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Line о	600W Lamp Ö „	10K^ л л/л
120 V 60Hz	MT2 A	yvV l G	-p monF
120 V 60Hz	MT1		-p monF
Neutral	MT1
	^-'

the di/dt of the line current has been adequately controlled before thishap-pens. The choke has a 2W loss at the dimmer"s rated current.

Smoothing Chokes

MPP is the best choice for smoothing chokes in switching power supplies where the ripple current is small compared to DC current. Where the ripple current is large, as in high power flyback type supplies, MPP losses may be loo large. In that case, gapped ferrites with Litz wire are better because of their lower hysteresis loss. For lower power flyback supplies, MPP cores are better than gapped ferrites.

Figure 3 la shows a forward converter 5V, 30A supply operating at 20kHz. The waveforms shown in Figure 37b assume that the input voltagecoulddrop by approximately 50% be fore regulation would be lost. The filter choke inductance will drop about 50% due to the 30A DC. This results in about 5 A peak-to-peak triangle current in the output filler capacitor (1.4Arms). This relatively low ripple current reduces heating in the output filter capacitor. The capacitor should be selected for lowest possible ESR to minimize output ripple.

For small load currents, the inductance returns to 22u.H, which reduces the ripplecurrent to 2.5 A peak-to-peak. The lower ripple allows the load current to go down to about 1.25A before regulation is lost. An MPP design for the filter choke is compared to a ferrite design. The cores used were a 1.06-in. diameter

Figure 35a Triac Dimmer, Dimmer Circuit

HF-090125-2 105 Turns #20 1mH @ 5А

Line

О—

120V 60Hz

Neutral

О-

То Dimmer

Figure 35b Triac Dimmer, Line Fitter Light Dimmer Filters

Triac lamp dimmers are infamous for causing AM radio interference. The circuit for a typical 600W incandescent lamp dimmer is shown in Figure 35a. These dimmers control the average power supplied to the (amps in the load by varying the duty cycle of the triac switch. Figure 36 shows the voltage across the switch for a 50% duty cycle.

At turn on, a voltage transient as great as 170V occurs across the terminals of the dimmer. The dv/dt of this transient is about 1,700V/u.sec. The current rise time is limited only by the leakage inductance of the utility transformer. With leakage inductance of 60uH, the di/dt is 2.8A/ usee. The high dv/dt and di/dt both radiate RFI from the lamp wiring.

Most commercial dimmers have a small inductor of about 60u.H in

series with their terminals but this is far too small to be effective against RFI. Usually, they have no shunt capacitor. Figure 35b shows an effective Rfi filter for a 600W dimmer. An 0.90-in. diameter 125 permeability HF core is used as a line choke and is rated at l.OmH at 5A. The dimmer terminals are shunted with a 47nF capacitor.

The capacitor reduces the dv/dt across the dimmer terminals at turn on to 2V/(.tsec. Overall voltage rise time is increased from 0.1u,sec to 50|jsec. The choke reduces the di/dt during turn on to 0.17 A/u sec. Due to the reduced di/dt, the triac can completely turn on before full current is applied. This increases its reliability during the high surge currents from cold lamp filaments. The choke saturates at the 7A peak of the cycle, but

Figure 36. Triac Voltage at 50% Duty Cycle Voltage: 50V/év. ; Sweep: 2 msec/div.

PCM ■ 199617