This application claims priority to and the benefit of European Patent Application No. 22275075.4, filed Jun. 2, 2022, the entire content of which is herein incorporated by reference.
The present disclosure relates to the field of solid state circuit breakers (SSCBs) and a snubber for an SSCB.
Circuit breakers are used in systems to isolate circuit functions and/or to prevent fault propagation in the event of a fault within the circuit. If a fault such as a short circuit occurs, an over-current condition is detected and the switch is automatically opened, cutting off power and preventing any damage to the circuit. In the past, this has been accomplished using electromechanical switches. These switches can experience problems with arcing during turn-off and bouncing during turn-on. Because of this, electromechanical circuit breakers can experience severe degradation over time. Further, electromechanical switches are often large and bulky, increasing the necessary size and weight of the circuit breaker.
In order to remedy the problems of electromechanical circuit breakers, solid state circuit breakers are often used. Solid state circuit breakers (SSCBs) utilise solid state power switches which provide relatively fast response times compared to electromechanical switches, and are very small, which is ideal for systems such as those on an aircraft. These solid state switches also do not suffer from problems of arcing during turn-off transient. However, solid state switches can encounter problems with EMI noise during switching and overvoltage stress on the solid state switching device during turn-off transients.
SSCBs are now being used more and more widely, for example in many aerospace and automotive power distribution systems. Applications requiring DC voltage isolation are often targeted for solid state implementation, more so than applications with multiplexed high power VSI motor loads or AC power distribution, but they can also be used in such applications. SSCBs are composed of semiconductor devices, and have the advantage of fast breaking, long contact life and a degree of intelligence, and therefore have high potential for use in the field of low-voltage protection. However, one problem with SSCBs is the risk of damage to the SSCB itself due to voltage surges from inductive loads. Inductive loads can store a large amount of energy. When the contact is broken, an inductive discharge spike occurs. This is less of a problem for mechanical switches, but does risk damage to a solid state switch e.g. a MOSFET which generally dissipates this energy in avalanche mode. Currently available MOSFETS are limited as regards the amount of energy that can be dissipated during avalanche mode and the selection of a suitable device creates challenges.
There is a need to provide a SSCB circuit which is capable of isolating supply phases during normal conditions and fault conditions, that protects the solid state device when an inductive load is disconnected by the SSCB whilst maintaining the benefits of SSCBs in terms of low cost, small size and weight, fast response times etc.
According to one aspect, the present invention provides a snubber circuit for a solid state circuit breaker (SSCB), the snubber circuit comprising a series connected capacitor and transient voltage suppressor, TVS, connected across switches of the SSCB.
Also provided is a bidirectional solid state circuit breaker comprising: a main SSCB circuit configured to be connected between a power supply and a load, and comprising first and second semiconductor switches connected in series, and a snubber circuit as defined above having a first end connected to a first end of the first semiconductor switch and a second end connected to a second end of the second semiconductor switch.
Examples of a SSCB circuit according to the disclosure will now be described with reference to the drawings. It should be noted that variations are possible within the scope of the claims.
Referring first to
At time t0, the circuit breaker switch Q1 is turned off and the inductive load current (L1_i) falls rapidly to a minimum. The second row of the time graph shows the switch voltage (Q1_vds) which rapidly increases, when the current falls, to the switch avalanche voltage. The bottom line of the time graph shows how the switch power (Q1_pwrd) increases at turn off and then rapidly falls to zero at t1. The fourth row shows the SSCB switch avalanche energy (Q1_energy) which quickly ramps up to a high level during the switch avalanche condition. The resulting high energy causes a significant increase in switch junction temperature (Q1_tj) shown in the third row, which gradually reduces once the switch power dissipation reduces to zero at t1. Thus it can be seen that when the current is interrupted abruptly by the SSCB, the inductive load creates a back emf from the stored energy. The energy is high and can drive the switch into avalanche and destruction if the switch is not sufficiently highly rated (which either limits the use of conventional switches or requires large and expensive components).
The present disclosure replaces the conventional RCD snubber circuit with a modified snubber design an example of which is shown in
Operation of the modified snubber will now be described with reference to the time graph of
At t0, the switch 70 turns off and the current falls during t0˜t1 period. Similarly to the arrangement in
For 3-phase applications using a conventional RCD snubber, a separate snubber would be required for each phase, each snubber requiring two suitably rated (I.e. relatively large) capacitors, so six capacitors in total or two high power rated TVS per phase (again, six in total) would be required.
The behaviour of the snubber of
Number | Date | Country | Kind |
---|---|---|---|
22275075.4 | Jun 2022 | EP | regional |