Block Diagram of Medical Equipments Power supplyInput Processing Output Device Unit Device Control
• The electric power is not normally used in the form in which it is produced or distributed.• Practically all electronic systems require some form of Power conversion.• A device that transfers electric energy from a source to a load using electronic circuits is referred to as power supply.• A typical application of a power supply is to convert utility AC voltage into regulated DC voltages required for electronic equipment.
Categories of Power SuppliesThere are two broad categories of power supplies:• Linear regulated power supply• switched mode power supply (SMPS) In some cases one may use a combination of switched mode and linear power supplies to gain some desired advantages of both the types.
Block diagram of power supplyParts of a power supply:
• The AC voltage is connected to a transformer, which steps that ac voltage down to the level for the desired dc output.• A diode rectifier then provides a full-wave rectified voltage.• This is initially filtered by a simple capacitor filter to produce a dc voltage.• This resulting dc voltage usually has some ripple or ac voltage regulation.• A regulator circuit can use this dc input to provide a dc voltage that not only has much less ripple voltage but also remains the same dc value even if the input dc voltage varies somewhat or the load connected to the output dc voltage changes.• This voltage regulation is usually obtained using one of the voltage regulator IC units.
Transformer• Transformer convert Ac electricity from one voltage to another with little loss of power.• Transformers work only with AC & this is one of the reasons why mains electricity is AC.
TYPES OF TRANSFORMER• Step-up Transformer• Step-down Transformer
• Step-up transformers increase voltage, step-down transformers reduce voltage.• The input coil is called the primary & the output coil is called the secondary.• There is no electrical connection between the two coils, instead they are linked by the alternating magnetic field created in the soft iron core of the transformer
• The two lines in the middle of the circuit symbol represent the core.• Transformers waste very little power , so the power out is almost equal to the power in. So, as voltage is stepped down current is stepped up.
RECTIFIER• In mains supplied electronic systems the AC input voltage must be converted into a DC voltage with the right value & degree of stabilization.• Rectifier does this work.• In other words a rectifier circuit is necessary to convert a signal having zero average value into one that has a nonzero average.• Two types of rectifiers : a. Half wave rectifier. b. Full wave rectifier.
• Figure above uses a center-tapped transformer with two rectifier diodes.• Figure below uses a simple transformer & four rectifier diodes usually known as a bridge rectifier.
SMOOTHING/FILTER• We need a way to smooth out the pulsations& get a much cleaner dc power source for the load circuit.• This is done by a filter circuit.• In power supply, a filter must remove or reduce the ac variations while still making the desired dc available to the load circuitry.• Any given filter involve capacitors, inductors,&/resistors in some combination.
Types of filters• Capacitor Filter : Used to obtain essentially a dc voltage with some ripple.• RC Filter : It is possible to reduce the amount of ripple across a filter capacitor by using an additional RC filter .• LC Filter : This reduces Hum & ripple (Used where noise should be reduced).
CAPACITOR FILTER• Smoothing is performed by a large value electrolytic capacitor connected across the DC supply to act as a reservoir, supplying current to the output when the varying DC voltage from the rectifier is falling.• The diagram shows the unsmoothed varying DC (dotted line) & the smoothed DC (solid line).• The capacitor charges quickly near the peak of the varying DC, & then discharges as it supplies current to the output.
Note: The smoothing significantly increases the average DC voltage to almost the Peak value(1.4×RMS value).Ex: A 6V RMS AC is rectified to Dc of about 4.6V RMS, with smoothing this increases to almost giving 1.4×4.6=6.4V smooth DC.
• Smoothing is not perfect due to the capacitor falling a little as it discharges, giving a small ripple voltage.• For many circuits a ripple which is 10% of the supply voltage is satisfactory & the equation below gives the required value of the smoothing capacitor. C= 5×Io ÷ Vs × f• A large capacitor will give less ripple .• Capacitor value must be doubled when smoothing half-wave DC.
RC filterIn order to reduce the ripple still more , we need to extend the filter a bit.
LC filterRC filter reduces ripple voltage with lot of resistivelosses, we can replace R with L as shown below.
REGULATOR• Regulator - eliminates ripple by setting DC output to a fixed voltage• Voltage regulator IC’s are available with fixed (typically 5,12 &15V) or variable output voltages.• They are also rated by the maximum current they can pass.• Negative voltage regulators are available, mainly for use in dual supplies.• Many regulators include some automatic protection from excessive current (overload protection) & overheating (thermal protection).
