Highly efficient linear power amplifier for driving fast slew rate capacitive loads

The modern Inkjet printer is able to place millions of tiny ink droplets precisely on a paper, fabric or even ceramic to create an image. The piezoelectric technique of inkjet printing employs a crystal that flexes whenever a voltage pulse is applied to the piezo transducer. A representative printing head configuration might employ a single Power Amplifier (PA) to drive up to 1024 nozzles with an equivalent load of approximately 100nF. The simplest voltage form that is used for driving the ink jet printer head is a trapezoidal waveform, which has controlled amplitude and ramps. Because of the needed voltage shape, linear power amplifiers (class B, AB) are normally employed, sacrificing efficiency for the quality of the voltage pulse. In this paper we present a highly efficient solution for the PA that drives an ink jet head with voltage pulses as high as 130V with slew rates of 45V/us and as fast as 110 kHz. The solution is based on an Envelope Tracking approach, i.e. the linear PA is supplied by a varying supply voltage and in that way avoiding high power losses in the linear PA, while preserving good characteristics of the output voltage. Due to the extremely fast slew rate (45V/us) and very high instantaneous power that must be supplied to the load (up to 800W), the varying voltage is implemented using a multilevel converter. By using the proposed solution the power losses can be cut down as much as 50% as it is shown by experimental results.