ST AN533 Application note

AN533

Application note

SCRs, TRIACs, and AC switches,

thermal management precautions for handling and mounting

Introduction

The behavior of a semiconductor device depends on the temperature of its silicon chip. This
is why electrical parameters are given at a specified temperature.

To sustain the performance of a component and to avoid failure, the temperature has to be
limited by managing the heat transfer between the chip and the ambient atmosphere. The
aim of this note is to show how to calculate a suitable heatsink for a semiconductor device
and the precautions needed for handling, mounting and soldering techniques.

March 2008 Rev 3 1/22

www.st.com

Contents AN533

Contents

1 Through-hole packages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.1 Thermal resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2 Thermal impedance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.3 Insulating materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.4 Insulated components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

1.5 Handling and mounting techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

1.6 Through-hole package wave soldering . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2 Surface mount packages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.1 Thermal resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.2 Thermal impedance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.3 Mounting techniques and R

2.4 Reflow soldering information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

th(j-a)

3 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2/22

AN533 Through-hole packages

1 Through-hole packages

1.1 Thermal resistance

1.1.1 Review

The thermal resistance of semiconductor assembly is the parameter which characterizes its
resistance to the heatflow generated by the junction during operation. A temperature
exceeding the maximum junction temperature curtails the electrical performance and may
damage the device.

The maximum dissipated power capability is:

T

P

max

Where:

● T

max

j

● T

● R

The R

is the ambient air temperature in degrees ( °C)

a

(j-a)

th

th(j-a)

An analogy between Ohm’s law and the thermal equivalent circuit can be made:

● Electrical resistance corresponds to thermal resistance

● Current corresponds to dissipated power

● Voltage corresponds to temperature

jmax-Ta

=

R

th(j-a)

is the maximum junction temperature of the semiconductor in degrees ( °C)

is the thermal resistance between junction and ambient air in °C/W

takes into account all materials between the junction and ambient air.

Thus: V = R . I corresponds to ΔT = R

th

. P

1.1.2 Dissipated power for a thyristor or a TRIAC

The maximum power dissipation versus average on-state current (for SCRs) or RMS on­state current (for TRIACs) is given in the datasheet for each product.

However, a more accurate result is obtained by using the V
following calculation:

P = V

. I

to

T(AV)

+ Rd . I

Where:

● V

● R

● I

● I

is the threshold voltage specified in the datasheet

to

is the dynamic on-state resistance specified as Rd in the datasheet

d

is the average on-state current

T(AV)

is the RMS on-state current

T(RMS)

Figure 1 shows the RMS

2

T(RMS)

and average values for different waveforms of current.

and Rd values with the

to

3/22

Through-hole packages AN533

Figure 1. RMS and average currents

T

I

T(AV)

A

I

p

t

0

B

I

p

T/2

C

I

p

t

0

T/2

1

= i(t)dt

∫

T

0

T

T

T

t

t

t

²

I

I

T(AV)

I

T(AV)

²

I

T(RMS)

I

T(AV)

T(RMS)

2 . Ip.t

=

2 . I

=

=

2 . I

=

π

T

1

i²(t)dt

=

∫

T

0

0

²

I

.Tπ

p

²

I

p

(

2

cos t0.

p .

T(RMS)

²

I

T(RMS)

2 . t

1 - + . sin

²

(

=

0

T

2 . π

π

T

π

(

²

I

.t

p

0

=

2 . T

²

I

p

2

1

4 . . t

(

π

0

((

T

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.

The following formula gives the total dissipated power versus I
TRIAC (see Figure 1 C with t

2 . 2√

P =

π

.I

T(RMS).Vt0

For a phase angle conduction the RMS

= 0):

0

+ Rd.I

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:

● T

● R

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

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

: ambient air temperature where the TRIAC is located

a

: thermal resistance between junction and ambient air given in the datasheet

th(j-a)

current

T

= P . R

j

th(j-a)

+ T

a

2

T(RMS)

current through the

T(RMS)

4/22

AN533 Through-hole packages

Figure 2. Thermal equivalent diagram

Junction

T

j

R

th(j-a)

Ambient air

1.1.5 TRIAC with external heatsink

If the estimated junction temperature is higher than the maximum junction temperature
specified in the datasheet, a heatsink has to be used.

Recommendation: this calculation has to be made in the worst case scenario i.e with the
maximum dissipated power, load and line voltage dispersions. We have to consider the
maximum ambient temperature around the component i.e. inside the box where the TRIAC
is located.

