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Single-Phase Half-Bridge VSI




For the half-bridge inverter circuit, the centre-tap of the DC supply is used as one of the load terminals. The centre-tap is created by the two series-connected equal-valued capacitors across the DC supply. The dc rail voltages are thus at +Vs/2 and −Vs/2 with respect to some fictitious ground potential.


The two switches, Q1 and Q2, are switched alternately, in a complementary fashion, at the desired output frequency, ƒo.



Power electronic converter: Single-Phase Half-Bridge VSI
Single-Phase Half-Bridge Inverter
  1. When Q1 is ON, Q2 is OFF and the voltage at the terminal a of the load is +Vs/2 with irrespective of the direction of current through the load. Similarly, when Q2 is ON, the Q1 is OFF and the potential at point a is –Vs /2.
  2. The load voltage is a square-wave of amplitude Vs /2. For a resistive load the current waveform follows the voltage waveform (as shown). For an inductive load the current waveform lags the voltage waveform by an angle which is, approximately, the load power factor angle.
RMS Output voltage and fundamental component of output voltage are given by:
{V_{0(rms)}} = \sqrt {\frac{2}{\pi } \smallint \limits_0^\pi  {{\left( {\frac{{{V_S}}}{2}} \right)}^2}d\theta }  = \frac{{{v_S}}}{2}
{V_{0(rms)}} = \frac{{2{V_S}}}{{\sqrt 2 \pi }} = 0.45{V_S}
The output voltage can be expressed in Fourier series as
{v_o}(t) = \sum\limits_{n - 1,3,5,..}^\infty  {\frac{{2{V_S}}}{{n\pi }}} \sin n\omega t
               = 0{\rm{    }}for{\rm{   }}n = 2,4,.....

Function of antiparallel diodes, D1 and D2

Load current may not reverse at the same instants as does the load voltage. Current may lead or lag the output voltage due to the presence of capacitance and/or inductance in the load circuit. Diodes D1 and D2 in anti-parallel with each transistors permit load current to flow if necessary.

In the leading current case, the output current reverses its direction at tX. Output voltage reverses its direction at T/2. Therefore, from tX to T/2 the output current will flow through D1.

In the lagging current case, the output current reverses its direction at tY. Output voltage reverses its direction at T/2. Therefore, from T/2 to tY the output current will flow through D2.

power electronic converter-DC-AC CONVERTER: Function of antiparallel diodes
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