Thermal Design

Since the VPS12 has the three-channel configuration shown in the circuit diagram on page 2, we first consider a single

channel. The chip temperatures of the transistors during operation can be determined from the following formula:

Tj = (Tri) =

θj-c (Tri) × Pc (Tri) + ∆Tc + Ta (°C) ........................(1)

θj-c (Tri): Thermal resistances of the transistor chips themselves

Pc (Tri): Collector loss of the transistors

∆Tc:

Increase in case temperature

Ta:

Ambient temperature

The

θj-c (Tri) for the individual transistors is:

θj-c(Tr1) to (Tr4) = 35°C/W.........................................................(2)

Although the loss in each transistor in the video pack changes with frequency and thus are not identical, assuming a

maximum frequency of 120 MHz (clock), the transistor with the largest loss is Tr3 in the emitter-follower stage. From

the Pd-f(clock) figure it can be seen that this loss is 22% of the total loss. Thus:

× 0.22 [W] .............(3)

Select a

θh for the heat sink so that the junction temperature (Tj) of this transistor does not exceed 150°C. Equation (4)

gives the relationship between

θh and ∆Tc.

∆Tc = Pd (TOTAL) × θh ..............................................................(4)

The required

θh can be calculated from this equation and equation (1).

No. 5309-3/4

VPS12