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# TPS23758EVM-080: Rcs Calculation

Part Number: TPS23758EVM-080
Other Parts Discussed in Thread: TPS23758

I must admit, I'm feeling rather dumb this morning but I'm not understanding the choice of resistors for the EVM.

• The EVM is supposed to support 2.3A
• The EVM has two 0.91Ω resistors in parallel
• Rcs = Vcsmax / Ipk
• Vcsmx-max is 0.6V according to the TPS23758 datasheet
• Rcs = 0.6 / 2.3 = 0.26Ω

I assume the EVM used 2 resistors in parallel for better heat dissipation but two  0.91Ω in parallel is 0.46Ω which is almost double what my calculation says it should be.

• Hello George,

I think the 2.3A you are thinking of is the output current.

The two 0.91ohm resistors are on the primary side.

The output current, or secondary current, of the transformer does not equal the primary side current.

To calculate Rcs, first calculate the  peak current through the primary side of the transformer.

But you are correct we use two resistors in parallel for heat dissipation.

Regards,

Michael P.

Applications Engineer

Texas Instruments

• Hi Michael,

Sorry, I forgot that the TPS23758 is on the primary side of the LDT1018 transformer. Looking at that the transformer spec I see that it has a 6:1 ratio between the primary and secondary (PIN 1-2 : PIN 6-7 = 6±3%:1) side.

So if I take the secondary current of 2.3A and divide by 6 this yields a Rcs value of 1.56Ω which still doesn't match the two 0.91Ω so I really must be having a bad day!

George

• Hello George,

The current calculation is not that simple. The flyback topology is switching. The primary side of the transformer acts like an inductor (technically it is an inductor). The switching voltage means that the current is ramping up and down. So the voltage on the source of the primary FET (which is integrated in the IC) is actually a ramp, like a sawtooth. you need to calculate the peak current.

I recommend reviewing the following application note to help with some of the calculations in the flyback. 