The same approach as presented in the previous section allows a suitable heatsink to be
defined. Figure 3 shows the thermal equivalent diagram.

Figure 3. Thermal equivalent diagram with external heatsink

Junction

Case

Heatsink

T

j

R

th(j-c)

T

a

Ambient air

T

c

Rth(c-h)

Th

Rth(h-a)

T

a

5/22

Through-hole packages AN533

The formula to calculate the thermal resistance between heatsink and ambient air is the
following:

Tj-Ta

R

th(h-a)

=

- R

P

th(j-c)

- R

th(c-h)

Where:

● T

● P is the maximum dissipated power in W

● R

R

is the junction temperature in °C

j

is the thermal resistance between junction and case in °C/W

th(j-c)

) is the thermal resistance between case and heatsink in °C/W, depending on the

th(c-h

contact case/heatsink.

Since the current alternates in a TRIAC, we have to consider the R
current which is different to the R

This difference is due to the die of the TRIAC. The first half of the silicon die works when the
current is positive, the second when the current is negative. Because of the thermal coupling
between these two parts, this gives the following equation.

R

th(j-c)AC

= 0.75 . R

1.1.6 Choice of heatsink

Choosing a heatsink depends on several parameters; the thermal characteristic, the shape
and the cost.

However, in some applications a flat heatsink can be sufficient. Figure 4 shows the curve
Rth(h-a) versus the length of a flat square heatsink for different materials and thickness.

Some applications need heatsinks with an optimized shape where the thermal resistances
are not known.

For this, the best solution involves measuring the case temperature of the component in the
worst case scenario and keeping to the following formula:

T

< T

c

Where:

● T

● T

● P is the dissipated power in the component

● R

is the case temperature

c

jmax

th(j-c)

- P . R

jmax

th(j-c)

is the maximum junction temperature

is the thermal resistance between junction and case.

th(j-c)DC

in direct current.

th(j-c)

in alternating

th(j-c)

6/22

AN533 Through-hole packages

Figure 4. R

100

50
30
20

10

5

3
2

1

100

50
30
20

10

5

3
2

1

R

0

R

0

th(h-a)

2

th(h-a)

2

th(h-a)

(°C/W)

(°C/W)

1.1.7 Forced cooling

For high power or very high power applications, a forced-air or liquid cooling heatsink may
be required. Heatsink manufacturers give a coefficient depending on the air or liquid flow.

versus the length of a flat square heatsink

(°C/W)

R

th(h-a)

100

(cm)

(cm)

50
30
20

10

5

0.5
1

3

2
5

2

1

0

2

Thermal model for calculation

e

0.5
1
2
5

8

based on square heatsink

- Semiconductor device
in the center

L

- Bare convector (no ventilation)

- Vertical position

COPPER

Thickness of
the plate in mm

Length

8

10 166

8

10 166

144

12

STEEL

Thickness of
the plate in mm

144

12

18 20

Length

18 20

10 166

ALUMINIUM

Thickness of
the plate in mm

144

12

Length

(cm)

18 20

0.5
1
2

5

However, in some applications like vacuum cleaners, dissipated power is only a few watts
and air flow cooling is available. This allows a very small heatsink to be used, very often a
flat aluminium heatsink. In this case it is necessary to measure the case temperature in the
worst case scenario and to check the following formula:

T

< T

c

jmax

- P . R

th(j-c)

1.2 Thermal impedance

In steady state, a thermal equivalent circuit can be made only with thermal resistances.
However, for pulse operation it can be useful to consider the thermal impedance, especially
when the component is on during a time lower than the time to reach the thermal resistance.
The thermal impedance value versus pulse duration is given in the datasheets (see an
example in Figure 5), in the form of the relationship Z

For example, BTA08-600SW is able to dissipate

T

jmax-Tamax

P =

Z

th(j-a) (1 s)

can be obtained from the datasheet by reading the value of the ratio Zth/Rth from the

Z

th(j-a)

curve (in the case of this product the ratio is 0.06 as seen in Figure 5) and multiplying the
ratio by the value of R

P =

125 - 25

60 x 0.06

from the datasheet. For this example R

th(j-a)

= 27.5 W

plotted against pulse duration.

th/Rth

≈ 27 W without heatsink during 1 s:

is 60 °C/W

th(j-a)

7/22