Hi,
Mosfets include a body diode. This is the same silicon as the mosfet's switch, obviously they have the same temperature.
A silicon diode has a temperature coefficient of about 2.1mV/°C in forward voltage.
Thus a temperature rise from 25°C to 150°C = 125°C results in about 260mV.
If you can find a way to measure the diode's forward voltage - eg in times when the PWM switches the Mosfet OFF - you durectly measure the junction temperature. ...almost perfect without adding time delay or added thermal mass.
This is not very easy, but If you want us to help you with this solution you should give us the half bridge schematic and the PWM timing.
*******
The theromcouple solution is more simple, but includes delays and can only read the case temperature instead of junction temperature.
The thermal model from junction to case may be simplyfied as a simple RC delay.
To get a clue about the junction temperature you need to do a multiplication (R part) and you need to take care about the rise rate of the temperature.
This can simply made in an EXCEL chart.
Klaus