在前面的教程中,在线UPS组装from its basic building blocks like Lead Acid battery Charger, Square Wave Inverter and Modified Sine Wave Inverter. Now in this tutorial, offline UPS will be designed. The Offline UPS is one of the topologies of UPS (Uninterruptible Power Supply). In UPS system, the term topology refers to the switching mechanism of the UPS system.
Contrary to online UPS, in offline UPS, the load circuit is initially directly powered by the AC mains. On the failure of power supply, the power failure is detected by a switching circuit and the power is provided to the load through the battery and inverter after the power failure has already occurred. So, in the offline UPS, the load experiences the power cutoff once before it receives the power backup from the UPS.
因此,离线UPS的切换机制power transfer from Mains AC to the Inverter and Vice-Versa. When the mains supply is available, the load circuit draws power from the Mains Supply directly. But, when there is a blackout or power cut off from the mains, the load circuit is connected to the inverter circuit for drawing power from the charged battery. The battery itself is charged from mains supply but it does not provide power to the load circuit until the power cut is detected. This operational algorithm of an offline UPS is represented in the following flowchart –
Fig. 1: Flowchart representing operation of offline UPS
The circuit of an offline UPS is not much different from the Online UPS except that the offline UPS has a switching mechanism to detect power failure and transfer power input of the load. The power failure detector and transfer switch has been designed in this offline UPS circuit using a relay circuit whose operation depends upon the status of the AC mains. The AC mains is rectified and stepped down to 12V DC to drive a relay circuit which in turns determine whether the load connects directly to the AC Mains or connects to the inverter circuit.
开关机构中使用了两个继电器。一个继电器将逆变器输入与电池和其他继电器连接起来，将负载与主电源连接起来。负载还连接到逆变器输出。当电源可用时，将逆变器输入与电池连接的继电器保持在状态下，因此，逆变器电路不会从电池中汲取任何电源当负载与其他继电器无点连接到主电源时，电源还保持在供电电源的条件下。随着电源的切断，将逆变器输入与电池连接连接的继电器变为OFF状态，使逆变器从电池中汲取电池，而电池通过NC点连接，而将负载与AC电源连接的继电器也处于OFF状态当负载通过无点连接到交流电源时，从交流电源中切断负载，因此负载然后从逆变器输出中汲取功率。
Components Required –
Fig. 2: List of components used in designing offline UPS
Block Diagram –
离线UPS的电路可以由以下框图表示 -
Fig. 3: Block Diagram of Offline UPS
Note that though there are two circuits designed for offline UPS, both circuits have the same block representation.
Circuit Diagram –
The Lead Acid battery is used in this offline UPS design. The battery charger has the following circuit diagram –
Fig. 4: Circuit Diagram of LM317 IC-based Lead Acid Battery Charger
添加了带有以下电路图的开关机构以使完整的离线UPS -
图5：离线UPS的电路图
Circuit Connections –
Since the battery charger circuit and inverter circuits have been already designed, now only these circuits need to be assembled together with the switching to make the offline UPS. For learning about the lead acid battery charger circuit, check out the following tutorial –
Designing 12V Lead-Acid Battery Constant Voltage Limited Current Charger for UPS
For learning about the design of square wave inverter, check out the following tutorial –
Designing Switching Mechanism with Step Up Transformer for Square Wave Inverter
For learning about the design of Modified Sine wave inverter, check out the following tutorial –
Completing Modified Sine Wave Inverter Design with Full Bridge Circuit and Step Up Transformer
In the offline UPS based on Square wave inverter, the 7812 voltage regulator IC is connected to the output of the full-bridge rectifier. The output of the 7812 Voltage regulator is connected to the relays of the transfer switch. In the transfer switch one relay connects the inverter inputs with the battery terminals while the other relay connects the load with the Mains Supply. The inverter inputs are connected to the battery terminals via NC point of the respective relay while load is connected to the Mains Supply via NO point of the respective relay.
Fig. 6:Prototype of Offline UPS built using Square Wave Inverter and Lead Acid Battery Charger
In the offline UPS based on Modified Sine wave inverter, circuit connections are same as that of the offline UPS based on Square Wave Inverter except that the inverter circuit used is a Modified Sine Wave Inverter.
Fig. 7: Prototype of Offline UPS built using Modified Sine Wave Inverter and Lead Acid Battery Charger
In the circuits, a green LED is connected between the primary coil of the relaay connecting the battery and the inverter. The green LED indicates the presence of mains power or normal mode, whenever mains power fails then this green LED turns OFF. The red LED then turns ON and this indicates the inverter mode. The red LED is connected between the common ground and the positive input of the inverter. It indicates that the inverter is now connected with the battery and drawing power from it to supply AC to the load.