Many of the fixed Voltage regulator IC’s have threeleads & look like power transistors, such as shown here. They include a hole for attaching a heat sink if necessary.
The regulated DC output is very smooth with no ripple. It is suitable for all electronic circuits.
Zener diode Regulators• For low current power supplies a simple voltage regulator can be made with a resistor & a zener diode connected in reverse as shown in the diagram.• Zener diodes are rated by their breakdown voltage Vz & maximum power Pz.• The resistor limits the current (like an LED resistor).• The current through resistor is constant, so when there is no output current all the current flows through zener diode & its power rating must be large enough to withstand this.
• Choosing a zener diode and resistor:• The zener voltage Vz is the output voltage required• The input voltage Vs must be a few volts greater than Vz (this is to allow for small fluctuations in Vs due to ripple)• The maximum current Imax is the output current required plus 10%• The zener power Pz is determined by the maximum current: Pz > Vz × Imax• The resistor resistance: R = (Vs - Vz) / Imax• The resistor power rating: P > (Vs - Vz) × Imax
• Example: output voltage required is 5V, output current required is 60mA. Vz = 4.7V (nearest value available)• Vs = 8V (it must be a few volts greater than Vz)• Imax = 66mA (output current plus 10%)• Pz > 4.7V × 66mA = 310mW, choose Pz = 400mW• R = (8V - 4.7V) / 66mA = 0.05k = 50 , choose R = 47• Resistor power rating P > (8V - 4.7V) × 66mA = 218mW, choose P = 0.5W
DUAL SUPPLIES• Some electronic circuits require a power supply with positive & negative outputs as well zero volts(0V).• This is called “dual supply” because it is like two ordinary supplies connected together as shown in the figure.• Dual supplies have three outputs +V, 0V, -V.
Some Common regulator CircuitsOp-amp based circuit with excellent regulation
Advantages of traditional Power supply• The linear regulator is the building block of nearly every power supply used in electronics.• The IC linear regulator is so easy to use that it is virtually foolproof, & so inexpensive that it is usually one of the cheapest components in an electronic assembly.
• Power supply discussed till now are also known as linear regulators and maintains desired output voltage by dissipating excess power .• Thus for proper operation of these supplies heat sinks are a must.• The power dissipated by a 15V regulator isPower dissipated=(dropout voltage) (current) =(18.95-15) (0.5)=1.98W• The power dissipated by a 5V regulator isPower dissipated=(8.91-5) (1.0)= 3.91W
Defects in the Traditional Power Supplies• Efficiency is very low save 45% to 55% only,
• A large amount of energy is wasted. 1. Unstabilized D.C voltage / current should be greater than the stabilized voltage / current. So, the energy loss is in the form of heat due to the power transistor, etc. 2. Energy loss in the form of heat etc. is in the main transformer. a. Due to eddy current loss. b. Copper loss.• The size & weight of the transformer is also large for high current. In transformer, number of turns is inversely proportional to frequency. So less frequency means more turns & high frequency means less turns.
• Very large value of capacitor is needed for reservoir for high current say 10,000 mfd to 50,000 mfd. Similarly at low frequency, capacitor value will be increased( so size will increase). At high frequency, capacitor value will decrease & hence size will also decrease.• The low frequency ripple from the main is always difficult to remove completely, even large capacitors are used.• Get shock from the chassis if accidentally touch it.
Answers to many of the problems formaking high current, lowdissipation, light weight, less spaceand without low frequency ripple Is….Switched Mode power Supply
• The SMPS owes its name to dc-to-dc switching converter for conversion from unregulated dc input to regulated dc output voltage.• Typical frequency range of SMPS is from 50 kHz to several MHz.
Requirements of SMPS• Less bulky Transformers• Less Energy loss / Eddy currents• Capacitors of small size
Switched-mode power supply• The input supply drawn from ac mains is first rectified & filtered using a capacitor.• The unregulated dc voltage across the capacitor is then fed to a high frequency dc- to dc converter.• Most of the dc-to-dc converters used in SMPS circuits have an intermediate high frequency ac conversion stage to facilitate the use of a high frequency transformer for voltage scaling & isolation.
