AN303

Latching current – the details

Doc ID 3654 Rev 3

5/13

2

Latching current – the details

The three examples in the previous section illustrate the importance of the latching current

parameter and the different issues considered in taken into account the latching current

requirements.

specified as a maximum value for a 25 °C junction temperature. Then corrections have to be

made according to temperature variations.

2.1

Effect of RC snubber circuit at turn on

For most applications, an RC snubber across TRIAC A2 and A1 terminals is used to

improve TRIAC immunity to fast transient voltages and also, in the case of inductive loads,

to ensure appropriate TRIAC turn off. (Refer to Figure 5 and AN437 for RC snubber circuit

design for TRIACs.)

Figure 5.

RC snubber circuit.

In the case of inductive loads, an RC snubber has another advantage at TRIAC turn on. The

energy stored in the snubber capacitor C, during the TRIAC off state, is fed back through the

TRIAC at turn on. The TRIAC current slope during the capacitor discharge is proportional to

the capacitor voltage before the TRIAC turn on and inversely proportional to the series

inductances of the board and the snubber resistor. Thus, the TRIAC current rise is faster

than the load current and reaches the device latching current in a shorter time (see

Figure 6).

Figure 6.

Snubber capacitor discharge at TRIAC turn on

A1

A2

G

IC

I

L

IT

C

R

I

t

IT1

IL current

level

IT

IC

I

dIT/dt

I : Current in the load

IC: Discharge current of capacitor C

IT:I +IC: Current in the TRIAC