In another thread, the question of cranking up the bias of an amplifier

to the absolute maximum has come up. As this is a tutorial opportunity,

I have started a new thread:

Attached is the portion of the IRF244 datasheet which lists the maximum

ratings. We see that the dissipation figure is 125 watts at 25 deg C, and

you must derate that by 1 watt per degree above that temperature.

The maximum dissipation calculation for this is predicated on the notion

that the maximum junction temperature is 150 deg C. This is pretty

universal for Silicon transistors.

The formula is:

Tj = Watts X (Rj + Rc + Rh) + Ta , where:

Tj = the temperature of the junction (deg C)

Ta = ambient temperature

Rj = thermal resistance of junction to transistor case (deg/watt)

Rc = thermal resistance of transistor case to heat sink

Rh = thermal resistance of heat sink to ambient (usually 25 deg)

Keep in mind that Rh is the amount of heat sink allocated to one transistor,

so if there are 2 transistors on the sink, the Rh figure is doubled.

From the 125 watt rating, we can infer that Rj = 1 deg/watt, and from

experience we know that the Rc value is also about 1 deg/watt.

If we simply know the temperature of the heat sink or the case, the

calculation is more simple:

Tj = Watts X (Rj + Rc ) + Th (Th is temperature of heat sink)

or

Tj = Watts X (Rj) + Tc (Tc is temperature of transistor case)

Let's take the case of a 65 deg heat sink. If the part is dissipating 50 watts,

then

Tj = 50 X (Rj + Rc) + 65

Tj = 50 X (1 + 1) + 65

Tj = 165

This a lot more than the manufacturer thinks is reliable. The rule of thumb

is that the lifespan doubles for every decrease by 10 deg C of the junction.

And contrariwise.

Lets say an Aleph 2 has 12 devices dissipating about 200 watts, or about

16 watts each. The temperature on the heat sink at the devices is about

60 deg C. This means that the Rh (experienced by each transistor) is

(60 - 25) / 16 or about 2 deg/watt

The maximum dissipation per device here can be estimated by

150 = W * (1 + 1 + 2) + 25

125 = W * 4

w = 125 / 4

w = 31 watts.

In our Aleph 2 example

Tj = 16 * (1 + 1 + 2) -25

Tj = 116 deg C.

And this is why Aleph 2's don't break.

to the absolute maximum has come up. As this is a tutorial opportunity,

I have started a new thread:

Attached is the portion of the IRF244 datasheet which lists the maximum

ratings. We see that the dissipation figure is 125 watts at 25 deg C, and

you must derate that by 1 watt per degree above that temperature.

The maximum dissipation calculation for this is predicated on the notion

that the maximum junction temperature is 150 deg C. This is pretty

universal for Silicon transistors.

The formula is:

Tj = Watts X (Rj + Rc + Rh) + Ta , where:

Tj = the temperature of the junction (deg C)

Ta = ambient temperature

Rj = thermal resistance of junction to transistor case (deg/watt)

Rc = thermal resistance of transistor case to heat sink

Rh = thermal resistance of heat sink to ambient (usually 25 deg)

Keep in mind that Rh is the amount of heat sink allocated to one transistor,

so if there are 2 transistors on the sink, the Rh figure is doubled.

From the 125 watt rating, we can infer that Rj = 1 deg/watt, and from

experience we know that the Rc value is also about 1 deg/watt.

If we simply know the temperature of the heat sink or the case, the

calculation is more simple:

Tj = Watts X (Rj + Rc ) + Th (Th is temperature of heat sink)

or

Tj = Watts X (Rj) + Tc (Tc is temperature of transistor case)

Let's take the case of a 65 deg heat sink. If the part is dissipating 50 watts,

then

Tj = 50 X (Rj + Rc) + 65

Tj = 50 X (1 + 1) + 65

Tj = 165

This a lot more than the manufacturer thinks is reliable. The rule of thumb

is that the lifespan doubles for every decrease by 10 deg C of the junction.

And contrariwise.

Lets say an Aleph 2 has 12 devices dissipating about 200 watts, or about

16 watts each. The temperature on the heat sink at the devices is about

60 deg C. This means that the Rh (experienced by each transistor) is

(60 - 25) / 16 or about 2 deg/watt

The maximum dissipation per device here can be estimated by

150 = W * (1 + 1 + 2) + 25

125 = W * 4

w = 125 / 4

w = 31 watts.

In our Aleph 2 example

Tj = 16 * (1 + 1 + 2) -25

Tj = 116 deg C.

And this is why Aleph 2's don't break.