All You Need to Know About Rectifier Circuit

2.1. Half-wave rectifier circuit

The half-wave rectifier circuit is shown in Figure 1. Among them, B1 is the power transformer, D1 is the rectifier diode, and R1 is the load.

B1 secondary is a sine wave voltage whose direction and magnitude change with time, the waveform is shown in Figure 2(a). The period from 0 to π is the positive half cycle of this voltage. At this time, the upper end of the B1 secondary is positive and the lower end is negative. The diode D1 is in forwarding conduction, and the power supply voltage is applied to the load R1, and the current flows through the load R1. The period from π to 2π is the negative half cycle of the voltage. The upper end of the secondary B1 is negative and the lower end is positive, the diode D1 is reversely cut off. No voltage is applied to the load R1, and no current flows in the load R1.

Repeat the above process in the subsequent cycles of 2π～3π, 3π～4π, etc., so that the waveform of the negative half cycle of the power supply is "cut" and a single direction voltage is obtained. The waveform is shown in Figure 2(b). Since the size of the voltage waveform obtained in this way changes with time, we call it pulsating direct current.

Suppose the secondary voltage of B1 is E, and the voltage across the load R1 in an ideal state can be calculated by the following formula:

Figure 3. Half-wave rectification calculation formula

The reverse peak voltage of the rectifier diode D1 is:

Figure 4. Calculation formula of the reverse peak voltage of half-wave rectifier diode

Since the half-wave rectifier circuit only uses the positive half cycle of the power supply, the utilization efficiency of the power supply is very low, so the half-wave rectifier circuit is only used in a few cases such as high voltage and small current, and is rarely used in general power circuits.

Although the half-wave rectifier circuit has a simple structure, it utilizes only the positive half-cycle of the alternating current, resulting in low power utilization and a large ripple coefficient of the output voltage. In order to improve power utilization and reduce ripple, full-wave rectifier circuits have emerged. Full-wave rectifier circuits are able to utilize both the positive and negative half cycles of the alternating current so that the frequency of the output voltage is twice the frequency of the input, thus making filtering easier.

2.2. Full-wave rectifier circuit

Because the efficiency of the half-wave rectifier circuit is low, people naturally think of using the negative half cycle of the power supply, so that there is a full-wave rectifier circuit. The full-wave rectifier circuit diagram is shown in Figure 5.

Compared with the half-wave rectifier circuit, the full-wave rectifier circuit uses a rectifier diode D2, and the secondary of the transformer B1 also adds a center tap. This circuit essentially combines two half-wave rectifier circuits together. During the period from 0 to π, the upper end of B1 secondary is positive and the lower end is negative. D1 is forward conducting, and the power supply voltage is applied to R1. The upper end of the voltage across R1 is positive and the lower end is negative. The waveform is shown in Figure 6(b), the current flow is shown in Figure 7.

During the period of π～2π, the upper end of B1 secondary is negative and the lower end is positive. D2 is forward conducting, and the power supply voltage is applied to R1. The voltage at both ends of R1 is still positive and the lower end is negative. The waveform is shown in Figure 6 (c), the current flow is shown in Figure 8. Repeat the above process in the subsequent cycles of 2π～3π, 3π～4π, etc., so that the positive and negative voltages of the two half cycles of the power supply are rectified by D1 and D2 and then applied to both ends of R1. The voltage obtained on R1 is always positive and negative. The waveform is shown in Figure 6 (d).

Suppose the secondary voltage of B1 is E, and the voltage across the load R1 in an ideal state can be calculated by the following formula:

Figure 9. The full-wave rectification calculation formula

The reverse peak voltage of the rectifier diodes D1 and D2 is:

Figure 10. Calculation formula of the reverse peak voltage of full-wave rectifier diode

The current flowing through each rectifier diode of the full-wave rectifier circuit is only half of the load current, which is twice as small as the half-wave rectifier.

2.3. Bridge rectifier circuit

Since the full-wave rectifier circuit requires a special transformer, it is more troublesome to manufacture, so a bridge rectifier circuit appears. This rectifier circuit uses an ordinary transformer but uses two more rectifier diodes than full-wave rectification. Since the four rectifier diodes are connected in the form of a bridge, this rectifier circuit is called a bridge rectifier circuit.

From Figure 12, it can be seen that during the positive half cycle of the power supply, the upper end of the B1 secondary is positive and the lower end is negative. The rectifier diodes D4 and D2 conduct. The current flows from the upper end of the transformer B1 secondary through D4, R1, D2, and returns to the lower end of the transformer B1 secondary.

