lower the no load power consumption?

The combined drain leakage current and drain supply current is about 330uA. Also the output load from error sense and preload resistor would make 80mW not possible.