Download (10 Marks)(June 2013)

UNIT-7
LINEAR POWER SUPPLIES & SWITCHED MODE POWER
1. Explain regulated power supply parameters: Load regulation, line regulation, output impedance.
Ripple rejection factor. Determine the output ripple of a regulated powersupply which provides a
ripple rejection of-80dB and a ripple voltage in the unregulated input were.
(10 Marks)(June 2013)
2. Explain buck regulator boost regulator and inverting regulator with neat diagram.
(10 Marks)(June 2013)
A switching regulator is a circuit that uses a power switch, an inductor, and a diode to transfer energy from
input to output. The basic components of the switching circuit can be rearranged to form a step-down (buck),
step-up (boost), or an inverter (fly back). These designs are shown in Figures 1, 2, 3, and 4 respectively,
where Figures 3 and 4 are the except for the transformer and the diode polarity. Feedback and control
circuitry can be carefully nested around these circuits to regulate the energy transfer and maintain a constant
output within normal operating conditions.
Switching regulators offer three main advantages compared to a linear regulators. First, switching efficiency
can be much better than linear. Second, because less energy is lost in the transfer, smaller components and
less thermal management are required. Third, the energy stored by an inductor in a switching regulator can
be transformed to output voltages that can be greater than the input (boost), negative (inverter), or can even
be transferred through a transformer to provide electrical isolation with respect to the input.
3. Describe the operation of Buck Regulator.
(10 Marks)(Dec 2013)
Switching Regulator:- Commonly used switching regulator configuration includes step-down or back
regulator, step-up or boost regulator and inverting regulator also called back-boost regulator.
Buck regulator or Step-down regulator:The fig(a) shows the basic circuit of step down switching regulators, which is also called back type
switching regulators. It uses an inductror L and series transistor Q1 which acts as a switch. The output is
feedback to error amplifier through potential divider, pulse width oscillator controls the operation of Q1 as
on or off depending on the load requirements.
Consider Q1 = ON, D1 = OFF capacitor charges more, Increasing the output voltage and Indcutor changes
from back e.m.f voltages to forward e.m.f voltage
4.
(10
Marks)(Dec 2013)
5. Define the following. i) Load Regulation ii) Output Impedance iii) Ripple Rejection Factor
(5 Marks)(June 2014)
Load regulation:- Load regulation in is defined as change in regulated output voltage of the power supply
as the load current varies of zero (No-load condition) to maximum rated value (full load condition). It is
usually expressed as percentage of
full - load voltage that it percentage load
regulation
Since load regulation may be expressed as a percentage of no-load voltage.
Line regulation:- Its the degree to which output voltage changes with input (supply) voltage changes as a
ratio of output to input change (for example typically 13V/V11) or the output voltage change over the entire
specified input voltage range (for example 'plus or minus 2% for input voltage between 90V and 260, 50 –
60Hz)
Output Impendance:- Output impedance is an important parameter of a regulated power supply.
•
It determine the load regulation of the power supply.
•
The regulated power supply may be represented by a thevenin's equivallent circuit comprising a
voltage source equal to the open circuit voltage across power supply output terminals in searies with
impendance equal to the output impendance of the power supply.
Ripple rejection factor:- Ripple rejection factor is defined as the ratio of ripple in the regulated output
voltage to ripple presnt is unregulated input voltage that is
6. Explain the construction and working of SMPS and mention different types of switching
Regulators.
(10 Marks)(June 2014)
The working of SMPS, Buck or Step down, Boost or Step up Regulator, Buck Boost or Inverting Regulator.
7. Define the terms load regulation, line regulation and output resistance for a voltage regulator. As
defined in the year June 2011. (10 Marks)(June 2014)
8. Name the constituent parts of a linear regulated power supply. Describe the following
terms.
9. Explain the working of a Buck regulator.
(5 Marks)(Dec 2014)
(5 Marks)(Dec 2014)
Ans.
10. Consider three terminal regulator circuit. Determine Load current, Current through
LM7812 Current through external transistor and Power dissipated in LM7812 with
Vbe=0.7V.
(5 Marks)(Dec 2014)
12. Explain the steps involved in custom design of mains transformer.
(6 marks )(June 2015)
Ans:
•
•
•
•
•
Inclusion of core loss
Selection of operating fluxdensity to optimize total loss.
Multiple winding designs: as inthe coupled-inductor case,allocate the available windowarea
among several windings.
A transformer designprocedure.
How switching frequencyaffects transformer size.
Figure: Transformer design.
13. What are SMPS? Compare linear power supplies with SMPS.
(7 Marks )(June 2015)
D.C. to DC convertor and DC to AC converter belong to the category of switched mode power
supply (SMPS).
• The various types of voltage regulator used in linear power supplies (LPS), fall in the category of
dissipative regulator, as they have a voltage control element usually transistor or zener diode which
dissipates power equal to the voltage difference between an unregulated input voltage and a fixed supply
voltage multiplied by the current flowing through it.
• The switching regulator acts as a continuously variable power converter and hence its efficiency is
negligibly affected by the voltage difference. hence the switching regulator is also known as “nondissipative regulator” in a SMPS,
• The input DC supply is chopped at a higher frequency around 15 to 50KHz using an active device
like the BJT power MOSFET or SCR and the convertor transformer.
• The oscillator in above figure allows the control element to be switched ON and OFF the control
element usually consists of a transistor switch an inductor and a diode.
• For each switch ON, energy is pumped into the magnetic field associated with the inductor which
is a transformer winding in practice this energy is then released to the load at the desirred voltage level.
There are three basic switch regulators
1. Step down or buck switching regulators.
2. Step up or boost switching regulator.
3. Inverting type switching regulator.
14. Explain the working of three terminal voltage regulator.
(7 marks )(June 2015)
• This series is typical of the three - terminal voltage regulator, the block diagram is shown in figure
the built in reference voltage VREF drives the non-inverting input of an op-amp.
• The operational amplifier consists of several large of voltage grain. Because of the high gain of the
op-amp.
• The error voltage between the two input terminals of the op-amp approaches zero. So the inverting
input voltage is approximately equal to VREF.
• This means that current flowing through the potential divider I =VREF/R21
• Incidentally, the prime attached to R21 indicates that this resistor in inside the IC itself rather than
being an external resistor. Since the same current flows through R11 the output voltage is given as Vout =
VREF / R21 (R11 + R21)