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LT8471 Flyback 2-pair output ref circuit

I wanna use one IC to output 2-pair output by flyback circuit.

LT8471 is the only one that has two output and two voltage control pin(FB1&FB2). But there's no flyback circuit to refer in LT8471 website.

I wanna use 1to2 transformer(like Sumida CEP1311E) and FB1&FB2 to change my output voltage. So I use LT8471's LTspice model to draw an abnormal circuit as below.

I'd like to know how to correct the circuit to have a 2-pair output(VPP_1/VNN_1 and VPP_2/VNN_2) circuit.

Thanks.

Parents
• Please see LT8302 datasheet or LT8331 datasheet for flyback design guidance.

• I checked LT8302, it's familiar with LT8304, but there's no ref pin for LT8471 to use.

LT8331 has flyback demo circuit, I try to follow the flyback circuit and draw a circuit as below.

From LT8471's spec I got the Vout formula. Vout=VFB*(Ra/Rb+1), and Vout = Vfw_1 in the circuit.

And then secondary side voltage formula comes from LT8331 as below.

From LTspice simulation result VPP=215V, it's seems right when R8=400k, R7=10k, D=0.4. But how can I know D? or how to estimate D?

Thanks.

• D is determine from desired Vin and Vout along with transformer turns. Use the equation you identified and solve for D. Remember:

• I think I find the right circuit from LT3958 as below. VFB comes from secondary side of transformer VPP and divided by RA&RB.

According to LT3958's flyback circuit, I modify my circuit as below, is the circuit right??

And then the secondary Vout can calculated by Vout=VFB*(RA+RB)/RB. Is this conclusion right? Thank you

• Yes this is correct however you will likely want a snubber across the primary winding as shown below.