UPS design

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As the name implies, The UPS stands for Uninterruptable Power Supply.

A typical UPS for computers has four basic protection roles: being able to cope with power surges, voltage shortage, complete power failure and wide variations in the electric current frequency. There are three types of UPS systems, depending on how the electric power is being stored and relayed to the electronic device connected to them: 

  • Offline UPS (also known as Stand-by UPS) 
  • Line-Interactive (or Continuous UPS) 
  • Online UPS (often called double conversion supply)

UPS Modules

An Offline UPS typically includes 3 modules as below :

  • DC to DC Converter Module
  • DC to AC Inverter Module
  • Battery Charger Module

Push Pull DC-DC Converter

Function : The Q1 and Q2 Transistors are switched alternatively using the PWM duty cycle and the transformer is used to step up the Voltage. V out = 2 x Vin x (N2/N1) x d.

N2/N1 is the secondary to primary windings ratio.

d is the duty cycle of the transformers.

Input : +36 Volts DC from the Solar panels, PWM control from the Microcontroller.

Output : +400 Volts DC.

DC-AC Inverter : Full bridge

Function : The full bridge inverter converts the 400 Volts DC input from the DC converter to 230 Volts AC output. It can be controlled by only 2 PWM channels from the Microcontroller which are used to control the switching MOSFET’s through the high voltage half bridge drivers. The Resistor R is used to sense overcurrent overvoltage conditions.

Inputs : +400 Volts DC, 2 x PWM channels

Output : 230 Volts AC

Battery Charge Controller

Function : A switch mode charger can be used for battery charge controller. It consists of a Rectifier, step down transformer and output filter. It can be controlled by an ASIC or a Microcontroller.

Inputs : 230 Volts AC, Logic Control

Output : DC Output to the Battery

Microcontroller Module

Function : The function of the MCU is to control all the modules by providing PWM function, Analog to Digital Conversion, Switching logic, fault sensing and prevention and user interface functions.

Firmware Modules

The firmware modules required for this project are

Analog to Digital conversion code

PWM control code

Power Conversion algorithm

Control Routines Code

UPS state machine Interrupt based

Offline UPS Block Diagram

Function : The Offline UPS system shown in the above Figure operates in Stand-by mode and in UPS mode. When AC line voltage is present, the system is in Stand-by mode until a failure occurs on the AC line. During Stand-by mode, the battery is charged and is maintained after becoming fully charged. When the battery is charging, the inverter works as a rectifier through the IGBT’s anti-parallel diodes. The flyback switch mode charger acts as a current generator and provides constant charging current to the battery.

After a power failure, the system is switched to UPS mode. In this situation, the DPDT relay is turned OFF to prevent power from being delivered to the AC line. The push-pull converter steps up the battery voltage to 380 VDC. The high DC voltage is then converted with the full-bridge inverter and filtered with an LC filter to create a pure sine wave 220/110 VAC output where load is connected. This power switchover sequence is made in less than 10 ms.

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Farees Ahmed

The aim of this blog is to serve as a quick reference guide for the Curious. Appreciate your feedback and comments !