# Please pay attention to voltage sharing when connecting diodes in series, and current sharing when connecting them in parallel!

When diode devices are used in series, attention must be paid to their static voltage sharing and dynamic voltage sharing.

Diode in series

When diode devices are used in series, attention must be paid to their static voltage sharing and dynamic voltage sharing.

Figure 1 Series application of devices

When the device is applied in series, at static state, the leakage current of each component in series is inconsistent, so that the component with the smallest leakage current can withstand the highest voltage, even reaching its rated limit, so it must be connected in parallel with a voltage equalizing resistor.

For a series circuit of n diodes, we can get a simplified formula for calculating resistance:

In the above formula: n – the number of series components; Vr – the rated voltage of the diode; Vm – the maximum value of the voltage in the series circuit; △Ir – the leakage current deviation value of the diode when running at the highest operating temperature.

Experience has shown that the solution to the dynamic pressure equalization problem is never the same as the static pressure equalization problem. If the carriers of one diode’s pn junction disappear faster than the other, it will also withstand voltage earlier during turn-off.

With n diodes of a given voltage of Vr in series, we can use a simplified formula to calculate the capacitance:

△QRR – Maximum deviation of reverse recovery charge between components in series, we can assume △QRR=0.3 QRR when the devices used are from the same manufacturing lot.

When the device is used in series, only when the static and dynamic of each device reach a fairly ideal symmetrical equilibrium state, can the rated parameters of each device in series be utilized to the maximum extent.

diodes in parallel

Usually in the parallel application of power devices, we should first consider current sharing. In the absence of special current sharing measures, the deviation of the on-state voltage of the devices connected in parallel should be as small as possible.

The temperature dependence of the device on-state voltage is an important parameter to measure the parallel application of devices. Some types of devices have a positive temperature coefficient of on-state voltage, and some devices have a negative temperature coefficient. When a device has a positive temperature coefficient, it is more suitable for parallel applications. However, because diodes always have some manufacturing variation, a large negative temperature coefficient (>2mV/K) in parallel diode applications may unbalance their operating temperature. This in turn makes the device permanently ineffective.

Figure 2 Dependence of different types of diodes on temperature