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Posted on 03 July 2019

Overcurrent Protection for Diodes and Thyristors

 

The term overcurrent refers to a current load acting on a power semiconductor that, given the existing cooling conditions, would lead to the destruction of the component unless the current is turned off in time by means of suitable devices. Unlike a short-circuit, this is not a current that rises steeply within a few milliseconds. Such overload can not be caused solely by unforeseen current increase (overcurrent) but can also be caused by an unintentional change in cooling conditions. As a result, the component can not cope with currents that are normally permissible when cooling is properly ensured. Examples of improper cooling are blocked fan slits, fan breakdown, or in water cooled systems, a malfunction in the coolant supply. A number of well established protective devices for overload conditions such as these are described below. There is a difference between protective devices which can be triggered as a result of unintentional current increase only and those which respond when the cooling system does not function as it should, as well as devices which provide protection in both cases.

Power circuit breakers

Power circuit breakers are the most common form of overload protection. They are switches with thermal, magnetic and thermo-magnetic tripping. Similar to fuses, their response times are dependent on the overcurrent and lie within the range of under 1s; the response time for high overcurrent is shorter. Manufacturers show this ratio in the form of a characteristic that can then be compared to the overcurrent capability (current rating) of the semiconductor component. Note that the trip current of power circuit breakers and the currents in the current/time curves for safety fuses are always effective values, while the maximum surge current of the semiconductor components are peak values of sinusoidal half waves. Thus, if comparisons are to be made, these values first have to be converted into effective values. Throughout the entire possible time period, the trip current of the power circuit breaker has to be lower than the permissible overcurrent of the semiconductor component in the case of error. If this cannot be achieved for the entire period, an additional protective device - normally a semiconductor fuse - has to be integrated for the period not covered.

Fuses

Fuses are intended first and foremost to provide short-circuit protection. In certain circumstances, however, the fuse may also provide protection from overload in the aforementioned sense of the word. To determine this, a comparison has to be made between the fuse current/time curve for the given period and the permissible overcurrents for the protected semiconductor component in the event of an error. It might be necessary to cover any remaining time in which the fuse in question does not provide protection with an additional protective device.

Suppression of driver signal

In controllable circuits, it makes sense to implement overcurrent protection by manipulating the driver unit. When overcurrents occur that are not intended for operation, the trigger pulses are either fully suppressed, or the driver unit is designed such that the current is limited to a permissible level in any given conditions. The requirement for the use of the driver unit as protection is, of course, that the thyristor controllability is maintained long enough (i.e. the maximum permissible virtual junction temperature is not exceeded) for the driver protection to take effect. For short-circuit currents that rise steeply within one semi-oscillation, for example, protection via the driver unit is not possible.

Bimetal thermostats

Bimetal thermostats contain bimetal discs which, at a certain factory-set temperature, snap from one position to the next, opening or closing a contact in the process . They normally have screw studs with which they are screwed onto the heat sink, establishing the closest possible thermal contact with the semiconductor components as possible. If several semiconductor components are to be protected by way of separate heat sinks, each of the heat sinks will in some cases require its own thermostat. The contacts are connected in series or parallel, depending on whether they open or close. Bimetal thermostats can be used for natural or forced air cooling, as well as for water-cooled systems. In the latter case, it makes sense to use an additional thermostat which suppresses or completely blocks the coolant supply if a certain heat sink temperature is not reached. This will ensure that no condensation accumulates on the isolated parts of the semiconductor components.

Bimetal thermostat for screw-connection to a semiconductor heat sink

Figure 1. Bimetal thermostat for screw-connection to a semiconductor heat sink; a) Design with open- ing contact in neutral position; b) Same design after the response temperature has been exceeded. The centre of the bimetal disc clicks upwards, opening the contact; c) Design with closing contact once the response temperature has been exceeded.

Temperature-Dependent Resistors

Temperature-dependent resistors have the advantage over bimetal thermostats that they respond to temperature changes more quickly, resulting in a minor safety margin between the operating current and the minimum current required to trip the protective device. An additional electronic circuit is needed to convert the change in resistance into a signal that can, for example, trigger a protective device. Temperature-dependent resistors can also be integrated into power modules - for example, soldered onto the insulating substrate. Integrated sensors such as these respond far more quickly than the sensors on the heat sink. However, these sensors are definitely not sufficient to detect increases in chip temperature caused by steep overcurrents before chip damage occurs. The silicon-based resistors used have either a positive temperature coefficient (PTC) where the resistance increases as temperature increases, or a negative temperature coefficient (NTC) where the resistance decreases as temperature increases.

High-speed DC circuit breaker

In rectifiers, where frequent shorts in the load are to be expected, a high-speed DC circuit-breaker is to be integrated on the load side, since the frequent replacing of fuses is costly and time-consuming. The right type of replacement fuses have to be available at all times, etc. The semiconductor fuses needed to provide additional protection in the event of failure of a semiconductor component, on the one hand, and the high-speed DC circuit-breaker, on the other hand, have to be selected such that if the load is shorted, the circuit breaker is activated before any of the semiconductor fuses melt. This is referred to as selectivity of the various short-circuit devices. In this case, selectivity is given if the time/current characteristic of the high-speed DC circuit-breaker runs below the pre-arc/current characteristic of the fuse throughout the entire range in question.

 

For more information, please read

General Voltage Surge Protection

Protecting Appliances with Inrush Current Limiters

Semiconductor Fuses: Terms and Explanations

Fuse Placement in Typical Converter Circuits

Dimensioning Semiconductor Fuses

 

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