How to calculate TVS diode SMA6J33CA for ambient 105 °C

[Reha]

Hello ST team,

I have trouble to calculate Tj and Rd for ambient 105 °C. What was my steps

Applications conditions for protection:

Vin_max = 30 V

Vin_absolute = 58 V

Tamb_max = 105 °C

Max surge current for 8/20 μs pulse wave:

Isurge_max = 1 kV / 42 Ohm = 24 A

Data sheet TVS diode SMA6J33CA (Tamb = 25 °C)

Vrm_DS = 33 V

Vbr_max_DS = 40.6 V

Ipp_DS = 57 A

Vcl_max_DS = 69 V @ Ipp_DS

Rd_max_DS = 0.512 Ω

TC_max_DS = 10 x 10-4/°C

Ppp_DS = 4000 W Peak pulse power dissipation @25 °C

For Tamb = 25 °C:

1) Standoff Voltage Vrm_DS = 33 V must be above Vin_max = 30 V. ---> OK

2) Clamping voltage Vcl_max_DS must be below Vin_absolute = 58 V ---> OK

Vcl_max = Vcl_max_DS - Rd_DS x (Ipp_DS - Isurge) = 69 V - 0.512 Ω x (57 A - 24 A) = 52.1 V ---> formula from AN5541Transient voltage protection

3) Power dissipation must be below Ppp_DS = 4000 W ---> OK

Ppp = Vcl_max x Isurge = 52.1 V x 24 A = 1250 W

For Tamb = 105 °C:

All data sheet values must be recalculated but we need the Tj. 

Rthja = 150 °C/W from Figure 12

 

Ppp_DR = Ppp_DS x (240 / 600) = 4000 x (240 / 600) = 1600 W for Tj = 175 °C from Figure 3

 

Tj = Tamb + ( Ppp_DR x Rthja ) = 105 + ( 1600 x 150 ) = ??? ---> thats not possible

Thanks

[MROUV.1 · Accepted solution]

Dear Reha,

Thanks for sharing your case.

All information to perform a TVS selection are present.

About calculations / comments:

1. Correct
2. Temperature must be considered because that increases Vcl max. The maximum ambient temperature impacts the TVS selection on your case
3. Figure 12 is useful for repetitive surges (additional copper surface can be needed to dissipate average power to have average junction temperature lower than Tj_max). It is not your case because you consider only a single pulse
4. Figure 13 is useful for single surge (your case). Your comment is fair but peak junction temperature can be higher than Tj_max for a single pulse. As example, with 175°C ambient temperature, it is possible the dissipate 240W (instead of 600W at 25°C) and that dissipation due to the single pulse will increase junction temperature.

To perform all calculation, you can use TVS Simulator.

According to your conditions, SMBJ30CA is recommended.

Best regards

Mathieu

[Reha]

Dear Mathieu,

thank you for your fast answer. The simulation tool is very easy to find out a suitable TVS diode.

Best regards