Table of Contents
Background (A Working Theory)
If tuning elements are added to a transducer, a high voltage swing situation can easily be created, potentially leading to an over-voltage failure. For example, a single passive component added to a transducer could create a DC short and very high voltage swing where previously, without that component, the DC resistance was extremely high and the voltage differentials were more well-defined by the intended pulse drive characteristics. With the added component, the voltage in a pulse could now swing from very negative (as intended with the pulse excitation) and then up to a highly positive voltage, with the “back swing”, like a pendulum, created by the passive component. Without the transducer’s tuning element, the pulse might only have been negative with a minimal positive overshoot as the pulse dissipates. When a transducer creates that highly positive back-swing, we are now talking about voltage differentials across components that are in the 700V+ range, while the design voltage maximums for the electronics might still be only for the nominal 200V-300V (with a 525V absolute maximum). The output pulse stage is designed for simplicity and low-cost, targeting a specific set of transducer characteristics. Handling high voltage back-swing, and thus high total voltage differences requires that addition of components and some larger bounding conditions for the transducers themselves. Such electronics features can and probably will be added in future design variants in response to the changing needs.
GXN General Reminders
- High Voltage: When power is applied to this board model and current revision, the high voltage supply is always on. So, it is important to be careful not to zap things during handling. As originally shipped, there was a conformal coating applied to minimize the likelihood of contact with the high voltage. However, with the rework on the board, the coating has been omitted to allow easy testing and rework. Thus, some high voltage contact points may be exposed again.
- “Cross-channel”: On this board: You can only transmit from Channel 1. Receive can be on either Channel 1 or Channel 2.
- No Tx Gain: There is no transmit gain (high voltage level setting) on this board. You set the voltage just by selecting which high voltage module you install, and by setting a load resistor value. If you want help adjusting the load resistor value, or further detail on how and why that works, just ask.
Verifying Over-Voltage Component Damage (Optional)
So far, it seems that the components that get damaged and need to be replaced in this kind of condition are (or have been): D8 and U36 primarily.
Who knows, maybe we’ll discover a new issue or failure mode. Until now however, this seems to be the case.
This verification step is optional because, well, assuming the failure mode is the same, you could also just try replacing the components and re-testing functionality without verifying. Anyway, the steps are below. Make sure the board is not powered up and has been allowed to sit for maybe a minute at least, such that there is no charge left anywhere on capacitors, for example. Cardinal direction references are made with the edge-launch SMA connectors pointing left (West) and the high voltage supply (HVS) at the top (North).
- Get a DMM capable of diode testing and get it set up to do so, i.e. leads in the correct jacks, settings knob correctly set.
- D8 is just South of the high voltage supply (HVS). Small black rectangular body, oriented North to South. To its left (or its West) are two resistors, one horizontal (East-West, marked R42) and one vertical (North-South). It’s exposed because the conformal coating was not re-applied, so as to make rework easier.
- Place your probes on the D8 pins. Once in one order, and then swap the probing points. That is, black on top (North), red on bottom (South) to start, and then switch to black on the bottom (or South) and red on the top (or North).
- Depending on your DMM readout, you should get something like 750 (give or take several tens) in one direction and something very high or even just nothing or “1” … or whatever indicates out of range in the other direction. That is, if the component is good. If you get like 750-ish in one direction and then like anything readable in that general range (450, 600, 830, whatever) in the other direction, the component (D8) is probably bad.
- U36 is located just caddy-corner Southwest to the HVS and it’s a surface mount TO-252 (D-PAK) type of package, also without conformal coating. Do the same diode type of test on the two small East-side pins on that package too. The results and interpretation are the same as above in point #4.
Fixing (Upgrading?) the Board with Rework
Locate D8 and U36 as noted in the previous section. With careful preheating and rework best practices, replace these components. Make sure you note the component orientation prior to removal. The part numbers and/or information for equivalent replacement:
- (D8) BZT52C15 in SOD-123 package
- (U36) You might want to replace this component with something roughly equivalent but with a higher voltage absolute max rating.
- Exact match: STD5N52U in D-PAK
- High voltage experimental rough equivalent: STD5N80K5 (not exact, but pretty close in key characteristics) If it were me, I’d want to try this component, in place of the other. Even if I ended up back with the original.
Preventing Future Damage
It’s possible that by replacing another component on the board with an equivalent component of different value, that you might be able to reduce the likelihood of future damage to this revision of board. One example is the new component for U36, as noted above. There are others. These types of things are quick, minimal cost, minimal time sorts of upgrades.
Another simple option is to reduce the value of the damping resistor (basically increasing the damping) R17 — also had its conformal coating removed for easy rework. Try half the value, like 20Ohms or experiment. The original value was 200 Ohms. The most recent value was about 41 Ohms. Example part number for the series: Mouser: 667-ERJ-P06F22R0V (Panasonic). There could be new issues that arise with an updated component value. But it would be a useful next step for testing anyway.
Those “damping” changes also change output impedance. Output impedance is also impacted by, for example, the choice of component for U36.
Actually, there are a few other options, including using a lower voltage high voltage supply (HVS) module. Or reducing the value of the load resistor for the HVS (mentioned above too), which will cause the module to drop its output voltage, since the module tends to output a higher voltage under light load, which is the case here; a lighter load that is.
Lastly, you could create a little protective and limiting module that connects to the SMA transmit channel on the board and to which you then connect the transducer lead. This module could include damping and clipping components. It might introduce other considerations, and also might still be worth a try. Just ask if you’d like details or even a prototype module or modules.