TPS92314A Single & Isolated Driver Reg;

Hello,

The numerical answer are a little out of the way?  Does this mean when it calculates an inductor it comes back with something like 20.6 uH which you would round up or down to 18, 20 or 22 uH or do you mean something else?  The calculator will get you close, electronics is never exact, there are tolerances for pretty much everything and delays that sometimes are not accounted for so if you calculate it to run at 65 KHz it may come back running at 60 KHz.  Your design should be robust enough for these tolerance as well as tolerances in input voltage, LED stack variation and a bunch of other things.

Turns ratio on an isolated design depends on how the designer wants it to work.  If the input voltage can go very high perhaps a lower turns ratio is needed to keep the voltage stress of the main MOSFET switch under its rating.  If the output diode voltage rating is of concern then the turns ratio wants to be higher.  Also have to keep in mind the rms and peak currents on the primary and secondary.  If the turns ratio is very low then the main MOSFET swtich duty cycle will be small and rms current for the MOSFET will be higher.  If the turns ratio is very high the rms current to the output capacitor will be high.  An example would be an offline 240 volt design.  The peak input voltage would be 240 * 1.414 * 1.1 = 373 volts.  If the output string is 40 volts I would probably start at around 4:1 since the reflected voltage would be 40V * 4 = 160.  The total voltage for the MOSFET would be 373V + 160V = 533V.  I would use an 800 volt MOSFET in this case.  This is due to leakage overshoot and headroom for clamping for an input surge with an MOV (most electronics will need to be protected for this if it's offline).

The datasheet doesn't have a non-isolated design calculated out but if it does show the schematic.  If you use turn ratio of 1:1 (if using the same winding as the input) the calculations in the datasheet will still work.  You can also do an non-isolated transformer design by simply connecting an isolated circuits primary and secondary grounds together.  This is a more complicated magnetic.  Also note the non-isolated common winding will reference the LED common to the rectified input voltage.

Thanks,

Dear Sir,
Thank You for reply !!
By calculations I meant the sample calculations given in the Data sheet itself

Thank You for the information on the Non Isolated Design...
I shall work upon as you have guided

Dear Sir,

How do we avoid the Auxilarry Inductance?
After going throught the SLVA663A Design I couldnt find the L_AUX Inductance
Please guide me,

Regards
Pratik

Hello Pratik,

I'm not sure what your question is. "How do we avoid the Auxilarry Inductance?" If you don't want an auxiliary winding you could just run it from a regulator from the rectified AC. If you want to calculate what the inductance is (it doesn't really matter, the main winding inductance is what determines the on/off times and switching frequency). But it will be n^2*Lpri. So if the aux winding is half the turns of the main winding and the main winding was 1 mH, then the aux winding would be (1/2)^2*1 mH or 0.25 mH. I think in the datasheet the turns ratio for the non-isolated was 1:1 so the aux and the same turns as the main winding.

Thanks,

Dear Sir,

Thanks you for your reply,

In the Non Isolated Design the Inductance (primary) Depends upon switching frequency, Ton (On time) and Toff( Off time),

the tweaking of on time and off time again depends upon switching frequency of the tps,

Please guide me upon frequency tuning, Is it perhaps true that proper switching frequency is the best way to optimise the thermal issues ?

Thanking You Regards

Pratik