There are two types of switching power supplies, namely:
- Primary switch (primary switching)
- Secondary switching
Figure 6.24: Block Diagram of Primary Switching Mode Regulator
In Figure 6.24 this DC voltage is switched at frequencies above audio frequency by a high voltage transistor to provide an alternating waveform in the primary transformer. The secondary AC is rectified and regulated by comparing the reference supply from the zener. The difference signal is used to control the duty cycle of the switching transistor. If the DC voltage drops when the load current increases, the balancing signal causes the modulator pulse width to switch the transistor ON for a long time and then OFF for half a cycle of the 20 KHz oscillator, so that the output voltage will rise again to a value very close to the previous value. The opposite happens if the load current is reduced. This primary switching mode is widely used in high power SMPUs.
However, you can replace the conventional linear regulator with a switched type using a secondary switch as shown in Figure 6.25. When the series transistor is switched ON, current flows through the LC filter. When the transistor is switched OFF, the inductor stores the current flowing as a reverse path action through the Fly Wheel Diode.
Figure 6.25 Block Diagram of Secondary Switching Mode Regulator
Various methods can be used to regulate the DC output. The duty cycle of the switching waveform or the frequency of the oscillator can be varied or a mixture of both methods. As long as the transistor is operated as a switch, one of them is OFF or ON so that the power dissipated by the transistor is lower. However, SMPU is more efficient and requires less space when compared to series regulators. SMPU, its main use is a unit that supplies large currents at low and medium voltages.
Switched Power Supply On Computer
For more details, the switched power supply on the computer will be explained below, because with the circulation of computers on the market, this power supply is the most widely used today. More details are given in the block diagram in Figure 6.26 below.
Figure 6.26 SMPU Block Diagram
Figure 6.27 Waveforms at Each Block Output Point
The function of each block can be explained as follows:
- RFI (Radio Frequency Interference) Filter. Its function is as a high frequency network filter where if there is a high frequency it will be suppressed and the low frequency (50 Hz) will be forwarded.
- Grid voltage rectifier and Filter capacitor. Its function is to change AC voltage to DC (unregulated) this circuit consists of rectifier diodes and filter capacitors. Before this circuit is usually installed NTC as a shock current holder (Isurge) when the power is first turned on due to the charging of the capacitor.
- Switching Element. Its function is to convert DC voltage into AC voltage in the form of voltage pulses that have a frequency much higher than the network frequency. Usually above audio frequency (> 20 Hz).
- I/O Isolating Power Transformer. The first function of this transformer is as an isolator between input and output where the input has a voltage of the same as the mains voltage, while the output needs to be lowered for safety. The second function is as a voltage reducer or increaser or as a multiple output maker.
- Output Rectifier. Its function is to rectify and filter the AC voltage from the transformer output into a DC voltage with very small ripple.
- Pulse Width Modulator (PWM). Its function is as a controller of output voltage stability by changing the pulse width for switching the switching transistor. If Vout drops, it will be detected by the Vsensor which changes the pulse width to increase so that it can increase the average output voltage. If it drops, the opposite is true.
- Isolation Transformer/Opto Coupler (Optical Coupling). Its function is to isolate the input output but can transfer PWM pulses to move the bases of the switch transistors.
- Auxiliary Power Supply. Its function is to supply the PWM circuit. This supply can be taken from the PC input or from the DC output. The wiring diagram of the computer power supply output is given in Figure 6.28 below:
Figure 6.28 Wiring the Power Supply on a Computer
Power Supply Wiring Methods and Problems
In some possible situations the power unit is required to supply the load through a fairly long wire as in figure 6.7. In the figure it can be seen that the load current flows from the supply and returns to the other wire, so that a voltage drop will occur causing the voltage along the load to be smaller than the power supply terminal voltage and consequently has a decrease in regulation.
