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3. BLOCK DIAGRAM OF 12V DC BOOST REGULATOR
The figure shows the simple block diagram of the 12V dc boost regulator. Each block can be separately assembled and checked. The functioning of each block is discussed below.
Figure 3.1 Block Diagram of 12V DC Boost Regulator
3.1 DC SUPPLY
It is the external dc input applied to the circuit. The boost regulator circuit corrects variations in the supply voltage. Additional current is drawn by the boost circuit to get the required output voltage.
3.2 LOW VOLTAGE PROTECTION CIRCUIT
The battery low voltage protection circuit is designed to protect the battery from being discharged too far. The circuit works by comparing a sample of the battery voltage to a reference voltage. The LM339 comparator is used to compare the battery voltage to the reference provided by the LM3524D. The controller circuit is shut down when a low voltage condition has been detected.
The LM339 consist of four independent voltage comparators designed to operate from single power supply over a wide voltage range. Device is ideal for system applications where it is desired to switch a node to ground while leaving it totally unaffected in the OFF state.
A simple battery voltage monitor circuit is used to monitor low battery conditions. The low voltage protection circuit shuts down the switching regulator IC in the event that battery voltage falls below a minimum level. A low voltage indication LED will glow in this case. The protection voltage is jumper selectable to 9, 10 or 11 V. The circuit uses an LM339 quad comparator in conjunction with a +5 V dc reference voltage provided by the controller IC. When the protection circuit is tripped, the supply boost function is disabled and battery voltage is present at the output of the supply. A reset of the battery protection circuit is accomplished by cycling the power switch.
3.3 SWITCHING REGULATOR IC
The switching regulator is the heart of the circuit. The regulator uses pulse width modulation to vary how long the switching transistors stay on for each switching cycle. By adjusting the pulse width of the switching transistors, the output voltage can be kept at a constant level. The switching regulator monitors the output through a voltage divider.
The controller used in this supply is LM3524D. The LM3524 Regulating Pulse-Width-Modulator is commonly used as the control element in switching regulator power supplies. The LM3524D family is an improved version of the industry standard LM3524. The LM3524D uses pulse width modulation to control the time that switching transistors are turned on.
3.4 SWITCHING TRANSISTORS
Switching transistors alternatively switch the legs of the primary winding of switching transformer to ground, creating an ac flux waveform in the transformer. The input to the switching transistors is given from the switching regulator IC, through Darlington pair for better impedance matching.
Switching transistors used must be high frequency transistors. Here we use IRF 3205 MOSFET transistors for switching. It has a ruggedized device design. This benefit, combined with the fast switching speed makes the device suitable for the purpose.
3.5 SWITCHING TRANSFORMER
A transformer is a static device that transfers electrical energy from one circuit to another through inductively coupled conductors, the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF) or "voltage" in the secondary winding. This effect is called mutual induction.
If a load is connected to the secondary, an electric current will flow in the secondary winding and electrical energy will be transferred from the primary circuit through the transformer to the load. In an ideal transformer, the induced voltage in the secondary winding (Vs) is in proportion to the primary voltage (Vp), and is given by the ratio of the number of turns in the secondary (Ns) to the number of turns in the primary (Np) as follows:
NS/NP=VS/VP
By appropriate selection of the ratio of turns, a transformer thus allows an alternating current (AC) voltage to be "stepped up" by making Ns greater than Np, or "stepped down" by making Ns less than Np..In the vast majority of transformers, the windings are coils wound around a ferromagnetic core, air-core transformers being a notable exception.
The transformer is based on two principles: first, that an electric current can produce a magnetic field (electromagnetism), and, second that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction). Changing the current in the primary coil changes the magnetic flux that is developed. The changing magnetic flux induces a voltage in the secondary coil.
It is common in transformer schematic symbols for there to be a dot at the end of each coil within a transformer, particularly for transformers with multiple primary and secondary windings. The dots indicate the direction of each winding relative to the others. Voltages at the dot end of each winding are in phase; current flowing into the dot end of a primary coil will result in current flowing out of the dot end of a secondary coil.
The positive battery terminal is connected to the center tap of the primary of the switching transformer T1. The secondary of T1 is also a center tapped winding, with its center tap also attached to the battery voltage. The voltages seen on the secondary legs of T1 are the battery voltage plus the voltage of the transformer windings. This configuration allows the transformer to supply only the difference between the output and battery voltages. In addition, the power requirements of the transformer and switching transistors are reduced. This also allows battery voltage to be present at the output of the supply when it is switched off.