Main characteristics and circuit description

AN3407

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Doc ID 018857 Rev 1

The divider R6, R9, R14 and R16 provides to the L6585DE multiplier the information of the

instantaneous mains voltage that is used to modulate the peak current of the boost. In

Figure 3 the characteristic curves of the multiplier are given. The resistors R1, R3, R7 with

R11 and C31 are dedicated to sense the output voltage and feed to the L6585DE the

feedback information necessary to maintain the output voltage regulated. The components

C7, R13 and C8 constitute the error amplifier compensation network necessary to keep the

required loop stability.

The resistors R2, R4, R5 with R8 are dedicated to detecting two different overvoltage

protections: dynamic overvoltage usually due to fast load transition, and static overvoltage

due to an excessive input voltage. The PFC boost peak current is sensed by resistors R23 in

series to the MOSFET source. The signal is fed into pin#12 (PFCS) of the L6585DE. The

protection is not latched, once the PFCCS falls below 1.7 V, the PFC driver restarts.

Figure 3.

Multiplier

2.3

Resonant power section

The resonant converter half-bridge topology works in ZVS. The resonant transformer T2,

manufactured by YuJing, uses the integrated magnetic approach. The leakage inductance is

used for resonant operation of the circuit. The T2 doesn't need an external coil for the

resonance. The T2 secondary winding configuration is the typical center tap, using a couple

of type D5 and D7 power Schottky rectifiers. The output capacitors are film type C15 and

C16 (4.7 µF/63 V). L2 and C17 filters have been added on the output, in order to filter the

high-frequency ripple.

The half-bridge driver oscillation is regulated by a current-controlled oscillator. It needs a

capacitor connected to pin#1 (OSC) of the L6585DE and uses the current flowing outside

pin#2 (RF) of the L6585DE as reference. Pin#2 (RF) of the L6585DE has a 2 V precise

voltage reference that lets the designer fix the run mode frequency simply by connecting a

resistor R17 between pin#2 (RF) of the L6585DE and GND. Each curve is related to a value

of the C13 capacitor and is depicted in Figure 4. Pin#3 (EOI) of the L6585DE is driven by

the internal logic in order to set the frequency during the startup.

Pin#4 (Tch) of the L6585DE is connected to the parallel of a resistor R18 and C11 and is

used to define the protection time. Pin#6 (EOL) of the L6585DE is the input of an internal

window comparator that can be triggered by a voltage variation due to a rectifying effect.

The reference of this comparator and the amplitude of the window can be set by connecting

a suitable resistor to pin#5 (EOLP) of the L6585DE. The reference of this comparator can be

set at a fixed voltage or at the same voltage as pin#7 (CTR) of the L6585DE.