MRF300 Max current rating

[Nan]

What is the safe max current when MRF300AN and MRF300BN transistors are configured as a wide band push pull HF linear amplifier? (50VDC operation) 

[QRN]

Pavel, your post here is interesting, because it implies that the device can be pushed well above 330w if the duty cycle is reasonably low and the cooling system is conservative and efficient.  I would add to your post by mentioning the MTTF chart in the datasheet.  Another way of stating your bottom line here is that no matter if you are pushing any power device hard to it's limits or if it is loafing, the more conservative and efficient you are in getting rid of the heat (heat sink, copper heat spreader plates, amount of forced-air cooling and airflow path, etc), the longer the device will last and the more reliability you will have.

[LPP · Accepted solution]

NXP doen't specify maximum current. For RF transistors, unlike the switching DMOS transistors, the peak power doesn't determines SOA. Instead the power is averaged over RF period and it is mainly a thermal reasons that defines safe operation.

There is no hard and fast upper limit to the max amount of saturated drain current that the device can handle. The only real limitation is the max rated junction temperature of 175°C. If we work backwards from this number and state that the max Tj is 175°C and the case temperature is a realistic 85°C, then we have 175°C-85°C or 90°C of internal temperature rise to deal with.

Knowing that the thermal resistance of the package is 0.55°C/W, then 90°C/0.55°C/W = 164W of thermal power dissipation allowed (datasheet specify 272W max @ 25'C).

NXP reference circuits provide about 330W RF power at 0.75% efficiency. The resulting thermal power dissipation is about 110W resulting in 60'C junction to case temperature rise. Power supply DC current is ~8.8A. Peak RF current is about twice of this value.

Change the case temperature, operating voltage or output power and that max Id for a 175°C max Tj can go up. The bottom line is that it is not the current that kills the parts, it is the heat. Keep them cool and they can handle lots of drain current.

Pavel

NXP TIC