Subject Name: Electronic Circuits Subject Code: 10CS32 Prepared By: Kavyashree.C,Supriya.V.Sullad Department: CSE Date:1/10/2014 12/4/2017

Agenda Introduction Constituents of power supply Designing Mains Transformer Linear IC Voltage Regulators Three terminal Regulators Regulated power supply parameters

Introduction Every electronic system requires one or more DC voltages for its operation. The regulated power supply converts the standard 220 volts, 50 or 60 Hz AC available at wall outlets into a constants DC voltage. The DC voltage produce by a power supply is used to power all the types of electronic circuits, such that television receiver, stereo system, CD players and laboratory equipment.

Constituents of power supply

Transformer: “Downconvert” the AC line voltage to a smaller peak voltage Vm, usually about 2-3 Volts larger than the ultimately desired DC output. Rectifier circuit changes AC to DC. Filter ideally eliminates the fluctuations in the output voltage of a half –wave rectifier and produces a constant-level dc voltage. Regulator circuit is a feedback circuit that ensures output DC does not change from its nominal value. All power supplies have built in protection circuit.

Designing Mains Transformer Transformer does not saturate at desired load current or output power. Ac=√p÷5.6 Ac –core cross section in square inches 2. The ratio of stack thickness (t) to the width of center limb (w) should be in the range of 1.1 to 1.5. 3. The turns per volt for different windings can be computed as turns per volt=10^8/(4.44*f*B* Ac )

5. Primary current= p/(efficiency*primary voltage) 4. Primary and secondary turns can be computed from primary and secondary voltages. 5. Primary current= p/(efficiency*primary voltage) 6. Secondary current=primary current/ɳ ɳ= Ns/Np

Linear IC Voltage Regulators

Functional block diagram into 4 blocks: Temparature compensator zener diode: constant current source offer zener diode to operate in fixed point. Error amplifier which controls Q1 which acts as a variable resistor. Series pass resistor: unregulated power supply source is connected to collector. Current limiter Q2: in short circuit condition. Frequency compensation controls frequency response of an amplifier.

High voltage regulator

Low voltage regulator

Three terminal Regulators They do not require any external components. These are available in both fixed and adjustable output voltage(both positive and negative). The well known IC regulators are: 1) The 78XX series - for positive regulators (2) The 79XX series - for negative regulators (3) The LM 317 - for adjustable positive regulators (4) The LM 337 - for adjustable negative regulators

The 78XX series - for positive regulators C1 and C2 are decoupling capacitors. C1 is generally used when regulator is located far from power supply filter.

There is an input, an output and an adjustable terminal. The floating regulator could be made into a variable regulator by replacing R2 with a pot. However, there are several disadvantages: Minimum output voltage is Vreg instead of 0 V. IQ is relatively large and varies from chip to chip. Power dissipation in R2 can in some cases be quite large resulting in bulky and expensive equipment. There is an input, an output and an adjustable terminal.

Adjustable voltage Regulator

Regulated power supply parameters Defines the quality of regulated power supply. Load regulation: change in regulated output voltage of power supply as load current varies from zero to maximum rated value. percentage load regulation= VNL-VFL *100 VFL 2. Line regulation: Variation of regulated output voltage for change in input voltage. line regulation=(0.2/10) *100= 2% 12/4/2017

3. Output Impedance: It determines load regulation of power supply. Regulator circuit is characterized by low output impedence. 4.Ripple rejection factor: Defined as the ratio of ripple in regulated output voltage to the ripple present in unregulated input voltage.

Calculated in decibels. Ripple is nothing but periodic variation in input voltage. V ripple(output)=Vripple(input)/1+loop gain

Switched Mode Power Supplies Linear v/s Switched Mode Power Supplies Linear Power Supplies have good line and load regulation, low output voltage ripple and negligible radio frequency interference. Switching power supplies have much higher efficiency(80-90%) and reduced size and weight for a given power – delivery capability. An improved efficiency and reduced size/weight Efficiency in switching supplies do not suffer as the unregulated input to the regulated output. In portable systems operating from battery packs and requiring higher DC voltages , the Switching supply is the only option.

