Dr John Fletcher John.Fletcher@unsw.edu.au Rectifiers Dr John Fletcher John.Fletcher@unsw.edu.au

Rectifiers Rectifiers can be uncontrolled (e.g. diode rectifier) or controlled (e.g. using active switches and diodes). They convert AC to DC. At power frequency (e.g. 50Hz) turn-on and turn-off processes can be assumed ideal. Let us look at a simple single-phase half-wave diode rectifier

Resistive Load Resistive load iL follows the input voltage Diode blocks reverse current Half-wave output voltage

Resistor-Inductor Load Inductor continues to force current around circuit beyond the zero crossing of the input voltage. Diode will not commutate off until load current falls to zero. iL does not follow the input voltage Half-wave output voltage (plus an extra bit at the end!)

Rectifier Analysis Currents and voltages are neither AC or DC They are transient. The circuit is non-linear because of the diode. This makes circuit analysis difficult! Variables typically required to be calculated in a rectifier circuit: Average load voltage Average load current RMS supply current RMS diode (or switch) current Peak voltage across semiconductor devices

Rectifier Analysis – Load Voltage Whilst the circuit is non-linear and the waveforms neither AC nor DC, the voltage and currents are cyclic. For example, the rectifier with RL load. Calculate the mean load voltage:

Rectifier Analysis – Load Current That was easy! Now find an equation for the current. The current waveshape is not sinusoidal. We need to perform transient analysis on the circuit.

Rectifier Analysis – Load Current

Rectifier Analysis - Example

Rectifier Analysis - Example This is a transcendental equation that is soluble by graphical or numerical methods (not by calculus). In the rectifier case, the trigger angle = 0o and with the parameters given φ=72.3o. From the graph, β=265o. We can now calculate the mean current

β=265o φ=72.3o

Rectifier Analysis - Example After a page of simplification gives: That is The DC current = DC Voltage/R A general result for all rectifiers supplying PASSIVE loads.

Rectifier with Freewheel Diode Sketch the load voltage and load current waveforms. Assume that the load current does not fall back to zero every cycle.