IC SG3525: Practical Circuit Applications, Working, Pinouts, Specifications

The IC SG3525 is a popular PWM controller IC which we can use for making different types of inverters, SMPS circuits, DC to DC converters and many other power electronic circuits. The main advantage of this IC is that it can generate two alternate PWM outputs. We can use these outputs for driving MOSFETs, IGBTs and other power switching devices through suitable driver stages.

We can also adjust the output frequency, PWM duty cycle, dead time and some other important functions by using a few external components. The IC also has an internal voltage reference, error amplifier, soft start facility and shutdown function.

In this article we will learn how the SG3525 works, understand its pin functions and see how we can use it for making different practical power electronics circuits. I have also included several SG3525 circuit designs from this website, so you can easily find the circuit which you may need for your project.

What is IC SG3525?

The SG3525 is a 16-pin PWM controller IC which is mainly used for controlling switching power supplies and inverter circuits. It has an internal oscillator, PWM comparator, error amplifier, reference voltage regulator and two alternate output driver stages.

The IC can generate two separate outputs which work alternately. This feature makes it very useful for making push-pull inverter circuits and transformer-based SMPS designs. For example in a push-pull inverter, we can use the two outputs for switching two sets of MOSFETs alternately. These MOSFETs then operate a transformer and convert the battery DC into AC.

In an SMPS circuit, the same IC can control the switching of a ferrite transformer or an inductor, so that we can get the required regulated DC output.

So the SG3525 can be used for many different types of power conversion circuits.

Main Features and Specifications of SG3525

Some important features of the SG3525 are given below.

Operating supply voltage: 8V to 35V.
Internal reference voltage: 5.1V.
Two alternate PWM outputs.
Adjustable oscillator frequency.
Adjustable dead time.
Internal error amplifier.
Internal soft start facility.
Shutdown control input.
Undervoltage lockout.
Separate oscillator synchronization input.
Internal PWM latching function.
Two source/sink output driver stages.

The oscillator frequency range and output drive specifications can slightly vary between manufacturers and different IC versions. So, you should check the datasheet of the actual IC that you are using.

SG3525 Pinout and Pin Functions

The SG3525 comes in a 16-pin DIP or surface mount package. Each pin has its own function. So, we should understand these pins properly before building any inverter or SMPS circuit.

Here is the basic pin arrangement:

Pin No.	Pin Name	Function
1	INV Input	Inverting input of the error amplifier.
2	Non-INV Input	Non-inverting input of the error amplifier.
3	Sync	Used to synchronize the oscillator with an external signal.
4	OSC Output	Provides the oscillator waveform output.
5	CT	Timing capacitor connection for setting oscillator frequency.
6	RT	Timing resistor connection for setting oscillator frequency.
7	Discharge	Provides the discharge path for the timing capacitor.
8	Soft Start	Connected to a capacitor to provide a gradual PWM startup.
9	Compensation	Used for connecting the compensation network of the error amplifier.
10	Shutdown	Used to stop the PWM outputs when a shutdown signal is applied.
11	Output A	PWM output A for driving an external power stage.
12	Ground	Common ground connection for the IC.
13	VC	Supply voltage for the output stage.
14	Output B	PWM output B for driving an external power stage.
15	VI	Main supply voltage input for the IC.
16	VREF	Provides a regulated 5.1V reference output.

Note: Pins 11 and 14 provide alternate PWM outputs, which are commonly used for driving push-pull and half-bridge inverter circuits.

One important thing to understand here is that pin 13 and pin 15 are not the same supply connections. Pin 15 supplies the main IC circuitry, while pin 13 supplies the output driver stages.

For more detailed information about the pin functions, internal circuit details and compensation network, you can read the following article:

Understanding SG3525 IC Pinouts:
https://www.homemade-circuits.com/understanding-sg3525-ic-pin-outs/

How SG3525 Works

The working of the SG3525 is quite simple when we understand its internal sections.

First, the oscillator section generates a fixed frequency signal with the help of the external timing resistor and capacitor connected to pins 5 and 6.

This oscillator signal is then processed by the internal flip-flop and PWM comparator sections.

