现代电力电子技术课件Chapter3-part2.ppt

1、1,3.3 Buck converter in DCM （Discontinuous Conduction Mode ）,Introduction to DCM Occurs because switching ripple in inductor current or capacitor voltage causes polarity of applied switch current or voltage to reverse, such that the current- or voltage-unidirectional assumptions made in realizing th

2、e switch are violated. Commonly occurs in dc-dc converters and rectifiers, having single quadrant switches. May also occur in converters having two-quadrant switches.,2,Introduction to DCM 3. Typical example: dc-dc converter operating at light load (small load current). Sometimes, dc-dc converters a

3、nd rectifiers are purposely designed to operate in DCM at all loads. 4. Properties of converters change radically when DCM is entered: M becomes load-dependent Output impedance is increased Dynamics are altered Control of output voltage may be lost when load is removed,Chapter3 The steady-state anal

4、ysis of converter,3,3.3.1 Origin of the DCM, and mode boundary,4,3.3.1 Origin of the DCM, and mode boundary,5,3.3.1 Origin of the DCM, and mode boundary,6,Mode boundary,3.3.1 Origin of the DCM, and mode boundary,7,K and Kcrit vs. D,3.3.1 Origin of the DCM, and mode boundary,8,Critical load resistanc

5、e Rcrit,3.3.1 Origin of the DCM, and mode boundary,9,Summary: mode boundary,3.3.1 Origin of the DCM, and mode boundary,10,3.3.2 Analysis of DCM buck converter M(D,K),Analysis techniques for the discontinuous conduction mode:,Inductor volt-second balance Capacitor charge balance Small ripple approxim

6、ation sometimes applies: because is a poor approximation when Converter steady-state equations obtained via charge balance on each capacitor and volt-second balance on each inductor. Use care in applying small ripple approximation.,11,3.3.2 Analysis of DCM buck converter M(D,K),12,Subinterval 1,3.3.

7、2 Analysis of DCM buck converter M(D,K),13,Subinterval 2,3.3.2 Analysis of DCM buck converter M(D,K),14,Subinterval 3,3.3.2 Analysis of DCM buck converter M(D,K),15,3.3.2 Analysis of DCM buck converter M(D,K),Inductor volt-second balance,16,3.3.2 Analysis of DCM buck converter M(D,K),Capacitor charg

8、e balance,17,3.3.2 Analysis of DCM buck converter M(D,K),Inductor current waveform,18,3.3.2 Analysis of DCM buck converter M(D,K),Solution for V,19,3.3.2 Analysis of DCM buck converter M(D,K),Buck converter M(D,K),20,3.4 Boost converter in DCM,21,3.4 Boost converter in DCM,Mode boundary,22,3.4 Boost

9、 converter in DCM,Mode boundary,23,3.4 Boost converter in DCM,Conversion ratio: DCM Boost,24,3.4 Boost converter in DCM,Subinterval 1,25,3.4 Boost converter in DCM,Subinterval 2,26,3.4 Boost converter in DCM,Subinterval 3,27,3.4 Boost converter in DCM,Inductor volt-second balance,28,3.4 Boost conver

10、ter in DCM,Capacitor charge balance,29,3.4 Boost converter in DCM,Inductor and diode current waveforms,30,3.4 Boost converter in DCM,Equate diode current to load current,31,3.4 Boost converter in DCM,Solution for V,32,3.4 Boost converter in DCM,Solution for V,33,3.4 Boost converter in DCM,Boost conv

11、erter characteristics,34,3.4 Boost converter in DCM,Summary of DCM characteristics,35,3.4 Boost converter in DCM,Summary of DCM characteristics,36,Summary of key points,The discontinuous conduction mode occurs in converters containing current- or voltage-unidirectional switches, when the inductor cu

12、rrent or capacitor voltage ripple is large enough to cause the switch current or voltage to reverse polarity. 2. Conditions for operation in the discontinuous conduction mode can be found by determining when the inductor current or capacitor voltage ripples and dc components cause the switch on-stat

13、e current or off-state voltage to reverse polarity.,37,3. The dc conversion ratio M of converters operating in the discontinuous conduction mode can be found by application of the principles of inductor volt-second and capacitor charge balance. 4. Extra care is required when applying the small-ripple approximation. Some waveforms, such as the output voltage,should have small ripple which can be neglected. Other waveforms, such as one or more inductor currents, may have large ripple that cannot be ignored. 5. The characteristics of a converter changes signifi