Viper318HD

[Rahmaa]

I have  design Buck  converter with Viper318HD ,With below specification:

Vin:(85 to 265)VAC

Vout=12VDC

Iout=500mA

And i not follow recommended   Commponents   value  which  Are mentioned in Edesign tool,  i use  components which  are availble  with me.

So  i need:

1- Know is this  design  will  be stable  to use it  or not ??

[Rahmaa]

when  entered  Input voltage with 265VAC  in Edesign  suit   for operating  condition  for Buck converter  with Viper318HD Controller, 

It mentioned in

1- Ton section

" Cycles skip"

2-IC TJ 300C

 Kindly support us to  Know What mean with these  warings   ,and  are   it will effect on Stability of Circuit  if i operate at 220VAC

[Peter BENSCH · Accepted solution]

Welcome @Rahmaa, to the community!

With your specification of 12V, 500mA, we are talking about an output power of 6W, which corresponds to an input power of 8.7W assuming an efficiency of 69%. With these power ratings and the voltage range, you will hardly be able to achieve stable operation, as I have already mentioned several times here in similar projects, e.g. here.

My recommendation: use a transformer and let the VIPER318 work as a flyback, then the design will be considerably simpler, more stable and easier to manage thermally, and you will also have galvanic isolation of the output voltage as a bonus.

Hope that helps?

Regards
/Peter

[Rahmaa]

 @Peter BENSCH ,  I very happy  with your reply ,Thanks  for your support , that's great  reply.

For  my information , kindly inform me what Is  maximum  power to use Viper318HD as buck converter  without any issues(More  stable)??

[Peter BENSCH]

At 12V, you can expect useful output currents around 250mA. At 265V with those 250mA, the VIPER318HD can get up to about 85DEGC, and at 265V with 300mA, it can reach around 115DEGC.

But you could try the VIPER318LD, which runs at 60kHz instead of 132kHz, so the losses are much lower. It does need physically bigger passive components (capacitors, coils), but even at 12V, 500mA and 265Vac, the VIPER318LD only gets (simulated) up to only about 96DEGC.

Regards
/Peter

[Rahmaa]

@Peter BENSCH , You mean  if  i  need to   design Viper318HD   as buck with Vout =12v, Iout =500mA , vin=265VAC, but decrease the switching frequency to 60kHZ, and increase value of inductance , this will reduce rms  current, and power dissipted will more lower  and  the converter will stable??

Also for below statement as you mentioned  , i need to know ,  you mean if i design on these condition (Vout=12,Iout=500, Vin=265VAC, Fs=60kHZ, high value of inductance) , this make JT=96 Degree, this degree is  will be actual  or may be higher   when Viper318HD  work at   actual system  conditions  ??

"but even at 12V, 500mA and 265Vac, the VIPER318LD only gets (simulated) up to only about 96DEGC"

[Peter BENSCH]

The VIPER318HD is working only at a fixed frequency of 132kHz, which is why I recommended the VIPER318LD with a fixed frequency of 60kHz.

So if you use an VIPER318LD with this lower frequency of 60kHz, I simulated with the eDesignSuite that a chip temperature of about 96DEGC will occur at the most unfavourable input voltage of 265V for maximum power dissipation. The temperature depends, of course, on environmental conditions such as convection cooling, etc., but it gives an initial indication of a possible chip temperature.

With its much higher switching frequency of 132kHz and at 12V, 500mA, the chip temperature would already be well above the permissible maximum, which is why the VIPER318HD is no longer suitable for a buck delivering 12V and 500mA.

Regards
/Peter

[Rahmaa]

@Peter BENSCH 

Thank you very much for the clarification, and thank you for your patience in answering my questions.

 This  below  graph mentioned in  data sheet of Viper31  IC, i need to know  this mean  , when  i increase  Copper to be area lager than  100mm^2 , this  will  reduce  the thermal resistance , and  also this reduce Junction temperture of Ic  ,then power dissiption will decrease ??

If  true   ??  i will consider it in next design  ,

 and  i need  to know , is  this will  help to  reduce the heat significantly? 

[Peter BENSCH]

The simulation assumes optimal conditions, meaning practically unrealistic cooling. A copper surface of 100mm² is still far from that, resulting in a significantly higher temperature difference. I estimate that for the simulated values, you must assume an unrealistic cooling copper area of at least 2000...3000mm², that is, 20 to 30 times larger, and additionally ensure sufficient air convection. Further increasing the copper area then has no significant effect. Additionally, the ambient temperature plays an important role, because the simulated chip temperature difference is not only added to it but also increases with the ambient temperature. As you can see, many influencing factors must be considered.

This effort is only necessary because you want to implement a buck regulator for the 12V, 500mA at all costs. A flyback regulator is not only significantly simpler but also cheaper to implement and does not require such an oversized switching regulator.