178 Integrated gate bipolar transistors IGBTs for motor drives

Present motor drives generally comprise integrated gate bipolar transistors (IGBTs) because they are capable of operating at the high power densities

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required for compactness. Each IGBT in commercial systems is configured as shown in Figure 17.11. Each module comprises several IGBTs with an equal number of diodes (six would be typical for a 75 hp motor drive). In steady operation, the heat, q, generated at the electronics in each IGBT may be as large as 6MW/m2. The flux is in one direction and is transferred to the coolant by a heat sink comprising a fin-pin array subject to flowing air generated by a fan. Conventional air-cooled heat sinks operate at fluxes, q < 2kW/m2 (Figure 17.12). The ratio q/q is accommodated by designing the sink with a cross-sectional dimension, bhs, that relates to that for the Si, bsi,

Figure 17.11 Conventional power electronic packaging

2bhs = 50 cm

Figure 17.11 Conventional power electronic packaging

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Surface heat flux (MW/m2) Figure 17.12 Performance domains for heat sinks

Surface heat flux (MW/m2) Figure 17.12 Performance domains for heat sinks

in accordance with:

With bsi = j cm, this requirement results in bbs = 25 cm, causing the overall system to occupy a large volume. The goal of the case study is to reduce bhs to 3 cm, by using a cellular metallic heat sink, resulting in an order of magnitude reduction in overall volume.

The power density at the electronics is limited by the temperature reached at the junction, Tj. For Si electronics, Tj must be less than 120°C to avert unacceptable degradation. The design of the system and the importance of the heat transfer coefficient at the sink interrelate through q, q and T,. The capacity of the fluid pumping system is another key factor. Such systems are characterized by an operating curve that connects the back pressure to the allowable fluid flow rate through the sink (Figure 17.13).

1 Heat spreader

2 Planar micro heat pipe

Figure 17.13 An example of integrated gate bipolar transistor design

Goal, mesocell

Goal, mesocell

Heat transfer coefficient (kW/m2K) Figure 17.14 Upper limits on power density and heat-transfer coefficient

Heat transfer coefficient (kW/m2K) Figure 17.14 Upper limits on power density and heat-transfer coefficient

This case study illustrates the benefits of using a cellular metal sink to cool power electronic devices. Analytical results provide upper bounds. Numerical simulations provide explicit operating benefits. The overall goal is to reduce the volume of the drive needed to operate, say, a 75 hp motor (relative to conventional IGBT modules) while increasing its durability and decreasing its cost.

Requirements

If two-sided cooling is used to double the heat flux achievable at the electronics, there is a maximum achievable flux at the sink, q*. This occurs when the device located between the electronics and the surface of the sink is designed to be nearly isothermal and equal to the junction temperature, Tj, q* = h(T j — Tf)

where h is the average heat transfer coefficient from the electronics into the coolant, and Tf is the average temperature of the fluid. The design used to approach this maximum entails the use of heat spreaders and planar micro-heat-pipes. Based on equations (17.1) and (17.2), for the heat sink to be reduced to 3 cm, a heat transfer coefficient exceeding about 3kW/m2K is needed. Such levels can be readily realized using liquids, but are well in excess of those now achievable with air cooling (Figure 17.12). The challenge is to determine whether cellular metals can attain such high h with air.

Cellular metal performance

For the airflows needed to realize these h, a fan/blower configuration with requisite operating characteristics must be designed. These characteristics typically exhibit a nearly linear interdependence between back pressure A p and volume flow rate, V (Table 17.1), with Ap* and V* as the respective perfor-

Table 17.1 Formulae for calculating achievable heat dissipation

(1) Heat extracted over sink area (4b2)

(2) Transfer length

(3) Biot number

'QfCp

0.4

vd

0.4

I kf \

.1 -a.

(where a = 0.37 Jp + 0.055p) (4) Pressure drop in sink

(5) Operating characteristics of pump

(6) Fluid flow rate

fv0f4

Figure 17.15 Maximum on-chip thermal performance using mesocell metal heat sinks

mance coefficients (Figure 17.15). These characteristics overlay with the pressure drop in the heat sink, also given in Table 17.1.

Equating the pressure drop with the operating characteristics results in an explicit flow rate for each heat sink (Figure 17.15): that is, for given cell size, relative density and thickness. With this defined flow rate, a specific heat flux, q, can be accommodated by the design. Accordingly, for a prescribed fan, the heat sink exhibits a heat flux domain wherein the relative density and cell size are the variables. One such domain is indicated in Figure 17.15, calculated for bhs = 3 cm and bsi = 0.5 cm. Note that there is a ridge of high heat flux coincident with an optimum cell size. At a cell size smaller than this optimum, the pressure drop is excessive: conversely, at a larger cell size, the diminished heat transfer limits the performance. Along the ridge, there is a weak dependence of heat flux on relative density in the practical range (P/Ps = 0.2-0.5).

By selecting cellular materials that reside along the heat flux ridge, the requirements for the fan can be specified, resulting in a relationship between heat flux, back pressure and fluid flow rate. Some results for a representative density (p/ps = 0.3) are plotted in Figure 17.16. The benefits of the cellular metal can only be utilized if the fan/blower assembly is capable of operating at back pressures of order 0.1 atm (10kPa), while delivering flow rates about one l/s. Upon comparison with Figure 17.12, it is apparent that these heat fluxes substantially exceed those normally associated with forced air convection. A corollary of the heat flux is that there must be a temperature rise in the cooling air.

Back pressure, Ap* (kPa)

Figure 17.16 Peak power density as a function of back pressure Device design issues

Back pressure, Ap* (kPa)

Figure 17.16 Peak power density as a function of back pressure Device design issues

To take full advantage of the heat transfer capabilities of the cellular metal, the thermal design of the device must establish nearly isothermal conditions: that is, minimal thermal resistance between the electronics and the heat sink surface. This can be achieved by combining a high thermal conductivity aluminum nitride dielectric with a copper planar micro-heat-pipe, fully integrated with the heat sink by brazing (in order to exclude high thermal resistance at the interfaces*). Simulated temperatures for this scenario indicate that the required isothermality can be realized: albeit that the associated manufacturing requirements are stringent and yet to be demonstrated.

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