Viper115 cannot produce the required current

[Mnemonic]

I am designing a new SMPS for my project I need 350mA at 15V but I could not get more than 100ma. when I reach to 100mA output voltage was start to decrease. when current goes to 200mA, output is like a sinusodial signal from 9v to 15V. I design my SMPS with E-design suit but its not working stable. I need urgent help

[Peter BENSCH]

What are the parameters of the coil L, particularly the saturation current, that you used with 330µH?

Regards
/Peter

[Mnemonic]

Hi Peter,

thanks for your fast reply. you can find parameter of coil at below and also I share its dataheet link

https://cdn.ozdisan.com/ETicaret_Dosya/527916_8583741.pdf

[Peter BENSCH]

For this type of coil, the manufacturer Core Master has only specified the DC current, but no saturation current, so that I cannot recommend such a coil for DCDC converters with your parameters. In addition, due to the small dimensions of the coil, only thin wire is used, which results in a very high DC resistance of 1.26 Ohm.

Please get a coil with a saturation current of at least 1A.

In addition, the output power is at the upper limit of the VIPER115HS. At full load, I calculate a junction temperature of approx. 124°C, so you might want to consider a larger VIPER.

Another question: your values for Rc, Cc and Cp lead to a low phase margin, so there may also be a stability problem. For your basic values, I calculate values of 150k, 7.5nF, 100pF instead of your 1k+100nF+10nF.

Regards
/Peter

[Mnemonic]

Thanks for your reply,

I will try your suggest. Also could you explain function of Comp pin ?

how could you calculate that values

[Mnemonic]

Hi again,

I have this coil. I was try with this but nothing was changed. Do you have any opinion for this

https://eu.mouser.com/ProductDetail/Bourns/SRN1060-471M?qs=UvjENeDOEefs7qNqJQPBYw%3D%3D

[Peter BENSCH]

The new coil has a saturation current of 900mA, which might just be enough, but the DC resistance is not much better at 0.95 ohms. The inductance is with 470µH also somewhat high, which would have to be checked again with the eDesignSuite. Nevertheless, you should be able to continue working with this coil for the time being.

Pin COMP:

As mentioned in the datasheet, the compensation network is to achieve stability and good dynamic performance of the control loop. Since it is not trivial to calculate it if you are inexperienced, you should use the values of the eDesignSuite for your environment parameters.

[Mnemonic]

Ok thanks alot for your information.

could you suggest a coil to me which is best for us ?

[Peter BENSCH]

As already mentioned, it should work with the coil from Bourns if you change the  compensation network as suggested by the eDesignSuite.

[AScha.3]

the design, buck, non iso, 15V , 400mA :

coil: 

Why not using 1,6A coil, as recommended by eDesignSuite ? (and other parts/values also...)