Single-phase half-bridge inverters are widely used in secondary power supply mainly with transformer load, for example, in small-scale inverter type devices [1].
The power part of such inverters represents an "artificial" bridge, two arms of which consist of two series-connected condenser groups, and the other two ones - of two series-connected thyristors or transistors in forward direction [2]. Together with the main circuit simplicity (two semi-conductor elements only) a high merit of the half-bridge inverter is the absence of the DC voltage component in the diagonal line of alternating current, i.e. under a load, at any unbalances and component values. It is this very merit that allows using a closed core transformer made of high induction material with rectangular hysteresis loop, for example, of anisotropic steel ("mopermalloy").
The demerit of the transistor variant of such inverters is the limited capacity (current).Thus, the inverter type welding units are produced for the current up to (150÷160) А. The welding process high-amperage currents require parallel joint of transistors, that complicates the inverter and reduces its reliability.
There is no such demerit in the thyristor half-bridge inverter; however, the known thyristor half-bridge inverters on one-operation (usual) thyristors are able to work only in a limited range of loads: from about 0,3 of the nominal one and higher. At light loads the cross-plugging of one-operation thyristors becomes impossible. To extent the operability zone is possible using the forced commutation or two-operation (gate-controlled) thyristors, but it complicates and makes the inverter more expensive as a whole; the losses growing as well.
In the power circuit the classical single-phase half-bridge thyristor inverter contains a half-bridge in the form of series-connected thyristors and another half-bridge in the form of series-connected condensers, which is hooked up by means of the DC current diagonal to the power supply shunted with smoothing condenser and hooked up by means of the alternating current diagonal to the load, and also containing a standard control system as part of active oscillator and the inverter control system, the output of which is connected with the control inputs of the inverter thyristors, one input is hooked up to the output of the abovementioned active oscillator, and the second one - to the comparer, to the inputs of which the definition and feedback signals are hooked up on the principle of degenerative feedback.
It is offered to introduce a supplementary half-bridge in the form of two series-connected transistors and two bypass diodes into the inverter circuit, which is hooked up parallel with the thyristor half-bridge and loaded by the diagonal of the alternating current to the restrictor; the common point of the transistors, bypass diodes and restrictor of the supplementary half-bridge being not connected with the common point of the thyristors; the restrictor loose end being connected with the common point of the condensers and the control inputs of the supplementary half-bridge transistors being hooked up to the supplementary output of the abovementioned active oscillator [3].
As the capacitor voltage has an intermittent nature, by the end of every half cycle the corresponding thyristor manages to switch off, and the transformer core hysteresis loop rectangularity provides a rapid excitation current fall-off of the transformer within the circuit of the corresponding thyristor. At the load current appearing the on-load voltage integral begins decreasing. The restrictor current decreases accordingly, i.e. the current hogging takes place: the more the load - the less the restrictor circuital current. It should be noted that the energy circulating within the restrictor circuit has a purely reactive nature and, with the neglect of the resistance losses in the capacitors, restrictor and transistor half-bridge this energy doesn´t practically magnify the current consumed from the net. If the device is used, for example, as an electric welding bug, then the transformer secondary is connected to the load (electrode - weldment) through the rectifier and current filter. Then the voltage regulation characteristic of the device guarantees welding at constant power, that is ideally suited for a welding process in the air. So, the supplement of the single-phase half-bridge thyristor inverter "classical" circuit with a restrictor and transistored half-bridge for the current of not more than 30% of the load current allowes providing the workability of the device through the whole load range and also lifting the current restrictions imposed by the parameters of the existing transistors in transistor inverters (avoiding the necessity to shunt connection of transistors in the devices with the current of more than 200A).
References:
- «Invertec» V - 130 - S-Linkoln, USA (catalog), 1998.
- Pryanishnikov V.A. «Electronics», Saint Petersburg, p. 400, 1998.
- Magazinnik L.T., Magazinnik A.G. Single-phase half-bridge thyristor inverter. Patentof the RF № 2294590, BI № 6 from 27.02.2007.