Here is a simple PWM dc to ac voltage inverter circuit based on IC SG 3524. The SG3524 IC chips is a fixed frequency PWM (Pulse Width Modulation) voltage regulator control circuit, with indifferent outputs for single ended or push pull applications. The SG3524 IC integrated circuit has all the functions necessary for the production of a regulating power supply, electrical inverter or switching regulator on a single chip.Moreover it can be used as the control element for high power output purposes. We know pulse width modulation PWM theory, i.e. a method of adjusting the width of the pulses in a pulse train relative to a control signal. If greater the control voltage, wider is the resultant pulses. Using a sinusoidal frequency as the control voltage for pulse width modulator circuits, it is likely to generate a high power waveform whose average voltage varies with the sine wave, which is suitable for driving ac loads.
PWM chip SG3524 application notes
- Regulating power supply.
- Electrical inverter or switching regulator on a single chip.
- Transformer-coupled dc-to-dc converters.
- Transformer less voltage doublers, and polarity-converter applications employing fixed-frequency, pulse-width modulation (PWM) techniques.
- The complementary output allows either single-ended or push-pull application. Each device includes an on-chip regulator, error amplifier, programmable oscillator, pulse-steering flip-flop, two uncommitted pass transistors, a high-gain comparator, current-limiting and shutdown circuitry.
Circuit schematic of PWM inverter
Click on the circuit diagram for enlarged view
- Power supply (12 V)
- Resistors (10kΩ; 4.7kΩ x 2; 100kΩ x 2; 1kΩ, 1W x 3; 100Ω x 2; 20kΩ; 0.1Ω; 200kΩ,2W)
- Electrolytic capacitors (0.1µF x 2; 0.001µF; 1µF; 100µF)
- PWM chip IC3524
- Transformer (12-0-12 V primary, 220V secondary)
Working of PWM inverter circuit
- The IC SG3524 operates at a fixed frequency, the oscillation frequency is determined by one timing resistor RT and one timing capacitor CT.
- RT set up a constant charging current for CT. So there exists a linear ramp voltage at CT, which is connected to the comparator.
- Comparator provides a linear control of the output pulse width (duration) by the error amplifier.
- The SG3524 contains an inbuilt 5V regulator that supplies as a reference voltage, also providing the SG3524 internal regulator control circuitry.
- The inside reference voltage is divided on the outside by a resistor network to give a reference to inbuilt error amplifier. (External reference can also be used).
- The output is sensed by a subsequent resistor divider network and the error signal is amplified. This voltage is then compared with the linear voltage ramp at timing capacitor CT, thus producing a pulse width modulation (PWM) pulse.
- The resultant PWM pulse from the comparator is passed to the corresponding output pass transistor (Q1, Q2 refer block diagram) using the pulse steering flip flop, which is synchronously toggled by the oscillator output.
Internal block diagram of PWM chip SG3524
- The oscillator output pulse also acts as an inhibiting pulse to make sure that both the transistors are never turned ON simultaneously. The duration of this pulse is determined by the value of CT.
- The pin 11 and 14 are connected to the TIP transistor for driving the transformer. T1 is a 12-0-12 V primary, 220V secondary, 300VA transformer.
- When signal at pin 14 is high, upper transistor is switched ON and current flows from the +12V source via the upper half of the transformer to the ground.
- When 11 of the IC pin goes high, lower transistor gets switched ON and current flows from the +12V source via the lower half of the transformer primary and sinks to the ground.
- Thus we got positive and negative half cycles of 220V AC supply.