Figure 6.7: Remote Load From Power Supply Terminals
Figure 6.8: Remote Sensing For Wire Resistance Compensation
Remote sensing techniques can only be used to provide optimum regulation for a single load. If the power supply is used to supply loads in parallel, then other techniques are used. A simple example is shown in Figure 6.9 below.
Figure 6.9: Regulators that use point of load
Each load is equipped with its own IC regulator circuit which is readily available and inexpensive. The main power supply unit that supplies the three separate regulators is usually unstable. In some situations, namely one regulated power unit supplies several circuits, then the arrangement must be connected in such a way that the interference caused by signal transmission from one circuit to the next circuit is minimal.
Figure 6.10 shows an example of a parallel connection, circuit C or B cannot be connected if the load is too heavy, as long as the current from the circuit can be set up by the interference signal in circuit A.
Figure 6.10: Parallel Distribution
Figure 6.11 shows an improved arrangement for Figure 6.10, in which the most sensitive circuit is A, supplied through a separate connecting wire that does not require a large wire. Circuits B and C are paralleled and positioned near the power supply.
Figure 6.11: Layout Improvement For Figure 6.10
Single point distribution, shown in Figure 6.12, is clearly the best solution, namely each circuit has its own supply wire.
Figure 6.12: Single Point Distribution of Best Solution
So the power distribution method should not be confused or disturbed during repair or testing. The appearance of the system will cause changes by rearranging the position of the supply wires or changing their resistance.
Switching Mode Power Unit (SMPU)
Switched power supply systems and switched mode regulators are used because they have high efficiency. Rapid developments over the last few years have shown the production of power supplies with maximum efficiency and small and light weight. Many of these circuits have been developed from the basic inverter (figure 6.3. In this circuit (figure 6.23) this is achieved by switching S1 and S2 back and forth continuously across the transformer primary. The transformer must be center tapped. In the first half cycle, current flows through the upper half of the primary coil and when the switch is turned off, current flows in the opposite direction, through the lower half of the primary. The result is that alternating current is produced in the secondary of the transformer.
Figure 6.23: Basic Inverter Circuit
The switch used is an electronic circuit (figure 6.23b) namely a transistor or thyristor controlled by a square waveform or pulse oscillator. Another method is to use a feedback coil on the primary so that the inverter transistor forms its own oscillating circuit. The frequency of this oscillating circuit is between 5 KHz to 25 KHz. This high frequency is used so that the transformer and its filter components will be relatively very small. If the frequency is very high, the start efficiency will drop to off. This pulse width will regulate the regulation of its output. Indeed, the switching power supply circuit is more complex than the linear regulated power supply circuit because here it produces more paths and electromagnetic interference, so it must be filtered carefully.
Linear Regulated Power Supply Parameters
Before conducting testing and repair of regulated power supply, important parameters must first be known to determine the next work steps, namely:
a. Area (Range)
namely the maximum and minimum limits of the power supply output voltage and current.
b. Load Regulation
that is the maximum change in voltage caused by a change in load current from no load to full load. The regulation percentage of a power supply is given by the formula.
Figure 6.5: Example of Load Regulation Curve for Linear Regulated Power Supply
This is illustrated in figure 6.5 and shows the load regulation graph for a 5 Volt power supply.
c. Line regulation
The maximum change in output voltage as a result of a change in the alternating current input voltage. Often expressed as a percentage ratio, for example a change in the main input voltage is ±10% causing a change in output of ±0.01%
d. Output impedance
The change in output voltage is divided by a small change in load current at some specified frequency (e.g. 100 KHz).
