TO-220 output transistors for power amp - diyAudio
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Old 12th June 2015, 04:05 PM   #1
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Default TO-220 output transistors for power amp

I'm looking at making a (physically) smaller power amp using TO-220 output transistors, i.e. Fairchild's 2SC5200 family.
I'm not sure how to estimate power output at 8/4ohms because SOA curves don't exist on the datasheets for the smaller parts. It might be the same die in different packages ? as the TO-220 and TO-264 parts have the same capacitance and Fairchild just cut'n pasted the specs. Only difference is the thermal resistance is 2-3x higher for the small parts.

"Same transistor is also available in:"
Power Diss.; RθJC Junction to Case
TO-264 package, 2SC5200/FJL4315 : 150 watts : 0.83°C/W
TO-3P package, 2SC5242/FJA4313 : 130 watts : 0.96°C/W
TO-220 package, FJP5200 : 80 watts : 1.25°C/W
TO-220-F package, FJPF5200 : 50 watts : 2.50°C/W

Any guidance on calculating pwr output from a pair or two of the smaller guys is appreciated.
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Old 12th June 2015, 04:52 PM   #2
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150 x 0.83 = 130 x 0.96 = 50 x 2.50 = 125C (150C max - 25C ambient)

80 x 1.25 = 100C
0.64 x 1.25 is not equal to 1 (aka look stupid)

For class AB, total max dissipation = (125 x n)/(Rjc + Rcs + Rsa x n)
n = number of output devices
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Old 12th June 2015, 05:07 PM   #3
Bigun is offline Bigun  Canada
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I've built several amplifiers using the T0-220 version of the 2SC5200 and had no issues by sticking to around 35W Class AB (single output pair). I found there is a limit to how hard you can bolt these things down when using top-hat washers etc. Nevertheless, they work just fine and my first ever DIY amplifier which uses them is still playing.

For some more recent designs I've discovered that the amplifier size is more limited by the size of the power rail smoothing capacitors I want to use so for those I stick with the learger TO-264 where possible.
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Old 12th June 2015, 05:28 PM   #4
wg_ski is offline wg_ski  United States
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You can dissipate about 20 watts with a TO-220 - period. The 3C/W case to sink Rth is the *****. And tightening the heck out of it around the mounting hole doesn't help. That's why car amp makers have gone to using a bar clamp that goes across the center of the package - where the die actually is.
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Old 12th June 2015, 06:40 PM   #5
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Quote:
Originally Posted by prairiemystic View Post
SOA curves don't exist on the datasheets for the smaller parts.
Estimation curves for the TO-220 versions can easily be drawn with the help of the C5200/C5242 ones.
(initial values are totally governed by package, peak values by the die)

(I'd use the best insulator material to minimise Rcs, definitely not mica and goop)
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Old 12th June 2015, 08:56 PM   #6
wg_ski is offline wg_ski  United States
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Quote:
Originally Posted by jacco vermeulen View Post

(I'd use the best insulator material to minimise Rcs, definitely not mica and goop)
High k silver-filled epoxy, directly to the heat sink. Leave the damn things hot and theramlly compensate them individually.

Not such a good idea in a switcing power supply. Not too much of a hazard with 35 rails in an audio amp.
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Old 12th June 2015, 09:31 PM   #7
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It seems you get about half the rated power dissipation on any device after you include thermal resistance of the insulating washer (0.2°C/W) and thermal grease (0.2°C/W), and a bit cooler 85°C rise (110°C in 25°C amb.) into an infinite heatsink... then you get half of that with a 1°C/W heatsink. About 32W for a TO-220.
My confusion was the hype on the datasheets and the derate is W/°C not °C/W. Oops.
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Old 12th June 2015, 09:36 PM   #8
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The SOA for a device is the same no matter what package it is in. So if there is a curve for a TO-3, for example, it can be used for the TO-220 as well.

paul
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Old 12th June 2015, 09:58 PM   #9
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Quote:
Originally Posted by prairiemystic View Post
insulating washer (0.2°C/W) and thermal grease (0.2°C/W)
For a TO-220, in your dreams only.
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Old 12th June 2015, 10:01 PM   #10
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Quote:
Originally Posted by phaselag View Post
The SOA for a device is the same no matter what package it is in.
I'd compare the SOA graphs of the linked 2SC5200/2SC5242 datasheets before posting.
(e.g. the DC and 100ms lines peak at Ic = Pd/10V. Which is 13A for the 130W device, and 15A for the TO-264)
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Last edited by jacco vermeulen; 12th June 2015 at 10:05 PM.
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