Series and parallel connection of power electronic devices

Series and parallel connection of power electronic devices

When the voltage and current of a single power electronic device cannot meet the actual requirements, two or more devices of the same type can be connected in series or in parallel to meet the requirements.

1. Thyristor connected in series

When the rated voltage of the thyristor is less than the requirements of the actual circuit, devices of the same type can be connected in series. Due to the dispersion of device characteristics, the volt-ampere characteristics of devices of the same type will also be different, resulting in uneven voltage distribution when connected in series. For this reason, voltage equalization measures must be adopted when the thyristors are connected in series.

The thyristor has 5 states in work: ① Forward blocking; ② Reverse blocking; ③ Turn-on process; ④ Turn-off process; ⑤ Complete conduction. Among them, the voltage equalization in states ① and ② belongs to static voltage equalization, and the voltage equalization in states ③ and ④ belongs to dynamic voltage equalization, and the voltage drop of the device after it is fully turned on is about 1V, so there is no need to consider the voltage equalization problem.

As shown in Figure 1a, after VT1 and T2 are connected in series, due to the slightly different volt-ampere characteristics of the thyristor, when the forward blocking, the same forward leakage current IR bears different forward voltages, and the voltage borne by T2 is much higher than that of VT1. If the voltage continues to increase, VT2 may be turned on hard, and VT1 will be turned on under the full voltage, causing both devices to lose control. Similarly, during reverse blocking, due to the uneven voltage division, the device subjected to high voltage will break down first, and then the other device will also have a chain breakdown. Aiming at the uneven voltage distribution caused by different static characteristics, the static voltage equalizing measure we take is parallel voltage equalizing resistor Rj. The resistance value of Rj is much smaller than the forward and reverse resistances when the device is blocked, so that the voltage when the device is blocked depends on the voltage division of the voltage-sharing resistor.

Series and parallel connection of power electronic devices
Figure 1 – Voltage distribution and voltage equalization measures when thyristors are connected in parallel

During the turn-on and turn-off of the thyristor, the devices that turn on later and turn off first will withstand the full voltage and cause damage to the thyristor. The measure of dynamic voltage equalization is to connect resistance-capacitance elements in parallel at both ends of the thyristor, and use the principle that the capacitor voltage cannot mutate abruptly to keep the voltage across the device from mutating during the dynamic switching process.

2. Thyristor in parallel

The static and dynamic parameters of the thyristor are inconsistent. Under the same voltage drop, the current flowing through the parallel devices is very different, as shown in Figure 2. In order to make the parallel devices achieve equal current distribution, in addition to selecting devices with relatively consistent characteristics, current sharing measures should also be adopted.

Figure 3a shows the series resistance current sharing. When the current of a certain branch is large, the voltage drop on the series resistance increases, thereby reducing the voltage across the thyristor connected in series with it, so that the current of the branch decreases. The series resistance value is the resistance at the maximum working current of the device, and the voltage UR is 1-2V. This method is suitable for small capacity occasions.

Series and parallel connection of power electronic devices
Figure 2 – Current distribution when thyristors are connected in parallel
Series and parallel connection of power electronic devices
Figure 3 – Current sharing measures when thyristors are connected in parallel

3. Parallel connection of other power electronic devices

The on-state resistance Ron of the power MOSFET has a positive temperature coefficient, the current increases, the heat generation increases, the on-state resistance also increases, and the current decreases. When the MOSFETs are used in parallel, it has a certain current balancing ability.

The on-state voltage drop of the IGBT generally has a negative temperature coefficient in the section below 1/2-1/3 of the rated current, and has a positive temperature coefficient in the above region. Therefore, when the IGBTs are connected in parallel, they also have a certain current automatic balancing ability, which is easy to use in parallel.

When actually using MOSFET and IGBT in parallel, try to keep the same in terms of device parameter characteristic selection, circuit layout and heat dissipation conditions.