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A PFC/Flyback has a PFC stage at the input, similar to the PFC buck converter. The PFC feeds the DC link capacitor C2. The flyback converter (mainly Q1, T1) is used for galvanic isolation. Galvanic

isolation is required for enhanced safety of the converter.

For example, if the secondary side output is touched without galvanic isolation, an electric hazard is present, despite only low voltage is used. The LED output may have a significant voltage potential to the ground level. With galvanic isolation, no direct connection between the secondary side and

4.6 Topologies with galvanic isolation 4 OVERVIEW DRIVER TOPOLOGIES LED L2 C1 Q1 D1 C2 D2 D3 D4 D5 L3 D6 Q2

Figure 34: The PFC generates a sinusoidal input current waveform to obtain a power factor close to one. The PFC is an additional circuit, which is placed upstream to the buck converter.

-400 -300 -200 -100 0 100 200 300 400 0 5 10 15 20-0.1 -0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 0.08 0.1

Voltage (V) Current (A)

time (msec)

Input current without PFC

Input current/voltage Voltage U_ac

Current Iac

Figure 35: The ideal input current of an ideal PFC converter has a perfectly sinusoidal shape. No phase lag or lead of the current can be observed.

4.6 Topologies with galvanic isolation 4 OVERVIEW DRIVER TOPOLOGIES

the human is possible. This safety measure increases electric safety significantly. Thus it is highly recommended for all LED power supplies, where the outputs may be touched by humans.

Q1 D1 C2 D2 D3 D4 D5 L3 D6 Q2 LED T1 D7 D8 C1

Figure 36: PFC/Flyback topology consists out of a standard PFC with a downstream flyback converter.

Operation principle The switch Q1 is closed by applying a PWM signal, the transformers inductor,

which is included in T1, is charged. As D8faces a negative voltage, it is blocking. The energy is stored

inside the transformers magnetizing inductance and primary side stray inductance. The current rises linearly until it crosses a specified threshold. Q1 is then turned off and the energy stored in the stray

inductance is dissipated at the zener diode D7. The energy stored in the magnetizing-inductance is

transferred to C1 - which then smooths the output current for the LEDs.

The transformer’s magnetizing inductor can store positive and negative currents. However, the flyback topology uses only positive currents. This is why only half of the cores potential is used. Therefore, the required core size will be larger.

Control One possible control method is the so called peak current mode. Every period, tc, the

switching cycle is started, until the turnoff current flowing over Q1is crossed. The energy stored in the

main inductance of the transformer can be calculated using the following equation:

E =LmainI

2 T1

2 (4.33)

As this procedure is repeated every switching period tc, the power transferred to the secondary side

can be calculated to:

Psec=

LmainIT12

4.6 Topologies with galvanic isolation 4 OVERVIEW DRIVER TOPOLOGIES

From (4.34) it can be observed that the power transfer is set by the square of the peak inductor current and the switching frequency. This means, the output power can be set and controlled by those two parameters.

Requirements for switches The switch utilized in a flyback converter must block the a significant

reverse voltage when the energy is transferred from the primary side by T1 to the secondary side: The

switch must withstand the DC link voltage, the reflected voltage and the voltage induced by stray inductance. This means that typically a fairly high blocking voltage is required. If the blocking voltage of a switch is doubled, the typical on resistance increases over-proportionally. Thus, switching and conduction losses are high in this converter topology.

Loss sources In this converter different loss sources are present. Some are individually discussed.

One loss source is the stray or leakage inductance. The loss of the stray inductance can be calculated analog to (4.34):

PLoss,Leakage=

LleakIP2

2tc

(4.35)

The relative stray inductor losses are independent of the transferred current and are exclusively defined by the inductors. This can be shown by the following calculus:

pLoss,Leakage,rel=

PLoss,Leakage

Psec

= Lleak

Lmain (4.36)

The parasitic winding capacitance of the transformer is shorted, when Q1 is turned on. The capacitor losses can be calculated using the following equation:

PLoss,TCap=

CTUC22

2tc

(4.37)

This means that the transformer capacitance CTand the capacitor voltage UC2 should be chosen

minimal to reduce losses.

Another loss source, which is comparatively high at low output voltages, is the losses of the secondary side output diode. The power loss of a diode can be calculated by the simplified equation:

PLoss,Diode= UD8Iout (4.38)

To reduce this type of loss, a low forward voltage schottky diode is used. However, to reduce secondary diode losses, so called secondary synchronous rectifiers are available that replace the diode

4.6 Topologies with galvanic isolation 4 OVERVIEW DRIVER TOPOLOGIES

by a mosfet. For this a special control IC is required. A significant improvement of efficiency can then be observed.

Typical efficiencies of flyback converters ranges from 70% to 93%. An example of typical efficiencies for a flyback/PFC converter is shown in Figure 37 [8]. It shows the efficiency over the output current at an output voltage of 24 V measured at a low line input voltages VI.

Figure 37: Efficiency of a flyback converter used for LED TVs. The efficiency increases with increasing output power. The higher the input voltage, the higher the efficiency. The measurement is reproduced from the reference design [8].