**2. Micropump module, based on mutually exclusive SMPS**

The high-voltage section of the analyzed circuit is designed around two boost SMPS power supplies, depicted in **Figure 1**. Each SMPS module provides corresponding positive and negative micropump excitation voltage: Positive voltage SMPS circuit comprises a switching transistor Q3, a diode D2, while the negative voltage SMPS comprises the switching transistor Q4, diode D1. If the switching operation between SMPS modules is alternated repeatedly, a rectangular shape of driving signal with RC charging and discharging transitions is synthesized. Micropump PZT actuator is represented by C1 and acts as a shared SMPS output capacitor in both circuits. Boost inductance L1 is also shared. While the driver is supplying negative voltage to the micropump C1, the transistor Q3 is fully open (its base is tied via R3 to VCC) and the Q4 with diode D1 performs the PWM (pulse width modulation) switching. For positive voltage, the roles of Q3 and Q4 are reversed: Q4 is fully open (its base tied via R4 to GND) and the Q3 with diode D1 performs the PWM switching. Transistors Q1 and Q2 in common-base orientation provide isolation between the two SMPS stages and prevent micropump capacitor C1 discharge through the inactive (i.e. fully open) transistor. During positive voltage switching, the Q4 is fully open - its collector-emitter voltage is

**Figure 1.** *High-voltage part of the mutually exclusive SMPS driver design.*

reduced to its saturation value, hence the Q1 isolation transistor is closed, thus effectively preventing any reverse current flow through diode D1. Similar analysis can be applied for transistors Q2, Q3 and the diode D2 during negative voltage switching cycle.

In order to synthesize a rectangular shape of driving signal with resistor-capacitor charging and discharging transitions, both positive and negative SMPS boost circuits have to be switched in the abovementioned sequence by providing the PNP\_DRIVE and NPN\_DRIVE signals.

To achieve this, an initial, simplified version of the micropump driving circuit was designed to assess the optimal PWM switching frequency range and establish the need for a PWM drive. High voltage part driving circuit, depicted in **Figure 2**, was based around a TTL 74HC257 4-channel 2/1 multiplexer for 5 V power supply version or its CMOS counterpart (40257) for 12 V version. Each 2/1 multiplexer comprises *a* and *b* inputs and an output *Y*. All multiplexers share the same selection input *s*. In **Figure 2**, only lower two multiplexers, indexed *0* and *1*, are used. High-frequency switching is provided by 2 AC voltage sources *V*2 and *V*4. Micropump excitation frequency signal is provided by *V*3, which is connected to the multiplexer selection input. Toggling speed of the multiplexer selection input thus corresponds to mutual exclusion of SMPS power supplies, which directly translate to micropump excitation frequency.
