Topology

APPLICATION NOTE

TOPOLOGIES FOR SWITCHED MODE POWER SUPPLIES

by L. Wuidart

I

INTRODUCTION the rectifier, different types of voltage converters can be made: - Step down “Buck” regulator - Step up “Boost” regulator - Step up / Step down “Buck - Boost” regulator II - 1 The “Buck” converter: Step down voltage regulator The circuit diagram, often referred to as a “chopper” circuit, and its principal waveforms are represented in figure 1:

This paper presents an overview of the most important DC-DC converter topologies. The main object is to guide the designer in selecting the topology with its associated power semiconductor devices. The DC-DC converter topologies can be divided in two major parts, depending on whether or not they have galvanic isolation between the input supply and the output circuitry. II NON - ISOLATED REGULATORS SWITCHING

According to the position of the switch and

AN513/0393

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APPLICATION NOTE

Figure 1: The step down “Buck” regulator

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The power device is switched at a frequency f = 1/T with a conduction duty cycle, δ = ton/T. The output voltage can also be expressed as: Vout = Vin . δ Device selection:

* Rectifier:

VRRM ≥ Vin max IF(AV) ≥ Iout (1-δ)

* Power switch:

Vcev or VDSS > Vin max Icmax or ID max > Iout + ∆I 2

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II.2

The “Boost” converter: Step up voltage regulator

Figure 2 : The step up “Boost” regulator

δ =

ton T

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In normal operation, the energy is fed from the inductor to the load, and then stored in the output capacitor. For this reason, the output capacitor is stressed a lot more than in the Buck converter. Vin 1-δ

Device selection:

* Power switch:

Vcev or VDSS > Vout Iout 1-δ + ∆I 2

Icmax or IDmax >

Vout =

* Rectifier:

VRRM > Vout IF(av) > Iout

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II - 3

“Buck-Boost converter: Step up/Step down voltage regulator

Figure 3 : The step up/step down “Buck-Boost” regulator

iL

∆

For a...