viper37 junction temperature

[PINI BITON]

Posted on August 16, 2016 at 18:44

How i calculation junction temperature in VIPER37HD/LD

and how the frequency(60k,115k hz) Affect on junction temperature?

[Patrizia BELLITTO]

Posted on August 18, 2016 at 14:56

Hello,

The junction temperature is given by the following formula:

Tj = Pdiss • Rth_ja + Ta

       where:

       Ta = ambient temperature

       Rth_ja = junction-to-ambient thermal resistance (reported on datasheet, its value can be reduced providing adequate copper area around DRAIN pins) </p>

Pdiss = Pbias + Pcond + Pswitch + PCAP is the power dissipated within the VIPer, in particular:

             Pbias = VDD • IDD1 (bias losses)

                         where:

                         VDD = voltage of the VDD pin (depends on the voltage delivered by the auxiliary winding of the transformer);

                         IDD1 = VIPer consumption, in switching conditions (reported on datasheet)

             Pcond = Rdson • Irmsp conduction losses

                         where:

                         Rdson = on resistance of the VIPer (reported on datasheet)

                         Irmsp = primary RMS current (depends on input/output conditions, primary inductance, reflected voltage)

</p>

Pswitch = (Vin + VR) • Ipk • Tc • fsw/3 switching losses is an approximate formula where

                          where:

                          Vin = input voltage

                          VR = reflected voltage

                          Tc = crossover time (the time during which, at turn-off, both DRAIN voltage and current are not zero), should be estimated and is in the range of some tens nsec

                          fsw = switching frequency

  Ipk = primary peak current

PCAP = Cdrain_par • Vds2 • fsw/2

                          where:

                          Cdrain_par = sum of all the capacitance of the DRAIN node (MOSFET output cap, reported on datasheet, plus stray capacitances due to the transformer)

                          Vds = DRAIN voltage value at turn-on

The components PCAP and Pswitch depends linearly on the switching frequency  

From: biton.pini

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Posted: Tuesday, August 16, 2016 6:44 PM

Subject: viper37 junction temperature

How i calculation junction temperature in VIPER37HD/LD

and how the frequency(60k,115k hz) Affect on junction temperature?

[PINI BITON]

Posted on August 19, 2016 at 09:06

thanks alot for your help

Pcond = Rdson • Irmsp conduction losses

you meant that  the current in Square ?

and if you  explain more what exactly  Tc

best regard

pinhas

[Patrizia BELLITTO]

Posted on August 22, 2016 at 10:12

-Iprms

is the root mean square of the primary current

-Tc

At the end of the MOSFETs ON time, the DRAIN current has reached its maximum, while the DRAIN voltage is zero, thus the product IDRAIN*VDRAIN = 0W

Some time after turn on, IDRAIN will be 0 and VDRAIN will be Vin + VR, so again IDRAIN*VDRAIN = 0.

But in between, since the switch is not ideal, there is a time interval Tc when both IDRAIN and VDRAIN are >0, and so their product results in power dissipation at turn off.