This switching regulator is highly efficient and can be used for AC and DC and requires no reactive L/C components. The regulator can provide a power factor very close to 1.
The circuit (see Figure 1) can be used to regulate a heating process with some thermal inertia such as a soldering iron, hot wire cutter, heater, and so on. The circuit is easily scalable for many more purposes from clothing irons to industrial heating and drying processes. It can be used as a dimmer for incandescent lamps as well.
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| Figure 1. | A simple regulator for heating or lighting purposes, the circuit can be used to regulate heating processes with thermal inertia such as a soldering iron, hot wire cutter, heaters, etc. |
Due to its low thermal inertia, the lamp is a special case. During a period of several milliseconds a load should tolerate the voltage pulses at full input amplitude. This may be too harsh for an incandescent lamp to survive. So, the nominal voltage of the lamp should not be lower than VIN.
The circuit has a capacitor-less rectifier at the input, hence the reverse voltage on the diodes is twice as low as it would be with a smoothing capacitor. This facilitates the usage of current-effective Schottky diodes in the bridge.
The output voltage is a train of monopolar pulses – thus the circuit regulates an effective voltage on the load. This resembles pulse width modulation (PWM), the difference is the non-constant amplitude of the pulses at the input in the case of AC.
The monopolar pulses the circuit produces are averaged by R1C1 and some part of the result (R2/R8, R9, R10) is compared by TL431(Q3) with its internal VREF. If this part is lower than the internal VREF (2.5 V), the transistor Q2 is closed, so the switch Q1 is closed as well, and the load is connected. And vice versa, when the input of TL431 is higher than 2.5 V, both Q2 and Q1 are open, and the load is disconnected.
When the input of TL431 is higher than 2.5 V, its cathode voltage (VKA) is not well documented; it’s only known it would be about 2 V. The diodes D3 reduce the gate voltage of opened Q2 to a value lower than 0.2 V. Diode D4 protects the circuit from an overvoltage caused by load inductance. The transistor Q1 may have no heatsink.
If the circuit uses AC as VIN, the time constant R1C1 must be more than the AC period. The values shown are for AC 50 Hz. Potentiometer R8 can have a scale graduated in volts. The simple circuit in Figure 1 is not well-suited for very heavy loads (~100 W).
The circuit in Figure 2 is more complex, it is intended for more heavy loads.
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| Figure 2. | Another regulator that is well-suited for heavier loads on the order of 100 W. |
It has several distinctions from the previous circuit including:
- A “half-driver” Q5 which accelerates the opening of Q1,
- A more effective source of auxiliary voltage (Q6, D1),
- A green LED D2 (~2 V) used as interface between TL431 and Q2 (instead of the diodes D3 in Figure 1), and
- A faster diode D4.
