How is AEPS-GROUP’s planar design of AC/DC units useful for hardware?

19/06/2023

Success is a projection of the goal on the plane of hard work.
Unknown author


In essence, a small power unit in a metal case is a box from which electrical power in the form of voltage and current comes in, and inside it dissipates power loss in the form of heat.

(The author suspects that the term “power supply unit” was coined by him and promulgated in 1990 to name the products of his first firms, Alexander’s… Subsequently, the term “power supply unit” was adopted by the global community of manufacturers of unified power supply devices. Let readers write what they know – this information will be needed for the author’s memoirs).

The most important indicator for power units from the position of layout in the equipment is structural specific power (energy density). With modern AC/DC units the value of 5-15 W/in3 is considered very good, but it does not say anything about the efficiency and the temperature inside the unit. If the cooling system in the hardware is highly efficient, almost perfect, it will cope with a large overheating of the power unit and the problem of low efficiency in terms of heat dissipation will be easily solved. The only remaining problem is the significant consumption from the incoming power supply network. In this case it is possible to realize a high specific power.

Unfortunately, in practice, to force heat without losses (i.e. through low thermal resistance) to leave the unit on the surface of the case, and then constructively ensure heat dissipation in the hardware is a very difficult problem, often crossing out all conceived miniaturization. In such a case, a value of 5-10 W/in3 is considered an acceptable design specific power.

 

HOW ARE THE HEAT DISSIPATION PROBLEMS SOLVED?


First, our team of developers and technologists at AEPS-GROUP (Alexander Electric Power Supplies Group) in the design of the power units of the new series tried to remove the heat on at least one edge of the power unit housing adjacent to the conventional surface of the heat sink. The most advanced companies (for example, Vicor) use even two faces of the housing – two-sided heat sink, and in devices operating in a special coolant, all six sides of the unit housing are involved.

Secondly, it is easy to see that close to a cube or a thick parallelepiped, the structural volume of the unit significantly loses in surface area for heat dissipation to a flat low-profile or, to be more precise – planar design of the power unit case (another author’s term, it seems).

Imagine that you take a conventional hammer (this is a joke of course, don’t try to repeat it) and start flattening the unit so as to reduce its profile. It’s okay that the unit will start to increase in length or width, or both. The main thing is that it will start to increase its cooling surface. At some point you as a designer will be satisfied – heat will start to be efficiently removed by a weak fan and ambient air and unit reliability will increase dramatically (reducing overheating by every 10°C allows about a doubling of MTBF!). If you continue with the same persistence, you will reach the ideal – the profile will become zero and the surface area will be infinitely large. What about a fan or air – cooling will be excellent even in a cosmic vacuum! Thus, we have invented a way to get a planar unit that does not require a cooling system at all.

 

WHAT GIVES PLANARITY


  • Consider planarity as a highly desirable design parameter. Power supply units are in most cases the thickest parts of a design and are very difficult to assemble with other microelectronic devices. A thin device is easier to find a place in modern hardware with typical low-profile components.
  • The layout is further complicated when a heatsink is attached to the power supply unit, which also takes up some space. Of course, it is best to use a heatsink with maximum thickness, spreading a planar unit on it.
  • Internal point heat sources, or heat concentrators, in a planar design are as close as possible to the cooling surface of the unit, so that their thermal resistance is significantly reduced.

The series of planar AC/DC power units consists of seven models: JETA100-LP, JETA150-LP, JETA300-LP, JETA700-LP, JETA1500-LP, JETA2000-LP and JETA3000-LP.

As can be seen from the unit designations, they cover a power range of 100-3000 watts.

JETA100-LP JETA700-LP and JETA3000-LP units

Fig. 1 – JETA100-LP JETA700-LP and JETA3000-LP units

 

Figure 1 shows JETA100-LP JETA700-LP and JETA3000-LP units as an example. Externally, they have the same design and differ (except for the output power) only in the size range of 100×51×18…299×169×38 mm. The figure shows the smallest unit of the JETA100-LP series, the medium in size and output power JETA700-LP and the flagship of the AEPS-GROUP series – the most powerful JETA3000-LP.

Let’s look at the characteristics of the new products – single-phase planar fanless AC/DC units.

