What Is a Full-Wave Rectifier? How It Works

A full-wave rectifier will direct both halves of an alternating-current waveform to send current through a load in one direction. This produces a voltage that is not smooth DC: it will rise as it approaches each input peak and drop back as it falls away from the input peaks, but the voltage across the load no longer reverses polarity.

You can create this function using either of two configurations: a four-diode bridge or a two-diode configuration with a center-tapped transformer. Even though both circuits create the ideal waveform for full-wave rectification, they will do so using different transformer connections, pathways for conduction, diode-voltage budgets and PIV references.

Because the circuit topology must be understood before you apply formulas or component ratings to a circuit, you should remember these points:

• When rectifying power from 50 Hz, the output will be in 100 Hz pulses; with 60 Hz, the output pulses will be 120 Hz.
• The values of 0.637Vm, 0.482 ripple factor, and 81.2% efficiency that are commonly quoted are based on an ideal, unfiltered resistive-load model; they do not represent an entire power supply.

What Does a Full-Wave Rectifier Actually Do?

During one half of each input cycle, the source drives current through the load in the desired direction; during the other half, the rectifier changes the conducting pathway so current flows through the load in the same direction.

Rectification, filtering and regulation are three distinct and separate functions. The intent of the rectification step is to establish the same output voltage polarity, while the function of the reservoir capacitor is to store energy when the voltage is near its maximum value and to release that energy into the load between peaks.

A regulator’s function is to maintain a specified output voltage within a specified voltage range as input voltage, load, temperature or transient conditions change. Because the rectification process produces pulsating DC through the load, this means that rectification alone cannot guarantee a constant supply voltage.

How Does Current Flow Through a Bridge Rectifier?

In a single-phase bridge, two of the four diodes will normally conduct current through the load during each half-cycle.

When the A terminal is positive with respect to the B terminal, the first diagonal pair of diodes carries current from terminal A to the positive output and through the load. The current will then return to terminal B through the negative output. The other diagonal pair is reverse-biased.

Once the AC power has reversed polarity, the B terminal will now be positive with respect to the A terminal. The first diagonal pair of diodes will turn off; now the opposite pair of diodes will conduct.

The current now enters the bridge through B, yet it still enters the same load terminal and returns through the same negative output. As the reference designators may change in bridge circuit schematics, be careful to use node polarity and current arrows rather than memorizing any specific designator sequence of D1-D4.

Figure 1. Bridge-rectifier conduction paths during the two input half-cycles.

How Does a Center-Tapped Full-Wave Rectifier Work?

Secondary windings in a center-tapped configuration provide two equal half-winding voltages with respect to the center tap. When the upper end of the secondary is positive with respect to the center tap, the upper diode conducts and the lower diode blocks, so current flows through the upper half-winding and into the load before returning to the center tap. Then, on the next half-cycle, the lower end becomes positive with respect to the center tap, and current will flow through the lower half-winding and the other diode, with the load current continuing in the same direction. Only one diode drop is present in the active load path, while the nonconducting diode experiences a different reverse-voltage condition from a bridge diode. The full-secondary RMS value should not be substituted for the peak value of one half-winding without conversion.

Figure 2. Center-tapped full-wave conduction through alternate half-windings.

Why Does the Output Repeat at Twice the Input Frequency?

An AC cycle is represented by a positive half-cycle and a negative half-cycle. Diode rectification of both half-cycles will produce the same-polarity output from both the negative and positive half-cycles; therefore, the repetition rate of the unfiltered pulse will equal 2fin. The diodes do not generate a new oscillation; they only redirect the half-cycle that would otherwise reverse load current.

When examining full-wave rectifiers, the numerical table assumes a sine wave source, as well as ideal diodes and a resistive load without filter capacitors.

Bridge or Center Tap: Which Topology Fits the Circuit?

A bridge rectifier is one type of full-wave rectifier, not a synonym for every full-wave circuit. The two most common single-phase implementations of full-wave rectification are compared using the same output-voltage objective and clearly defined secondary-voltage references.

For a standard two-wire secondary or direct AC input, the bridge rectifier will be the simplest configuration. The full secondary is used on both half-cycles and will require a lower ideal PIV from each diode when operating with the specified voltage reference. In situations where a transformer is already included in the design/assembly, and it would be advantageous to have only one diode drop, a center-tap circuit may be preferred, especially when working with low-voltage/high-current applications. Although the center-tap configuration appears to minimize the number of diodes needed to rectify AC, there is no guarantee it will also have a smaller or less costly full-wave power circuit because of the inclusion of the transformer.

