AL6562 datasheet(10/16 Pages) DIODES

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AL6562

Document Number: DS37542 Rev. 2 - 2

10 of 16

www.diodes.com

February 2015

AL6562

Application Information (cont.)

, the distortion can further be defined by:

Where

and

are the RMS (Root Mean Square) value n-th fundamental component of the current respectively. If the current and

voltage are in phase, then θ= 0, which will lead to

and the PF will be simplified as:

ZCD (Zero Current Detection)

The ZCD feature detects when the transformer primary current falls to zero, as the voltage across the inductor reverses, to initiate a new cycle

that switches on the power MOSFET. The signal for ZCD is obtained by an auxiliary winding on the boost inductor, as shown in Figure 2.

Multiplier

The internal multiplier takes two inputs, one from a portion of the instantaneous rectified line voltage (via pin 3, MULT) and the other from the

output of the E/A (via pin 2, COMP), to feed the PWM comparator to determine the exact instant when the MOSFET is to be switched off. The

output of multiplier is a rectified sinusoid, similar to the instantaneous rectified line voltage, multiplied by the scaling factor determined by

output of the Error Amplifier. The MULT output is then fed into the PWM comparator and is compared to the current sense voltage VCS, to

switch the Power MOSFET off. The formula governing all parameters is given by:

Multiplier Output:

Where: k is the multiplier gain. VMULT is set by external resistors R1 and R2.

OVP (Output Overvoltage Protection)

The output voltage can be kept constant by the operation of the PFC circuit close to its nominal value, as shown by Figure 2, which is set by

the ratio of the two external resistors R3 and R4. Neglecting ripple current, current flowing through R3, IR3, will equal the current through R4,

IR4. As the non-inverting input of the error amplifier is biased inside the AL6562 at 2.5V, the current through R4 is:

(1)

If any abrupt change of output voltage, ΔVO > 0 occurs due to a load drop, the voltage at pin INV will be kept at 2.5V by the local feedback of

the EA. The network connected between INV and COMP introduces a time constant to achieve high PF. The current through R4 will remain

equal to 2.5/R4, but IR3 will become:

(2)

The difference current

will flow through the compensation network and enter the error amplifier output via pin

COMP. The AL6562 monitors the current flowing into the error amplifier output pin. When the detected current is higher than 40µA, the

dynamic OVP is triggered. The IC will be disabled and the driver signal will be stopped.

The output ΔVo that is able to trigger the Dynamic OVP function is then:

(3)

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