MOSFET specs, very confused

[Simon]

I'm refering to the datasheet of the IRFZ44N: http://www.irf.com/product-info/datasheets/data/irfz44n.pdf

Figure 8 shows output characteristics and it looks like the headlined 49A is only achieved at 2V DS, what exactly does this mean ? the overall supply or the voltage across the D/S while under that load of 49A which should be fine if it is 17.5mR and the load has a decent resistance.

[Kremmen]

Why the confusion? If you think about it, it should be pretty obvious. The SOA just reflects the fact that you cannot really have an arbitrarily huge dissipation in the transistor die and expect the heat to escape to subspace. Take say 50 amps and 50 volts drain to source. The device would now dissipate a respectable 2,5 kW. The waste power would have to emerge like a welding torch flame to get rid of that kind of power.
So, the maximum currents are always specified for deep saturation and then the low channel resistance is relevant.

Edit: At 17.5 mohm and 50 amps the device would dissipate 42.5 W, a much more reasonable figure. In that operating point the Vds would be just .85 V

[Simon]

ok, just making sure there's no huge catch. Basically the graph is a representation of what you can calculate given the current passed, the on resistance and the max power you can dissipate, although it seems to also indicate how much peak can be coped with etc.

[Kremmen]

Basically it is a representation of the heat capacity of the device body. The loci in the diagram outline constant energy per impulse resulting in the maximum permissible temperature of the device. At one limit is the DC operation where you are basically in a continuous state and need to see that the heat is put away as quickly as it is generated. That is where the Rdson is most relevant. The single pulses are supposed to heat the device to max and then you need to cool it down before the next jolt.

[Simon]

yea, that's what I meant, the graph has calculated the max V/I for the given wattage plus added rule of thumb for the heat capability of the mosfet's body in absorbing a pulse of high heat (current) for a short period without overheating.

[Marco]

Basically it is a representation of the heat capacity of the device body. The loci in the diagram outline constant energy per impulse resulting in the maximum permissible temperature of the device.

The 160 A limit is a additional limitation of course, otherwise the 100 us line would extend above it instead of flattening out. MOSFETs are not generally short circuit rated, regardless of pulse length (unlike some IGBTs).

[Simon]

well most have a pulse current spec but it is mere uS so not of much practical use and more of a safe guard to protect the device

[poorchava]

Figure 8 shows output characteristics and it looks like the headlined 49A is only achieved at 2V DS, what exactly does this mean ?

It means that at any time in steady condition Id*Vds should not exceed quoted power dissipation rating of 94W. 49A*17.5mR= 42W. No problem here. Problems you would have if you drove the gate with too low voltage, and had ~35mR. the you would dissipate about 90W of power and approach the SOA. Other way to calculate that is 49A*17.5mR=0.85V, which is way below the 2V limit.

As long as you drive your gate hard. Whenever I have a power mosfet like this i try to achieve Vgs=10..15V. This will ensure the minimum Rdson in probably every mosfet.