Phase power regulator on a key field-effect transistor. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Phase power regulator on a key field-effect transistor. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Regulators of current, voltage, power

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Typically, phase AC power regulators are built on the basis of a thyristor or triac. These schemes have long become standard and have been repeated many times both by radio amateurs and on a production scale. But thyristor and triac controllers, as well as switches, have always had one important drawback - limiting the minimum load power. That is, a typical thyristor regulator for a maximum load power of more than 100 watts cannot well regulate the power of a low-power load that consumes units and fractions of watts.

Key field-effect transistors differ in that the physical operation of their channel is very similar to the operation of a conventional mechanical switch - in the fully open state, their resistance is very small and amounts to fractions of an ohm, and in the closed state, the leakage current is microamperes and this practically does not depend on the voltage on the canapes.

That is why the key cascade on the key field-effect transistor can switch the load with power from units and fractions of watts, up to the maximum current allowable value. For example, the popular IRFS40 field-effect transistor without a heat sink, operating in a key mode, can switch power from almost zero to 400 watts.

In addition, the switching FET has a very low gate current, so very low static power is required for driving. True, this is overshadowed by the relatively large gate capacitance, so at the first moment of switching on, the gate current can turn out to be quite large (current per charge of the gate capacitance). This is combated by turning on a current-limiting resistor in series with the gate, which reduces the speed of the key, since an RC target is formed consisting of this resistance and the gate capacitance, or the output of the control circuit is made more powerful.

The power regulator circuit is shown in the figure.

Phase power regulator on a key field-effect transistor

The load is powered by a pulsating voltage, as it is connected through a diode bridge VD5-VD8. This is suitable for powering an electric heating device (soldering iron, incandescent lamp).

Since the negative half-wave of the pulsating current is "turned" upwards, ripples with a frequency of 100 Hz are obtained. But they are positive, that is, a graph of change from zero to a positive amplitude value of the voltage. Therefore, adjustment is possible from 0% to 100%.

The value of the maximum load power in this circuit is limited not so much by the maximum current of the open channel VT1 (it is 30 A). how much is the maximum forward current of the rectifier bridge diodes VD5-VD8. When using KD209 diodes, the circuit can operate with a load of up to 100 watts. If you need to work with a more powerful load (up to 400 W), you need to use more powerful diodes, for example, KD226G, D.

On the inverters of the D1 chip, a driver of control pulses is made, which open the transistor VT1 in a certain half-wave phase. Elements D1.1 and D1.2 form a Schmitt trigger, and the remaining elements D1.3-D1.6 form a powerful output inverter.

The output had to be boosted to compensate for the troubles caused by the current surge to charge the capacitance of the VT1 gate at the moment it was turned on.

The low-voltage power supply system of the microcircuit by means of the VD2 diode is divided into two parts - the actual supply part, which creates a constant voltage between terminals 7 and 14 of the microcircuit, and the part that is a mains voltage phase sensor. It works as follows.

The mains voltage is rectified by the VD5-VD8 bridge, then it is fed to the parametric stabilizer on the R6 resistor and the VD9 zener diode. Since there is no smoothing capacitor in this circuit, the voltage on the zener diode is pulsating.

The R1-R2-C1 circuit, together with the VD1 diode, sets the phase of the pulsating voltage at which the voltage across the capacitor C1 reaches the switching threshold of the Schmitt trigger. By changing the resistance of this RC circuit, we change the opening delay time of the key transistor from the moment when the voltage in the network reaches a value of 8-10V (the voltage value of the switching threshold of the Schmitt trigger). Since the mains frequency is quite stable, the opening moment of the key transistor relative to the phase of the mains voltage is maintained sufficiently stable relative to the set resistor R1.

The diode VD1, together with the resistor R5, forms a circuit for the accelerated discharge of the capacitor C1, which is necessary for this capacitor to be discharged when the phase of the mains voltage comes to zero.

In this case, the Schmitt trigger switches to the zero state and the key transistor closes. Thus, by adjusting the resistance R1, we change the phase of the opening moment of the key transistor, and the voltage is supplied to the load only in the period from this point to the amplitude value. Thus, phase power control occurs. In general, the principle is almost the same as in a thyristor regulator.

Now about the power supply of the microcircuit. In practice, the microcircuit is powered by the voltage stored in the capacitor C2. At each half-wave, this capacitor is charged through the diode VD2. Then, when the phase goes to zero, this diode closes and the microcircuit is powered by the charge of capacitor C2. Therefore, the supply voltage of the microcircuit is constant, stable and not subject to ripples. All parts except for the resistor R1 on the printed circuit board with one-sided metallization.

Since the author's version is designed to work with a load with a power of not more than 100W, no radiators are provided and KD209 diodes are used in the bridge rectifier. However, the FET does not need a radiator even with a rated load power of up to 400 watts. But the diodes will have to pick up more powerful.

Chip K561LN2 can be replaced by K1561LN2. Zener diode. D814G can be replaced with another zener diode for a voltage of about 10V.

During the adjustment process, it may be necessary to select the resistances of the resistor R2 (to provide the necessary width of the adjustment range) and the resistor R5 (to ensure the discharge of C1). The resistance R5 must be chosen as large as possible, but such that at the minimum power set by R1, the transistor does not open at all.

Author: Kapachev D.E.

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