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SLB0587

Dimmer IC for Halogen Lamps

Infineon Technologies 

SLB 0587

Functional Description

With the SLB 0587 it is possible to generate one defined current pulse per line half cycle.

Together with a triac and a few extra passive components, a line-powered phase-control circuit

can be designed. The phase-control angle (turn-ON time of the triac) can be set on the two

control inputs, pins 5 and 6, of the IC.

The voltage supply to the IC in a two-wire connection is ensured by limiting the angle of current

flow to approx. 152°. This makes it simple to exchange mechanical wall switches in conven-

tional lighting installations. The IC’s internal logic is synchronized with the line by PLL. Thus a

phase control range independent of the line frequency is obtained.

Operation with Low-Voltage Halogen Lamps

In normal, resistive operation of a phase control circuit there is alternately part of the positive

and negative line-voltage half cycle applied to the load via the triac that has started to conduct

because of the trigger pulse. Operation of the circuit with a transformer and low-voltage

halogen lamp connected is largely identical to the operation of a normal filament lamp due to

the primarily resistive nature of the load. In operation with resistive and inductive portions of

load, the zero crossing of the current compared to that of the line voltage line is delayed.

In operation with heavily inductive loads (eg an idling transformer after lamp failure), a highly

lossy state (half cycle operation) can occur after a fault, leading to thermal destruction of the

transformer. Control mechanisms integrated into the SLB 0587 serve to protect the load from

this situation.

If, for instance, a trigger pulse is missing in a half cycle because of a fault, there will be a con-

siderable increase in current in the transformer into the line shortly after the zero crossing of a

voltage wave – after the next firing of the triac at large phase-control angles. If the next trigger

pulse comes into phase when the triac is still conducting because of the inductive current lag,

it has no effect. It is only the subsequent trigger pulse that will fire the triac again.

The case described above, where only one trigger pulse per line cycle leads to firing of the

triac, can turn into a steady-state condition in the absence of further measures.

The SLB 0587 provides the following features to prevent Steady-State Half-Cycle

Operation:

1)

Allowance for the conducting state of the triac when setting the trigger pulses. If a

trigger pulse, determined by the set firing angle and status of the internal PLL, coin-

cides with the conducting phase of the triac, the trigger pulse will not be output to the

triac until after the zero crossing of the current wave.

2)

Detection of high saturation currents at angles of current flow of more than 180° by

sampling the synchronizing input levels.

If the frequency of such peak situation current exceeds a value defined in the IC,

there will be a safety cut-out.

Semiconductor Group

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