TPS23753A: Power dissipation

Hi Jieun,

Please see the measured power of TPS23753AEVM-004 (www.ti.com/.../TPS23753AEVM-004):

48-V RJ45 input at open load condition: Vin = 48V, Iin = 14.03mA, Vout = 3.3V

12-V VDD-RTN adapter input at open load condition: Vin = 12V, Iin = 21.91mA, Vout = 3.3V

Best regards,

Diang

Hi Diang, 

Thanks for your reply, 

When I see the TPS23753AEVM-004, I couldn't find the input current information.

I was wondering if I could know the Iin value when the output voltage is 5V under the same input condition?

And, if I calculate the power dissipation of TPS23753A, which value should I check in the datasheet?

(under the same condition as I mentioned

(1) 48V POE voltage is applied to VDD pin

(2) DC 12V aux adapter voltage is applied to VDD1 pin)

Best regards,

Hi Jieun,

Thanks for your reply. 

Normally 60 – 70% power loss comes from the DC/DC converter: the FET switching & conduction loss, snubber losses, gate drive circuit loss, transformer / inductor core & winding losses. These losses depend on the converter design. The TPS23753A's internal hotswap and power interface rectifier losses take <30% normally. The other losses such as quiescent current loss could be neglected for a rough estimation. 

Assuming a 13-W PD input power and 48-V input from RJ45. I = 13W/48V = 0.27A. With a 0.7-Ω hotswap on-resistor, the internal hotswap power loss is 51 mW (less than 0.4% of 13-W total power).  The rectifier loss is 0.27A * 0.7V * 2 = 378 mW (less than 3% of 13-W total power). The rest is dc/dc converter loss.

If using 12-V VDD-RTN adapter input, the hotswap is not conducting and there is no hotswap power loss and rectifier loss. There will be a additional oring diode loss (13W/12V*0.7V = 758 mW). The rest is dc/dc converter loss. While the converter may has lower efficiency consider the current will increase when voltage drops.

Best regards,

Diang

Hi Diang, 

I want to estimate Tj data from below equation

Tj = Tc + ψjc(℃/W) * W

I could find ψjc(℃/W) value in datasheet, 1.9(℃/W) but, I have no idea how to get power dissipation of TPS23753A. 

That is why I ask you TPS23735A's power dissipation in two cases. 

1) 12V VDD-RTN adapter input

2) 48-V input from RJ45

Best regards,

Hi Diang, 

Thanks for reply, 

I have more question.

- A certain percentage of the gate drive power loss: 0.5 * Cgs * VCC^2 * fsw

From this equation, Is VCC voltage Vc or GATE voltgae? and what is Cgs from below circuit?

Best regards,