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# CSD17571Q2: MOSFET Zth

Part Number: CSD17571Q2
Hello,
I am concerned about the increase in temperature of a MOSFET.
Example:
I drive a power resistor, with a single pulse with ton=300ms; the current is 7A.
From the datasheet I extract Zth=0,2*190=38K/W and Rds=24mOhms.
Power dissipation of the MOSFET=7*7*0,024=1,176W
That means an increase in temperature of 1,17*38=45K
Anyway, during the ON pulse the Rds will increase accordingly, due to temperature.
But I calculated Power dissipation considering the Rds@25=24m.
Is that ok? Does the Thermal Impedance Graph already consider the increase of Rds during the on pulse or should I apply a derating factor?
Best regards, Gheorghe
• Hi Gheorghe,

Thanks for your interest in TI FETs. The transient thermal impedance curves are based on power dissipation and not current. The temperature variation of on resistance is not really relevant to the curves in the datasheet. Your power dissipation calculation should take into account the temperature variation of on resistance. Multiply the I2R loss by the normalized on resistance shown in Figure 8 of the datasheet at the operating temperature of the FET. For example, at 100degC & VGS = 4.5V, the normalizing factor is about 1.4. That should give you a better estimate of the power loss.

• Hi John,

thank you anyway for the answer! But I didn't say that the transient thermal impedance curves are based on current!

It is still not clear to me, therefore I will repeat my calculation and I will ask again my questions:

Temperature = 25 grad; ton=300ms, single pulse; I=7A
From the datasheet: Zth=0,2*190=38K/W and Rds=24mOhms, worst case;
P_loss=7*7*0,024=1,176W;
Delta_temperature=1,17*38=45K;