Control-Integrated circuit for SMPS SG3525: Pulse frequency modulation and pulse width modulation are two types of techniques used in control integrated circuits for switch-mode power supplies. In Pulse frequency modulation technique on time of pulse remains constant but the frequency increases with the increase in load. But the Pulse frequency modulation technique causes some issues in switch-mode power supplies. On the other hand in Pulse width modulation technique frequency remains constant but the width of pulse changes with the change in load. The width of the pulse or on time increase with the increase in load. Pulse width modulation also resolves the issues which occur in Pulse frequency modulation. Therefore, Pulse width modulation is a preferred technique for control circuits of switch-mode power supplies.
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Types of PWM controllers
There are many PWM ( Pulse width modulation ) controller integrated circuits available in the market just like SG3525. These PWM controllers ICs you can use very easily in your SMPS projects by connecting some external circuitry with them according to the selection of frequency. I will discuss these things in detail later in this article. There are two types of PWM controller integrated circuits:
The voltage control method used a feedback voltage by comparing it with a reference value to set a duty cycle of PWM. In return to control the output voltage of SMPS. While the current control method uses Output current from output inductor to compare it with the reference value and to set the duty cycle of PWM. I have used a voltage mode PWM controller in pure sine wave inverter. Sg3525 is used in Dc to DC converter part to control the output voltage and for switching of MOSFETS connected to push-pull converter chopper.
Introduction to SG3525
- It is a pulse width modulation controller IC which has 16 pins.
- It provides two pulse width modulator signal which is a complement of each other.
- It is used to generate a PWM signal for power electronics projects and also for switch-mode power supplies.
- It offers feedback circuitry to control the output voltage by comparing the feedback signal with a reference voltage.
- It has a protection circuitry that shutdown the PWM signal based on the feedback current limit.
SG3525 pinout diagram
This is a pin configuration diagram and the functionality of each pin is provided in the next section.
Features of SG3525
- It can operate with a supply voltage of between 8 and 35 volts. It may damage above 40 volts.
- It also has an external oscillator synchronization pin.
- It has a pulse to pulse shutdown capability.
- It can operate with a frequency range of 100 to 400KHz.
- It also provides a feature of versatile dead-time control between switching signals to turn on or off devices like MOSFETs, IGBTS and power transistors.
- The maximum power dissipation is about 1000mW.
- Other similar integrated circuits are SG2525, UC3525.
- For further features and specifications check datasheet.
How and Where to use SG3525
- Sg3525 is a voltage mode PWM controller integrated circuit. It is used in maximum inverters available in the market.
- Even top inverters manufacture companies also use Sg3525 in dc to dc converter part of the inverter. It is a 16 pin integrated circuit.
- It has two PWM outputs both are an inversion of each. Another advantage of SG3525 is that it has a built-in totem pole base PWM driver.
- If you want to know about the internal architecture of SG3535. Search its datasheet.
- Output pins can driver semiconductor devices up to a current range of 50mA. For example, if you are using a switch that requires more than 50mA to operate, then you will have to use driver IC as MOSFET driver IR2110.
- The description and functionality of each pin are given in the next section on the pinout of sg3525.
Working with the functionality of each pin
The above diagram shows the pin configuration of SG3525. Description of each pin is given below:
- Pin 1 is an inverting pin and pins 2 is a noninverting pin. If the voltage on the inverting pin is greater than the voltage on the non-inverting pin, the duty cycle increases and if the voltage on the non-inverting pin is greater than the inverting pin, the duty cycle decreases. So you can use one pin for feedback through the voltage divider and one pin for the setting of a reference voltage.
- pin 3 is used for the synchronization of two waves.
- pin 4 is the output of an oscillator.
- pin 5, 6 and 7 is used to set the frequency of PWM. Frequency can be calculated by using the following formula:
f= 1 / CT (.7 * RT+ 3 * discharge)
By adjusting the values of CT capacitor, RT resistor, and discharge resistor. you can adjust the frequency of PWM.
- Pin 8 SS is used for soft starting for enabling output after some time. Greater the value of capacitance connected to pin 8, greater the soft-start time.
- pin 9 is a compensation pin used with feedback to avoid rapid fluctuations in output voltage with the change in load or input voltage.
- Pin 10 is shut down pin. IF shut down pin = 0 it will work and if shut down pin = 1 means connected with 5 volts it will remain in shut down mode.
- Pins 11 and 14 are output pins. These pins provide input to MOSFETs and as I have already mentioned, there is no need to connect any Mosfet driver because sg3525 has a built-in MOSFET driver circuit.
- Pin 13 and 15 are power pins. Vc should be between 5-35 volt and Vin should be between 8-35 volt.
- Pin 16 is a reference pin and it is used to set reference voltage through pin 1 or 2. It can also be used to give 5 volts to shut down the pin in case you want to shut down sg3525 through a push-button.
Example Circuit diagram SG3524
Circuit diagrams of example below show the circuit diagram of sg3525 which generates two inverted PWM signals. Users can adjust the width of PWM using a variable resistor shown in the feedback circuit. You can change the variable resistor value to adjust the resolution of PWM.
- In the above circuit diagram feedback from the output is used to get regulated voltage.
- It is discussed in detail in Dc to dc converter using a push-pull topology part.
Inverter Circuit diagram Example
In this second example, this PWM controller IC is used to generate a 220 volt AC supply from 12 volts dc. This circuit is based on SG3525A which is the same IC. Two Mosfets and ferrite core transformer are used in push-pull configuration mode. This example circuit provides power regulated voltage output with the help of the feedback circuit. Variables resistor R10 provide voltage regulation feature.
- It is used for power electronics applications like pure sine wave inverters.
- It is used to generate regulated voltage for dc to dc convert circuits like a buck converter, boost convert, cuk converter, and many others.
Other articles of pure sine wave inverter.