中国电力电子网讯：The maximum permissible junction temperature (Tjmax) of an

IGBT is fixed and a suitable heat sink must be selected to

keep the junction temperature (T

j

) below this maximum. If

this junction temperature is exceeded damage may occur to

the IGBT.

1. SILICON SEMICONDUCTOR LIFE EXPECTANCY:

The relationship of life expectancy and operating junction

temperature for a typical silicon semiconductor is shown

below.

As can be seen, average life expectancy for a silicon

semiconductor is greatly increased if a lower junction temperature

is maintained. It is very important to consider these

effects when choosing adequate cooling.

2. COOLING:

The electrical connections of IGBT modules are usually

electrically insulated from their base plates, allowing several

devices to be mounted on the same heatsink. The

method of cooling must ensure that the (Tjmax) of each

module is not exceeded.

An equivalent circuit can be drawn to represent the heat

conduction in a semiconductor device, in this case a single

IGBT chip in a module. For the purpose of the equivalent

circuit assume that power loss (W) is generated in the

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thermal time constant of the heatsink. The case to heatsink

junction responds to the average heating effect. The

switching losses need to be taken into account, and can

affect the choice of heatsink.

Note; The heatsink temperature and the device case temperature

(Tc) are measured directly below the IGBT module.

The thermal capacity of the heatsink affects the overload

current capability of the module and needs to be considered

when selecting a heatsink.

3. CHOOSING THE RIGHT HEATSINK:

When cooling is required several questions need to be

asked;

What is the maximum permitted junction temperature of

your device?

What is the maximum junction temperature for an acceptable

semiconductor lifetime, if applicable?

What overload current capability is required?

What cooling methods are available?

What are the costs of the cooling methods?

How reliable is the cooling method?

How much physical space is there for cooling?

etc.

If there are no physical constraints on the size of the

heatsink a reliable cooling solution is a Naturally Air

Cooled Heatsink (AN), the only failure that can occur in

such an assembly is the module. This method of cooling can

be relatively cheap but the bigger the heatsink required the

higher the cost. Orientation of the heatsink is important.

4. FORCED COOLED ASSEMBLIES (FC) :

These are very similar to naturally air cooled assemblies,

the difference being the air is forced along the heatsink fins

instead of relying on air convection currents. This method

saves space but the heatsink needs to be enclosed so that

unless a thermostat is fitted. Another problem is possible

water leaks, water and electricity do not mix and any

leakage can be a problem.

6. MOUNTING RECOMMENDATIONS:

Users should read these recommendations prior to mounting

Mitel Semiconductor modules. Failure to follow the

recommendations may lead to damage of the module and

reduced performance.

The heat sink surface must be smooth and flat; a surface

finish of N6 (32min) and a flatness within 0.05mm (0.002")

are recommended. If the surface does not meet this standard

there will be an increase in the thermal contact resistance

between the base of the module and the heat sink

(Rth(c-h)) and damage may occur to the module.

Immediately prior to mounting, the surface of the heatsink

should be lightly scrubbed with fine emery cloth or a mild

chemical enchant and cleaned with solvent to remove oxide

and foreign material. Care should be taken to ensure no

foreign particles remain.

An even coating of jointing compound (e.g ÔUnialÕ) should be

applied to the heatsink and module mounting surfaces. This

should ideally be 0.05mm (0.002") per surface to ensure

optimum thermal performance.

After application of thermal compound, place the IGBT

module on the heatsink near the mounting holes and twist

the module 2 or 3 times, to bed the surfaces together. Place

the recommended fixing bolts in the holes and finger tighten.

Using a torque wrench, slowly tighten the fixing bolts rotating

no more than a 1/4 of a revolution at a time. Continue

until the specified torque is reached on each fixing bolt.

Note; When mounting a single module placeit in the centre

of the heat sink with the module lengthways in the direction

of the heatsink fins. This reduces the effect of heatsink

distortion due to temperature change. When mounting

more than one module on the same heatsink care must be

taken to allow adequate area for modules that require

greater cooling to maintain the desired junction temperature.

7. THERMAL EFFECTS OF PARALLELING IGBT

DEVICES:

When operating IGBTÕs in parallel consideration should be

given to current sharing, so that individual units operate

within their limits. The most important parameters to con-

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8. MOUNTING COMPOUNDS:

It is important to use a suitable interface compound between

a semiconductor device and its' heatsink.

Two basic types of compound are available:

i. As designed for interfaces which are both thermally

conducting and current carrying; e.g. with disc type thyristors.

This type was originally developed for electical busbar joints

and the thermal resistance value is not stated.

ii. Optimised for good thermal performance only, with the

thermal resistance value specif

9. INTERFACE COMPOUNDS:

Ôvirtual junctionÔ (J) of the IGBT chip. The equivalent circuit

used is shown below.

Using the above we can calculate the average junction

temperature as follows:

Tj = W x {Rth(j-c) + Rth(c-h) + Rth(h-a)} + Ta

The module power loss (W) includes the switching loss of

the IGBT. Power losses generated during conduction and

switching periods are usually short in comparison with the

air flow is channelled down the fins. A fan is needed to force

the air through the fins and the power of the fan depends on

the level of cooling required. The fan is an extra cost and if

the fan fails the heatsink will become less effective. A

thermostat may be fitted to the heatsink which can be used

to perform a system shutdown, if the heatsink goes over

temperature.

An example of a heatsink which Mitel Semiconductor use for

Forced and AN cooling is the EM series heatsink. For high

power applications, the EM heatsink by Mitel Semiconductor

has been found to be suitable for both AN and Forced

cooled applications. The heatsink profile is shown in fig 4.

Fig.4

Honeycomb heatsinks are another form of forced cooling,

the air is blown down holes (a honeycomb) in the heatsink.

This is a more efficient heatsink than the EM but more

expensive.

The thermal resistance of FC and AN cooled heatsinks can

be improved by black anodising the heatsink. The thermal

resistance of the heatsink surface to air, Rth(h-a), is improved

because of increased radiation from the heatsink surface.

Radiation effects are more significant at higher surface

temperatures. The overall effect is that thermal resistance

appears to increase with increases in power dissipation.

5. WATER COOLED HEATSINKS:

Water cooled heatsinks are very efficient and have a good

thermal capacity, but there are disadvantages and they can

introduce problems regarding reliability. The water pump

used could possibly fail which may lead to module failure

sider are on-state voltage (Vce), current and temperature.

The V

ce

versus Ic characteristic varies as a function of the

temperature and for this reason it is good practice to mount

several modules on the same heatsink, to ensure they have

similar a T

j

. Using separate heatsinks can cause larger

current unbalances due to different heatsink temperatures.

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