Solid-state devices -- from pocket calculators to personal computers to SCR power controls to solid-state motor starters -- generate heat during operation because there is a voltage drop across conducting silicon semiconductors. That voltage drop shows up as waste heat, which can be calculated by using Ohm's Law:
Volts x Amps = Watts
Assuming a 2 V drop (typical for power SCRs), 2 W of heat are generated for every amp of current -- waste heat that must, without exception, be dissipated to prevent the semiconductors generating it from overheating, degrading and ultimately failing.
With low power products, the manufacturer usually is able to design in a heat-dissipation mechanism that will function without any special precautions. But, when large amounts of power are involved -- such as on a single-phase 150 A power control or a 100 hp motor starter -- keeping semiconductors cool is more difficult. Power SCRs have a zero-current temperature rating of 257oF (125oC). Rate an SCR control for a 122oF (50oC) case temperature rise and a 122oF (50oC) ambient temperature, and there is little room for error: If either goes over the 122oF limit, the lifetime of the SCRs in question becomes very short indeed.
The life expectancy of an SCR control, then, is affected by:
- The manufacturer's design (ability to dissipate waste heat to limit the case temperature rise).
- The user's installation (installed so that the ambient does not exceed 122oF).
A few precautions can ensure that your SCRs operate reliably.
Choosing an SCR ControlWhen selecting an SCR control, certain key points about the design and construction of power electronic devices should be kept in mind.
Heat Sinks. Heat dissipation via airflow over aluminum heat exchangers called heat sinks is the most common, most economical and -- when properly designed -- most reliable way to cool SCR controls. Convection heat transfer (air moving across the fin surfaces) accounts for 85 percent of total heat transfer from a heat sink. Its design must minimize surface irregularities that create turbulent airflow across the heat sink because such disturbances greatly reduce the cooling ability of a heat sink. Furthermore, these losses cannot be offset by additional fins or mass. Fins must be spaced so that dust and dirt cannot accumulate and cut down airflow.
Fans are not necessary to achieve a 122oF ambient temperature rating below 600 A per phase when the heat sink is well designed. As mechanical devices with a nominal 2,000 hr mean time before failure, fans are a weak link in any electronic cooling system and should be avoided.
Semiconductors. Individual semiconductors mounted separately on heat sinks are easier to cool than encapsulated semiconductors blocks and solid-state relays. At current levels above 25 A per phase, dissipation of waste heat is very difficult from encapsulated blocks because the silicon pellets are only a few millimeters apart. Power SCRs should be replaced by diodes whenever possible in three-phase controls. Diodes generate less waste heat and have a higher temperature withstand rating than equivalently rated SCRs.
Using SCR ControlsHeat is the number one threat to industrial solid-state equipment. Even the most careful design can be nullified by failure to observe basic installation guidelines.
Enclosures. SCR controls, unlike their electromechanical counterparts, are not susceptible to (nonconductive) dust or dirt, so the traditional dust-tight electrical enclosure is not necessary; in fact, it is to be avoided at all cost because it will trap waste heat and lead to overheating failure.
Enclosures for SCR controls must be ventilated with 10 in2 of inlet and outlet area for each 50 A power phase. The inlet vent should be in the bottom of the enclosure; the outlet should be openings at the top of each side. Louvers and fine-mesh screens over the openings reduce the volume of airflow and should be avoided.
A common misconception is that an oversized enclosure is an acceptable substitute for ventilation. Field studies clearly show that regardless of how large an enclosure may be, if not ventilated the temperature inside will eventually reach levels dangerous to most solid-state components.
Location. Often overlooked is where an SCR control is installed. Computers are infamous for their inability to tolerate even the slightest overtemperature conditions, so they are placed in special air-conditioned rooms; but SCR controls, which are just as vulnerable to -- and generate much more -- heat are seldom given such special consideration.
Well-ventilated enclosures are meaningless if the enclosures are mounted next to or above a furnace that keeps the air around it at 122oF or more. Similarly, putting a solid-state control in an enclosure on a roof in direct sunlight or in a metal building in the middle of a field will counteract cooling.
Likewise, a well-ventilated enclosure will not help if one solid state control is mounted directly over another. Heat rising from the lower one will make it very difficult to keep the upper one cool. And, a ventilated enclosure in a mild ambient area may not be enough if other heat-generating equipment is placed inside without ensuring that there is enough ventilation to get rid of the extra waste heat.