Load-Independent Class-E Power Conversion

The Class-E topology was presented as a single-switch power amplifier with high efficiency at the optimum condition, where the switch enjoys zero-voltage switching (ZVS) and zero-voltagederivative switching (ZDS). It is also used in MHz dc-dc converters, and in inverters for wireless power transfer, induction heating, and plasma pulsing. The load current in these applications usually varies over a range. Efficiency of a conventional Class-E design degrades dramatically due to the hard switching beyond the optimum conditions. Keeping ZVS with load change in a ClassE topology is preferred within the load range. Soft switching with load variation is realized by duty cycle modulation with additional transformer, matching network, or resistance compression network. Since two ZVS requirements need to be satisfied in a conventional Class-E design, at least two parameters are tuned under load variation. Thus, changing switching frequency, duty cycle, and component values were used. Impressively, a load-independent Class-E inverter design was presented in 1990 for maintaining ZVS and output voltage under a given load change without tuning any parameters, and it was validated with experimental results recently. The operating principle of this special design (inconsistent with the conventional design) is not elucidated in the published literatures. Load-independency illucidation by a Thevenin Model – A Thevenin model is then established (although Class-E is a nonliear circuit) to explain the load-independency with fixed switching frequency and duty cycle. The input block of a Class-E inverter (Vin, Lin, Cin, and S) behaves as a fixed voltage source vth1 and a fixed capacitive impedance Xth1 in series at switching frequency. When the output block (Lo and Co) is designed to compensate Xth1, the output current phase is always equal to the phase of vth1 with resistive load (satisfies the ZVS requirement of a load-independent design). Thus, soft switching is maintained within load variation. Output voltage is equal to vth1 since Xth1 is canceled, so that the output voltage is constant regardless of output resistance. Load-independency is achieved without adding any components or tuning any parameters. Sequential design and tuning of a load-independent ZVS Class-E inverter with constant voltage based on Thevenin Model Based on the model, it’s found that each circuit parameter is linked to only one of the targeted performance (ZVS, fixed voltage gain, and load range). Thus, the sequential design equations and steps are derived and presented. In each step, the desired performance (e.g. ZVS) now could be used to check and tune component values so that ZVS and fixed voltage gain in the desired load range is guaranteed in the final Class-E inverter, even when component values vary from the expectations. The Thevenin model and the load-independent design is then extended to any duty cycles. A prototype switched at 6.78 MHz with 10-V input, 11.3-V output, and 22.5-W maximum output power was fabricated and tested to validate the theory. Soft switching is maintained with 3% output voltage variation while the output power is reduced tenfold. A load-independent ZVS Class-E inverter with constant current by combining constant voltage design and a trans-susceptance network A load-independent ZVS Class-E inverter with constant current under load variation is then presented, by combining the presented design (generating a constant voltage) and a trans-susceptance network (transferring the voltage to current). The impact of different types and the positions of the networks are discussed, and LCL network is selected so that both constant current and soft switching are maintained within the load variation. The operation principle, design, and tuning procedures are illustrated. The trade-off between input current ripple, output current amplitude, and the working load range is discussed. The expectations were validated by a design switched at 6.78 MHz with 10-V input, 1.4-A output, and 12.6-W maximum output power. Soft switching is maintained with 16% output current varying over a 10:1 output power range. A “ZVS” Class-E dc-dc converter by adding a diode rectifier bridge and compensate the induced varying capacitance at full-load condition The load-independent Class-E design is extended to dc-dc converter by adding a diode rectifier bridge followed by the Class-E inverter. The equivalent impedance seen by the inverter consists of a varying capacitance and a varying resistance when the output changes. As illustrated before, ZVS and constant output can only be maintained with resistive load. Since the varying capacitance cannot be compensated for the whole load range, performance with using different