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TPS23750EVM-108: How is an ESD spark on the secondary side of the transformer managed?

Part Number: TPS23750EVM-108
Other Parts Discussed in Thread: STRIKE

In the case of an isolated design; How is an ESD spark on the secondary side handled by the circuit? I assume the gnd of the secondary side is connected to the chassis. Thus, when an ESD spark is applied on the chassis, this needs to be handled, right? I assume that C10 transfers the energy to the primary "gnd" but what happens then? Thus, it should be important to use capacitors with low ESR. Is that why C1 and C2 are not electrolytic type of capacitors? 

Please explain why this design i ESD proof? 

  • Hi Mats,

    Today is a national holiday in the US. We will be able to have answer for you on this as early as Friday. Thanks!

  • Hi Mats,

    During an ESD strike, the goals is to have a controlled path for the energy to find its way to earth ground. Majority of designs will connect the secondary to the bob smith termination which is connected to chassis (which should connect to earth ground).

    Otherwise, the ESD will find the path of lowest impedance. In a typical PoE design, this will probably be through the common mode caps near the transformer. If the PD controller is ON, then it can go through the PD's internal pass FET to the PSE (which connects to earth ground). Ideally, you don't want the ESD to get the IC as it can damage the part, or the noise can cause unusual behavior of the IC which is why we recommend the secondary ground connection to earth.

    Thanks!

  • Hi Darwin, let us discuss the schematics described in http://www.ti.com/lit/an/slua469/slua469.pdf . Assume an LCD is connected to the secondary side (+3V) and its ground is connected to the secondary gnd. The LCD mechanical housing is connected to that gnd too. What happens then if the PD is sourced by a PSE via an unshielded ethernet cable and the PD is mounted in a way, without proper grounding. How to handle ESD then? 

  • Also, let us say that we need an isolated DC/DC converter since our low voltage circuitry also contain an USB-port. Does it mean that I should connect the USB port shield to the overall shield and not to the system gnd? The image I refer to is from https://e2e.ti.com/support/power-management/f/196/t/483623?PoE-grounding-isolation-conundrum 

    BR/Mats 

  • Hi Mats,

    If there is no true earth ground on the PD system, then there are two possibilities. One, if completely floating, then the impedance could be high enough such that ESD does not discharge through the PD since there is no common mode path to ground. The second, if there is an earth connection on the PSE side (most likely), then the ESD can find its way from secondary ground through the bob smith termination (if it is connected to secondary ground) and also through isolation cap near the transformer, through the PD, through the cat5 cable, and to the PSE's earth ground. The second reason is why it's good to have a controlled path to earth ground (connected at the secondary) to prevent the energy from finding its way to the PD controller.

     

  • Hi Mats,

    Typically for USB applications, you'll see the chassis of the USB port connected to secondary GND and connected to earth ground of the PD and connected to the bob smith terminations.