basics_of_smpc class note

basics_of_smpc class note - Basic Principles of Switch Mode...

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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. Bi-positional switch The most basic component of a switch mode power converter is the bi-positional switch. A mechanical bi-positional 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, on-off 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 bi-positional 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. Bi-positional 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. Bi-positional 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 bi-positional switch is in position 1 (positive terminal) in each period is defined as ON T . The duty ratio of the bi-positional switch is then defined as ratio of on-time to total period, as expressed in (1). dutyratio ON S T , d T = ( 1 ) Fig. 3a shows the actual electronic implementation of a complete bi-positional switch using t t q A ( t ) 1 T ON T S v A ( t ) V in Fig. 2. Waveforms corresponding to a bi-positional 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 bi-positional 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 bi-positional switch. (a) for bi-directional poleElectronic implementation of bi-positional switch....
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basics_of_smpc class note - Basic Principles of Switch Mode...

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