AN319 APPLICATION NOTE

2/7

A solution is shown in Figure 2, circuit c, in which a Transil diode is put in series with the recovery device.

A much higher voltage is developed across the diode-Transil combination than across a single diode, thus

accelerating the current decay.

Figure 2. Relay Drive Protection

Note: (b) Recovery Diode.

(c) Diode and Transil.

Transil

The Transil is an avalanche diode specially designed to clamp overvoltages and dissipate power in impul-

sive mode. It also offers the possibility of considerable average power absorption. It is available in different

cases covering a wide range of voltages (5 to 600V).

Moreover, all the necessary data to calculate power parameters and to carry out temperature evaluation

(Zth, Rth, etc...) can be found in technical notes. So the Transil is perfectly suited to this application.

In the case of a steady state power supply we can consider three hypotheses:

Circuit d (Figure 3). This is an economic solution which requires a unidirectional Transil only, but the cur-

rent released by the inductance goes through the power supply and may affect the ground points.

Circuit c (Figure 2) and circuit e (Figure 3). The inductive discharge current only goes through the

LRD loop, thus is does not create disturbances in the ground points. This is a more expensive solution

since two diodes or one bidirectional Transil are necessary. In the case of an alternating sinusoidal supply

for low power, circuits e and f (Figure 3) are well suited to limit inductive overvoltages.

Comparison of the Current Decay Time in the Inductive Load Between the Recovery Diode and the

Transil Diode Schematics (see Figure 4)

b

L

R

A

D

c

L

R

A

D

T

⁄

1

11 4

,

⁄

⋅

+

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=