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.

Va=(ton/T)Vs

VRMS=k1/2Vs

Pout=(kVs2)/R

k=ton/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: VL=L(di/dt)

“Off” mode: Assume current decreases at a constant rate. Then

Vo=Vs+VL

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• 1st and 2nd quadrant

converter• 3rd and 4th quadrant

converter• Four quadrant converter

Second Quadrant Converter

1-2 and 3-4 quadrant converters1st quad: S1, D4

2nd quad: S4, D1

3rd quad: S3, D2

4th quad: S2, D3

Polarity of the load EMF is reversed.

Four quadrant converter