AN2794 datasheet(7/39 Pages) STMICROELECTRONICS

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AN2794

System description

Doc ID 14827 Rev 2

7/39

on and off in order to transfer power to each primary of the center tapped transformer.

Contemporary conduction of both devices must be avoided by limiting the duty cycle value

of the constant frequency PWM modulator to less than 0.5. The PWM modulator should also

prevent unequal ON times for the driving signals since this would result in transformer

saturation caused by the "Flux Walking" phenomenon.

The basic operation is similar to a forward converter. In fact, when a primary switch is active,

the current flows through the rectifier diodes, charging the output inductor, while when both

the switches are off, the output inductor discharges. It is important to point out that the

operating frequency of the output inductor is twice the switching frequency.

A transformer reset circuit is not needed thanks to the bipolar flux operation, which also

means better transformer core utilization with respect to single-ended topologies.

The main disadvantage of the push-pull converter is the breakdown voltage of primary

power devices which has to be higher than twice the input voltage. In fact, when voltage is

applied to one of the two transformer primary windings by the conduction of a transistor, the

reflected voltage across the other primary winding puts the drain of the off state transistor at

twice the input voltage with respect to ground. This is the reason why push-pull converters

are not suitable for high input voltage applications.

For the above mentioned reasons, the voltage fed push-pull converter, shown in Figure 4, is

chosen to boost the input voltage from 24 V to a regulated 350 V, suitable for optimal

inverter operation. The high voltage conversion ratio can be achieved by proper transformer

turns ratio design, taking into account that the input to output voltage transfer function is

given by:

Equation 1

The duty cycle is set by a voltage mode PWM regulator (SG3525) to keep a constant output

DC bus voltage. This voltage is then converted into AC using a standard H-bridge converter

implemented with four ultrafast switching IGBTs in PowerMESH™ technology, switching at

16 kHz. The switching strategy, based on PWM sinusoidal modulation, is implemented on

an 8-bit ST7lite39 microcontroller unit. This allows the use of a simple LC circuit to obtain a

high quality sine wave in terms of harmonic content.

The power supply section consists of a buck-boost converter to produce a regulated 15 V

from a minimum input voltage of 4 V. The circuit can be simply implemented by means of a

L5973 device, characterized by an internal P-channel DMOS transistor and few external

components. In this way, it is possible to supply all the driving circuits and the PWM

modulator. A standard linear regulator, L7805, provides 5 V supply to the microcontroller

unit.

out

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