1. A Unified Model for the ZVS DC-DC Converters With Active Clampby
N.Lakshminarasamma, B. Swaminathan,
Prof V. Ramanarayanan, IISC
Department of Electrical Engineering
Indian Institute of Science
Bangalore

2. Linear Regulator Best dynamic performance Very good regulation
Series Regulator Efficiency = K
(1-K)Vg
KVg R + - Vg
Best dynamic performance
Very good regulation
Poor efficiency and bulky

3. Switching Regulator Ideal losses zero Output discontinuous
TON
TOFF Vg + - R
Switching Voltage Regulator
Ideal losses zero
Output discontinuous
Smoothing filter needed

4. Typical Converter Switches control power flowVg Vo
Switches control power flow
Reactive elements smoothen power flow
Both are non-dissipative elements

5. Classification of SMPS

6. Hard Switching ConverterP T1 T2 V I t I V t

7. Soft Switching Converter
ZVS
ZCS
OFF/ON
Transient
ON/OFF

8. Active Clamp ZVS Buck ConverterCC LR I D V DC
Throw1
Pole CR
Clamp Capacitor
Clamp Switch s 2 S 1 +

9. Interval T1 - Zero-voltage Turn-onS2 S1 CR D1 D2 LR D VC
i(t)
i(0) = -I*
i(T1) = I
IN - Normalized current

10. Interval T2 - Resonant CommutationVo S2 S1 CR D1 D2 LR D VC
i(t)
i(0) = I
v(t)
v(0) = V
v(T2) = 0

11. Interval T3 - Power-on DurationVo S2 S1 CR D1 D2 LR D VC
i(t)
S1 turned off at end of T3 and CR almost instantly charges to V+VC.

12. Interval T4 – Assisted Turn-offVo S2 S1 CR D1 D2 LR D VC
i(t)
i(T4) = I

13. Interval T5 – Resonant CommutationVo S2 S1 CR D1 D2 LR D VC
i(t)
v(0) = 0
v(T5) = V
V ( T5 ) = V

14. Interval T6 – Power Freewheeling DurationVo S2 S1 CR D1 D2 LR D VC
i(t)
At the end of T6 interval current i(t) has reversed and Flows through MOSFET of S2. CR almost instantly discharges to zero. Now S1 may be switched on with zero voltage across the same.

15. Theoretical Waveforms
I Active Switch S1 I* T2 T1 T3 T6 I T4 T5 t
I(T5)
kI(T5)
Resonant Inductor LR Current
I Freewheel Diode
Pole Voltage Vo Vg

16. Clamp Ratio and Clamp Voltage
I(T5)
kI(T5)
Resonant Inductor LR Current t
Clamp Capacitor Current I
kI(T5)

17. Steady State Equivalent Circuit Model
(1+k)LR/Ts
Steady State Equivalent Circuit for Active Clamp Buck converter

18. Equivalent Circuit Models of Other ConvertersRd Rd
1:D
1-D: 1
Rd1
Rd2
1-D: 1
1: D
Equivalent circuits of the active clamped ZVS boost, buck-boost and
cuk converters

19. Spread Sheet Design..\pesc04\work\spreadsheetdesign.xls

20. Steady State Definitions Of Base Voltages And Currents
Buck
Boost
Buckboost
Cuk v Vg Vo
Vg+Vo
Vg+Vo
Ig + IL I Io Ig IL Rd M

21. Dynamic Model Of Active Clamp Buck Converter
Perturbation of the nonlinear circuit averaged model about a quiescent operating point. L + -
1:D Rc C
Small signal ac model of active clamp buck converter
1:D Rc L C R - +

22. Simulated Active Clamp Buck Converter
Output power = 60 watts
Input voltage = 50 volts
Output voltage = 20 volts
Switching frequency = 250 KHz

23. Resonant Inductor Current Waveform

24. Clamp Capacitor Current Waveform

25. Steady State Performance Of Active Clamp Buck Converter

26. Experimental Waveforms Of Active Clamp Buck Converter
Vgs and Vds of S1 showing ZVS; Vgs and Vds of S2 showing ZVS

27. Experimental Waveforms Of Active Clamp Buck Converter
Pole voltage and Inductor current waveforms; Pole voltage and Clamp capacitor current waveforms

28. Dynamic Performance Of Active Clamp Buck Converter
Measured output impedance of Hard-switched buck converter and Active clamp buck converter

29. Conclusions – Active Clamp Converters
Derived from Hard-Switched Converters by the addition of few Resonant elements following the simple rule.
Circuit equations governing these sub-intervals are identical when expressed in terms of pole current; throw voltage and freewheeling resonant circuit voltage (I, V, and VC).
Steady state and Dynamic equivalent circuits are obtained from this idealized analysis.
The resonant sub-interval introduces lossless damping in the converter dynamics.

30. Advantages – Active Clamp Converter
High Efficiency - ZVS
Simple Dynamic Model
Wide Variety of Topologies

31. Thanks