"Mastering Power Conversion: Designing an Inverter Circuit with Feedback Control"

Mastering Power Conversion: Designing an Inverter Circuit with Feedback Control

In this article, we will look at two inverter circuits that use automatic feedback control to ensure that the output does not exceed the normal stated AC output level or the specified overload circumstances.

What is Inverter Feedback Control?

Inverters typically include a feedback loop to regulate the output voltage and current and keep them from exceeding unsafe limits.

In this system, the output AC mains voltage is first reduced to a correspondingly lower level before being sent to the control IC's shut down pin. The stepped-down feedback voltage now follows the output AC and changes up and down proportionally.

The control ICs shut down circuitry compares and monitors this feedback signal with a fixed fer obtained from the inverter's battery voltage.

If the output voltage tends to climb over the predetermined value and beyond the reference level, the error amplifier is activated, which shuts down the inverter output PWM. When this occurs, the output voltage immediately drops, causing the feedback signal to fall below the reference value. This condition causes the IC's shut down feature to be disabled, and the IC to resume normal operation.

If the output tries to increase above the unsafe level again, the above procedure is repeated in the same manner, and this continues indefinitely and rapidly, guaranteeing that the output voltage is never permitted to exceed the stated unsafe threshold.

Inverters SG2524/SG3524/SG3525 Feedback Control

The first example circuit explains how to add an automatic feedback control to an SG2524 inverter circuit. The same idea may be applied to any other inverter versions that use the ICs SG3524 and SG3525.

The feedback control loop is configured as follows, which can be understood as follows:

A 4 diode bridge rectifier circuit is used to first rectify the 220V AC output.

The rectified high voltage DC is reduced to a lower DC level, typically 5V to 10V, via a voltage divider network constructed of 220K resistors and a 10K preset.

The 10K preset is used to fine-tune the feedback voltage until the output voltage is precisely controlled.

The feedback is obtained from the centre arm of the 10K preset and supplied to the error amplifier's non-inverting input pin #1 of the IC 2524.

This error amplifier is nothing more than an internal opamp set for managing the PWM of output pins 11 and 14.

The inverting or (+) input pin#2 of the op amp is held at a fixed reference level of +2.5V by a pair of voltage divider resistors placed around the IC's pin#2 and pin#16. The +5V reference potential is obtained from IC pin #16 and then reduced to 2.5V via the two voltage divider resisters.

Because pin#2 of the error amplifier is fixed at 2.5V reference, if the pin#1 of the opamp climbs over the 2.5V level, the IC's PWM feature is activated, causing the output PWM to the transistors to narrow.

The feedback 10k setting is set so that when the output voltage reaches the stated unsafe high voltage level, the feedback voltage at pin#1 becomes 2.6 V.

When the pin#1 receives a 2.6 V, the internal error amp activates, restricting the output PWMs to the transistors, causing the output voltage to drop to the safe lower values.

Feedback in an IC 555 Inverter

You may have previously read the piece that describes how to make basic 555-based inverters.

Although all of these inverters are well-designed and will produce the necessary 220 V or 120 V from a simple IC 555 configuration, they lack a built-in feedback system to ensure a steady output voltage.

The diagram below shows how a regular IC 555 inverter may be changed into an improved inverter using a simple feedback loop control network.

In this circuit as well, the 220V output from the transformer is rectified to a DC level before being stepped down via a resistive network consisting of a 220K resistor and a 10k capacitor.

The 10k preset centre lead is configured with the NPN transistor BC547, the collector of which can be seen attached to the IC's pin #5, which is the control input.

We know that when pin#5 is open, the PWM at the IC's output pin#3 is at its maximum; however, as the potential at pin#5 is reduced, the output PWM decreases proportionally.

When pin#5 is grounded, the output PWM at pin#3 becomes exceedingly narrow, with essentially no average voltage at this pinout.

When the output voltage of the IC 555 feedback circuit tends to grow over the unsafe high voltage threshold, as established by the 10k preset, the base of the BC547 gradually begins to bias. When this happens, the BC547 begins to conduct, causing the IC's pin #5 to gradually ground. The grounding of the IC's pin #5 causes the output PWM at pin #3 to narrow, causing the output voltage to return to normal values.

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