Next: Protection of the galvanic
Primary controlled switching power supplies are being used increasingly for the supply of electronic circuits. Their advantage is that they are small and lightweight, but there is also another interesting feature, i.e. their ability to process different mains voltages in order to produce a controlled secondary voltage. This means that switching power supplies with small power can be designed for a secondary voltage of for example 8 V and a load of 100 mA (800 mW output power), that are able to withstand input voltages of between 20 V to 350 V (AC and DC voltages) without any switching of the input voltage range.
This is especially interesting for measurement amplifiers, which are used as stand-alone control cabinet devices for different mains voltages in many different applications around the globe (see Fig. 1).
Many switching power supply applications mainly describe measures indicating high efficiency, good control response, etc. However, in certain industries such as the chemical or petrochemical industry, it is also important in the use of switching power supplies that the galvanic barrier of such electronic transformers withstands under all circumstances, even if IC-internal electronic protection fails. Thus, it is critical that devices used in explosive environments (like fuel tanks in refineries) are fail-safe and fully developed. If the galvanic barrier blew, this would result in an explosion caused by high-energy sparks, which would in turn result in enormous economical damage and injury.
Fig. 1: PT100 measurement amplifier for explosion-proof (Exi) usage in a multi-module casing.
In such safety concepts it is preferable to use reliable working principles based on physical-elementary fundamentals instead of on electronically controlled systems.
To prevent the insulation body of a DC/DC converter from melting through, the thermal energy at the transformer must be kept low or completely interrupted by a blowout fuse.
Power supplies with a wide input voltage range have the problem that a relatively high input current must flow at low supply voltages and a low input current must flow at high voltages in order to achieve the same power.
Therefore the blowout fuse must be designed in such a way that it will not blow even at the lowest supply voltage at this high current (e.g. 125 mA). Naturally, this fuse also works at a voltage of 250 Vrms, and so the developer of such wide range voltage supplies will encounter difficulties in protecting the supply unit from overheating even at high voltages.
In many cases electronic fuses in the converter ICs take on this protective function against thermal overload, thermo fuses (Fig. 3) or voltage limitation circuits are used in bigger transformers; whereby the latter do not apply due to their wide input range. Since the current transformers are very small, it is difficult to fit thermo fuses in the coil chambers.
TURCK has developed a safety procedure for DC/DC transformers that takes advantage of the behavior of magnetic ferrites, i.e. that they lose their permeability at a certain temperature. From this temperature on, which is substance-specific for each ferrite and known as the Curie temperature, the relative permeability µr((B,T) suddenly decreases to 1.
Next: Protection of the galvanic