Input power networks: JETA-LP series (low profile) units are designed for American standard AC mains 120V/60Hz (~100-127V) and European standard 220-240V/50Hz (~198-242V). These applications use 230W (~100-242V) input mains at 50-60 (optional 400) Hz. Transient deviations of 1 s and supply from DC voltage of any polarity are possible; ~100-264 V, or 140-340 V rectified voltage is allowed. Taking into account requirements for special applications, 230V AC (~182-242V, deviation ~176-264V for 1 second) or 115V AC (~81-138V, deviation ~81-150V for 1 second) with frequency of 50-60 (400 on request) Hz are available on request. Power can also be supplied from any polarity 256-342 V DC for 230 V mains input and 113-195 V DC for 115 V mains input.

Note that the numbers 100, 150, 300, 700, 1500, 2000 and 3000 in the unit designation define the maximum output power in watts. However, in order to ensure long-term reliability, it is recommended that the load factor of the units be in the 0.7-0.8 range. This means that the units in this series are optimized for an average output of 80, 120, 240, 600, 1200, 1600, and 2400 W, respectively.

Electrical advantages: All JETA-LP single-channel units contain synchronous rectifiers up to an output voltage of 27V, which greatly increases their efficiency. The possibility of realizing a two-channel output with galvanically isolated channels gives a unique schematic realization of the output in all four possible variants: two independent channels, channels connected in parallel, channels connected in series to provide a high-voltage output, output with a midpoint. With the exception of JETA100-LP and JETA150-LP all units have a power factor corrector. From JETA300-LP to JETA3000-LP the units include all service functions up to parallel operation.

Design advantages: The JETA-LP units are extremely compact and have a high level of efficiency. This gives them an unprecedented structural power density in the 17-25 W/in3 range. The aluminum or copper base of the enclosure has stiffening ribs and also serves to conductively dissipate heat. Given that all components with noticeable weight are structurally bonded to the base and encapsulated in a rigid, thermally conductive compound, the resistance and robustness to mechanical stress is very high.

Thermal characteristics

Fig. 2 – Thermal characteristics

 

Figure 2 shows how the planarity of the power units affects the reliability of the hardware when hermetically sealed volumes are used.

Let’s imagine that in the sealed volume of block 1, apart from electronic devices practically not generating heat (they are not shown in the figures), four highly efficient modern power supply units of XXX company are used, each with output power of 225 W and efficiency equal to 90%. This corresponds to a power dissipation of 25 W per unit.

The units are of conventional design, not planar (“chest-shaped”), i.e. of great thickness. The figure shows the metal bases of the units, through which they give off heat to the case-radiator from inside the sealed volume. The red circles conventionally show the heat concentrators inside each “chest”. These are heated transformers, separately installed power semiconductors, i.e. strongly warming components.

The 25 watts of heat generated by each unit is divided into two parts: the heat of 15 watts goes into the case-radiator, and the second part goes into the internal hardware volume of the case, heats everything inside and indirectly goes into the case-radiator as well. As a result, 4×15 = 60 watts of heat will go out through the case-heatsink through the base of the units, and 4×10 = 40 watts indirectly through the internal volume; a total of 100 watts of heat power. In this case, the internal temperature of the unit will be, for example, 120°C, which will determine a MTBF of 5,000 hours.

The author cautions that, for simplicity, the problem is considered when the useful output power leaves the sealed volume, not dissipated internally.

Figure 2b in Box 2 illustrates the heat distribution in the case of planar units with all heat concentrators as close to the bases as possible and as a result, 20 watts of dissipated power are transferred from each unit to the housing-radiator, while only 5 watts are delivered to the internal volume.

The situation is significantly different. As in the first case, the same 100W will be transferred to the outside of the heatsink, but the internal temperature will be set at 80°C, which will significantly increase MTBF to about 30,000 hours! And this is exactly due to the planar design of the new AEPS-GROUP JETA-LP series units.

Our website lists 11 applications. The AEPS-GROUP units of the JETA-LP series are most effective in areas 3, 4, 5, 7, 9, 10, 11 – for flying objects at altitudes up to 5 km in unpressurized bays. These objects include airplanes, helicopters, drones, flying balloons, gliders, meteosondes. Units are also designed for operation in ground transport of all kinds – in railway locomotives and wagons, cars, tracked mobile vehicles. The units are optimal for use in supercomputers, radars and environmental display screens. Finally, the units work successfully in the Arctic and Antarctica, in the mountains at all altitudes, in cold and red-hot deserts.

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