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Compatibility of LED lamps with phase-cut dimmers Disclosure Number: IPCOM000195169D
Publication Date: 2010-Apr-22
Document File: 3 page(s) / 101K

Publishing Venue

The Prior Art Database


ID672552, ID672554

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ID 672552, ID 672554

Compatibility of LED lamps with phase-cut dimmers

    In many markets LED products are starting to replace other light-sources, these so-called retrofit products have to be compatible with existing systems.

Many Lighting Systems use a dimmer to adjust the light-output of a lamp or module; most dimmers are designed to work with filament lamps or halogen-transformers. Typically LED-circuits are not compatible with this type of dimmer.

This publication focuses specifically on trailing edge dimmers that are often used in the professional market.

In most Mains-Voltage Lighting Systems the dimmer is of the phase-cut variety, the RMS voltage presented to the lamp or module is reduced by cutting off the input voltage for a part of the (half-) mains cycle. Figure 1 shows a typical waveform for a trailing edge dimmer.

1 i(v3)

Figure 1, Typical Trailing-edge dimmer waveform

Figure 2 Simplified Dimmer Topology






i(v3)in amperes



52.0m 56.0m 60.0m 64.0m 68.0m time in seconds

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Figure 2 shows a (simplified) trailing-edge dimmer topology.

The dimmer uses an electronic-switch that is controlled by a control-circuit that sets the conduction-angle. The electronic LED driver senses the resulting RMS voltage to determine the desired dim level.

    The mains, the dimmer and the lamp are connected in series. Two conditions have to be met for the control-circuit to keep operating correctly. The switch should never conduct continuously (i.e. if the switch were continuously ON the voltage for control-circuit would be 0) and there should be a path to conduct the supply current for the control circuit, this path must be through the load (in Figure 2 the lamp closes the current loop).

    A problem with trailing-edge dimmers in combination with LED-Drivers is that the supply current for the control-circuit inside the dimmer is not sufficient because the LED-driver does not close the current loop when its internal rectifier bridge is not conducting. This typically results in severe light-output flicker.

    The LED lamp needs to offer a low-impedance path to discharge the capacitor and keep the dimmer power-supply alive when the dimmer-switch is not conducting. Prior-Art solutions use so-called bleeder circuits to pr...