The internal flip-flop activates output A and output B alternately. Because of this, the two outputs produce alternate switching pulses.

The PWM comparator controls the width of these pulses according to the control voltage received from the error amplifier.

This means we can change the ON time of the output pulses without changing the basic switching frequency.

When the PWM pulse width increases, the power transferred through the external switching stage generally increases. But when the PWM pulse width decreases, the power transfer is reduced.

The error amplifier can be used for automatically adjusting the PWM according to the output voltage or another feedback signal.

The IC also has a soft start facility. This helps the PWM output to increase slowly during startup, instead of suddenly applying maximum pulse width.

Understanding the SG3525 PWM Outputs

The SG3525 has two separate outputs at pins 11 and 14. These outputs work alternately. This means when output A is active, output B remains inactive, and after this output B becomes active.

This arrangement is very useful for push-pull inverter circuits.

For example suppose we connect two MOSFET banks to the two outputs through suitable gate driver circuits. The first MOSFET bank switches during one half-cycle, while the second MOSFET bank switches during the next half-cycle. This allows the transformer primary winding to receive alternate switching pulses.

One important thing we should understand is that the oscillator frequency and the individual output pulse repetition frequency are not always the same. In the normal alternate push-pull arrangement, each output operates at approximately half the oscillator frequency.

The maximum duty cycle of each output is also restricted. This helps to provide the required non-overlapping switching interval.

But the actual dead time must be selected according to the switching devices, driver circuit and transformer requirements.

How to Set the SG3525 Oscillator Frequency

The oscillator frequency is mainly controlled by the external resistor and capacitor connected to pins 5, 6 and 7.

Pin 5 is connected to the timing capacitor, while pin 6 is connected to the timing resistor.

Pin 7 provides the discharge path and also helps to determine the oscillator dead time.

A commonly used approximate frequency relationship is:

f = 1 / [CT (0.7RT + 3RD)]

Here:

f = Oscillator frequency in Hz.
CT = Timing capacitor in Farads.
RT = Timing resistor in Ohms.
RD = Discharge resistor in Ohms.

This is an approximate design relationship. For accurate frequency selection, we should check the values using the particular manufacturer's datasheet. The frequency should be selected according to the transformer, MOSFETs, switching losses and the intended application.

For example a low-frequency inverter and a high-frequency ferrite transformer SMPS will need completely different oscillator settings.

Important Design Considerations When Using SG3525

Although the SG3525 is a useful controller, still we should take care of a few things when using it in practical power circuits.

MOSFET gate driving

The two internal outputs can drive moderate gate loads, but when we are using high-power MOSFET banks or IGBTs, suitable external gate driver circuits may be required. This helps to provide enough gate charging current and proper switching performance.

Dead time adjustment

Dead time is very important in push-pull and bridge circuits. If the switching devices conduct at the same time in the same bridge leg, it can create a dangerous shoot-through condition.

So we must provide sufficient dead time according to the power stage and driver specifications.

Proper feedback control

The internal error amplifier can be used for regulating the output voltage. But when we need both adjustable voltage and current regulation, a properly designed external feedback circuit may be more suitable.

Soft start

The capacitor connected to pin 8 controls the soft start action. This helps to reduce sudden switching stress during startup.

Power supply decoupling

The main supply and output driver supply should have suitable local bypass capacitors. The PCB layout should also keep the high-current switching paths away from sensitive feedback and oscillator connections.

Different SG3525 Inverter and SMPS Circuit Applications

Now let us look at some practical circuits which we can build using the SG3525. I have arranged these designs according to their applications, so you can easily select the circuit which matches your requirement.

SG3525 Basic Inverter with Low Battery Cutoff and Automatic Voltage Regulation

This is a useful application of the SG3525 for making battery-operated inverter circuits. The circuit uses the alternate PWM outputs of the IC for driving the inverter MOSFETs.

It also includes additional control circuits for detecting low battery voltage and regulating the inverter output voltage.

The low battery cutoff feature helps to prevent excessive battery discharge, while the output feedback arrangement helps to maintain the required AC voltage when the load or battery condition changes. This type of design can be useful when we need a basic inverter with some additional protection and voltage correction features.