At low frequencies the above formula for load current changes very slowly, so the resistive part of Zout stands out. Rout can be read from the load regulation graph (see Figure 6.5) and for a suitable power unit is at most a few hundred milliohms.
e. Ripples and Noise
that is the peak to peak or rms value of any alternating or random signal that enters the DC voltage with all operating and environmental parameters held constant. Ripple will come out at full load or possibly at a specified value of the load current.
f. Transient Response
that is, the time taken by the DC output voltage to obtain a voltage of 10 mV from the steady state value (hereinafter referred to as the steady state) following a sudden application at full load.
g. Temperature Coefficient
namely the percentage change in the output voltage in line with temperature at specified values of the main input alternating current and load current.
h. Stability
namely the change in output voltage over time, assuming that the heat gained by the unit is balanced and the alternating input voltage, load current and ambient temperature are all constant.
i. Efficiency
namely the ratio of output power to input power expressed in percent. For example, a 24 volt power supply that has a main voltage of 240 volts, the required alternating current is 200 mA, if then the power supply is loaded with an output current of 1.2 A, then its efficiency is:
j. Current limiting
namely the method used to protect the power supply components and circuits supplied by the unit from damage caused by overload currents. The maximum steady state output current is limited to several safe values (see figure 6.5).
k. Foldback current limiting
namely an improvement on simple current limits. If the value of the load current exceeds the specified value, the power supply will switch to limit the current to a smaller value (see figure 6.6).
Figure 6.6: Backflow Boundary Characteristics
Using the above parameters, an example of a typical specification for a simple power supply unit is as follows:
- input voltage 110 V/220 Vac frequency 50 Hz/60 Hz;
- output voltage + 24 V;
- maximum output current 1.2 A;
- temperature area --5 oC to 45 oC;
- temperature coefficient 0.01 %/ oC;
- regulation line 10% of the main change produces a 0.1% output change;
- load regulation 0.2% from zero to full load.
Understanding Linear Regulated Power Supply
We have known that almost every system or electronic equipment uses a power supply circuit in it and the circuit varies greatly, but has the same basis. From the diagnosis of errors found are generally located in the power supply section, therefore it is very important to first learn about the various types of power supplies. The power supply is used to operate the system or instrument, it can be a battery but generally uses a single-phase alternating current main power source that is converted into a stable direct voltage.
There are two main methods used to regulate and stabilize direct current (dc) voltage, namely:
- Linear series regulator: used for simple/small power requirements (see figure 6.1).
- Switching Mode Power Unit (SMPU): for large power requirements (see figure 6.2).
Switching systems are more efficient because they conduct less heat and take up less space, compared to conventional linear regulators.
Figure 6.1: Example of a Linear Series Regulator Circuit
Figure 6.2: Example of a Switching Regulator for a Computer
There are 2 (two) types of power units, namely:
- Inverter. An inverter is a power unit that produces alternating current power output from a direct current input voltage. Its output frequency can be 50 Hz to 400 Hz (figure 6.3) Examples: emergency lights, UPS.
- Converter. A converter is basically an inverter followed by a rectifier, or in other words, a change in direct current to direct current again (figure 6.4). Example: A portable instrument for obtaining a direct voltage of 1 KV with a current of 1 mA to supply a tube from a 9 Volt battery.
Figure 6.3: Emergency Light as an Inverter Circuit
Figure 6.4: Converter circuit
Understanding Linear Series Regulators
Linear series regulator is a circuit that is generally used for medium power needs and even though the circuit is simple, it is able to provide better utility. The block diagram is given in Figure 6.13 as follows:
Figure 6.13: Block Diagram of Linear Series Regulator
Unstable input (Vi) is fed to generate a reference voltage and bias the error amplifier, the output voltage (Vo) that occurs is compared with the reference voltage by the error amplifier. This error signal is given to the series element, which is usually an NPN power transistor. If the output voltage decreases, it will cause the error signal to be amplified by the error amplifier which causes the series path element to increase the output voltage. Conversely, if the output voltage is too high, the error signal with the opposite polarity is also amplified by the error amplifier which causes the series path element to reduce its output current and output voltage.
This series element is a power transistor connected as an emitter follower that provides a low output impedance to control the load. While an example of a regulated power supply available on the market is as shown in Figure 6.14.
