TDA 4862

Semiconductor Group

4

1998-02-16

Functional Description

Introduction

Conventional electronic ballasts and switching power supplies are designed with a

bridge rectifier and bulk capacitor. Their disadvantage is that the circuit draws power

from the line when the instantaneous AC voltage exceeds the capacitor’s voltage. This

occurs near the line voltage peak and causes a high charge current spike with following

characteristics: the apparent power is higher than the real power that means low power

factor condition, the current spikes are non-sinusoidal with a high content of harmonics

causing line noise, the rectified voltage depends on load condition and requires a large

bulk capacitor, special efforts in noise suppression are necessary.

With the TDA 4862 preconverter a sinusoidal current is achieved which varies in direct

instantaneous proportion to the input voltage half sine wave and means a power factor

near 1. This is due to the appearance of almost any complex load like a resistive one at

the AC line. The harmonic distortions are reduced and comply with the IEC555 standard.

Operating Description

The TDA 4862 contains a wide bandwidth voltage amplifier used in a feedback loop, an

overvoltage regulator, an one quadrant multiplier with a wide linear operating range, a

current sense comparator, zero current detector, a PWM and logic circuitry, a totem-pole

MOSFET driver, an internal trimmed voltage reference, a restart timer and an

undervoltage lockout circuitry. These functional blocks are described below.

Voltage Amplifier

The voltage amplifier is internally compensated and yields a gain bandwidth of 0.8 MHz

and a phase margin of 80 degrees. The non-inverting input is biased at 2.5 V and is not

pinned out. The inverting input is sensing the output voltage via a resitive devider. The

voltage amplifier output

V

way via an external capacitor. It forms an integrator which monitors the average output

voltage over several line cycles. Typically the bandwidth is set below 20 Hz. ln order to

keep the output voltage constant the voltage amplifier output is connected to the

multiplier input for regulation.

Overvoltage Regulator

Fast changes of the output voltage can’t be regulated by the integrator formed with the

voltage amplifier This occurs during initial start-up, sudden load removal, or output

arcing and leads to a current peak at the voltage amplifier input while the voltage

amplifier’s differential input voltages remains zero. The peak current is flowing through

the external capacitor into

V

regulation circuitry to reduce the multiplier output voltage.