Thermal Design

Since the VPS13 includes three channels as shown in the circuit diagram on page 2, we first consider a single channel.

The chip temperature of each transistor under actual operating conditions is determined using the following formula.

Tj = (Tri) =

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

θj-c (Tri) : Thermal resistance of an individual transistor

Pc(Tri)

: Collector loss for an individual transistor

∆Tc

: Case temperature rise

Ta

: Ambient temperature

The

θj-c (Tri) for each chip is: θj-c (Tr1) = 35°C/W

θj-c (Tr2) to (Tr4) = 30°C/W.......................................................(2)

Although the loss for each transistor in a video pack varies with frequency and is not uniform, if we assume the

maximum operating frequency, f = 130 MHz (clock), then the chip with the largest loss will be transistor 3 (Tr3) of the

emitter-follower stage. From the Pd-f (clock) figure we see that loss will be 22% of the total loss for a single channel:

(3)

Here, we must select a heat sink with a capacity

θh such that the Tj of these transistors does not exceed 150°C. Equation

(4) below gives the relationship between

θh and ∆Tc.

∆Tc = Pd (Total) ×

θh..................................................................(4)

The required

θh is calculated using this equation and equation (1).

No. 5548-3/4

VPS13