Block diagram of SMPS High freq. dc to dc converterAC dc to ac ac to dc converter convertermains
DC to DC conversion in SMPS• One method is to use a High frequency transformer for voltage scaling & isolation.• Another popular method is to use simplified switching circuits that omits the transformer action. In such SMPS, the unregulated input dc voltage is fed to a high frequency chopping circuit which switches between “ON” & “OFF” states.
Types of SMPSSMPS can be classified into four types according to the input & output wave forms as follows:• AC in, DC out: rectifier, off-line converter input stage.• DC in, DC out: Voltage converter or current converter, DC to DC converter.• AC in, AC out: Frequency changer or cyclo converter.• DC in, AC out: Inverter
AC in DC out (Ordinary SMPS)This type of power supply previously used in TV receivers / instruments having used the main at full voltage to provide a power for an oscillator / inverter whose output is in turn is fed to high frequency transformer, then to rectifier, switcher, stabilizer and then to regulator. The correct of the error signal and output signal is also done by negative feedback loop from output to the input switch.
Input Rectifier• The first stage is to convert AC input to DC output (Rectification).• The rectifier produces an unregulated DC voltage which is sent to a large filter capacitor.• The current drawn from the mains supply by this rectifier circuit occurs in short pulses around the AC voltage peaks.
• These pulses have significant high frequency energy which reduces the power factor.• Special control techniques can be employed by the following SMPS to force the average input current to follow the sinusoidal shape of the AC input voltage thus the designer should try correcting the power factor.
INVERTER• This converts DC, Whether directly from the input or from the rectifier, to AC by running it through a power oscillator, whose output transformer is very small with few windings at a frequency of tens or hundreds of kHz.• The frequency is usually chosen to be above 20 kHZ, to make it inaudible to humans.
OUTPUT TRANSFORMER• If the output is required to be isolated from input, (as is usually the case in mains power supplies) the inverted AC is used to drive the primary winding of a high- frequency transformer.• This converts the voltage up or down to the required output level on its secondary winding.
Output rectifier & filter• If a DC output is required, the AC output from the transformer is rectified.• For output voltages above ten volts or so, ordinary silicon diodes are commonly used.• For lower voltages Schottky diodes are commonly used as rectifier elements (they have faster times than silicon diodes, thus allowing low-loss operation at higher frequencies).• For even lower output voltages, MOSFETs may be used as synchronous rectifiers.
• The rectified output is then smoothed by a filter consisting of inductors & capacitors.• For higher switching frequencies, components with lower capacitance & inductance are needed.
Main High Rectifying & OutputVoltage Oscillator frequency Smoothing Switching SmoothingSupply transformer Control Signal Block Diagram of Switch Mode Power Supply
SMPS having opto-isolator in the feedback loop. Main Inverter High Rectifying & 15 V Supply Oscillator Frequency Smoothing transformer Output Opto Isolator Transformer Control Signal Feedback LineBlock Diagram of SMPS Using Opto Isolator Transformer
Features & drawbacks• A lamp from output to Optocoupler.• When Optocoupler fails, the light will fall on the photocell, the mains will flow through feedback circuit & reflect in output. Hence a shock hazard.• So, now SMPS using pulse transformer. Feedback will have mark& space modulation.
Other types of feedback loops in SMPS• SMPS having isolation Pulse High Rectifying & Output transformer Main Supply Amplifier Frequency Transformer Smoothing (small size) in the feedback loop with pulse Pulse Transformer Pulse Width Modulated Oscillator. width modulator. Block Diagram of S.M.P.S Using Pulse Transformer
SMPS having a single package (single IC) is presently used for electronic circuitry supply. T1 (20 KHz) ~ + Rectifier 2 20 KHz ~ + ~ -230 V SwitchedAC + Mode + RegulatedMains Rectifier 1 Voltage Load DC OutputInput - C1 Regulator - Voltage ~ - Switch Mode D.C. Regulated Power Supply
Different Topologies of SMPS• fly-back• forward• push-pull• C’uk• Sepic• Half bridge• H-bridge circuits.
• A particular topology may be more suitable than others on the basis of one or more performance criterions like cost, efficiency, overall weight and size, output power, output regulation, voltage ripple etc.• All the topologies listed above are capable of providing isolated voltages by incorporating a high frequency transformer in the circuit.