Switching Regulators

The buck Converter circuit consists of the switching transistor, together with the flywheel circuit (Dl, L1 and C1). While the transistor is on, current is flowing through the load via the inductor L1. The action of any inductor opposes changes in current flow and also acts as a store of energy. In this case the switching transistor output is prevented from increasing immediately to its peak value as the inductor stores energy taken from the increasing output; this stored energy is later released back into the circuit as a back e.m.f. as current from the switching transistor is rapidly switched off.

Transistor Switch ‘on’ Period In Fig. 3.1.2 therefore, when the switching transistor is switched on, it is supplying the load with current. Initially current flow to the load is restricted as energy is also being stored in L1, therefore the current in the load and the charge on C1 builds up gradually during the ‘on’ period. Notice that throughout the on period, there will be a large positive voltage on D1 cathode and so the diode will be reverse biasedand therefore play no part in the action.

Transistor Switch ‘off’ Period When the transistor switches off as shown in Fig 3.1.3 the energy stored in the magnetic field around L1 is released back into the circuit. The voltage across the inductor (the back e.m.f.) is now in reverse polarity to the voltage across L1 during the ‘on’ period, and sufficient stored energy is available in the collapsing magnetic field to keep current flowing for at least part of the time the transistor switch is open.

The back e.m.f. from L1 now causes current to flow around the circuit via the load and D1, which is now forward biased. Once the inductor has returned a large part of its stored energy to the circuit and the load voltage begins to fall, the charge stored in C1 becomes the main source of current, keeping current flowing through the load until the next ‘on’ period begins. The overall effect of this is that, instead of a large square wave appearing across the load, there remains only a ripple waveform, i.e. a small amplitude, high frequency triangular wave with a DC level of: VOUT = VIN x (On time of switching waveform (tON) / periodic time of switching waveform( T))

Therefore if the switching waveform has a mark to space ratio of 1:1, the output VOUT from the buck Converter circuit will be VIN x(0.5/1) or half of VIN. However if the mark to space ratio of the switching waveform is varied, any output voltage between approximately 0V and VIN is

Operating principle The key principle that drives the boost converter is the tendency of an inductor to resist changes in current by creating and destroying a magnetic field. In a boost converter, the output voltage is always higher than the input voltage. A schematic of a boost power stage is shown in Figure 1. (a) When the switch is closed, current flows through the inductor in clockwise direction and the inductor stores some energy by generating a magnetic field. Polarity of the left side of the inductor is positive.

(b) When the switch is opened, current will be reduced as the impedance is higher. The magnetic field previously created will be destroyed to maintain the current flow towards the load. Thus the polarity will be reversed (means left side of inductor will be negative now). As a result two sources will be in series causing a higher voltage to charge the capacitor through the diode D. If the switch is cycled fast enough, the inductor will not discharge fully in between charging stages, and the load will always see a voltage greater than that of the input source alone when the switch is opened. Also while the switch is opened, the capacitor in parallel with the load is charged to this combined voltage. When the switch is then closed and the right hand side is shorted out from the left hand side, the capacitor is therefore able to provide the voltage and energy to the load. During this time, the blocking diode prevents the capacitor from discharging through the switch.

The switch must of course be opened again fast enough to prevent the capacitor from discharging too much.

Boost Regulator

Ideal Waveform

The circuit diagram of a step up operation of DC-DC converter is shown in Figure . When the switch S1 is closed for time duration t1, the inductor current rises and the energy is stored in the inductor. If the switch S1 is openerd for time duration t2, the energy stored in the inductor is transferred to the load via the didode D1 and the inductor current falls. The waveform of the inductor current is shown in figure.

When the switch S1 is turned on, the output voltage is given by: Po=1/2x(L1XIp2 xf)+Vinx(Ip/2)x(tOff/T)                                                                                                                                                      

• The voltage across the load can be stepped up by varying the duty ratio D • The minimum output voltage is Vs and is obtained when D = 0 • The converter cannot be switched on continupusly such that D = 1. For values of D tending to unity, the output becomes very sensitive to changes in D

Connecting Power Converters in Series If power converters are connected in series greater load output can be expected While if one power converter fails to operate ,other will fail and the whole system shuts down

Connecting Power Converters in Parallel When Power converters are connected in Parallel ,the system works even if one circuit fails. While Parallel Converters suffers from unequal Load Sharing.