Switching Power Supplies
In this article, the fundamental theory of inverter operation is discussed. The basics covered here provide a better understanding of the operation of a typical single-phase inverter.
- Battery, Battery Input, Charger, and DC Filter The purpose of this block is to provide the DC power from a bank of batteries, connect the batteries together, provide a charger for replenishing the battery charge, and filter any unwanted AC from the battery charger output.
- Power Switching This block is where the inverter function is performed. This block contains the power switching devices (SCRs) used to convert the DC into AC.
- Controls This block is the control panel that allows the user to operate the inverter. This block also contains the circuits that generate the gating pulses. The gating pulses are used to switch on and off the power switching devices. Also, this block contains circuits used to synchronize the inverter output with an alternate power source.
- Isolation, Output, and Harmonic Filter This block provides for transformer isolation of the output for safety and reliability. The transformer used is a constant voltage transformer (CVT) that provides for output voltage regulation. Also, since the inverter produces square waves and they contain a large number of odd-ordered harmonic frequencies, a harmonic filter is used to remove these undesired signals from the output.
- Static Switch This block contains the high-speed automatic electronic switch that is used to transfer a critical load from the output of an overloaded or failed inverter to an alternate power source or standby inverter without interruption of power.
Figure 1: Typical Inverter Block Diagram
During the years since the invention of the transistor, a wide variety of solid-state switching devices has been produced. Thyristors are a family of switching devices whose members include the silicon controlled rectifier (SCR), the triac, and the diac. The operation of these extremely useful devices was discussed in another article.
Other more exotic power switching devices include gate turn-on devices (GTOs), metal oxide semiconductor field effect transistors (MOSFETs), and insulated gate bipolar transistors (IGBJTs). These devices are also very useful as switching devices. The theory behind their operation is beyond the scope of this article.
No matter what power-switching device is used, it must be able to take the DC input and re-shape it into a square-wave AC output. For the purpose of explanation, we will examine the operation of a simple power switching circuit using mechanical switches. This circuit is for the purpose of understanding only and would not be practical to use for a real world inverter. This circuit is shown in Figure 2 below.
The SCR gate circuits are the triggers that actually turn the SCRs on at the proper times. The gate circuit applying a positive voltage to the gate terminal of the SCR accomplishes SCR turn-on. The gate circuitry is complex, but it is fundamentally an oscillator circuit. This circuit also includes the rectifier diodes, D1, D1', D2, and D2'. These diodes are not part of the inverter process, but they are used to clamp the amplitude of the load voltage to a value approximately equal to the magnitude of the source voltage. Figure 3 shows the diagram of a four-SCR parallel-commutated inverter bridge. Figure 4 is a two-SCR bridge. The operation of the two-SCR bridge is similar to that of the four-SCR bridge. It differs in that half the number of controlled rectifiers is used, and each must hold off a voltage approximately equal to twice the supply voltage. The center-tap of the transformer provides a return path for the direct current.
The bank of batteries must be capable of providing a filtered DC source for the power switching circuit. This ensures that the output of the power switching circuit is transient-free. Usually, an AC source, a transformer, a rectifier, additional filtering, and a logic diode are used to maintain the batteries in a fully charged state. If a power failure occurs that results in a loss of AC power, the batteries will continue to power the inverter for a time. This feature is what makes the inverter so useful in uninterruptible power supplies (UPS).
Since the AC power that is used to charge the batteries may vary in voltage at different times of day and under different load conditions, the battery charger must be capable of producing a constant output voltage. This voltage regulation is accomplished by using a specially designed rectifier whose output waveform is controllable. Figure 5 shows a basic block diagram of the battery charger/rectifier. The circuit is supplied by an AC input. The function of the blocks is described below.
Figure 5: Block Diagram of Battery Charger/Rectifier
- Input Transformer The input transformer is designed to step up or down the incoming AC supply voltage to the proper value for the rectifier. The transformer also provides isolation between the supply and rectifier circuits for safety and ground reliability.
- Rectifier This block is where AC is converted into a pulsating DC output. Typically, the rectifier is made up of three diodes and three SCRs (or 6 thyristors) connected in a full-wave array. The conduction interval of the SCRs is adjusted by firing at the proper time during each half-cycle. This change in the conduction interval allows the rectifier to maintain a constant output with varying input conditions. A longer conduction period increases the DC voltage output, and conversely, a shorter conduction period lowers the DC output voltage. This method of regulation is called phase angle control.
- Output Filter This block contains the circuits that smooth the pulsating DC output from the rectifier. The filter is made up of a choke (inductor) and a capacitor bank. These are connected together in an "L" configuration. The capacitors oppose changes in voltage, and the inductor opposes changes in current. After filtering, the DC output voltage will be relatively smooth and ripple-free. The capacitor part of the filter may consist of the capacitors located in the inverter.
