DIODE IN SERIES AND PARALLEL

When designing high voltage, high power rectifiers or other types of high voltage converters where diodes are necessary, diodes can be used in series or parallel blocks to increase ratings.

Because the exact characteristics of each individual diode cannot be guaranteed to be the same it is prudent to include additional simple circuitry to ensure reverse voltage and forward current sharing occurs.

Without protection circuit-individual diodes can be destroyed due to over voltage or over current conditions.

1. DIODES IN SERIES:

If diodes are connected in series the combined effect is to increase the reverse blocking capability. When forward current flows in the forward direction both diodes conduct the same current and the forward voltage drops are very similar.

However, reverse voltages across each individual diode could vary drastically dependant on the characteristic of each diode. In this figure it can be seen that the voltage drop across D2 will not cause breakdown however, avalanche breakdown will occur in diode D1.

  • Forward voltage:

    The direct current is:

  • Reverse voltage:

    The effect of using diodes with different characteristics on the reverse blocking capability

    The simplest protection circuit is to connected high value resistors in parallel with each diode. Theoretically, if the exact characteristics of each diode are known it would be possible to design the resistors so that exact voltage division is achieved. Practically however, this is not possible and a simple design can be used.

Example: Reverse series voltage protected

If U = U1 + U2 = 400 V and the reverse leakage current for each (same) diode is specified as .

Then,

A standard value of 4.7 kΩ is preferred.

The power loss in each resistance is:

For further transient voltage equalization an additional series connected RC network can be placed across each diode in addition to R1 and R2.

2. DIODES IN PARALLEL:

Connecting diodes in parallel will increase the current carrying capability.

If it is possible to match the diodes so that approximately equal current sharing is achieved this should be done.

  • Forward voltage:

    I = I1 + I2 + I3 + I4

  • Reverse voltage protected: