AN2590

Board description

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2.2

Dimming function

The two modes for dimming function are frequency modulation and voltage modulation.

In this solution, the dimming function is obtained using two methods: frequency modulation

and voltage modulation. When the trigger angle of the triac changes from 0° to 90°, only

frequency modulation works. When the angle changes from 90° to 180°, both of them work,

and voltage modulation dominates.

Frequency modulation depends on the operational amplifier in L6574D. In Figure 6, R1, R2,

R3 and C10 were used to simulate the trigger angle of the triac. When the angle changes

from 0° to 180°, the voltage across C10 which is the positive input of the amplifier (Pin 7,

here pin number is for L6574D), decreases accordingly. The output (Pin 5) of the amplifier

decreases with Pin 7, and the negative input (Pin 6) of the amplifier automatically

decreases. The equivalent resistance between Pin 4 and GND of the IC decreases, so the

switching frequency increases, hence lamp power decreases. If the angle changes from

180° to 360°, lamp power increases.

Voltage modulation means that the voltage across C1 (see Figure 6) decreases when the

firing angle of the triac changes from 90° to 180°. This makes the lamp power decrease.

2.3

Principle of the main circuit

In order to simplify the analysis, the main circuit is shown in Figure 7. There are two

independent circuits, power factor correction (PFC) circuit and half bridge resonant circuit.

The PFC which is labeled as A (dotted line) consists of Cf1, Cf2, L1, Db1, Db2, S1, S2 and

C1. The other one is a half bridge resonant circuit, which is labeled as B (solid line). It

consists of C1, S1, S2, Cb, Lr, Cr and the Lamp. S1 and S2, the Power MOSFETs operate

complementarily at almost 0.5 duty ratio.

Figure 7.

Main circuit

Since the switching frequency was high enough, the voltages across the divider capacitors

(Cf1&Cf2) can be treated as constant during one switching cycle. Thus the voltage across

C1 is always higher than the line peak. In Figure 8 per switching cycle shows the model of a

four stage current flow through L1.