Top switch selection
Hello
I used Pi Expert to generate a power supply it suggested a TOP254YN because the application uses about 20 Watt in a adapter situation. But as mention this spec of 20 Watt is achieved at max 50 degrees Celsius. And out application goes up to 70 degrees Celsius. Therefore I selected the TOP 257 which we used in an eelier design.
Is it safe to conclude I can achieve the 20 Watt with the Top-257YN at 70 degrees Celsius ambient ?
Do i also have to choose a bigger transformer because of my temperature spec ?
When I optimize for the Top-257YN i get a suggested input Cap of 39 µF but in the datasheet 3 µF / W is suggest as starting point. Do use about 60 µF or start with the suggested 39 µF (The 39µF is a calculated value of the used T filter)?
If a input 1% voltage accuracy on the mains output are there any disadvantages on the regulation of the second power output of the design ? Or on the load response ?
We have universal power in and 15V 0.6A regulated and 12V 0.75 A unregulated outputs. The two power outputs must maintain isolation between each other and to the power input.
Comments
We need galvanic isolation between the two outputs. So I just remember I must not stack my output windings but keep them floating.
As mentioned in the application notes this will influence cross regulation and the most on the unregulated power output. what can I suspect from the non regulated 12 volt output when the 15 V is regulated?
On the the 12 volt output we connect two DC/DC convertors:
12V -> 3.3 V 1.2A max
12V -> constant current 60 mA @ max 22V
If I choose the DC/DC converter to accept an input voltage of 10 - 15 V is it sufficient?
I am not sure how much isolation is needed but galvanic isolation is a must. So several kV
My first guess is 10-15V is sufficient.
If the loads on both outputs don't vary much, then the regulation will be good.
I have selected:
39µF 5 mH 39µF Pie filter
TOP257 (instead of TOP254)
EEL22 core (instead of EEL19)
The program was not able to optimise the design for the bigger core seize.
I increased the Secondary Main Number of Turns (Manual Input) up to 17.
Is this number acceptable? Original value was 11.
I increased the value because the Peak Flux Density was to high.
Pls. post the PI Expert design file.
I have uploaded the file.
please rename the file the uds file was not allowed for upload.
I suggest the following changes to your design:
- update your PI Expert to the latest version
- Use TOP-JX family (TOP26x)
http://www.powerint.com/en/products/topswitch-family/topswitch-jx
- Use TOP265EG or VG. It is capable of 26W in an adapter application (50*C internal ambient). See above link, look in the power table in the right side of the page, under the 85-265V, adapter column. Worst case use TOP266. TOP267 is probably overkill.
- in PI Expert under "design options" change "optimization type" from "cost" to "efficiency"
- under "Design" "key parameters", set VOR to 105V and KP to 0.57 These settings tend to maximize efficiency but tend to require a larger TOPSwitch
There are multiple ways to reduce the temperature rise in order to operate in a high ambient temperature environment.
1) Use a larger TOPSwitch as you mention
2) Increase the input cap to 3 uF/W
And yes you will need a larger, more efficient transformer.
All the above increase efficiency so as to reduce temperature rise.
The 2 outputs will track each other closely if you follow the general guidelines for good cross-regulation. For example, the 2 outputs' windings should be wound next to each other.
With 1% regulation necessary on one output, it means the TL431 feedback probably should only monitor one output. The other output can probably stay within 5%.
How much isolation is needed between the 2 outputs?