Fig. 8: Image showing blue LED glowing in Square Wave Inverter based Offline UPS
图9：Image showing red LED glowing in Square Wave Inverter based Offline UPS
Fig. 10: Image showing blue LED glowing in Modified Sine Wave Inverter based Offline UPS
Fig. 11: Image showing red LED glowing in Modified Sine Wave Inverter based Offline UPS
Here both the LEDs consumes around 20mA current which is sufficient enough for decent brightness of the LEDs. For limiting the current, the current limiter resistances R1 and R2 are connected in series with the LEDs. Let’s calculate the values of R1 and R2 as per the current requirement.
Calculating value of resistance R1
The Green LED gets power by the 7812 IC so input voltage of green LED is 12V.
(Input voltage of green LED), Vg = 12V
As per the ohms law, Vg = Ig* R1
(desired current for green LED), Ig = 20mA
By putting all the values in the above equation
12 = 0.02* R1
R1 = 600E
Calculating value of resistance R2
The Red LED gets power by the battery and the battery is rated for 14.4V so input voltage of red LED is 14.4 V.
(Input voltage of red LED), Vr = 14.4V
再次应用欧姆法律，
Vr= Ir* R2
(desired current for green LED), Ir = 20mA
14.4 = 0.02* R2
R2 = 720E
While assembling these circuits, the following precautions must be taken care of –
1. Make the proper connection of relay, do not connect common pin of RL1 relay to other relay RL2. As this will short the ground of DC and to one of the AC lines which may cause a serious damage to the surrounding as well as the circuit.
2.请勿在输出处连接DC负载，否则负载可能会损坏。
3. Never common the line of the mains AC and AC from the inverter when both are ON. This will cause a serious damage to the surroundings.
4. Diode D1 should be used as a Protection diode so that it can prevent the external capacitor from discharging through the voltage regulator IC during an input short circuit. This will save the 7812 IC from the back current.
5. The Flyback diode (D2 and D3 in the circuit diagram) should be used across both the relays for preventing the circuit from any back current.
6. The output of voltage regulator IC should be equal or greater than the voltage rating of the relay. Otherwise, the relay will not be activated.
7. Do not exceed the input voltage limit of the voltage regulator IC as this can damage the IC. For this refer to the datasheet of the IC.
8. The capacitor C1 at the input of voltage regulator can handle mains noise. The ceramic capacitor C2 in parallel with this capacitor C1 is used to reduce the overall ESR.
9.调节器输出处的电容器C3保持输出处的快速瞬态变化和噪声。
10. The capacitor used in the circuit must be of higher voltage rating than the input supply voltage. Otherwise, the capacitor will start leaking the current due to the excess voltage at its plates and will burst out.
11. Make sure the filter capacitor should be discharged before working on a DC power supply. For this, short the capacitor with a screwdriver wearing insulated gloves.
How the circuit works –
The offline UPS are one of the simple and cheap UPS among other UPS designs. The offline UPS are also known as standby UPS. If any system is in its standby mode that means that system is not performing its primary function but it is also not completely cut off from the power supply line. This kind of system consumes little power in standby mode as in the case of Offline UPS. The Offline UPS is not always connected to the AC load and does not provide continuous power to the load appliance which is the primary function of the UPS. But in this state, they are consuming some power from the mains supply for charging the battery used as backup. That’s why they are also known as Standby UPS.
在离线UPS中，交流负载直接连接到主电源线（如框图中所示）。这些UPS监视电源线电压，并且每当电压降低到某个预设电压时，电池就会开关到逆变器。同时，将设备从电源线断开，然后切换到逆变器，然后逆变器为负载设备提供功率。每当电源供应返回时，负载设备都会再次切换回电源电源，并且逆变器与电池断开。因此，在这些UPS中，负载设备在电力故障期间以及在电源返回过程中再次会中断。根据所使用的传输开关，中断可以从几毫秒到微秒。
在这个离线UPS设计、转换开关是used for switching mechanism. The transfer switch can be static bypass switch or mechanical switch. In the circuits designed above, the mechanical switches i.e. relays are used. There are two relays used to make the transfer switch. One of the relay is used for switching the battery and the inverter. Another relay is used for switching the appliance from the mains power line and the inverter. As the relay is of 12V DC rating, for supplying a constant power to the relay, 7812 voltage regulator is used which can provide a constant 12V for driving the relays. The power is drawn by the transfer switch from the AC mains. The AC mains is rectified and stepped down to 18V level using a full-bridge rectifier circuit and the 18V DC is then regulated to 12V DC using the 7812 voltage regulator.
