Altair04-900

[Yathavakannan]

Hi,

       Can I utilize this IC in a buck converter configuration to step down from +400Vdc to +15Vdc? If so, could you please provide the application guide for buck converter design? Thank you!

[Peter BENSCH · Accepted solution]

Welcome @Yathavakannan, to the community!

The ALTAIR04-900 cannot be used as a buck converter because of e.g. the variable switching frequency, so you should consider using one of the VIPerPlus instead. However, I would like to point out that even these can only be used to a limited extent for non-isolated buck, because extremely small duty cycles occur with the usual large voltage differences, which are difficult to regulate and strongly limit the reachable output power. It is sometimes overlooked that these regulators are optimised for flyback or quasi-resonant operation and can only work as a buck occasionally and under certain conditions.

With the search text "viper buck converter" you will find some tools on our website that may be suitable for testing.

Hope that helps?

Regards
/Peter

[Yathavakannan]

Hello Peter,

             Thanks for your response. As per your suggestion, I reviewed the Evaluation Tools list and noticed that the  max current is specified as 200mA whereas we required 350 - 400mA. Can I use the Viper38 IC in a buck converter configuration to step down from +400Vdc to +15Vdc at 400mA output current? If that's feasible, could you please provide the application guide for buck converter design?

[Peter BENSCH]

Well, that may already be at the limit of what is feasible. You can find application notes in the buck converter designs mentioned above (STEVAL-xxx). Otherwise I recommend the eDesignSuite, with which you can simulate the design quite well:

1. Power Supply Design Tool > Select block AC/DC > Start a new project
2. Fill the input and output parameters, e.g. MinV=200, MaxV=300, Output Voltage=15, Current=0.4
   (note that the voltage must be entered as AC, even if you already have DC and do not need a rectifier)
3. Click Features (left of the long light blue bar) > Topology: Buck > Click No filter > Select Include

You will then receive a series of design suggestions, which may not necessarily fit perfectly. You can look at them one by one (click Start Design under the design to be viewed, which will open this design in a new window/tab, switch back to the overview window and repeat this procedure with the next design to look at).

I recommend taking a closer look at two examples for your specific case: VIPER319XD and VIPER115HS. Here you can see very clearly how, for example, the switching frequency influences the phase margin (stability), the chip temperature or the efficiency.

The VIPER319XD is probably best suited to your application.

Good luck!
/Peter

[Yathavakannan]

I have reviewed the VIPER319XD datasheet and the Power Supply Design Tool, and I have a few questions about this IC:

1. The datasheet only provides RthJA (80°C/W) and does not specify the RthJB value. In another datasheet for the same package, I found RthJB listed as 10°C/W without the bottom pad provision. How is this thermal resistance possible, considering that components with a bottom pad typically exhibit the same thermal characteristics? Could you please confirm the RthJB value?

2. In the Power Supply Design Tool, there is a mention of IC capacitive loss, but I’m not clear on what that entails. If the output inductance increases, does this loss also increase? Could you please provide more details on this?

Thank you in advance for your assistance!