THVD8010: Is it possible to use THVD8010 in 24VAC(50Hz/60Hz) power line?

Hi Gabriel,

So yes you can do that and customers already use this type of implementation - there are a couple more things that need to be done but nothing too complicated.

1. You use the standard design approach (inductors + capacitors as in a DC application) but at the output of the AC source (before the inductor) there needs to be a capacitor to the AC ground - this capacitor is to filter out the high frequency OOK signal from the THVD8010 without sinking too much current from the power source.

For example if you are using a modulation frequency of 125Khz and a power frequency of 60Hz the AC caps to ground need to be high enough impedance that it doesn't request too much current from the source during operation. A 10uF cap would be <.15 Ohms at 125KHz and it would be ~265 Ohms at 60Hz(about 91mA of current draw from the source) - a 1uF cap would be ~1.15 Ohms at 125KHz and 2.652K Ohms at 60 Hz (about 9.1mA of current draw from the source). This can be balanced to minimize ook noise on the power rail while also minimizing the current draw on the source. This is actually assumed to be there in DC circuits due to high decoupling capacitance that exists - so adding this cap is more or less what the device expects its more just balancing the current draw on the source. These caps should also exist on the power loads.

2. For the risk of damage - there is a slight higher chance of damage but there are simple mitigating solutions that can be used (that we typically still recommend in DC applications)

2.1.) First is to use a TVS diode between A and B that will prevent any EOS on the A/B lines as during harmful transients this diode can help shunt current to bring down the voltage - we suggest this on DC applications as well.

2.2) The design of the series capacitor. This needs to be set to 5 Ohms at mod frequency. For 125Khz - that is 255nF (pick the closest value available if 255nF is not) at 60Hz this is ~10K Ohms - which will greatly attenuate the low frequency power signal. I'd suggest picking as high of mod frequency as possible - for the 8010 that is 300K which will give 106nF - which is 25K Ohms at 60Hz. This should keep the THVD8010 safe during operation but the TVS diodes will help if there is a transient that needs to be watched for.

2.3) Instead of a TVS diode - a surge rated diode may also be warranted in these situations. 

Please let me know if have any other questions!

Best,

Parker Dodson

Hi Gabriel,

1. Termination Resistors are strongly recommended; they help mitigate reflections that cause EMC issues (when termination resistance and conductor characteristic impedance is matched - we recommend 120 Ohms as 2 120 Resistors as this is seen as 60 Ohms by the driver with tolerance we will hit the 54 Ohm differential resistance spec from RS-485). Customers do use these systems without termination - but I'd strongly recommend it for most use cases (typically if it is a shorter and slower application termination resistor aren't as necessary).

2. For EMC issues there are couple solutions that customers have had success with:

2a) Ensure proper impedance matching using termination resistors and the characteristic impedance

2b)  Ensure proper differential coupling - meaning that there is very little (ideally none) variation between the spacing of the A and B lines as well as that both the A and B conductor pathways are the same length.

2c) Using a Split termination instead of a normal termination - please see below drawing, keep in mind it says CAN transceiver - but its the same architecture with respect to  the split termination:

Where R_TERM is 120 Ohms; R_TERM/2 = 60 Ohms; and CSplit value is picked based on modulation frequency (CSplit should be high impedance at OOK modulation frequency but low impedance for frequency bands larger than ~3x of the modulation frequency)

2d) The use of common mode chokes is also a popular method of mitigating common mode noise on the bus. 

2e) If using a cable to transmit data between nodes - a 120 Ohm Twisted pair is suggested.

2f) Any traces on a PCB should be as straight as possible to minimize any angles/points of radiation on the traces. This should also have a characteristic impedance of 120 Ohms (this can be controlled with the trace thickness, trace width, and dielectric properties - it is best practice to have this as a top/bottom layer trace with at a minimum a ground pour running under the top trace - but it is preferred to have an entire ground plane below/above the 120 Ohm trace to  minimize characteristic impedance changes)

Please let me know if you have any other questions!

Best,

Parker Dodson