CCS/PGA460-Q1: PGA 460 Full-Bridge Transformer

Hi Reinaldo,

The "PGA460 Full-Bridge Driver Solutions for Ultrasonic Transducers" app note shows how to design a +/-45V full-bridge direct-drive option on the PGA460 using the DRV8870 as a pre-driver substitute to a transformer.

The PGA460 accepts a maximum of 15V at the center-tap of the transformer (due to a max voltage rating of 30V at the OUTA and OUTB pins), and most push-pull transformers are configured in 1:1:10 turns ratio, which enables +/-150V or 300Vpp at the transducer.

In your case, you will need to consider one of two options to reach up to 500Vpp with the PGA460:

1. Use a transformer with a higher turns ratio. If you can find a transformer that supports a 1:1:20 (push-pull) or 1:20 (single-ended) turns ratio, then this will enable a transducer excitation voltage of up to 600Vpp. I am currently unaware of any transformers configured with these turns ratios that also have a secondary inductance of 1~5mH. You may need to contact some transformer manufacturers for their recommendations. You can find a list of transducer and transformer suppliers here: PGA460 Transducer & Transformer Listing
2. Use a pre-driver option, similar to what is proposed with the DRV8870. Except, instead of using the DRV8870, use a discrete configuration of transistors to allow for a higher center-tap voltage at the transformer. This way, rather than sinking current through the PGA460's integrated low side nFETs, you will use external nFETs rated for a larger voltage and current. So, if you were to apply 25V at the center-tap of the transformer, and use external nFETs rated for >=30V V_GS, then you should be able to achieve upwards of 500Vpp using the traditional 1:1:10 transformers already available. The gate driver used in the DRV8870 full bridge circuit would be tied to the gates of the external discrete nFETs instead (replacing the DRV8870) as shown in the "PGA460 Full-Bridge Driver Solutions for Ultrasonic Transducers" app note.

It is important to note that though a transducer may be rated for 800Vpp, it is not always required to drive the transducer at this high voltage to generate sufficient sound pressure level. In many cases, I have worked with transducers that have a voltage rating of several hundred Vpp, but even a drive voltage of 120Vpp is often able to work for the application's performance requirements, especially if the application is targeted short range measurements.