DC-DC Converters (choppers) The objective is to convert a fixed DC voltage to a variable DC voltage It is possible to step up and step down voltage.

Voltage step down (buck converter) First, suppose L=0, E=0. The diode is not needed. V a =(t on /T)V s V RMS =k 1/2 V s P out =(kV s 2 )/R k=t on /T is the duty cycle

Step down converter with RL load

Pspice schematics Step down dc-dc converter

Voltage step up (boost) converter “On” mode: V L =L(di/dt) “Off” mode: Assume current decreases at a constant rate. Then V o =V s +V L To ensure continuous current flow, a capacitor is included.

Performance of a step up converter with resistive load (C=0).

DC-DC Converter classification First quadrant converter Second quadrant converter 1 st and 2 nd quadrant converter 3 rd and 4th quadrant converter Four quadrant converter

Second Quadrant Converter

1-2 and 3-4 quadrant converters 1 st quad: S1, D4 2 nd quad: S4, D1 3 rd quad: S3, D2 4 th quad: S2, D3 Polarity of the load EMF is reversed.

Four quadrant converter