1 Introduction

An ultrasound is one of the most routine examinations in the hospital. Generally, these tests are done by cart-based or portable ultrasound equipment. The advent of the smart probe makes all these processes easier because only a wired or wireless probe connected to a smart phone or tablet is needed. As Figure 1-1 shows, this small-size device integrates the same units as common ultrasound equipment, including a transmitter (TX), receiver, FPGA connection, and power module (high- and low-voltage circuit). The whole device is usually powered by a Li-Ion battery. To extend the battery life, the device requires a highly-efficient power supply, low power consumption, and good thermal performance. This application note provides a new strategy to reduce quiescent dissipation for the power supply of the transmitter.

For the pulsers in an ultrasound smart probe, high- and low-voltage power supplies are required to transmit the desired voltage pulse for the transducer to generate an ultrasound signal. Generally, there are two modes, Brightness (B) mode and Continuous (CW) mode, which requires bipolar high voltage (+75 V and –75 V) and low voltage (±2.5 to ±15 V), respectively. In the B mode, the whole working period consists of a pulse mode period and a no load mode period as shown in Figure 1-2. During the former period, the high-voltage pulses are transmitted for a short time (several microseconds) with a large current (about 1 A). To meet the power requirement, a switched-mode power supply (SMPS) is applied in most ultrasound systems. However, the switching frequency (Fsw) of SMPS is often fixed by a resistor which causes the power loss during the no-load period. If simply shutting down the SMPS during the no-load period, the device cannot wake up in time before the next pulse. Thus, slowing down the Fsw is a good way to reduce the power consumption. If synchronizing the frequency with an external clock, the change of frequency must be within +25% and –30% of the desired Fsw, which cannot save the power significantly. In this application note, the external GPIO is input to program the switching frequency to 100 kHz at standby mode while 250 kHz at normal operation, achieving the lowest no-load power consumption at the maximum voltage output. This approach can also scale to other applications that need SMPS with low power dissipation.