Thermal Design for the VP503

We recommended that the VP503 be used with the operating case temperature, Tc, under 100°C. Since the VP503

includes three channels, 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 (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 that the

maximum operating frequency, f = 85 MHz (clock), then the chips with the largest loss will be transistor 3 and that loss

will be about 1/4 of the total loss.Thus from the Pd for a single channel we have:

Pc (Tr3) f = 85 MHz = Pd (1CH) f = 85 MHz

× 0.25 [W] ..................... (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. 4933-8/11

VP503