Referance design at 105C ambiant temprature

Posted by: oner_hatipoglu on 05/26/2009

Hi,

I am working on designing an over temprature smps design. My smps outputs are 12V/120mA and -5V/100mA. Before of all, I made a -12V/150mA buck-boost source and now it's still being tested. I am trying to calculate junction temprature in buck boost but i cant. Can you give that calculation for LNK306 and a reference buck-boust design at 105°C?

However, i saw using flyback designs in high-temprature designs when i was searcing about that subject. Can you tell me difference between buck-boost and flyback design for High temprature designs?

Thank you.

Comments

I do not have a specific calculation for buck boost losses. In the LNK306, there are two main loss factors. The switching losses that are dependent on the input voltage, the device switch rise/fall time and inductor value. Using an oscilloscope, measure the switching time and the drain current and voltage. Calculate the energy dissipated by the internal FET for the rise and fall time for each switch cycle. Multiply this by the switching frequency. This will give you the switching losses in Watts. The other loss is the conduction loss. For this loss you need to calculate the RMS current through the switch element. This current squared times the RDSon will give you the conduction loss through the device.

A comparison between buck boost and flyback depends on the application. As a rule of thumb, buck boos does not have the leakage inductance, inter-capacitance of the windings and core losses of a flyback. Depending on the input to output voltage ratios, the flyback usually has lower RMS currents on the primary side. Especially if the input voltage/output voltage ratio is large.

Z. Cochrane

Thank you for your answer. Thr last question, are there any referance design in flyback topology at 105°C?

Öner Hatipoğlu

ARGE Mühendisi
Elektronik Tasarım I
34950 Çayırova Kampüsü - Tuzla / İstanbul
Tel : 90 216 - 585 8092
Mail : oner.hatipoglu@arcelik.com

There are no reference designs for 105 deg. C operation. My advice is to pay close attention on heat sinking the power electronics to keep the junction temperature at a safe level. Also if you are using aluminum electrolytic capacitors, use 105 deg C rated parts. If the power supply is to be operating at this temperature, the capacitor's life will be low.

Z. Cochrane