The power diode
The power diode is a nonlinear passive electronic component composed of two terminals: the anode and the cathode. It makes the electric current pass in only one direction and blocks it completely in the other direction (see Figure 1). It is a semiconductor component and therefore acts as a one-way current switch. Its behavior, as mentioned before, is not linear, as the signal at its output does not always follow the trend of the signal at its input. The diode’s characteristic curve graphically represents the current value by varying the voltage between the anode and the cathode of the power diode. If the voltage is positive, the component is directly polarized. If the voltage is negative, the power diode is reverse-biased.
There are three types of power diodes:
• Diodes for general applications, direct current, or mains frequency circuits. Because their operation is almost static, they can withstand thousands of volts and amperes, with the possibility of parallel connections between them.
• Schottky diodes. They are used when a low-voltage drop is required. In fact, their potential difference is about 0.2 V or 0.3 V. Their maximum reverse voltage is rather limited.
• Fast-recovery diodes for high-speed operation. They are usually used in conjunction with fast electronic switches, such as MOSFETs. Their recovery time is less than 1 µs.
Some types can withstand currents of thousands of amperes and voltages of thousands of volts. Consider, for example, power applications in the automotive, rail, and transport sectors in general. A power diode can conduct current when the voltage at its anode is higher than that of the cathode. During operation, the potential difference between these two terminals is quite low, between 0.2 V and 2 V (a difference of 0 V exists only in ideal diodes). If the voltage on the cathode is greater than that on the anode, the power diode is off and does not conduct current. Its usefulness, therefore, covers the functionality of a switch, a commutator in rectifiers, switching circuits, insulation, charging capacitors, recirculation devices, and many others. The characteristic of the graph just seen is described by the Shockley equation, according to the following formula:
where Id is the direct current on the diode, Vd is the forward bias voltage, Is is the inverse saturation current, and n is the ideality factor, between 1 and 2 (germanium: 1; silicon: 2)
In power and high-frequency applications, a very important parameter is the recovery time, which is the time between the instant the current passes through zero and the moment the reverse current drops to 25% of its maximum peak. Unfortunately, it is a limiting factor of the amount of direct current and of the working frequency.
Transformation from alternating power to continuous power
In all homes, electricity arrives in alternating forms, for reasons of its transportation along the power lines. Most power applications work with DC voltage. Therefore, it is necessary to convert this sinusoidal alternating voltage (with a frequency of 50 or 60 Hz) into direct voltage. It is very simple to obtain a direct voltage. A transformer and a diode rectifier convert the alternating current input into direct current. If the powers involved are high, the dimensions of the transformer could be very important. But today, we always try to avoid big and expensive transformers.
The continuous output component must be perfectly leveled and free from undulations and harmonics, using high-capacity capacitors with filter functions. If the theory says that a system of this type allows obtaining a perfectly continuous voltage, in practice, various phenomena occur that unfortunately slightly deteriorate the output signal. The function of the capacitor is to charge up to a value close to the peak of the alternating voltage and to maintain this level as much as possible. Only the potential difference of the diodes prevents the exact achievement of the maximum peak value. The low impedance of the load discharges the capacitor, creating a “ripple” signal with a double frequency to that of the alternating input signal. In Figure 2, it is possible to observe the classic circuit for transforming alternating current into direct current. It consists of the following electrical and electronic components:
• Sinusoidal alternating power source with V0p of 325 V (RMS of about 230 V)
• Ideal 40-V voltage transformer
• Rectifier with Graetz bridge, consisting of four super-fast recovery diodes RFN20TF6S (Vr = 600 V, ID = 20 A)
• Electrolytic smoothing capacitor (filter) of high capacity
• 10-Ω resistive load, with average dissipation of 90 W
The circuit, with a suitable leveling electrolytic capacitor (about 47,000 µF) has the following electrical voltages at the various nodes:
• V (a, b): 650-Vpp 50-Hz AC power source (–325 V, 0 V, 325 V)
• V (c, d): voltage transformed and lowered by the transformer equal to 64 Vpp (–32 V, 0 V, 32 V)
• V (e): DC voltage of 30 V (with smoothing capacitor)
• V (e): pulsating voltage of 40 V0p (without smoothing capacitor)
It is advisable to carefully examine all the oscillograms present in the graph of the same figure, referring to nodes A, B, C, D, and E. The efficiency of this circuit is about 75%, and the transformer must obviously be able to withstand the power required by the load. Sometimes, the distorted currents absorbed by a load can cause distortions in the power supply voltage waveform.