- Control Board The control board supplies the pulses that gate the SCRs at the proper times. The exact point of pulse generation is a function of the output voltage of the rectifier. The control board compares the output of the rectifier to an internal reference and then generates an error signal, which in turn adjusts the firing angle of the SCRs. If the output of the rectifier begins to drop, a signal is generated that increases the conduction interval of the SCRs and thus returns the output voltage to its normal value. In addition, the control board has built-in overvoltage protection. If the DC output should go abnormally high, the gate pulses to the SCRs are immediately reduced. This feature protects the load against the high DC voltages that may occur if a drastic increase in AC supply voltage were to occur.
To better understand the operation of the rectifier portion of the charger, a simple half-wave rectifier using an SCR for phase control will be examined. See Figure 6.
Figure 7: Rectifier Input and 90 Output Waveforms
Suppose for example, that the AC input voltage increased. The gate circuit would sense this and adjust the conduction angle so that it would be less than the 90 angle shown in Figure 7 above. The conduction angle may decrease to 45; this output waveform is shown below (see Figure 8).
Figure 8: Rectifier Input and 45 Output Waveforms
If, for example, the AC input voltage decreased, the gate circuit would detect this and generate a new firing angle that would cause the SCR conduction angle to increase from the 90 steady-state value to a value of 135. This is shown in Figure 9 below. Now the output is receiving power for a larger percentage of each sine-wave input, so the average output voltage of this waveform after filtering would be higher than the steady-state value shown when the conduction angle was 90. This increase in average output voltage would compensate for the decrease in the AC input voltage.
Figure 9: Rectifier Input and 135 Output Waveforms
Using a full-wave rectifier instead of a half-wave rectifier increases the efficiency of the circuit. This allows the entire sine-wave input to be used. Also, full-wave rectification causes the peaks to be moved closer together in time, which again makes the output waveform easier to filter.
The control board controls the firing of the SCRs in the proper sequence. This is important so that the average DC output of the rectifier can be controlled.
At some value of volt-seconds across saturating transformer ST1, its magnetic core will become saturated, which destroys the resonant condition and limits the voltage rise across the capacitor. In effect, this saturating element is clamping the voltage across the capacitor to a specified value that is dependent on the number of turns on the winding and the cross-sectional area of the core of ST1. Since the voltage across ST1 may not be exactly the level desired at the output, a separate transformer, labeled T2, is used to adjust the level of the output voltage. In addition, a small transformer labeled T1 has been included. The purpose of this transformer is to compensate for a slightly rising output voltage that is directly proportional to the input voltage. This voltage is developed in the secondary of T1 and is added to the output voltage out of phase so that, in effect, it adds to the output negatively and tends to flatten the regulation curve.
The filter components, which are labeled CH1 and CH2, are used to improve the waveform so that the load is presented with an approximate sine wave. Choke CH1 is a linear device and is tuned together with C1 to the third harmonic of the input frequency. These components, therefore, present very low impedance to the third harmonic and reduce its amplitude in the output to a very low value.
Choke CH2 is allowed to saturate and, therefore, generates harmonic currents that are transformer-coupled out of phase into choke CH1. The addition of these out-of-phase harmonics in CH2 tends to cancel the higher order harmonics that appear across the primary winding of choke CH1. The result of using this type of arrangement is the generation of a sine wave with a total harmonic content of approximately 3%. A short circuit on the output will have a current level determined by the magnitude of input voltage divided by the reactance of L1. This component is responsible for the current-limiting characteristic of the ferroresonant transformer in the overload region.
The static switch has been designed to detect the only two failure conditions possible with ferroresonant inverters. These are:
- Loss of square wave resulting from a power or control circuit failure
- Loss of inverter AC output voltage resulting from ferroresonant transformer failure
Provisions have also been made to detect and initiate transfers of load from the inverter to an alternate source upon load faults and overload conditions. This mode is very useful for clearing load circuit branch fuses in the shortest possible time by relying on the superior fault clearing capabilities of the alternate source. Figure 11 shows a block diagram of the typical output switching arrangement.
Failure of the inverter square wave is detected virtually instantaneously, causing the static switch power SCRs to switch before depletion of the stored energy within the output filter of the ferroresonant transformer. This results in a truly uninterrupted, make before break, transfer of power to the load.
Transfers that result from failure of the inverter output transformer and load fault or from overcurrent are initiated by simulating a failure of the inverter. This is done by removing power to the reed relay coil when failure or abnormal conditions have been detected. In the case of a voltage loss resulting from transformer failure, an interruption of cycle maximum will be experienced because no energy is available to maintain the load during the switching interval.
A two-position manual switch is often used to bypass the inverter and static transfer switch for maintenance purposes. It can also be used as a manual transfer switch when a static switch has not been included as part of the uninterruptible power system. The switch is available with "make before break" contacts when used with a system employing either a static transfer switch or an inverter line synchronizing accessory. In Figure 11, a single pole switch is shown for simplicity. In three-phase units, a 3PDT or 4PDT switch would be used. One pole of the switch may be used to activate an alarm.