AL6562

Document Number: DS37542 Rev. 2 - 2

13 of 16

www.diodes.com

February 2015

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AL6562

PFC Pre-Regulator

Another major application of AL6562 is to implement a wide-range mains input PFC pre-regulator, which acts as the input stage for the

cascaded DC-DC converter and can deliver above 350W in general.

There are two methods, in general, to design a pre-regulator stage:

1.

With fixed frequency

or

2.

With fixed ON time

The AL6562 can easily be implemented with fixed ON time due to its simplicity, while the fixed frequency technique is more complicated.

In fixed ON time mode, AL6562 is also working in transition mode where the inductor current will be turned on when zero crossing is detected.

By using boost-switching technique, the AL6562 shapes the input current by drawing a quasi-sinusoidal current in-phase with the line voltage.

A simplified circuit, shown in Figure 5, explains the operation as follows:

Figure 5 ZCD Pin Synchronization without Auxiliary Winding

The AC mains voltage is rectified by a diode bridge and delivered to the boost converter which boosts the rectified input voltage to a higher

The error amplifier compares a portion of the output voltage with an internal reference and generates a signal error proportional to the

difference between them. The bandwidth of the internal error amplifier is set to be narrow within 20Hz; the output would be a DC value over a

given half-cycle. Output of E/A fed into multiplier, multiplied by a portion of the rectified mains voltage, will generate a scaled rectified sinusoid

whose peak amplitude depends on the rectified mains peak voltage as well as the value of error signal.

The output of the multiplier is fed into the non-inverting pin of the internal PWM comparator. As the output from the multiplier, a sinusoidal

reference for PWM, equals the voltage on the current sense pin CS(4), the MOSFET will be turned off. As a consequence, the peak inductor

current will follow the envelope of a rectified sinusoid. After the MOSFET is turned off, the boost inductor discharges its stored energy to the

load until zero current is detected and then the MOSFET will be turned on again.

In the case where there is no auxiliary winding on the boost inductor, a solution can be implemented by sconnecting the ZCD pin to the drain

of the power MOSFET through an R-C network: in this way the high-frequency edges experienced by the drain will be transferred to the ZCD

pin, hence arming and triggering the ZCD comparator.

The resistance value must be properly chosen to limit the current sourced/sunk by the ZCD pin. In typical applications with output voltages

operation is ensured even with a few volts difference between the regulated output voltage and the peak input voltage.