compensation is discussed. With the selected fullload compensation, ZVS is achieved at full load condition and slight non-ZVS occurs for the other load conditions. The expectation was validated by a dc-dc converter switched at 6.78 MHz with 11 V input, 12 V output, and 22 W maximum output power. ZVS (including slight non-ZVS) is maintained with 16% output voltage variation over 20:1 output power range. Design of variable Capacitor by connecting two voltage-sensitive capacitors in series and controlling the bias voltage of them The equivalent varying capacitance in the Class-E dc-dc converter can be compensated in the whole load range only with variable component. The sensitivity of a Class-E power conversion can also be improved by using variable capacitors. Thus, a Voltage Controlled Capacitor (VCC) is presented, based on the intrinsic property of Class II dielectric materials that permittivity changing much with electric field. Its equivalent circuit consists of two identical Class II capacitors in series. By changing the voltage of the common point of the two capacitors (named as control voltage), the two capacitance and the total capacitance are both changed. Its operation principle, measured characteristic, and the SPICE model are illustrated. The capacitance changes from 1 μF to 0.2 μF with a control voltage from 0 V to 25 V, resulting a 440% capacitance range. Since the voltage across the two capacitors (named as output voltage) also affects one of the capacitance when control voltage is applied, the capacitance range drops to only 40% with higher bias in the output voltage. Thus, a Linear Variable Capacitor (LVC) is presented. The equivalent circuit is the same as VCC, while one of the capacitance is designed much higher to mitigate the effect of output voltage. The structure, operational principle, required specifications, design procedures, and component selection were validated by a design example, with 380% maximum capacitance range and less than 20% drop in the designed capacitor voltage range. This work contributes to • Analytical analysis and Thevenin Model in load-independent Class-E power conversion • Variable capacitance with wide range Load-Indepedent Class-E Power Conversion Lujie Zhang General Audience Abstract The Class-E topology was presented as a single-switch power amplifier with high efficiency at the optimum condition. Efficiency of a conventional Class-E design degrades with load variation dramatically due to the hard switching beyond the optimum conditions. Since two requirements need to be satisfied for soft switching in a conventional Class-E design, at least two parameters are tuned under load variation. Impressively, a load-independent Class-E inverter design was presented for maintaining Zero-Voltage-Switching (ZVS) and output voltage under a given load change without tuning any parameters, and it was validated with experimental results recently. A Thevenin model is established in this work to explain the realization of load-independency with fixed switching frequency and duty cycle. Based on that, a sequential design and tuning process is presented. A prototype switched at 6.78 MHz with 10-V input, 11.3-V output, and 22.5W maximum output power was fabricated and tested to validate the theory. Soft switching is maintained with 3% output voltage variation while the output power is reduced tenfold. A load-independent ZVS Class-E inverter with constant current under load variation is then presented, by combining the presented design and a trans-susceptance network. The expectations were validated by a design switched at 6.78 MHz with 10-V input, 1.4-A output, and 12.6-W maximum output power. Soft switching is maintained with 16% output current varying over a 10:1 output power range. The load-independent Class-E design is extended to dc-dc converter by adding a diode rectifier bridge, inducing a varying capacitance. With the selected full-load compensation, ZVS is achieved at full load condition and slight non-ZVS occurs for the other load conditions. The expectation was validated by a dc-dc converter switched at 6.78 MHz with 11 V input, 12 V output, and 22 W maximum output power. ZVS (including slight non-ZVS) is maintained with 16% output voltage variation over 20:1 output power range. The varying capacitance in the Class-E dc-dc converter needs variable component to compensate. Thus, a Voltage Controlled Capacitor (VCC) is presented. The capacitance changes from 1 μF to 0.2 μF with a control voltage from 0 V to 25 V, resulting a 440% capacitance range. The capacitance range drops to only 40% with higher bias in the output voltage. Thus, a Linear Variable Capacitor (LVC) is presented, with 380% maximum capacitance range and less than 20% drop in the designed capacitor voltage range.