Rectifier for high currents with low losses. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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The described unusual AC rectifier is designed for use where low regulated voltages are required at relatively high currents and low losses. An example of an application is the power supply of Peltier elements used in cooling systems, where, in addition, it is necessary to regulate the temperature. Electroplating baths and low-voltage soldering irons are other examples of applications for a similar rectifier.

When obtaining low supply voltages in rectifiers, the problem arises of a voltage drop across the rectifier semiconductor diodes, due to the semiconductor material used in the diodes (0,6 ... 0,9 V in silicon diodes), which has the greater effect, the lower the rectified voltage. There is a problem of heat removal at high load currents. When it is also necessary to adjust the output voltage, they resort to using a series voltage stabilizer, the voltage drop at the junction of the regulating transistor of which is, in addition to the drop on the rectifier diodes, a few more volts, which leads to useless power dissipation, the efficiency of the device, while, does not exceed 50%. Figure (Bild 1) shows a rectifier circuit taken from the collection of GDR patents [1], which can significantly reduce power losses.

Fig. 1.

This is primarily a full-wave rectifier with a mid-point, which is characteristic and known as a rectifier having two diodes and a tap from the middle of the transformer winding. Here, the rectifier diodes are replaced by emitter-collector junctions of the regulating transistors (VT1 and VT2). This provides an advantage over diodes, since the voltage drop at the emitter-collector junctions for modern high-power planar transistors is only 0,1 ... In addition, when using transistors as controlled elements, it becomes possible to adjust the output rectified voltage, namely, by phase truncation.

Fig. 2

During the positive half-cycle, the current flows through VD1, switch contacts S (S - first in the far right, according to the diagram, position), resistor R and diode VD4 in the base-emitter circuit VT2. VT2, at the same time, is controlled, as a result of which the lower branch of the rectifier opens, and the capacitor C is charged. During the negative half-cycle, the transistor VT1 is controlled through the diode VD2, S, R and VD3, which opens the upper branch of the rectifier. Since we are talking about a full-wave rectifier, in which the residual voltage drop across the emitter-collector junctions of transistors is very small, the power dissipated on the transistors is also small, equal to the voltage drop at the emitter-collector junction multiplied by the current flowing in this circuit. If the dissipation power is low, the heat sink can also be small, and if the negative pole of the rectifier can also be connected to the metal case of the powered device, then the control transistors can be screwed with collector leads directly to the chassis without insulating gaskets.

Now let's consider the possibility of adjusting the output voltage of the rectifier using a chain of diodes VD5 ... VDn, switched by the switch S, which cut off the phase (Bild 2). Transistors, in this case, begin to conduct not immediately from the beginning of the corresponding half-cycle of the alternating voltage, but after some time, when the instantaneous value of the voltage amplitude in the half-cycle exceeds the sum of the direct voltages of the diodes on. Accordingly, the shorter the time the transistors are open, the lower the voltage can be charged by the filter capacitor C. Of course, the effect of later opening and earlier closing of transistors depends on the direct voltage drop across the diodes VD1 ... VD4 and on the opening voltage of transistors VT1 and VT2. Here it is best to use germanium diodes due to the small forward voltage drop across them, for example, 0,1 A or 1 A diodes from the GY series. Diodes with a Schottky barrier turn out to be more modern here, but the results obtained with them are no better, but worse than with the good old germanium diodes, especially since not everyone can still get Schottky diodes.

Particular attention should be paid to the maximum allowable reverse voltage of the base-emitter junctions VT1 and VT2. If this voltage is exceeded, the current from the corresponding outer end of the secondary winding of the power transformer will flow through the locked emitter-base junction (as a stabilization current (or "avalanche breakdown current") in the zener diode) and from there through the base-collector junction switched on in the forward direction of current flow, - directly to the rectifier output. In this case, of course, there can be no question of any regulation by transistors and they are damaged. The peak voltage value on any half of the secondary winding must not exceed the allowable reverse voltage of the emitter-base junction (Ueff * 3 2), which must be within 6 ... 9 V.

It is recommended to measure the allowable reverse voltage of the base-emitter junctions before installing transistors in the circuit (and, probably, since the circuit is symmetrical, select a pair of transistors with the same parameters). The way to measure this voltage is simple: you need to turn on the base-emitter junction in the opposite direction (blocking the passage of direct current) through the resistor and measure the voltage at the junction in the same way as the stabilization voltage is determined on a conventional zener diode. We increase the voltage supplied to the resistor connected in series (for example, with a resistance of 1 kΩ) and the base-emitter junction ("plus" to the emitter if it is an npn transistor), on a voltmeter connected in parallel with the junction, we observe the value of the maximum reverse voltage, when it ceases to noticeably increase with increasing supply voltage. The latter circumstance (rather low allowable reverse voltage of the base-emitter junction) limits the maximum output voltage of the driven rectifier circuit to 5 volts. The resistance value R = 200 ohm was chosen as a compromise for an output voltage of up to 5 V at load currents of 1 ... 2 A: its too small value leads to excessive losses in the resistor itself (uneconomical), while a large one does not for which the losses also increase (now on the regulating transistors).

Transistors should have as much base-emitter reverse voltage as possible and have the highest possible current gain. If pnp transistors (for example, KT818) are used, all diodes and an oxide filter capacitor should be "flipped" and the polarity of the output voltage will change.

You can go further and instead of discrete adjustment of the output voltage, apply a smooth one by installing instead of diodes VD5 ... VDn and switch S, the same conductivity as VT1 / VT2 (collector to the junction point of diodes VD1 and VD2, emitter to resistor R) and a potentiometer, the output of the engine of which should be connected to the base of an additional transistor, and the extreme conclusions to the collector and emitter of this transistor. Other inclusions with a falling characteristic are also possible (analogue of a dinistor). For the experimenter, there is a large field of activity.

Literature

1. Patent DDR-WP HO2 313189.7
2. Dipl.-Ing. M. Franke
3. FUNKAMATEUR 1988, no. 11, p. 554.

Translation: Viktor Besedin (UA9LAQ) ua9laq@mail.ru, Tyumen; Publication: cxem.net

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