AN2208 datasheet(20/51 Pages) STMICROELECTRONICS

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5 Thermal stress tests

AN2208

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Thermal stress tests

The thermal model of a generic Intelligent Power Switch (IPS) can be exemplified as shown in

Figure 18. RthJC and RthCA represent the junction-to-case and the case-to-ambient thermal

resistance, whereas CthC is the predominant thermal capacitance and is basically related to the

package itself.

Note:

Case thermal time constant of 48 ms without external cooling.

RthJC = Rth2+Rth1/Nb of channels

If Nb = 8, RthJC = 1.34°C/W

If Nb = 4, RthJC = 2°C/W

The aim of the designer will be to provide the lowest possible junction–ambient thermal

impedance, in order to minimize the chip temperature jump-up.

Example VN808:

I = 0.5 A, L = 130 mH, f = 0.5 Hz, TA = 60°C, Duty cycle = 0.5, VCC = 24 V,

8 channels active, 4 channels working at the same time.

Conduction losses:

Losses due to ISON (supply current): 24V* 12mA(max.) = 288 mW

PowerMOS losses at ON State: 280 m

Ω(max.)*(0.5)^2*0.5*8 = 280 mW

Switching losses:

Switching losses are due to inductance discharge:

PDOFF = 8* EOFF*F = 8* 21mJ * 0.5 = 84 mW

Total losses and Junction temperature:

Total power losses = 652 mW (0.652 W)

Figure 18. Simplified thermal models

VN808 (8-channel IPS)

th1 = 1.9˚ C/W

th2 = 1.1˚ C/W

thCA

thC

36 mJ/K

Ai11610

VN340 (4-channel IPS)

th1 = 1.33˚ C/W

th2 = 1.67˚ C/W

thCA

thC

16 mJ/K

AN2208 Datasheet(PDF) 20 Page - STMicroelectronics