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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Synchronous rectifier. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Voltage converters, rectifiers, inverters The voltage drop across the rectifier diodes does not allow increasing its efficiency above a certain limit. By shunting or replacing each diode with an electronic key, this limit can be exceeded. However, due to the complexity of the electronic key control unit, synchronous rectifiers have found application only in professional power supply equipment. The proposed article describes a simple design of a synchronous rectifier, available for repetition in amateur radio conditions. One of the most important tasks facing the designers of modern power supplies is to achieve high efficiency. Typically, rectifiers are made on silicon diodes or Schottky diodes, less often on germanium diodes. The typical voltage drop on silicon diodes is 1 V, on germanium and Schottky diodes - about 0,5 V. Significantly less energy loss in synchronous rectifiers on powerful key field-effect transistors, where diodes are replaced by field-effect transistors. The open channel resistance of modern field-effect transistors is reduced to a few milliohms. This makes it possible to reduce the voltage drop and, accordingly, the heat dissipation by an order of magnitude. But the use of field-effect transistors in rectifiers has a number of features. The first is the presence of an internal diode in the field-effect transistor. If a voltage of reverse polarity is applied to the field-effect transistor, then the internal diode will open. With a synchronous supply to the gate of the transistor relative to the source of the voltage of the opening polarity of a sufficient value, the channel of the field-effect transistor opens, connected in parallel with this diode. Since the channel resistance of an open field effect transistor is much less than the resistance of an open diode, almost all of the current will flow through the channel. Another feature of the field-effect transistor is the turn-on and turn-off delay due to the presence of gate-source and gate-drain capacitances. These capacitances are highly voltage dependent. They are large at low voltage and decrease when it is increased. To ensure that the transistor opens, it is necessary to charge the input capacitance to 10 ... 12 V. This process is hindered by the Miller effect, which increases the equivalent input capacitance. More details about the features of high-power field-effect key transistors can be found in the book by B. Yu. Semenov "Power electronics: from simple to complex" (M .: "SOLON-Press", 2005).
On fig. 1 shows a diagram of a full-wave synchronous rectifier designed to rectify a rectangular and sinusoidal voltage. The rectifier is connected to the secondary winding of the transformer with a tap from the middle. Pins 1 and 3 - to the beginning and end of the winding in any order, pin 2 - to the winding outlet. For rectification, transistors VT1 and VT2 with internal diodes are used. Capacitor C1 - smoothing. The node for the formation of control pulses applied to the gates of transistors is assembled on microcircuits DA1, DA2, DD1, DA4, diodes VD1, VD2 and resistors R1-R6. This node receives a 10 V supply voltage from a voltage regulator on the DA3 chip. If control pulses are not supplied to the gates of the transistors, for example, if the pulse shaping unit is disabled, the rectifier operates as a normal (asynchronous) rectifier on the internal diodes of the transistors. The principle of generating a control pulse at the transistor gate: the pulse voltage should open the transistor channel when the voltage at the cathode of the internal diode is less than the voltage at its anode, which is connected to a common wire - minus the output voltage. That is, when the voltage at the cathode is negative polarity, an opening voltage of positive polarity must be applied to the gate of the transistor relative to its source. The rest of the time, the voltage between gate and source must be zero for the transistor to be off. It is very important that the opening pulses must not overlap in time so that both transistors are not open at the same time. The pulse shaping unit works like this. The voltage at the drains of the transistors is monitored by comparators DA1 and DA2. On the DD1 chip, a node is assembled that excludes the overlap of the opening pulses. The inverters on the DA4 chip provide up to 1,5A of output current, which quickly charges the input capacitance of the transistors despite the interference of the Miller effect. Let a positive voltage half-wave act on the drain of transistor VT1. A voltage of +0,7 V from the diode VD1 is applied to the inverting input of the comparator DA1 relative to its non-inverting input, as a result of which a high level appears at the DA1 output. This leads to a high voltage level at pin 2 of the DA4 driver, and therefore, its output will be a low voltage level. Transistor VT1 is closed. Let a negative half-wave of voltage act on the drain VT1, opening its internal diode. At the non-inverting input of the DA1 comparator, the voltage is greater than at the inverting one, as a result of which the comparator output will have a low voltage. This will cause pin 2 of the DA4 driver to go low and the output to go high. Transistor VT1 opens and shunts its internal diode, resulting in reduced rectification energy losses. The transistor VT2 is controlled in a similar way. On the DD1 chip, a control unit for the correct operation of the rectifier is made. It contains four logical elements "exclusive OR". The fact is that at the moment the sinusoidal voltage passes through zero, the outputs of comparators DA1 and DA2 will simultaneously have low voltage levels. If these outputs were connected to the inputs of the DA4 chip, this would lead to the simultaneous opening of both transistors VT1 and VT2, which is unacceptable due to the through current through them. Therefore, between the outputs of the comparators DA1 and DA2 and the inputs of the DA4 chip, a node on the DD1 chip is included. Let's take a look at his work. Let the outputs of both comparators have low voltage levels. This combination of input signals at the input of the element DD1. 1 corresponds to a low voltage level at its output. An inverter is made on the DD 1.2 element, for which the supply voltage (high level) is applied to pin 13. Thus, pin 6 of the DD1.3 element and pin 9 of the DD1.4 element have a high voltage level, and they will also work as inverters. As a result, both inputs of the DA4 driver have a high voltage level, the gates of both transistors VT1 and VT2 have a low level, so they are closed. There will be no through current through them. In the case of anti-phase signals at the outputs of the comparators and, accordingly, at the inputs of DD1.1, a high voltage level will operate at pin 3 of DD1.1. After the inversion in the logic element DD1.2, a low voltage level translates the logic elements DD1.3 and DD1.4 into signal repeaters. Therefore, the signals from the outputs of the comparators DA1 and DA2 will pass without change to the outputs of the driver DA1. One of the transistors will be open, the other closed. A stabilized supply voltage of 10 V is generated by the L4810CV (DA3) microcircuit, which has 1,5 A output current overload protection and an automatic shutdown unit when the temperature rises above the maximum permissible value. This microcircuit maintains the voltage stabilization mode when the voltage difference between the input and output decreases down to 0,5 V. It is powered by the output voltage of the rectifier.
The synchronous rectifier is assembled on a printed circuit board made of 1,5 mm thick fiberglass foiled on one side, its drawing is shown in fig. 2. All parts are installed on it, except for the smoothing capacitor C1. If transistors VT1 and VT2 get very hot, they are installed on heat sinks. A place is provided for their placement on the board. The author uses a synchronous rectifier to rectify the voltage from the secondary winding of the Feron ET105 electronic transformer. The secondary winding is wound in it with two wires, which facilitated the task of performing a tap from its middle. To reduce voltage ripple at twice the mains frequency, a smoothing oxide capacitor with a capacity of 10 μF and a rated voltage of 400 V is installed at the output of the rectifier bridge inside the electronic transformer. The frequency of the output voltage of the transformer is about 45 kHz. These transformers have a minimum power limit that must be taken into account to ensure reliable operation. The synchronous rectifier allows this electronic transformer to obtain an output voltage of 12 V at a load current of 9 A. The smoothing capacitor C1 of the capacitance indicated in the diagram is used to rectify the voltage with a frequency of 45 kHz. Of course, a synchronous rectifier can also be used to rectify a voltage with a frequency of 50 Hz by calculating the capacitance of the smoothing capacitor in the same way as for a conventional (asynchronous) full-wave rectifier. Author: V. Kalashnik
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Comments on the article: Alexander Can I use other transistors? These have diodes. It will be okay what or k.z.
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