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Thermal Considerations for Hybrid DC- DC Power Converters


VPT’s DV series Hybrid DC-DC converters are rated for the full military temperature range of  -55°C to +125°C and can be operated at full rated power with in that range as long as the power dissipation and temperature rise is properly addressed. This document discusses the thermal management considerations for various assembly configurations of hybrid DC-DC converters.Solid state DC-DC power converters always have an efficiency less than 100% and therefore always waste a percentage of their input power. This wasted power is dissipated as heat and will cause the temperature of the DC-DC converter to rise above the ambient system temperature. The temperature rise of the DC-DC converter must be considered during the system mechanical and thermal design to ensure the converter does not exceed its maximum rated operating temperature. 


Thick film hybrid packaging technology uses bare semiconductor die and high thermal conductivity materials to achieve high temperature operation. A diagram of the typical hybrid package is shown in Figure 1. In its basic form, the bare silicon die is mounted to a ceramic substrate, usually Al2O3 (alumina), which is mounted to the metal package, usually steel or Kovar. Power is dissipated in the semiconductor die, which may be an IC,  power  transistor,  or  power  rectifier.  The  die  has  a  maximum  semiconductor  junction  operating temperature, typically 150°C or 175°C, as specified  by the manufacturer. 
Figure 1. Internal Hybrid Construction
The semiconductor junction temperature inside the hybrid, Tj, is determined by the following formula: 
Tj = Tcase + ΔT                                                                                                                                         (1)
ΔT = Pd · θjc                                                                                                                                          (2)
Tcase is the case temperature of the hybrid; ΔT is the temperature rise from junction to case; Pd is the power dissipated in the die; and θjc is the thermal resistance from the junction to the case. θjc is the sum of any intermediary thermal resistances, in this case the ceramic substrate, the attachment materials, and the case itself.

The operating temperature of the hybrid should be verified by both analysis and measurement. For design purposes the operating temperature can be calculated using computerized finite element analysis methods or a simple thermal resistance model. A thermocouple mounted on the baseplate of the hybrid in the actual system is a good method of verification but usually must wait until late in the development cycle. Basic thermal resistance calculations will be presented in this paper for several mounting configurations. These calculations, usually approximate, are a good design tool early in the development cycle before full system thermal models are developed. 


Figure 2.   DVFL mounted directly to metal heat spreader.

Figure 3.   Mechanical stackup and thermal resistance model for DVFL.

Figure  2 shows  a  side-leaded  DVFL  hybrid  mounted  directly  to  a  heatspreader.  Figure  3  shows  the mechanical stackup and equivalent thermal resistance model, assuming the heatspreader is mounted to a chassis with a known ambient temperature. 

The case temperature of the DVFL is calculated similarly to (1) and (2): 

Tcase = Tamb + Pd · ∑θ                                                                                                                                     (9) 

Tamb is the known ambient temperature of the system chassis. Pd is the total power dissipation of the hybrid and can be calculated from (7) where the efficiency has been measured or read from the datasheet graph at the correct operating condition. The total thermal resistance  ∑θ is the sum of all intermediate thermal resistances from the hybrid to the ambient, in this case the thermal pad and the heat spreader. 

∑θ = θthermalpad + θheatsink                                                                                                                               (10)

The thermal resistance of the thermal pad can be read from Table 1. The thermal resistance of the heatsink can be obtained from its manufacturer or calculated  The power dissipated internal to the hybrid can be assumed to be spread evenly across its baseplate, so the area in (3) would be the area of the DVFL baseplate, not the entire area of the heat spreader. If the heatsink is unusual or nonrectangular in shape, its thermal resistance can be approximated by breaking it up into rectangular blocks which are in series with respect to the heat flow. The thermal resistance of each block can be calculated individually and summed to obtain the total thermal resistance. 

Figure 4 shows an example of “deadbug” style mounting for the DVTR hybrid in the flanged package. This is a common mounting configuration for severe vibration environments. Electrical connections to the pins can be made with discrete wires or a flexible or rigid printed circuit board.


Figure 4.   “Deadbug” Style Mounting for Downleaded Flanged Units.

Figure 5. “Deadbug” thermal model.

In this case, heat is transferred only through the mounting flanges. The maximum temperature is assumed to be in the center of the hybrid and there is an additional thermal resistance and temperature rise from the center to the mounting flange. For this configuration since power is dissipated across the surface of the baseplate, finite element methods were used to obtain an effective thermal resistance, R-deadbug, from the center of the package to the flange, as shown in Figure 6. This effective thermal resistance will give a valid hot spot case temperature when used in conjunction with the total power dissipation of the hybrid. This effective thermal resistance is given in Table 2 for various flanged packages.

For applications with high power dissipation or high ambient temperature, the case temperature of the hybrid can be lowered by adding a heat spreader to the basic  “deadbug” mounting configuration, as shown in Figure 6. The heat spreader should have a thermal conductivity greater than that of the hybrid package (cold rolled steel, K = 1.318W/in-C). For example, for the DVTR package an aluminum bar 0.6” x 2.9” x 0.1” thick could be used. To be effective, the spreader must clear the electrical pins and maintain thermal contact along its entire length; a thermally conductive adhesive is recommended.

Figure 6.   Heat Spreader Attached to DVTR to Reduce Thermal Resistance. 


Lower power hybrids can often be mounted directly to the circuit board or PCB as shown in Figure 7. Good thermal  contact  should  be  maintained  between  the  hybrid  and  the  board.  An  adhesive  is  often  used. Mounting flanges or a mounting strap across the top of the hybrid can also help maintain good thermal contact. Typical  PCB materials  are  not  good  thermal  conductors.  Copper planes  are often employed to improve thermal conductivity along the length of the PCB. Likewise, thermal vias are used to improve thermal conductivity through the PCB, usually under the hybrid or at the board mounting locations.

Figure 7.   DVSA Mounted to a Circuit Board.

When the PCB alone is not sufficient to carry heat away from the hybrid, a heat spreader can be added to the assembly as shown in Figure 8. In this case, the thermal path through the PCB can usually be ignored and the case temperature of the hybrid can be calculated directly from (9). Additionally, intentionally isolating the  thermal  spreader  and  hybrid  from  the  PCB  can  serve  to  lower  the  temperature  of  the  PCB  and surrounding components.

Figure 8.   Converter/Heat Spreader/PCB Assembly.
Another option is to cut a hole in the PCB, and allow a heat spreader to protrude up and make contact with the heat spreader. The mechanical mounting should again be sufficient to ensure good thermal contact between the baseplate and the heat spreader
Proper system thermal design is necessary to allow hybrid DC-DC converters to operate reliably over the full military temperature range. To ensure maximum ratings are not exceeded, it must be recognized that the hybrid operating temperature will be greater than the ambient or heatsink temperature. The hybrid operating temperature is specified at the bottom center of the baseplate. It can be determined either by analysis or measurement.  Knowing  the  actual  temperature  will  allow  accurate  reliability  calculations  and  proper tradeoffs between design complexity and reliability.