Figure 6.14: Examples of Regulated Power Supplies on the Market
There are many linear series regulator circuits available on the market, but not all of them will be discussed here. There are three important series regulator circuits that have safety features, namely:
Series Regulator Current Limiter:
The basic circuit of series regulator current limiter is shown in figure 6.15. The simple circuit using the above components does not reduce the reliability of the power supply. Rsc is a resistor to monitor the load current. If something causes more, the voltage on Rsc rises to 600 mV, Tr2 conducts and diverts the base current out of Tr1, so that its characteristics will be like Figure 6.5. For example Rsc is 1 Ohm, then it will limit the load current to about 600 mA and the voltage on Rsc is sufficient to operate Tr2.
Figure 6.15: Series Regulator Current Limiting Circuit
Foldback Current Limiting Circuit
A useful property of the power supply is that it will provide an output voltage close to zero, if the value of the load current is excessive, for that it is necessary to have an additional circuit in the form of a foldback current limiting as shown in Figure 6.16. Resistor Rm is installed in the return line, and the voltage formed across the resistor is used to switch ON the thyristor as soon as the overload trip current is exceeded, the thyristor is ON and the voltage across the thyristor is dropped to about 0.9 volts. This is not enough to forward bias the diodes D and Tr, so the output voltage will be zero. An LED can sometimes be installed to indicate that an overcurrent fault has occurred. The foldback current load is very effective in preventing damage to the series pass transistor when a short circuit occurs between the + and - terminals.
Figure 6.16: Reverse Current Load Protection Circuit
Over Voltage Protection Circuit
It is also very important that the series regulator supplies a sensitive IC load, such as TTL. With TTL, if the power supply exceeds 7 volts, the TTL IC will be damaged, for this reason an overvoltage safety circuit is needed as in Figure 6.17.
Zener diode is used to sense the output voltage of the power supply. If the voltage increases, so that the zener conducts and the SCR will be turned on causing the current to flow almost entirely through the SCR and cause the fuse to burn. Then the voltage at the collector of Tr1 (series element) drops very quickly to zero because the fuse burns. So here what is sacrificed is the fuse, the fuse will break when there is an increase in voltage at the output but the regulator circuit will not be damaged and the circuit supplied by this type of regulator will not be damaged either.
Figure 6.17: Overvoltage Protection Circuit
Most modern power supplies use IC regulators, so that the circuit becomes simpler and if damage occurs it is easier to fix. The most popular IC regulator today and is cheap and versatile is the IC regulator ?A 723 A.
a.PIN Configuration
b. Circuit in Figure 6.17: Regulator IC ?A 723 A
The internal circuit of this IC consists of a reference supply, a bias amplifier, a series pass transistor and a current limiting transistor. Connections for various variations can be made on this IC depending on the user to plan it flexibly according to their needs. The reference voltage is the voltage given to pin 6 with a voltage of 7.15 volts ± 0.2 volts, and this can be connected directly to the non-inverting input or through a voltage divider. A basic regulator circuit using the IC 723 is shown in Figure 6.19, which provides an output voltage of 7 volts to 37 volts.
Figure 6.19: 7 V to 37 V Regulator
The equation for calculating the output voltage is:
Vout = ( R1 + R2 ).Vref / R2. So the output voltage price can change according to the ratio of R1 and R2 which can be adjusted from the potentiometer. The output current capacity of the circuit above is very limited, to increase the current strength up to 2 Amps can be done simply by adding a power transistor without having to change the circuit much. The method is with a 2N3055 transistor connected to the circuit above where the transistor base is connected to IC pin 10 (output), then the emitter is connected to IC pin 2, while the collector is connected to the input with IC pins 11 and 12. So now the circuit will still be able to regulate the output voltage with the current strength increasing to 2 Amps.
Two important things to know regarding the IC ?A 723 A are as follows:
- The voltage must always be at least 3 V or greater than the output voltage.
- A low voltage capacitor must be connected from the compensation frequency pin to the inverting input. This is to ensure that the circuit does not oscillate at high frequencies.