Thermal design and pin layout of prototype ETD29-16-10 core

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Hello, I have got two questions on a PI Expert’s design proposal for a 120W power supply based on a TOP258Y TOP Switch HX: 1. Thermal design of the power supply: PI Expert states a primary RMS current of 1140 mA . Taking into account the voltage drop stated in the data sheet of 2.9V @ 100°C junction temperature I would use a heat sink with a thermal resistance of less than 13 K/W (as TJC is 2 K/W) to allow operation up to an ambient temperature of 50°C. PI Experts has calculated a secondary RMS Current through the output diode of 15.53A. According to the datasheet of the proposed MBR20100CT the forward voltage drop is about 0.85V leading to a power dissipation of 13.2 W. As the maximum allowed junction temperature is 175°C and the power supply is supposed to be used at ambient temperature not higher than 50°C I intend to use a heat sink with a thermal resistance of less than 7.5 K/W (as TJC is 2 K/W). Is that correct or did I miss something? 2. I am little confused with the pin layout of the proposed ETD29-16-10 core. The data sheet from your homepage (ETD29_14pin_YihHwa_YW-166-00B.pdf) shows that pin 1 is in the upper left corner and pin 14 is in the lower left corner of the transformer. PI Expert’s transformer construction data states that the primary windings cover pins 1, 2 and 3, pins 8 und 9 are used as outputs for the rectifier diode and pins 4 and 5 are used for the bias supply. With this pin layout it gets tricky to implement the PCB layout example shown in figure 18 in your Application Note AN-43 as therefore the connection to the positive terminal of the input capacitor would have to be on the opposite side. Is the data sheet correct on the pin assignment of the core? Or can the prototype transformers be assembled that way (by you), that I can stick to the reference design of AN-43? Thanks in advance for helping me out! Ruediger

Your question has 3 parts.
1) Dissipation in MOSFET of TOPSwitch -
This analysis is correct

2) Dissipation in output diode
The output diode can be modelled as a voltage source (VF) and a series resistance (Rd)
With this approach the power disspation in diode (Pd) is given by
Pd = VF(t)*IO(t) + Rd*IRMS^2
where VF(t)*IO(t) is the instantaneous power integrated over a switching cycle. This approach will give you a dissipation below 13 W I guess.

3) Regarding the transformer bobbin
You can update the pin assignments in the software from the "Winding Construction" ---> "Secondaries" dialog. Once this is updated the RTSS technicians can assemble according to your specification.