This preview shows pages 1–3. Sign up to view the full content.
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
Unformatted text preview: Basic Principles of Switch Mode Power Conversion This chapter discusses some of the basic principles that are common to the steady state analysis of all switch mode power converters. 1. Bipositional switch The most basic component of a switch mode power converter is the bipositional switch. A mechanical bipositional switch may be realized using a single SPDT (single pole, double throw) switch shown in Fig. 1a, but the electronic implementation always consists of two SPST (single pole, single throw, or simply, onoff switches) as shown in Fig. 1b. The two positions of the switch 1 and 2, are invariably connected across a dc voltage source or across a big capacitor whose voltage is close to a constant dc (we will see later that the pole of the switch, denoted as A in Fig. 1 is in series with a dc current source or a big inductor whose current is close to a constant dc). The bipositional switch, which is also referred to as a switching power pole, switches at very high frequencies, and is controlled by a control signal, ( ) A t q as explained below. Referring to both Fig. 1a and Fig. 1b, A q = 1 implies that switch is in position 1 (positive terminal) and A i n v V = A q = 0 implies that switch is in position 2 (negative terminal) and A v = Fig. 1. Bipositional switch (a) using a SPDT, (b) using two SPSTs. 1 2 A ( ) A t q + _ V in + _ v A ( t ) ( ) A t q ( ) A t q (b) 1 2 A ( ) A t q + _ V in + _ v A ( t ) (a) i A ( t ) i A ( t ) Fig. 1. Bipositional switch (a) using a SPDT, (b) using two SPSTs. 1 2 A ( ) A t q + _ V in + _ v A ( t ) ( ) A t q ( ) A t q (b) 1 2 A ( ) A t q + _ V in + _ v A ( t ) (a) i A ( t ) i A ( t ) 1 2 A ( ) A t q + _ V in + _ v A ( t ) ( ) A t q ( ) A t q (b) 1 2 A ( ) A t q + _ V in + _ v A ( t ) (a) i A ( t ) 1 2 A ( ) A t q + _ V in + _ v A ( t ) (a) i A ( t ) i A ( t ) Duty ratio: As illustrated in Fig. 2, the switching period, which is the inverse of the switching frequency, S f , is denoted as S T . The time for which the bipositional switch is in position 1 (positive terminal) in each period is defined as ON T . The duty ratio of the bipositional switch is then defined as ratio of ontime to total period, as expressed in (1). dutyratio ON S T , d T = ( 1 ) Fig. 3a shows the actual electronic implementation of a complete bipositional switch using t t q A ( t ) 1 T ON T S v A ( t ) V in Fig. 2. Waveforms corresponding to a bipositional switch t t q A ( t ) 1 T ON T S v A ( t ) V in t t q A ( t ) 1 T ON T S v A ( t ) V in Fig. 2. Waveforms corresponding to a bipositional switch 1 2 A i A ( t ) 1 2 A i A ( t ) + _ V in + _ v A ( t ) + _ V in + _ v A ( t ) Fig. 3. Electronic implementation of bipositional switch. (a) for bidirectional poleElectronic implementation of bipositional switch....
View Full
Document
 Spring '11
 Ayynar

Click to edit the document details