Can fast recovery diodes with different withstand voltages be used in series?

Can fast recovery diodes with different withstand voltages be used in series?

They cannot simply be connected in series, because the reverse resistances of all fast diodes connected in series must not differ much. Otherwise, diodes with large reverse resistances will have to withstand a larger reverse voltage than other diodes. If they exceed their own withstand voltage, they will be broken down. If irreversible breakdown occurs, a chain reaction will be caused, which will be broken down one by one, so the diodes in series must be connected in parallel with their respective voltage-equalizing resistors. The diodes in the high-voltage silicon stack are screened before being packaged into the silicon stack. They are produced in the same batch.

Of course, as long as the total withstand voltage after series connection is much higher than the actual voltage, voltage sharing can not be considered, because fast recovery diodes are non-linear devices. Among them, diodes with large reverse resistance will soft breakdown first, and after soft breakdown occurs, the voltage at both ends will no longer rise, but the current can increase in a small range without secondary breakdown. The effect is equivalent to reducing the reverse resistance, so the voltage will be redistributed. If two diodes with a withstand voltage of 100 volts are connected in series, The two diodees will enter a soft breakdown state one after another, and the final withstand voltage is 200 volts. Of course, this is a static state, but it is different if it is high-frequency rectification, because the reverse recovery time of the diode under soft breakdown becomes longer, the power consumption will be greatly increased, and it is prone to severe heating and damage.

It is possible to use high back-voltage diodees in parallel. As long as the margin is large enough, current sharing can not be considered when the performance of each diode is not much different. For example, if 2A output is required, 5 FR107s can be used in parallel. The reason is also due to the non-linearity of the diode. The forward voltage drop through the diode with large current will inevitably increase, causing the current to be shunted to the diode with small current. When the performance difference is not very large, the diode will not be damaged.

When reversing, it has to withstand the peak value of the entire voltage. Even if the current is zero, all voltages are applied to all diodes. As for the equalization and unevenness, it depends on the respective divided voltages. If the reverse leakage current of individual diodes is smaller than others, then it will withstand most of the voltage, while other diodes have a large leakage current equivalent to a small resistance, which may cause some diodes to break down first. This is absolutely not allowed in diode strings or thyristor strings in inverters for ultra-high voltage DC transmission, and it is not necessary to eliminate voltage grading resistors as some people say.