Historically, dc-dc converters rated less than 25 W have been treated like most other pc board mounted components because they are typically rated for a given amount of power up to a certain ambient temperature. Converters classified as “high density,” however, are more closely associated with power levels above 25 W and range up to a few hundred watts. These high-density converters, unlike their lower power predecessors, are specified like power semiconductors and are rated in terms of their maximum allowable baseplate temperature, regardless of the ambient.

With LED lamps and fixtures, the heat removal challenges are more complex because of the required form factors. That means even at power levels as low as 7-10 watts, these designs need more attention so that LED chips do not overheat.

From a thermal management standpoint, a high-density dc-dc converter, like virtually all power supplies, can deliver substantial power, but only if excessive heat is removed by transferring internal heat through a number of materials to a surface exposed to surrounding air. Heat from that outer surface is then transferred to the surrounding air in proportion to the air turbulence immediately adjacent to that surface, whether that “turbulence” results in the form of fast-moving air (active cooling) or very tiny air “eddy currents” in a passively cooled system.

The reference to “outer surface” needs a further comment because no mention was made of a “metal” surface. The reason for this is that in a perfect situation all of the internal heat would originate from every cubic millimeter within the module and would be evenly distributed to all parts of the outer surface. Whether the outer surface is plastic or metal makes no difference as long as there is no effort to attach one of the hot surfaces to a heat sink; that is, as long as there is only (surface-to-air) convection cooling.

In a real world power module or light fixture, specific components (which can be either traditional power semiconductor or power LEDs) are the principal heat generators. The area adjacent to these components is usually hotter than other areas. An outer metal case or plate spreads the heat and reduces point-to-point temperature variations. With a high-density converter having a metal baseplate, the thermal conductivity of the plate material is an indispensable link to a cooling technique, which is better than simple surrounding air.

The baseplate has two functions: to spread the heat generated by small-surface-area power semiconductors and to conduct that heat to a cooler place. For the remainder of this discussion, the concentration will be on heat removal from the power semiconductors within the converters (or light fixtures) because they usually represent over 85% of the internally generated heat.

The process which final surface-to-air heat makes transfer is what determines the ultimate ability of a converter to deliver its maximum possible power. The barrier to perfect heat transfer is defined as “thermal resistance,” that is, the lower the thermal resistance, the better the heat transfers. Stated another way, the higher the thermal conductivity, the better.

However, it has become the convention in addressing power semiconductor and power supply thermal management to talk in terms of “thermal resistance” instead of thermal “conductivity.”

Fig. 1. Thermal transfer cone.

In practice, internal heat is never perfectly distributed and there are hot spots. Fig. 1 shows how heat from a point source, such as the chip within a MOSFT or power rectifier package reaches its destination. Heat travels outward in a conical path that is rarely a perfect cone because of the physical configuration of heat generators. Fig. 2 depicts the same process in a way that reflects the typical arrangement for heat removal from the power semiconductor in a dc-dc converter.

Fig. 2. Typical dc-dc converter thermal transfer path.

In an LED light fixture there is typically another set of thermal barriers between the chip tab or substrate and the heat sink.  It is usually in the form of a metal-core PC board and related thermal interface material.

The ability of heat to get from one place (such as the chip) to another (the surrounding air) is determined by:

  • The distance (thickness)
  • The area (a large glass window transfers or loses heat faster than a small windoww.
  • Thermal resistance of the various materials and interfaces through which the heat must pass.

Often not appreciated is the fact that a material of given thermal resistance has a more adverse effect the closer it gets to the point source of heat. Conversely, the actual thermal resistance of a given thickness of material becomes less important the more it is at the wide end of the het travel cone. This means that an infinite amount of heat sinking and airflow cannot keep the chip cool if the chip-to-case and case-to-heat sink surfaces have excessive thermal resistance. In some applications, the simple change to a chip of larger area and correspondingly lower junction-to-case thermal resistance could have more effect than an exotic fan and heat sink system.

Table 1 compares a number of materials that often play a role in thermal management. It may be surprising that all epoxies, plastics and silicones fall into a narrow band from 10 to 30. There are no units for thermal resistance. The figures are normalized, using air as 1.0. Thermal resistance, unlike electrical resistance has a few conditions attached to it. It is influenced by the thickness and area of material, so Table 1 shows the relative difference in thermal resistance for different materials to be employed in an identical physical configuration.

Also, there is no such thing as a “thermally conductive” epoxy unless it is filled with a material higher up on the scale, such as aluminum oxide. One epoxy might be relatively more thermally conductive than another, but the thickness and area can often outweigh the actual material type.