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

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

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

Typical Converter Switches control power flow Vg Vo Switches control power flow Reactive elements smoothen power flow Both are non-dissipative elements

Classification of SMPS

Hard Switching Converter P T1 T2 V I t I V t

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

Active Clamp ZVS Buck Converter CC LR I D V DC Throw1 Pole CR Clamp Capacitor Clamp Switch s 2 S 1 +

Interval T1 - Zero-voltage Turn-on S2 S1 CR D1 D2 LR D VC i(t) i(0) = -I* i(T1) = I IN - Normalized current

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

Interval T3 - Power-on Duration Vo 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.

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

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

Interval T6 – Power Freewheeling Duration Vo 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.

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

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

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

Equivalent Circuit Models of Other Converters Rd 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

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

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

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 - +

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

Resonant Inductor Current Waveform

Clamp Capacitor Current Waveform

Steady State Performance Of Active Clamp Buck Converter

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

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

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

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.

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

Thanks