ELECTRONICS, PHOTONICS AND MICROSYSTEMS

Andrzej DZIEDZIC, Piotr MARKOWSKI

Autonomous Power Supplying Systems

Topic 3. Practical power supply units

1. Blocks of practical power supply units

2. Transformer

3. Rectifier

4. Smoothing

5. Regulator

6. Pulse Width Modulated (PWM) power supply units

7. Typical block diagram for the energy harvesting circuit

Practical power supply unitsThere are many types of power supply. Most of them aredesigned to convert high voltage AC mains electricity toa suitable low voltage supply for electronics circuits and otherdevices. A power supply can be broken down into a series ofblocks, each of which performs a particular function.

An example – block diagram of a 5 V regulated supply:

Role of particular blocks

a) Transformer - steps down high voltage AC mains to low voltage ACb) Rectifier - converts AC to DC, but the DC output is varyingc) Smoothing - smooths the DC from varying greatly to a small rippled) Regulator - eliminates ripple by setting DC output to a fixed

voltage

Description of particular blocks together with their circuit diagrams and signals of their outputs

Transformer (1)Transformer converts AC electricity from one voltage to another withlittle loss of power. Transformers work only with AC and this is one ofthe reasons why mains electricity is AC.

Step-up transformers increase voltage, step-down transformers reducevoltage. Most power supplies use a step-down transformer to reducethe dangerously high mains voltage (230 V in Poland) to a safer lowvoltage.

The input coil is called the primary and the output coil is called thesecondary. There is no electrical connection between the two coils,instead they are linked by an alternating magnetic field created in thesoft-iron core of the transformer. The two lines in the middle of thecircuit symbol represent the core.

Transformer (2)Transformers waste very little power so the power out is (almost) equalto the power in – when voltage is stepped down current is stepped up.The ratio of the number of turns on each coil, called the turns ratio,determines the ratio of the voltages. A step-down transformer hasa large number of turns on its primary (input) coil which is connected tothe high voltage mains supply, and a small number of turns on itssecondary (output) coil to give a low output voltage.

Vp = primary (input) voltage Vs = secondary (output) voltage Np = number of turns on primary coil Ns = number of turns on secondary coilIp = primary (input) current Is = secondary (output) current

Transformer symbol and output voltage

The low AC voltage output is suitable for lamps, heaters and special AC motors. It is not suitable for electronic circuits unless they include

a rectifier and a smoothing capacitor.

Bridge rectifier (1)

A bridge rectifier can be made using four individual diodes, but it is alsoavailable in special packages containing the four diodes required. It iscalled a full-wave rectifier because it uses all the AC wave (both positiveand negative sections). 1.4 V is used up in the bridge rectifier becauseeach diode uses approximately 0.7 V when conducting and there arealways two diodes conducting, as shown in the diagram below. Bridgerectifiers are rated by the maximum current they can pass and themaximum reverse voltage they can withstand (this must be at leastthree times the supply RMS voltage so the rectifier can withstand thepeak voltages).

Bridge rectifier (2)

Bridge rectifierAlternate pairs of diodes conduct, changing over the connections so

the alternating directions of AC are converted to the one direction of DC.

Output: full-wave varying DC(using all the AC wave)

Single diode rectifierA single diode can be used as a rectifier but this produces half-wavevarying DC which has gaps when the AC is negative. It is hard tosmooth this sufficiently well to supply electronic circuits unless theyrequire a very small current so the smoothing capacitor does notsignificantly discharge during the gaps.

Single diode rectifier

Output: half-wave varying DC(using only half the AC wave)

Transformer + Rectifier

The varying DC output is suitable for lamps, heaters and standard motors. It is not suitable for electronic circuits unless they include a

smoothing capacitor.

SmoothingSmoothing is performed by a large value electrolytic capacitorconnected across the DC supply to act as a reservoir, supplying currentto the output when the varying DC voltage from the rectifier is falling.The diagram shows the unsmoothed varying DC (dotted line) and thesmoothed DC (solid line). The capacitor charges quickly near the peakof the varying DC, and then discharges as it supplies current to theoutput

Smoothing is not perfect due to the capacitor voltage falling a little asit discharges, giving a small ripple voltage. For many circuits a ripplewhich is 10% of the supply voltage is satisfactory and the equationbelow gives the required value for the smoothing capacitor. A largercapacitor will give less ripple. The capacitor value must be doubledwhen smoothing half-wave DC.

Smoothing capacitor for 10% ripple:

C = smoothing capacitance in farads [F]IO = output current from the supply in amps [A]VS = supply voltage in volts [V], this is the peak value of the unsmoothed DCf = frequency of the AC supply in hertz [Hz] (50Hz in Poland)

Smoothing (2)

Transformer + Rectifier + Smoothing

The smooth DC output has a small ripple - it is suitable for most electronic circuits

Regulator

Voltage regulator ICs are available withfixed (typically 5, 12 and 15 V) orvariable output voltages. They are alsorated by the maximum current they canpass. Negative voltage regulators areavailable, mainly for use in dualsupplies. Most regulators include someautomatic protection from excessivecurrent ('overload protection') andoverheating ('thermal protection').

Choosing a Zener diode and resistor

1. The Zener voltage VZ is the output voltage required2. The input voltage VS must be a few volts greater than VZ

(this is to allow for small fluctuations in VS due to ripple)3. The maximum current Imax is the output current required plus 10%4. The zener power PZ is determined by the maximum current:

PZ > VZ • Imax

5. The resistor resistance: R = (VS - VZ) / Imax

6. The resistor power rating: P > (VS - VZ) • Imax

Transformer + Rectifier + Smoothing + Regulator

The regulated DC output is very smooth with no ripple - it is suitable for all electronic circuits

Pulse Width Modulated (PWM) power supply

PWM (Pulse Width Modulation) power supplies are a type ofswitching power supply. Pulse Width Modulation is used to helpregulate the voltage in a switching power supply. This is necessarywhen the current demand on the power supply or the chargingsystem's supply voltage is not constant.

PWM signal – the PWM is actually a square wave modulated. Thismodulation infects on the frequency (clock cycle) and the duty cycle ofthe signal. The PWM signal is characterized from the duty clock andthe duty cycle. The amplitude of the signal remains stable during time(except of course from the rising and falling ramps). The clock cycle ismeasured in Hz and the duty cycle is measured in percent (%).

Clock cycle and duty cycle parameters

All three signals are square wave oscillations modulated as per theiroscillation width, so called "duty cycle". They have the samefrequency (t1), but they differ on the width of the positive state (t2).The duty cycle is the percentage of the positive state compared to theperiod of the signal.

PWM in voltage and power control

How can PWM control the voltage? The idea is simple. A PWM signalwith 100% duty cycle would deliver 100% of the voltage. It would belike a DC power supply. By altering the duty cycle it is possible toreduce the area of the power delivered to the load.

The total power delivered to the connected load each time, is thearea under the positive state of the PWM (the drawn area on thedrawing).It is clearly seen that by altering the duty cycle, we can alter thepower delivered by the supply. For square wave the power suppliedeach time is calculated as

PDELIVERED = PSUPPLIED • DutyCycle

Lets consider PWM generator working at high frequency andconnected with the capacitor in the following manner:

The resulting output voltage of the above circuits operating with dutycycle 10% (left waveforms) and with 90% (right waveforms) is asfollows:

The above example is the principal of the operation for the switchingpower supplies

Block diagram for the energy harvesting circuit

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