Popular types of SMPS Non Isolated SMPS:• Non isolated power supplies contain an inductor instead of a transformer. This type includes Boost converters Buck converters Buck-boost converters• These belong to the simplest class of single input, single output converters which utilize one inductor & one active switch (MOSFET).
Buck converter• This reduces the input voltage, in direct proportion to the ratio of the active switch “on” time to the total switching period, called the duty cycle. For ex: An ideal buck converter with a 10V input operating at a 50% duty cycle will produce an average output voltage of 5V.• A feed back control loop is employed to maintain (regulate) the output voltage by varying the duty cycle to compensate for the variations in the input voltage.
Boost & Buck-boost converter• The output voltage of a boost converter is always greater than the input voltage.• The buck-boost output voltage is inverted but can be greater than, equal to, or less than the magnitude of its input voltage.• There are many variations & extensions to this class of converters but these three form the basics of almost all isolated & non isolated DC to Dc converters.
Other types of SMPS• By adding a second inductor the Cuk & SEPIC converters can be implemented or by adding additional active switches various bridge converters can be realised.• Other types of SMPSs use a capacitor-diode voltage multiplier instead of inductors & transformers.• These are mostly used for generating high voltages at low currents.• The low voltage variant is called charge pump.
Chopper controller/Regulation• A feedback circuit monitors the output voltage & compares it with a reference voltage, which is set manually or electronically to the desired output.• If there is an error in the output voltage, the feedback circuit compensates by adjusting the timing with which the MOSFETs are switched on & off.• This part of the power supply is called the switching regulator.
• Depending on design/safety requirements, the controller may or may not contain an isolation mechanism (such as opto-couplers) to isolate it from the DC output.• Switching supplies in Bio-medical instruments have these opto-couplers to tightly control the output voltage & isolation of patient .• Open regulators do not have a feedback circuit. Instead, they rely on feeding a constant voltage to the input of the transformer or inductor, & assume that the output will be correct.
Advantage & disadvantage of SMPSAdvantage:• Greater efficiency because the switching transistor dissipates little power in the saturated state & the off state compared to the semiconductor state.• Small size & lighter weight ( elimination of low frequency transformers)& low heat generation.Disadvantage:• Greater complexity• Generation of high amplitude, high frequency energy that a low pass filter blocks.
• Due to large electronic circuitry and feedback from output to input it become difficult to repair as compare to ordinary Power supplies.• The Transformer is small in size but it is tuned to particular high frequency (20 kc/s – 50 kc/s). So, whenever the transformer gone defective, it must be replaced with the original one.• The output load current / voltage must not be more than 15% up or down the mention standard value.
Application of SMPS in Biomedical circuits• Isolation of patients in diagnostic instruments like ECG etc. (using opto- couplers in control circuitry)• SMPS battery chargers for battery based instruments.
• To avoid shocks, in SMPS the input is isolated from the output. This is achieved at two places in SMPS. Once at mains & another at feedback using either transformer or optocoupler.• +ve feedback for oscillation & -ve feedback for stabilization.• To reduce losses, quality of transformer must be better.• At high frequency, no iron core will be used. Ferrite core is used. Basically small particles of ferrite are bound under pressure, so no shock hazard.• Now Integrated SMPS are available in LSI & MSI packages (1394)
Comparison of LPS & SMPS• Unlike a linear power supply, the pass transistor of a switching mode supply switches very quickly (typically between 50 kHz and 1 MHz) between full-on and full-off states, which minimizes wasted energy. Voltage regulation is provided by varying the ratio of on to off time. In contrast, a linear power supply must dissipate the excess voltage to regulate the output. This higher efficiency is the chief advantage of a switched-mode power supply.• Switching regulators are used as replacements for the linear regulators when higher efficiency, smaller size or lighter weight are required. They are, however, more complicated, their switching currents can cause electrical noise problems if not carefully suppressed,
• SMPS is an electronic power supply unit (PSU) that incorporates a switching regulator.• A linear regulator maintains the desired output voltage by dissipating excess power in a “pass” power transistor, the SMPS rapidly switches a power transistor between saturation (full on) & cut off (completely off) with a variable duty cycle whose average is the desired output voltage.• The resulting rectangular waveform is low- pass filtered with an inductor & capacitor.