Through-hole packages

AN533

4/22

Figure 1.

RMS and average currents

1.1.3

Dissipated power in a TRIAC

A TRIAC is made up of two thyristors connected back to back. This means we consider the

sum of the dissipated power of both thyristors.

For a phase angle conduction the RMS current is given in Figure 1 C.

1.1.4

TRIAC without external heatsink

Figure 2 shows the thermal equivalent diagram for a TRIAC without external heatsink.

In practice the imposed parameters are:

●

●

●

P: dissipated power in the TRIAC depending on the used TRIAC and on the load

current

The following equation defines the junction temperature depending on these parameters:

Ip

A

B

C

Ip

Ip

t0

T/2

T/2

T

T

T

t0

t

t

t

IT(AV) =

2 . Ip .t0

.T

π

IT(AV) =

IT(AV) =

1 -

+

. sin

2 . Ip

π

2 . Ip .

I T(RMS) =

²

Ip²

2

T

0

IT(AV) =

i(t)dt

1

T

I T(RMS) =

²

i²(t)dt

T

0

1

T

I T(RMS) =

²

Ip².t0

2 . T

I T(RMS) =

²

Ip

2

²

1

2 .

π

2 . t0

T

4 . . t0

T

π

∫

∫

(

( (

(

(

(

cos

t0 .

²

π

T

π

P =

2 .

2

√

π

.IT(RMS) .Vt0 + Rd .I T(RMS)

2