Which Full-Wave Rectifier Formulas Apply?

Assumptions about Formulae: Sinusoidal input; ideal diodes (unless otherwise specified); resistive load; unfiltered output; Vm is the peak of the active source path; and Im is the corresponding peak load current.

The 81.2% value is based on waveform-modelling and should not be taken as the full power supply efficiency. Factors outside that ideal definition include transformer copper and core losses, wiring resistance, diode forward losses, capacitor RMS currents, regulator losses, switching losses, and cooling power.

Real diodes do not allow a single universal voltage subtraction. A bridge rectifier has two junctions in its conducting path, while a center-tapped circuit normally has one; forward voltage drops vary by type of diode, instantaneous current, junction temperature, and pulse shape. For example, capacitor-input bridge configurations can see their unloaded capacitors charge near Vm – 2VF, but that is a peak-based estimate and should not be substituted for the unfiltered average value of 2Vm/π.

What Changes When a Reservoir Capacitor Is Added?

A charged reservoir capacitor stores energy when the rectified source voltage exceeds the voltage stored in the capacitor plus the forward voltage drops in the conducting diodes. After the rectified voltage drops below that of the charged reservoir capacitor, the stored energy in the reservoir capacitor powers the load until the next peak recharges the reservoir. This causes the average load voltage to rise closer to the rectified peak value and reduces the valley depth, but also compresses the time interval during which diode and transformer currents flow into shorter charging pulses.

Small-ripple estimate: ΔVpp ≈ Iload/(frippleC) = Iload/(2finC). This approximation assumes a nearly constant load current and a capacitor that discharges approximately linearly between closely spaced peaks.

The peak-to-peak ripple across the load when operated from a 60 Hz source, with a load current of 200 mA and a filter capacitor capacitance value of 2200 μF, will have a ripple frequency of 120 Hz and an approximate peak-to-peak ripple voltage of 0.20/(120 × 0.0022), or about 0.76 V. If this load and capacitance combination is operated at a 50 Hz frequency, the peak-to-peak ripple would be approximately 0.91 V because the charging of the capacitor occurs at 100 Hz. These values are first-pass estimates, not guaranteed measurements.

The amount of ripple voltage and average load output can be affected by a variety of variables, including source impedance, transformer regulation, line tolerance, capacitor ESR, leakage, diode conduction angle, and transient changes in load. Increasing the capacitance of the filter capacitor will reduce the amount of calculated ripple voltage, but at the same time increase the inrush current, charging-pulse amplitude, bridge heating, transformer RMS current, and stress on switches or fuses.

Figure 3. AC input, unfiltered full-wave output, and capacitor-supported voltage.

How Do You Select the Diodes or Bridge Module?

For line-frequency inputs, the main source of stress for the diode is surge current, while low-voltage, high-current outputs are dominated by forward-voltage loss, and high-frequency secondaries are dominated by switching performance. For each application, matching a diode family with the dominant stress avoids unnecessarily over-specifying other parameters that do not have a significant effect in that application.

50/60 Hz Power Input

For most line-frequency or transformer-secondary bridges, first check repetitive peak reverse voltage, average forward current, non-repetitive surge current, forward-voltage drop at the expected pulse current, and thermal resistance. The peak current through the diodes in the bridge may be several times greater than the DC load current because of the use of a reservoir capacitor. The load-current label for a bridge should never be the only source of information when selecting a bridge.

With 50/60 Hz powered equipment, reverse recovery will typically have less impact than surge, conduction loss, and temperature. However, the specific datasheet for any diode will ultimately determine how to choose the appropriate device for the application.

Low-Voltage, High-Current Output

In applications where the DC rail has only a few volts, two bridge diode drops can have a large impact on both the DC voltage level and the amount of power available to the output load. Silicon PN diodes, Schottky devices, and a center-tapped path should be compared using forward voltage at the actual current and temperature. Although Schottky diodes may help reduce power loss, their reverse-voltage rating and reverse leakage current at elevated temperature must still meet the circuit requirements.

High-Frequency Secondary Rectification

At the secondary side of high-frequency converters, reverse-recovery time or charge, junction capacitance, switching loss, and ringing will become significant. A general-purpose line-frequency rectifier may have an adequate current and voltage rating, but may dissipate an excessive amount of power or produce EMI at the switching frequency. A high-frequency converter will generally use a rectifier device such as fast, ultrafast, Schottky, SiC, or synchronous devices, chosen based upon the waveform of the converter, not based upon a generic “rectifier” label.

Where Are Full-Wave Rectifiers Used?

