Chapter 4 Phase-controlled Rectifiers Read Chapter 5 of "Principles of Power Electronics" (KSV) by J . G . Kassakian, M. F. Schlecht, and G. C. Verghese, Addison-Wesley, 1991. Thyristor Devices: SCR (Silicon Controlled Rectifier) K Figure 4.1: Thyristor SCR: Acts like a diode where you can select when conduction will start, but not when it stops. Stay off until a gate pulse is applied while VAK> 0. Once on, behaves like a diode and does not turn off until i + 0. To stay off (after VaK > 0 again) must have i stay at 0 for a short time t, (10 - loops) So the device is semi-controlled: we control the turn on point, but only turns off when circuit conditions force it to. Simple example: Figure 4.2: Example Phase of thyristor turn on (with respect t o line voltage) is termed firing angle a . Consider a full-bridge converter (inductive/current load). Diode version: CHAPTER 4. PHASE-CONTROLLED RECTIFIERS A ot 2- Dl, D2 / - Conduct D3, D4 ./ - - Conduct Figure 4.3: Diode Version Thyristor (phase-cont rolled) version (firing angle a ): Con uct ( Ql,Q2 Conduct Figure 4.4: Thyristor Version Lets analyze the output voltage < v, >: == Q3,Q4 Conduct Id > 0 by necessity (conducbon of thyristor) A Rectification a z > ( Rectification >o Power Flows AC -> DC Inversion - - Power AC > ..................... Quadrants of 0 ration in ol,17 Figure 4.5: Output Voltage So with a phase controlled converter, we can regulate the output voltage by varying firing angle a . We can even cause power flow from dc-side to ac-side as long as Id > 0 (e.g., pull power out of inductor and put into line). Consider the power factor of a phase-controlled converter: V,i Figure 4.6: Power Factor CHAPTER 4. PHASE-CONTROLLED RECTIFIERS Phase shift of fundamental of square wave in phase with square wave, therefore, = a . So the power factor of a phase-controlled converter varies with firing angle a. Consider the effect of ac-side reactance: No LC + r r Q~A Ql LC il 44 il A VsSin@t) Qz r No LC +~d \ - a+u ot > n a -Id QLQZ Al- +?-= e Q3,Q4 Al- +?-= e + e Q3,Q4 Figure 4.7: AC-S ide Reactance Similar t o the diode rectifier case, a commutation period exists during which all devices are on, while current in LC switches between +Id and -Id (between and Q3IQ4. Q1/Q2 A similar analyze to the diode case shows t h a t for the full-bridge thyristor converter: 2K < v, >= -[cos a n- - cI d 1-XK Note t h a t the need to commutate devices places a limit on how negative the output voltage can be made as a function of 5. (require a X$-d and a . This is analyzed in KSV, Chapter + u < n-) . Summary: VdO ! Commutation Limit Figure 4.8: Summary

Phase-Controlled Rectifiers

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