**Introduction**

**Bridgeless Boost Converter **is used to convert the AC (Alternating Current) voltage to DC (Direct Current) voltage without using bridges. In normal boost converter only the variable DC voltage is converted to required DC voltage, but here AC is directly converted to DC.

**Battery Charging.**

There are three stages of charging a battery.

- Bulk Charging State: The charger provide constant current around 10% to 30% of the AH rating at 14.8V (AGM batteries). This stage is used to charge the battery to 80%.
- Absorption Charging: The charger provides constant voltage around 14.4 (AGM batteries). This stage provides the remaining 20%.
- Float Charging: The charger provides constant voltage of around 13.5V to 13.8V (AGM batteries). This stage maintains the battery at 100%.

Project details is described below..

I did a lot of research on the internet I could not find how to successfully implement a battery charger using the bridgeless boost converter topology (using h bridge). To effectively charge a battery, the charger should be able to;

- Provide a constant current at 14.8V while the current drops to minimal (bulk charging stage)
- Provide a constant voltage of 14.4V while the current drops to zero (absorption charging stage)
- Provide a constant voltage of 13.8V while the current is maintained at zero (float charging stage)

All the resources I saw, only made emphasis on how to increase the output voltage of the converter by changing its duty cycle. While some literature showed that the maximum duty cycle is 50%, others claimed that it can go higher than 50%. Based on these information only the bulk and float charging stage can be achieved (i.e. changing the duty cycle to generate different voltages). This means that the battery can only be 80% charged.

The big question now is, **how is the bridgeless converter used in implementing the three stage charging in power inverters?**

**Change the duty cycle to generate the required voltage****Change the switching frequency to generate the required current**

**Challenges**

- How to provide constant current at 14.4V for bulk stage. The input of the converter is not stable or constant, this is due to the voltage changes in mains especially in Nigeria.
- How to provide the constant voltage for the absorption stage.
- How to provide the constant voltage for the float stage.

** ****Solutions**

**Providing constant current at 14.8V (Bulk stage)**

Recall equation 1.

Vout = Vin / (1 – D)

D = 1 – Vin/Vout ……equation 2

Vout = output voltage of the converter (i.e bulk stage voltage)= 14.4V

Vin = input voltage of the converter = output voltgae of the transformer = ?

D = duty cycle = ?

But recall that transformer

Np/Ns = Vp/Vs = TR (transformation ratio) ………………………. equation 3

Np= primary turns

Ns = secondary turns

Vp = primary voltage = 220V

Vs = secondary voltage = 6V

Therefor

TR = 220/6 = 36.67. …….equation 4

**Note**

Vin of the converter is the Vout of the transformer, we cannot assume its 6V since the mains voltage cannot be constantly 220V, and we cannot measure the Vout of the transformer. Therefore it is safe to measure the Vin of the transformer (mains voltage) and estimate the Vout of the transformer using it transformation ratio.

TR = Vp/Vs = 36.67

Vs = Vp/TR = Vp/36.36

Where Vp (Vin transformer) = Vmains …..equation 5

Vin (converter) = Vs(Vout transformer) ….equation 6

Vin(converter) = Vs(Vout transformer) =Vmains /TR … equation 7

Therefore

Substitute equation 5 and 6 in equation 2

D = 1 – Vin(converter)/Vout(converter)

D = 1- (Vmains/TR) / Vout (converter)

D = 1 – Vmains / (TR * Vout ) …… equation 8

Equation 8 is the equation we need to generate the 14.8V needed for the bulk stage.

Where Vmain is the mains voltage and Vout is 14.8V.

To generate the required current we simply change the switching frequency. As at the time I am compiling this literature I have not found any mathematical equations relating switching frequency to current.

I had to practically measure the charging current for different frequencies and different voltages.

SN |
Switching Frequency in KHz |
Current in Amp |
Mains Voltage in Volts |

1 | 100.00 | 20.00 – 25.00 | 200.00 – 238.00 |

2 | 90.00 | 25.00 – 30.00 | 200.00 – 238.00 |

3 | 65.00 | 30.00 – 35.00 | 200.00 – 238.00 |

4 | 35.00 | 35.00 – 40.00 | 200.00 – 238.00 |

5 | 20.00 | 40.00 – 45.00 | 200.00 – 238.00 |

6 | 10.00 | 45.00 – 50.00 | 200.00 – 238.00 |

7 | 5.00 | 50.00 – 60.00 | 200.00 – 238.00 |

Table 1. Current Measurement at different switching frequencies at constant Voltage

SN |
Switching Frequency in KHz |
Current in Amp |
Mains Voltage in Volts |

1 | 100.00 | 20.00 – 25.00 | 225.00 – 248.00 |

2 | 90.00 | 20.00 – 25.00 | 215.00 – 224.00 |

3 | 20.00 – 25.00 | 20.00 – 25.00 | 195.00 – 214.00 |

4 | 35.00 | 20.00 – 25.00 | 175.00 – 194.00 |

5 | 20.00 | 20.00 – 25.00 | 155.00 – 174.00 |

6 | 10.00 | 20.00 – 25.00 | 135.00 – 154.00 |

7 | 5.00 | 20.00 – 25.00 | < 135.00 |

**Table 2.** Current Measurement at different switching frequency and different voltages

From table 1 and 2, it is observed that changing the switching frequency affects the current in a boost converter, like I said earlier I have no found any mathematical equation to prove that yet.

The bulk charging state is completed when the battery voltage gets to 14.8V. , and the current is at minimum.

**Providing constant voltage of 14.4 (absorption charging stage)**

To generate the constant voltage of 14.4V for the absorption charging stage, we need to measure the output of the converter and adjust the duty cycle, i.e. we increase the duty cycle when Vbat (battery voltage or converter output voltage) is less than 14.4V or decrease the duty cycle when Vbat is less than 14.4V until the current falls to zero.

**Note.**

I saw some equation that estimate a time required for the absorption stage to complete. So rather than wait for the current to fall to zero, we just wait for the time to elapse.

2.**Providing constant voltage of 13.5V (float charging stage)**

This stage is similar to the stage above (absorption charging stage). This difference is that the stage is endless. The converter output voltage is maintained at 13.5V.

Fig 4:Boost charging Test part oneFig 4:Boost charging Test part two

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## gamefly

Hi there just wanted to give you a quick heads up and let you know a few of the images aren’t loading correctly.

I’m not sure why but I think its a linking issue. I’ve tried it in two different web browsers and both show

the same results.