Full-wave rectification converts an AC power source to a unipolar DC bus and is used for many applications, including LPSs, SMPSs, and battery chargers. However, each application has a different load requirement and thus a different requirement for the rectifier stage. The decision of what type of topology to use cannot be made independently of what comes afterward.

Linear DC Power Supplies

The standard configuration for a traditional LPS consists of a transformer preceding the rectifier, a reservoir capacitor downstream of the rectifier, and a linear voltage regulator after the capacitor. The transformer provides isolation and sets the secondary voltage, while the voltage regulator provides regulation to compensate for line and load fluctuations. A bridge configuration is commonly used because it works with a standard secondary, whereas a center-tapped circuit may be used when the transformer already provides the required winding and a single diode drop improves a low-voltage output.

Offline Switch-Mode Power Supplies

An offline SMPS will generally have the bridge rectifier located after the input protection and EMI network and feeding into a high-voltage DC bus capacitor or PFC stage. When rectifying 120 or 230 V AC, there is hazardous stored energy in the high-voltage DC bus. The negative side of the bus is not inherently protective earth or a safe reference for an oscilloscope; the distinction is identified in the ground-reference guide. The rectifier is only viewed as the front end of the SMPS; switching, isolation, feedback, and regulation occur in the downstream portion of the SMPS.

Battery Chargers and Industrial DC Buses

Similar to offline SMPS, battery chargers, control supplies, motor drives, and industrial DC buses commonly create intermediate DC bus voltages from the incoming AC power source via full-wave rectification. The use of the rectifier to determine a “safe” charging profile or motor-control waveform is not applicable here. A charger will still require current and voltage control, battery-specific termination, reverse-current protection, and fault handling; similarly, motor drives convert DC bus power via choppers or inverters. These devices require rectifiers to be selected based on bus voltage, continuous and surge current, thermal cycling characteristics, and ripple-current requirements for the following stage.

What Changes in a Real PCB Implementation?

The circuit schematic depicts four ideal diodes; however, the PCB carries narrow, high-amplitude charging pulses from the AC connector to the bridge, reservoir capacitor and return path. It is very important to keep this loop compact, to size the copper and vias considering RMS current and temperature rise, and to place the bulk capacitor so that the charging current does not share an avoidable path with sensitive control or measurement grounds. The broader placement and return-path process is documented within the PCB layout guide.

Integrated bridges must also have a verified thermal path. Use the power loss and thermal data from the package to estimate junction temperature for the expected ambient, enclosure, airflow, copper area, and mounting conditions. Mains-related circuits must also have established creepage, clearance, fuse coordination, surge protection, isolation barriers, connector ratings, and test access requirements from the applicable equipment standard and must not use those from a low-voltage example.

How Can a Rectifier Be Checked Safely?

Remove and isolate power, verify that voltage is not present with appropriate equipment, and control the stored capacitor energy before using the diode mode of a tester. The capacitor-discharge procedure describes how discharge resistance, time, pulse energy and final voltage verification work together. This section does not represent a procedure for live-mains probing.

An isolated, healthy junction normally conducts in one lead orientation and blocks in the other; however, semiconductor type, meter stimulus, temperature and parallel circuit paths may alter the reading. An open, short, intermittent connection, thermal failure or load-dependent breakdown may go undetected by a single in-circuit diode check. If the symptom represents a complete power stage, then follow the board-repair diagnostic process rather than replacing the last component tested on the board.

Common Interpretation and Design Mistakes

• Treating all full-wave rectifiers as four-diode bridges
• Using full-secondary voltage where a center-tapped formula defines Vm as one half-winding peak
• Applying 2Vm/π after a reservoir capacitor has altered the waveform and average level
• Selecting diode current ratings from DC load current without considering charging-pulse and surge conditions
• Assuming the drop is always fixed at 0.7 V or ignoring its dependence upon current and junction temperature
• Calling the rectified output regulated DC before the filtering and control stages have been defined

Conclusion

The term full-wave rectification refers to the use of both AC half-cycles while keeping the load current flowing in one direction. A bridge configuration changes between two conducting diode pairs and normally has two forward drops. A center-tapped configuration utilizes half-windings alternately and will only pass through one active diode path; however, a suitable transformer is required and the PIV calculation must be done differently.

The ideal unfiltered waveform has an average of 0.637Vm, a ripple frequency of 2fin, and a ripple factor of 0.482, with a maximum theoretical rectification efficiency of 81.2%. A capacitor will change both the average and the current waveform, so practical design must also take into account ripple, inrush, surge, thermal loss, reverse voltage, load type, grounding, and safety requirements.