Author Topic: Choosing a right FET or IGBT for this application  (Read 528 times)

0 Members and 1 Guest are viewing this topic.

Offline XaviPachecoTopic starter

  • Regular Contributor
  • *
  • Posts: 243
  • Country: do
Choosing a right FET or IGBT for this application
« on: August 23, 2019, 01:25:32 pm »
Some caveats about the circuit attached:

1. The input ranges from 100 to 600VAC.
2. The coil L1 has a DC resistance of 300 Ohms.
3. It's oversimplified to show the concept. SW1 is a SCR. When SW1 is open, the load should not draw more than 10 mA at worst. Normally about 4 mA. When SW1 quickly closes, the load gets bypassed, and a higher current will flow through the coil L1 until the supply is cut. The coil should trip the power supply as soon as possible to avoid FET overheating. Once the supply is cut off, the only way to turn it on is manually. I mean, the coil has to be reset manually.
4. The FET is configured as a current limit. This mean it's always On, not switching. So I have to make sure I don't go outside the minimum and maximum current limits.

How do I do the right power calculations to find a suitable transistor? This is what I think:

- I would need a high VDS breakdown. At least 1200V for the case when the input is 600VAC. It gives about 848VDC. I can add TVS diodes across VDS to protect the FET.
- When SW1 is open, VDS is high. Supposing the load draws 5 mA, P = 5 mA x 800V = 4W
- When SW1 closes, VDS gets a bit smaller, about 400V. This yields P = 1.2A x 400V = 480W!! But this is a very short pulse. Let's suppose the trip coil cuts the supply in less than a second.
- Adding a resistor in series with the coil would reduce the stress over the FET I think.

What would you suggest?
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf