As the name suggests, Boost Regulator is a power supply component that boosts the input voltage to a desired value. Let us take a alkaline battery which gives a voltage of 1.5V. In case, the circuit has to be operated at 3.3V, we have to use a boost converter in between to boost to a desired voltage of 3.3V. The basic boost converter circuit principle is based on the following circuit:
As shown in the above basic circuit of Boost converter, the main component in the Boost circuit are Inductor, Switch and a diode. The switch must be a low resistance semi-conductor device like a MOSFET.
Operation of the above circuit:
1. When the switch 'S' is closed, the current passes through the inductor through the switch 'S'. The inductor charges with + at the source side and - at the switch side. The inductor generates a magnetic field around it.
2. Once the switch is open, no current passes though switch 'S'. The charged inductor slowly discharges and the current now passes through the Diode 'D' to the load. The resultant current to the load increases as the source will be adding to the inductor current.
3. By the time inductor fully discharges, if the switch is closed, the inductor charges again. As the inductor is not allowed to discharge fully, the voltage across the load seen is sum of source voltage and inductor voltage. This meant that the output voltage is always greater than the source voltage.
4. The capacitor 'C' supplies to the load when the switch is closed. the diode in this case acts as blocking diode not allowing any current from capacitor pass through to the switch side.
5. So, when switch is closed, capacitor supplies to the load and when the switch is open, source+inductor supplies to the load and also charges the capacitor.
6. Input filter requirements are little relaxed as the supply sees a constant inductor load and there isn't discontinuous currents and need for filtering.
The diode used in this circuit is called "fly-back" diode. The fly-back diode is sometimes replaced by another low-resistance switch in low power applications. This is to avoid the diode losses.
The following parameters are to be checked while selecting a Boost converter:
1. Input voltage range
2. Output voltage (fixed/adjustable)
3. Output current
4. Switching frequency
5. Efficiency
6. Maximum switch current
7. Mosfet internal resistance
8. Quiescent current
All the above parameters are available from the datasheet of the boost converter ic.
Some of the important points of boost converter:
1. As the input voltage is reduced for a given fixed output voltage, the current drawn by the switch of the Boost converter increases, in this case the efficiency of the boost converter is less. The below figure from the data sheet of boost converter IC LM2621 illustrates the same:
Operation of the above circuit:
1. When the switch 'S' is closed, the current passes through the inductor through the switch 'S'. The inductor charges with + at the source side and - at the switch side. The inductor generates a magnetic field around it.
2. Once the switch is open, no current passes though switch 'S'. The charged inductor slowly discharges and the current now passes through the Diode 'D' to the load. The resultant current to the load increases as the source will be adding to the inductor current.
3. By the time inductor fully discharges, if the switch is closed, the inductor charges again. As the inductor is not allowed to discharge fully, the voltage across the load seen is sum of source voltage and inductor voltage. This meant that the output voltage is always greater than the source voltage.
4. The capacitor 'C' supplies to the load when the switch is closed. the diode in this case acts as blocking diode not allowing any current from capacitor pass through to the switch side.
5. So, when switch is closed, capacitor supplies to the load and when the switch is open, source+inductor supplies to the load and also charges the capacitor.
6. Input filter requirements are little relaxed as the supply sees a constant inductor load and there isn't discontinuous currents and need for filtering.
The diode used in this circuit is called "fly-back" diode. The fly-back diode is sometimes replaced by another low-resistance switch in low power applications. This is to avoid the diode losses.
The following parameters are to be checked while selecting a Boost converter:
1. Input voltage range
2. Output voltage (fixed/adjustable)
3. Output current
4. Switching frequency
5. Efficiency
6. Maximum switch current
7. Mosfet internal resistance
8. Quiescent current
All the above parameters are available from the datasheet of the boost converter ic.
Some of the important points of boost converter:
1. As the input voltage is reduced for a given fixed output voltage, the current drawn by the switch of the Boost converter increases, in this case the efficiency of the boost converter is less. The below figure from the data sheet of boost converter IC LM2621 illustrates the same:
2. At low load current conditions, the ripple current causes the inductor to discharge more quickly. This is the condition during which the modern step-up converters manage by varying the switching frequency.
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