UCPD TCPP01-M12 Hardware Design Question

[Jason927]

I'm using tcpp01-m12 and stm32u5 borad to develop a UCPD sink application.

The UCPD module seems works fine, and I can request selected voltage from source.

But when I closed the MOS which controlled by tcpp(mcu set tcpp ctrl pin high), the vcc output voltage still have about 500mV.

Is this a Characteristics of MOS tube, or any hardware design suggestion?

[Peter BENSCH · Accepted solution]

Well, the gate threshold voltage of the STL100N10F7 is between 2.5V and 4.5V, meaning that an unfavourable specimen may not be able to conduct (see data sheet, fig. 8, Transfer characteristics).

Instead of the STL100N10F7, you could use a low gate threshold MOSFET (known as a logic level types), e.g. the STL160N6LF7, whose threshold voltage is between 1.2V and 2.5V and which typically reaches its maximum current at 3.8V (see data sheet, fig. 6, typical transfer characteristics).

However, I also noticed that you are apparently connecting the voltage divider R30/R31 with its impedance of 180k||20k = 18k directly to the ADC input of the MCU. Please note that this only makes sense for the STM32U5 at resolutions of max. 8 bits; for higher resolutions, an impedance converter or lower-impedance resistors are strongly recommended (see data sheet, search term ‘input impedance’).

Regards
/Peter

[Jason927]

hi, peter

I'm already changed R30/R31 to 33K/4.7K, MOSFET to STL160N6LF7, but the VCC_OUTPUT still have about 500mv voltage with float pin(no load).
Any chance the voltage leaked from MOSFET's GATE?How can I fix it?

[Peter BENSCH]

Ah, so you measured 500mV with an unloaded output, which you didn't mention before.

What happens when you connect a load resistor of, say, 1k to the output?

[Jason927]

When connected to a load, VCC_OUTPUT is 0V, should I add a resistor e.g. 1M between MOSFET source and GND?

[Peter BENSCH]

Well, MOSFETs have leakage currents (zero gate voltage drain current) that are not negligible and can increase by a factor of 100 at high temperatures. If you are bothered by the voltage resulting from this leakage current and the load connected to the output (internal resistance in the case of your measuring device), you can certainly connect a permanent load resistor.

However, in the worst case scenario of 125°C (yes, the TCPP01-M12 is only guaranteed to work up to 85°C), the full input voltage could occur at the output with a load of 1M – but to measure this voltage, you would need a measuring device with at least ten times the input resistance, or better still, even higher, because otherwise the measurement result would be drastically distorted.

The permanent load resistance could be selected as follows, for example:

• 100k for max 100mV at an assumed leakage current of 1µA, or
• 1k for max 100mV at an assumed leakage current of 100µA.

Regards
/Peter

[Jason927]

Hi, peter

After removed MOSFET, still have the voltage, check the TP9, found the 500MV is from power monitor(INA219BID).|
THANK YOU!

[Peter BENSCH]

Thank you for letting us know.