tny274pn 5v5w design output problem

Posted by: 168 on 07/29/2013

dear pi engineer:

i designed a smps with tny274pn, but there exists some problems with the output, please see the attached pic.

test condition: 230vac input, 1a output(with 5ohm resistor load)

as u can see from the pic: the output is very bad. the lowest point is 4.58v which is not ok; the highest point is about 5.16v.

and there exists a lot of noise at the output.

can u tell me how to improve it? thanks in advance.

by the way: 1: the smps has no post filter;

2: the output cap is 330uf/35v electrolytic capacitor;

3: input bulk cap is 10uf/400v

4: choose tny274pn

5: np:107turns; ns:5turns ee16 core. lp equals to 1684uh

Comments

Hi,

The switching frequency ripple in a flyback converter output is dependent on the ESR of the output capacitor. You may need to use low ESR output capacitors. Alternately you may be able to use a post filter. Kindly verify.

I request you to use our design software PIExpert to verify your design.

If you need additional support, a PIExpert file will be very useful, please provide the file.

Regards

PI-Sarek

hello, please see the attached pic. as u can see, tl431 performs the feedback circuit. but i dont use r6 , c8, l3 and c7. and in my circuit, resistor r7 and r8 is 12k, r5 is 150ohm.

by the way, can u tell me how to determine the resistor r5, do u have some equations?

if i want to add post lc filter, how can i choose the inductor and capacitor?

i have increased the output electrolytic caps, it seems that the output become a little good, but its not what i want.

dear pi engineer:

one question:

please see the attached pic, this is the ds voltage waveform. and my question is : why the frequency of the ds voltage is not

constant? sometimes it is 65k, sometimes it is 132k which i think is right. from my oinion, the frequency of the ds voltage

is constant which is 132k.

Hi,

You must use the components R6 and C8 in order to achieve stable operation. your power supply appears to be unstable based on the waveforms that you have provided.

Please refer to application note AN-42 for which I am providing a link below. This application note explains selection of components for the feedback loop for a ON/OFF control power supply controller. This application note also provides guidance for the selection of components for the post filter.

http://www.powerint.com/sites/default/files/product-docs/an42.pdf

The TinySwitch uses ON/OFF control. This is a technique in which the output voltage is regulated by skipping pulses. This effectively reduces switching frequency as a function of load which means you will typically see the part operating at close to full switching frequency only at rated full load and the lowest rated input voltage when the power supply is designed correctly.

ON/OFF control has the advantage of keeping the efficiency high even at very low input power due to the drop in switching losses as a result of drop in switching frequency. You should refer to the datasheet for the part and you will find a detailed explanation for this method and the techniques used in the architecture of the part to optimize the operation.

The resistor in series with the opto coupler sets the gain and the application note AN-42 provides guidance for selection of the value.

Note that unlike a typical fixed frequency linear PWM control power supply, the ON/OFF controlled power supply is inherently stable and allows considerable flexibility in selection of these component values.

Regards PI-Sarek