# Electronic – How is the lower voltage on a transformer calculated

power supplyswitch-mode-power-supplytransformervoltage

I was reading the data sheet for the LT8301 and laid on eyes on one of their example schematics.

The input range (Vin) in the example states 2.7V-36V, and even gives measurements for when Vin = 5V. However, looking at the Sumida 12387-T041 inductor (T1) shows that the application voltage range on the primary size is 18V-42V.

Presumably allowing less than 18V (i.e. Vin < 18V) enter the transformer is fine, otherwise Linear wouldn't give this example schematic.

So this begs the question: how is the lower voltage limit calculated, and at what point does the transformer no longer functions (output power on the secondary becomes insignificant (I realize this is a bit ambiguous, but I can't figure out how to word it differently))?

That lower voltage value is listed under the general heading "target application" and I'm sure if you dug deeper you'd find that target application and be able to show a circuit diagram then all would become obvious.

It also says (under that target application) that the output voltage is 5V yet the LT circuit shows 15 volts so you need to dig a bit to find that circuit.

The LT circuit is a flyback converter and the ratio of DC input to DC output voltage has very little to do with turns ratio and much more to do with duty cycle. For instance, the transformer (aka coupled inductor) in question is ratio 1:1 so, as far as duty cycle is concerned I'd expect input voltage to equal output voltage at 50:50 (this is a broad brush impressionist type remark of course) and if you do the sums (below) it is not that far off....

Input voltage range is 2.7 volts to 36 volts and this averages about 19 volts so having 15 volts at the output is "about" right (generalisms used with caution of course) for the mid range of input DC voltage.

Like I said earlier, it's a flyback circuit and duty cycle is king and basically turns ratio is the parlour maid: -

She's the one at the back on the left and he's the one at the front calculating the duty cycle.