When mains power is present then both the relays get energize by the 7812 IC and the relays common pin(C) then get connected to the Normally open pin(NO). The inverter inputs are connected to the battery via NC point, so the inverter gets disconnected from the battery. The load is cnnected to the AC mains via NO point of the other relay, so it gets power directly from the AC mains while inverter is not drawing any power from the battery.
When mains supply goes out then the rectifier turns off which further shut down the 7812 IC. So both the relays now get de-energized and the common pin of both the relays switch to the (NC)normally closed pin. This connects the battery to the inverter and the load appliance to the secondary source i.e inverter. Whenever mains power return to the normal mode then again the load appliance switches back to the mains supply.
Testing the Circuits –
在离线UPS中，逆变器的输出波形和电源电源之间仍然存在短暂的时间差。这是一个重要因素，需要在离线UPS设计中确定。一般图显示了当电源故障失败时负载的过渡时间如下 -
Fig. 12: Graph showing transient time difference in offline UPS output waveform
Practically, the transient time of the load when the mains power fails can be measured by observing the input waveform of the load on a cathode ray oscilloscope (CRO).
As the Offline UPS has been designed using both square wave inverter as well as Modified wave inverter, transient time for both circuits have been observed. The following graph shows the switching time when mains power shuts down and the load is connected with the Square Wave Inverter.
Fig. 13:Graph showing Transient Time in Voltage waveform of Square Wave Inverter based Offline UPS Circuit on Power Shutdown
在图中，可以观察到有一个时间段，负载电压为零（大约）。这次是负载的瞬态时间，大约是200毫秒。下图显示了关闭后电源电源恢复并再次与主电源连接的切换时间。
Fig. 14: Graph showing Transient Time in Voltage waveform of Square Wave Inverter based Offline UPS Circuit on Power Resumption
There is a transient time of 3 ms on power resumption as observed from the voltage waveform on the CRO. The time from switching the load from the inverter to mains AC is less as compare to switching from mains to the inverter. This is due to the fact that the square wave inverter takes some time to give output voltage.
The following graph shows the switching time when mains power shuts down and now the load is connected with the Modified Sine Wave Inverter.
Fig. 15:Graph showing Transient Time in Voltage waveform of Modified Sine Wave Inverter based Offline UPS Circuit on Power Shutdown
在图中，可以观察到有一个时间段，负载电压为零（大约）。This time is the transient time of the load and it is around 1.8 s. The following graph shows the switching time when mains power recovers after a shutdown and the load is again connected with the mains power.
Fig. 16:图显示了经过修改的正弦波逆变器的电压波形中的瞬态时间，基于电源恢复时电路UPS电路
This time is the transient time of the load and it is around 3ms. The time from switching the load from the inverter to mains AC is less as compare to switching from mains to the inverter. This is due to the fact that the Modified sine wave inverter takes some time to give output voltage.
The other important factor in the UPS design isits efficiency. The efficiency of the UPS is same as of the inverter used in it. For square wave inverter based offline UPS, the efficiency was 52 percent while the modified sine wave inverter based UPS had an efficiency of 40 percent only.
离线UPS具有自己的优势。它需要较少的工作温度的零件。设计离线UPS的价格较低。整流器的瓦数仅减少，因为它仅为电池提供电源。低成本的另一个原因是，逆变器并非始终为负载提供电力。UPS并非连续，因此由于低热量耗散，不需要典型的冷却过程。因此，与在线相比，这些因素降低了离线UPS的成本。电池充电器的功率处理能力降低。由于逆变器并非始终存在，因此离线UPS的效率会提高。离线UPS的功耗低，这降低了UPS的长期成本。
也有一些缺点的离线UPS system. First, it has a finite switching time from load to inverter(inverter mode) and again from the load to the AC line(normal mode). The output of the Offline UPS is not reliable. It does not have protection from dropout, voltage spikes, and brownouts as the load is directly connected to the mains supply when mains power is present. Due to voltage distortion in AC line, the battery is used frequently. There is no isolation between input and output due to the direct connection of the load to the mains supply. there can be variable output frequency as the load is switching between mains and inverter so the frequency from both the power source may vary in a configurable range. There is no protection of the load from the unwanted voltage spikes and ripples when the load is connected with mains line. There can be unregulated output voltage as the load switches between direct AC and the inverter.
The Offline UPS can be used with devices which can afford power interruption like in printers, scanners and home desktop. It can also be used as emergency power supply at home.
This completes the UPS design series. Also, check out the following tutorials which may help in assembling the UPS circuit in a better way –
1) High and Low Side Switching of MOSFET
2) Testing IR2110 IC
3) Improving Switching Time of Relay
4) Testing MOSFET
5) Bootstrap circuit for High side MOSFET
