The Difference Between Synchronous Switches and Hybrid Switches

There are many of the articles related on the Synchronous Switches and hybrid Switches,
most of the articles are one-sided description of the synchronous switch is superior than the hybrid switch, where I think it is necessary to write an article about it.

Synchronous Switches
Synchronous Switches

There are synchronous switches in fact after the invention of the microcontroller, just with with the development of science and technology and the cost decline, the synchronous switches have become more and more popular. It control the mechanical switch contacts making and breaking at zero point by a single-chip controller, for the synchronous switch to control the capacitors, is to close at the point when the voltage in both ends of the moment is zero, so that to realize the no inrush current input of the capacitor, to break at the current zero point to achieve the no arc breaking of the switch contacts.

In order to achieve synchronous switching function, the micro controller must accurately detect the cycle and phase of the power supply. Since the mechanical switch contacts action is slow, It is usually need several milliseconds of delay time from sending a drive single to the contact act in place, so the microcontroller must be able to determine the action delay time of the contacts in order to send the action signal in advanced to ensure that the contacts act in place in the time required. The normal switch is different in the delay time of of making and breaking, and the action delay time of the mechanical contact is affected by many factors such as the ambient temperature, the driving voltage, each closing and breaking of the electromagnetic mechanism,the mechanical mating deviation, and etc. In order to minimize its impact, the driving voltage of the mechanical switch is generally more than double of the normal pull-in voltage (The impact is large on the mechanical switch mechanical structure, affecting mechanical life), used to improve the mechanical making and breaking speed, to ensure the synchronous operation can be realized in a variety of environmental conditions.

Advantages: low cost, good resistance to overvoltage, relatively no arc, can be completely electrically isolated after disconnection.

Disadvantages: due to the existence of mechanical switch bounce time, while the mechanical switch can not reach the absolute zero connected, zero break, there is a certain turn on and off arc, there is a certain risk of mechanical switch adhesion when switching capacitors, the making inrush current is bigger than hybrid switches, the Capacitive load secondary input time interval is longer than the hybrid switch, the electrical life of the synchronous switch mechanical contact is less than 1/2 of that of the hybrid switch, the mechanical impact of mechanical switch making and breaking operation is large, the synchronous switch mechanical operating life is less than 1/2 of the hybrid switch.

Hybrid Switches
Hybrid Switches

The hybrid switch is a parallel combination of thyristor and mechanical switch. In the process of switching in the capacitor, first use the thyristor zero-crossing conduction to achieve the capacitor no inrush current switching in, and then turn on the mechanical switch to keep the capacitor running continuously, thus avoiding the inrush current when switching in the capacitor, also to avoid the loss of the the continuous operation of the thyristor. In the process of switching off the capacitor, first send a trigger signal to conduct the thyristor, then disconnect the mechanical switch, and finally cancel the trigger single to make the thyristor current shutdown at zero-crossing, thus avoiding arc of the mechanical switch breaking, improve the lifetime of the mechanical switches, the thyristor of the hybrid switch only works in the capacitor making and breaking moment, the loss is very small, no need heat sink.

Advantages: it can achieve absolute no arc, the making current is very small, capacitor load secondary switching-in time interval is short, the mechanical switch contact life is long; the mechanical impact of the mechanical switch making and breaking operation is small, the mechanical operation life of the mechanical switch is long.

Disadvantages: high cost, complex circuit, the voltage endurance capability is lower than the synchronous switch, there is over-voltage breakdown risk of the thyristor, can not be completely electrical isolated after the disconnect.

So, the synchronous switch and hybrid switch each have their own advantages, the engineers should select the switches according to the actual situation.