The ripple
In this type of circuit, there is a ripple signal, which depends on the value of the filter capacitor and that of the load (see Figure 3). The ripple percentage is inversely proportional to the load’s resistive value and the filter capacitor’s capacity. Often, the voltage supplied by a power supply retains some “traces” of the alternating mains voltage in the form of more or less large ripples superimposed on the direct voltage. These ripples have the shape of a sawtooth and double the input frequency. The ripples are due to the current absorbed by the load. The capacitor’s discharge phase occurs between one-half wave and the next. The lower the current absorbed by the load and the greater the capacitance of the capacitor, the lower the ripple voltage value. In simpler power supplies, the component that smooths the output voltage is the electrolytic capacitor. There are some formulas for determining its ideal value. The value of the capacitor, of course, depends on the percentage of ripple that the user is willing to accept. A capacitor with a low capacitance value determines, in fact, a higher percentage of a ripple than a capacitor with high capacitance. A simple formula that returns the capacitor value to use is the following:
where C is the capacitance of the capacitor, expressed in farads (for microfarads, just multiply the value by 1,000,000), Ioutput is the maximum output current, f is the frequency of the positive half wave (in this case, 100 Hz), and Vripple is the peak-to-peak ripple voltage to be obtained.
For example, the above electrical circuit has the following ripple value, depending on the following filter capacitors:
• 4,700 µF: ripple of about 10 V (not acceptable)
• 47,000 µF: ripple of about 1 V (quite acceptable)
• 860,000 µF: ripple of about 0.05 V (very acceptable)
From the formula, it can be deduced that the greater the capacitance of the capacitor, the greater the current absorbed by the load and the smaller the ripple amplitude. In practice, 10,000 µF is calculated for each ampere of current on the load.
The ripple is expressed as a percentage and refers to the average VCC value of the voltage that powers the load. The amount of allowed ripple must be determined in the design phase. The formula for calculating this parameter as a percentage is the following:
A low percentage of ripple is synonymous with a high-quality power supply. It is interesting to observe some FFT analyses on the continuous output signals, concerning the type of filter capacitor used. Figure 4 shows, in fact, three analyses of the FFT using, respectively, electrolytic capacitors of 860,000 µF, 47,000 µF, and 4,700 µF at the output. Note that a large capacity allows obtaining a clean signal with a very low number of harmonics. Obviously, an excellent filter equipped with high-capacity capacitors significantly increases the cost of the project.
Conclusion
The AC/DC conversion circuit is the most used, especially in recent years. For its optimal functioning, all the components used are of excellent quality, which could significantly increase its cost. With this solution, the efficiencies involved are not very high, and the power losses may not be tolerable. In the next articles, you will discover further transformation techniques using the latest-generation electronic components.
Written by Icey Ye from AIChipLink.
AIChipLink, one of the fastest-growing global independent electronic component distributors in the world, offers millions of products from thousands of manufacturers. Whether you need assistance finding the right part or electronic components manufacturers for your design, you can contact us via phone, chat or e-mail. Our support team will answer your inquiries within 24 hours.
Disclaimer: This article is provided for general information and reference purposes only. The opinions, beliefs, and viewpoints expressed by the author of this article do not necessarily reflect the opinions, beliefs, and viewpoints of AIChipLink or official policies of AIChipLink.
.png&w=256&q=75)