AN4376 datasheet(6/37 Pages) STMICROELECTRONICS

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Theory of operation

AN4376

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DocID025380 Rev 1

Theory of operation

Most applications for an LED driver are designed using a common isolated flyback for

decoupling the secondary side and the load from the input mains, but sometimes the use of

transformers could be costly and is not always needed. In the bulb replacement market the

buck-boost topology without isolation and a transformer is often considered when isolation

is not requested by regulations.

Figure 2 and Figure 3 show the differences, the flyback is derived from the basic buck-boost

topology and in fact it could be considered as a flyback with a unity primary to secondary

turn ratio. As a consequence the output of the buck-boost is floating and the negative side of

the output capacitor is not at GND but connected to the input mains.

Note then that the transformer of Figure 2 has been substituted by a simple inductor in

Figure 3.

Then in both situations, when the switch T is ON, the energy is stored in the inductor for the

BB or primary side for the FL, when the switch is OFF, the energy is transferred to the output

through the output diode. The primary current coming from the mains is in red and the

current flowing into the output is in blue.

Approaching the real LED driver, the LED load has been represented by the average LED

current in black.

Considering n as the transformer turn ratio, if n = 1, it is easy to represent the current and

waveforms for the buck-boost (Figure 5).

Figure 2. Flyback (FL)

Figure 3. Buck-boost topology (BB)

Figure 4. TM (transition mode) flyback currents

Figure 5. TM (transition mode) buck-boost

currents

AN4376 Scheda tecnica(PDF) 6 Page - STMicroelectronics