Hi there SachinAbeysinghe25,
We can provide some guidance. Fast charging shouldn't affect the temperature of your iPhone any more than regular charging does. Are you using your iPhone while it's charging? We ask because it's normal for your iPhone to get warm in certain situations:
Your device might get warm
You might notice that your device feels warmer when you:
- Set up your device the first time
- Restore from a backup
- Wirelessly charge your device
- Use graphics-intensive or processor-intensive apps, games, or features, including augmented-reality apps
- Stream high-quality video
These conditions are normal, and your device will return to a regular temperature when the process is complete or when you finish your activity. If your device doesn't display a temperature warning, you can keep using your device.
Learn more here: If your iPhone, iPad, or iPod touch gets too hot or too cold - Apple Support
Also, you may want to move your iPhone to an area that isn't as warm:
Your iPhone might get slightly warmer while it charges. To extend the lifespan of your battery, if the battery gets too warm, software might limit charging above 80 percent. Your iPhone will charge again when the temperature drops. Try moving your iPhone and charger to a cooler location.
Have a look at this support article for details: If your iPhone or iPod touch won't charge - Apple Support
Take care.