From Figure 13, it can be seen that during the negative half cycle of the power supply, the lower end of the B1 secondary is positive and the upper end is negative. The rectifier diodes D1 and D3 conduct, and the current returns from the lower end of the transformer B1 secondary to the upper secondary of transformer B1 through D1, R1, and D3. The voltage across R1 is always positive and negative, and its waveform is consistent with that of full-wave rectification.

Suppose the secondary voltage of B1 is E, and the voltage across the load R1 in an ideal state can be calculated by the following formula:

Figure 14. Bridge rectifier calculation formula

The reverse peak voltage of the rectifier diodes D1 and D2 is:

Figure 15. The formula for calculating the reverse peak voltage of bridge rectifier diodes

The current flowing through each rectifier diode of the bridge rectifier circuit is half of the load current, which is the same as full-wave rectification.

Under normal circumstances, the bridge rectifier circuit is simplified to the form shown in Figure 16.

Bridge Rectification vs. Full Wave Rectification

Bridge rectifier circuits have several advantages over full wave rectifier circuits:

1. No need for a center-tapped transformer, using an ordinary transformer can be, reducing cost and complexity

2. The output voltage is the same as the full-wave rectifier, which is about 0.9 times the input peak voltage

3. Each diode withstand reverse peak voltage only input peak voltage, while the full-wave rectifier for the input peak voltage of 2 times

The disadvantages are that four diodes are required instead of two, which increases the component cost, and each diode has a voltage drop, which may result in a large energy loss in low-voltage applications.

2.4. Voltage doubler rectifier circuit

The output voltage of the three rectifier circuits introduced above is less than the effective value of the input AC voltage. If the output voltage is required to be greater than the effective value of the input AC voltage, a voltage doubler circuit can be used, as shown in Figure 17.

It can be seen from Figure 18 that in the positive half cycle of the power supply, the upper end of the transformer B1 secondary is positive and the lower end is negative, D1 is on, D2 is off, and C1 is charged through D1. After charging, the voltage across C1 is close to the peak value of the secondary voltage of B1. The direction is positive on the left and negative on the right.

It can be seen from Figure 19 that in the negative half cycle of the power supply, the upper end of the transformer B1 secondary is negative and the lower end is positive, D1 is off, D2 is on, and C2 is charged through D1. After charging, the voltage across C2 is close to the voltage across C1. The sum of the peak values of the secondary voltage of B1, the direction is the lower end is positive and the upper end is negative. Since load R1 is connected in parallel with C1, when R1 is large enough, the voltage across R1 is close to twice the B1 secondary voltage.

There is another way of drawing the double voltage rectifier circuit. As shown in Figure 20. The principle is exactly the same as that in Figure 17, but the form of expression is different.

The double voltage circuit can also be easily expanded into an n double voltage circuit. The specific circuit is shown in Figure 21.

2.5. Filtering techniques for rectifier circuits

The rectified pulsating DC contains a large AC component, which requires a filter circuit to reduce the ripple and obtain a smooth DC. Commonly used filtering methods include:

Capacitive Filtering

The most commonly used filtering method utilizes the charging and discharging characteristics of capacitors to smooth the voltage waveform. Capacitor filter circuit is simple and low cost, but the ripple suppression ability is limited.

LC Filtering

Combination of inductor and capacitor filtering, the filtering effect is better than capacitor filtering alone, but the cost is higher and the volume is larger.

π-Type Filtering

Consisting of two capacitors and one inductor, it has better filtering effect and is often used in demanding power supply circuits.

The choice of filter circuit depends on factors such as load requirements, cost budget and space constraints.

2.6. Limitations and solutions of rectifier circuits

Ripple problem

The rectified voltage contains obvious ripple components, which will lead to unstable load operation. The solution is to add a filtering circuit. Commonly used filtering circuits include capacitor filtering, inductor filtering and LC filtering.

Voltage Drop Problems

The forward voltage drop of a diode (about 0.7V) can cause significant energy loss in low voltage applications. The solution is to use Schottky diodes (voltage drop of about 0.3V) or synchronous rectification techniques.

Efficiency Issues

Conventional rectifier circuits typically have efficiencies of around 80%. Modern power supply designs can increase efficiency to over 95% by using techniques such as synchronous rectification and resonant rectification.