IRS2505LPBF

10

2015-9-2

Functional Description

The following topics are described in more detail below:

1) PFC Control Circuit

2) Multi-Function PFC Pin

3) On-Time Modulation

4) Over-Voltage Protection (OVP)

5) Over-Current Protection (OCP)

The IRS2505L is primarily intended for but not limited to pre-regulator PFC Boost converter front end

stages in general power supplies and lighting applications such as LED power supplies, fluorescent and HID

ballasts. The IRS2505L may also be used in other converter topologies such as Buck, Buck-Boost and Flyback,

where its small size and functionality can offer simplicity and cost advantage. It can also be used in converters

where PFC is not required.

1. PFC Control Circuit

The IRS2505 is a high frequency SMPS PWM controller designed for Boost PFC pre-regulators operating in

critical-conduction mode (CrCM), also known as transition (TM) or boundary mode (BM). The PFC Boost converter

provides a regulated high voltage DC output over a range of AC line input voltage and load, while at the same time

shaping the input current so that it follows the sinusoidal profile of the voltage resulting in high power factor and low

total harmonic distortion (THD).

Figure 1 shows the main elements of the PFC Boost converter; the input capacitor (CIN) is for high frequency

bypass and does not provide smoothing at low frequency under steady state operation. When a load is connected at

the output the rectified input voltage across CIN is full wave rectified. During the on-time of the PFC MOSFET

(MPFC), the IRS2505 gate drive output (PFC) is high so the inductor (LPFC) current ramps up linearly to a peak

current value. During the off-time the gate drive output is low and the inductor current linearly discharges back down

to zero at which point MPFC is turned on again. The control loop response has to be slow compared to the line

frequency so that the on time remains effectively constant over a single line half cycle; except for some gradual

increase as the voltage approaches the zero crossings, which improves THD. The off time varies during the AC line

half cycle being longest at the peak.

CIN

CBUS

LPFC

MPFC

DPFC

(+)

(-)

(-)

(+)

Rectified

AC Line

Input

DC BUS

ILPFC

t

ON-TIME

OFF-TIME

Figure 1: Boost-type converter and inductor current during critical-conduction mode.

The result is a triangular shaped inductor current that varies in frequency depending on the level of the

instantaneous rectified AC line voltage shown in figure 2. The maximum switching frequency occurs at the zero-

crossings of the rectified AC line voltage and the minimum at the peak of the rectified AC line voltage. The average

inductor current is equal to half of the peak, which is equal to the instantaneous line input current after the high

frequency elements have been filtered out.