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I designed below circuit to provide 45V/200mA for a LED ceiling lamp.

schematic First, I built it on breadboard. Everything was fine. No voltage drop. MC34063 was warm and transistor TIP31C was hot but can cool down by heatsink.

After some tests, I made a stripboard version with the same components but cannot work normally... 1. MC34063 became very hot after powering up. (~5sec) 2. TIP31C became cooler than breadboard version (touchable). 3. Output dropped to ~39V.

I guess the driver/output transistors inside MC34063 wasted too much energy and generated too much hot. Should I lower the value of input resistor of TIP31C or do something else?

By the way, I tried to replace the inductor with netshop's product during breadboard version test and found the circuit's performance decreased (They have the same rating 470uH/3A). Origin inductor's core is yellow and netshop one looks like: netshop I can't explain why. Can anyone explain it for me?

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  • \$\begingroup\$ As an aside here, does anyone know the material characteristics used for the typical Chinese color coding of cores? \$\endgroup\$ – Spehro Pefhany Oct 27 '16 at 13:01
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You need a resistor in series with the drive signal to the base of the TIP31 transistor - the base/emitter region on the transistor is just a forward biased diode so it will try and clamp the controller output and make it get hot. The current limit resistor that feeds (also) the collector of the internal transistor in the chip is too low to prevent significant (and too large) drive current into the TIP31 base. Alternatively try inserting a resistor in pin 8 - see figure 9, 9a and 9b of data sheet and here's what I mean pictorially: -

enter image description here

Note R2 in the picture.

For the question about the inductor, links to both data sheets are required.

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  • \$\begingroup\$ Thank you very much. I added a 100 ohm resistor in front of the base pin of TIP31 and replaced original input resistor with a 47 ohm resistor. Now the IC is no longer heating. \$\endgroup\$ – Bruce Hsu Oct 27 '16 at 18:27
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According to Andy aka's answer, I revised my design. It now works fine. (Converting performance is not good enough yet...)

enter image description here

Things about the inductor:

The net shop (China's taobao site) I purchase inductors doesn't provide any datasheet or information more than rating & current. The only advantage was "very very cheap". The page also said it is made of MnZn.

enter image description here

The inductor mounted to PCB is OK. The other causes bad performance and audible noise. They have the same rating.

I guess the green one is easy to saturate on high frequency. This might be the root cause of bad performance and noise.

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your noise reason is low frequency from very high inductance.is also caused heating and low efficiency. if you need corect check current please use analog current meters or test on big distance from load. - and + give different measuring (lowest measuring mostly real).

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