I’m guessing the N-FET isn’t being turned on all the way. It's being biased in linear mode and so has a substantial IR drop across it, which is being shed as heat. You don't want that.
How bad is it? Let's assume the FET Vgs threshold is about 4.5V. Then when it's on:
- Vd = 60V
- Vg = 36V - 2V = 34V (limited UC3845B output swing)
- Vs = (Vg - Vth) = (34 - 4.5V) = 29.5V
- Vds = (Vd - Vs) = (60 - 29.5V) = 30.5V
If you're drawing, say, 1A from the supply, that approximately 30-35W you're dissipating in the FET, peak.
What keeps it from frying immediately is the stepping ratio that determines the on-time of the FET:
- Vout/Vin * W = 12/60 * 35W = 7W
And that's at only 1A. Clearly, this isn't workable.
To fix it, the N-FET gate needs to be brought all the way above 60V, to at least 65V to 70V, to ensure the FET is fully turned on to its lowest Rds(on).
What happens when you do that? Here's the peak wattage shed in the FET:
- Vd = 60V, Vg = 65V
- Vs = about 60V (transistor is fully on)
- Vds = (Vd - Vs) = 0, or close to it
So in theory almost no power gets shed in the FET. In reality, this will be:
- Iout^2 * Rds(on)
- 10^2 * 0.020 ohm = 2W peak at 10A output
With stepping ratio being 12/60, the FET is on about 20% of the time:
That's very manageable for a TO-220 FET.
How to do this? You need a bootstrap circuit to generate the higher gate drive voltage (about 5-10V above Vin), and a gate driver that accepts that voltage to make the above-Vin gate signal. The bootstrap voltage can (and usually is) generated from the inductor flyback via a diode and capacitor.
Problem is, the UC3845B is not a bootstrapped high-side driver. It's really designed to be a low-side driver for a flyback topology. Further, it's limited to +36V. For both reasons it’s a poor choice for this application.
You could mess about with making a bootstrap + level shifter, but why? Select a different device instead. Example: this 75V input dcdc controller from TI (bonus: it’s synchronous so your supply will be more efficient): http://www.ti.com/lit/ds/snvsai4/snvsai4.pdf