Do It Yourself

DC Power Supply

28V to 50V 15A DC Power Supply

The accident happened during the test :

Transistor explosion during test

¡@

Break to two pieces

¡@

During testing

I started the testing after all the PCBs and components had been assembly. I used my digital multimeter to check the variable output voltage. It worked so fine from DC 3V to 50V. After that, I used a 1ohm 20W resistor for the output loading to check the output current. I also put this loading resistor inside a cup of water because the power handle of this resistor was not enough. I always use this method for a heat sink and also it works quite good. First of all, I adjusted the output voltage at the minimum position. The output is DC3V. Just 3A current will transfer from the supply to the loading. I think it is safety!!! BUT the transistor was explosion after three minutes. One power transistor protection diode was broken down and short circuit. Three of middle transistors was failed. One explosion.

Before testing

Before assembly, the circuit was also be tested. The driver circuit was no problem for DC50V 1A and worked more than 5 hours. Without any power transistor here because I think the 4 pcs of darlington power transistors are too enough for 1A base current to produce 15A output.
Explain what's  happened It was too many voltage dropped at the regulator during large current output. For the rectifier output, the voltage is DC 62V. When the output voltage is DC50V, the voltage drop at the regulator circuit is DC62V - DC50V = 12V. For DC50V 1A output, drop is DC12V x 1A = 12W. For DC50V 15A, the power drop is 12W x 15 = 180W. It is no matter for Vce = 10V and Ic = 15A. But for DC3V voltage output, the voltage drop at circuit is DC62V - DC3V = DC59V. For DC3V 3A output, drop is DC59V x 3A = 177W. Although it is just 177W and also less than before but for the transistor specifications, the total power dissipation is fixed. In this case, it is so dangerous for Vce = 59V and Ic = 3A. This means that if the power dissipation is 100W, Vce = 10V then Ic = 10A but for Vce = 100V, Ic just equal to 1A.
Solve the problem Although I used 4 pcs of 30A darlington power transistor for output stage, the problem still happened. It is the best way to modify the transformer with 4 different voltage output. From AC 10V to 40V. For less than DC10V output, use choice 1 - AC10V. For DC50V output, use choice 4 - AC40V. The choice will be changed by tracking the DC voltage output adjust variable resistor. It can more effective to reduce the voltage drop at the regulator circuit.
Summary But now, I cannot modify the transformer and also don't want to buy a new one. I will change the operation range to just DC50V 15A. It is ok for my next project 6m 300W linear amplifier. Or else, from DC28V to 50V 15A. Because of the later project, the 70cm 100W linear amplifier. It operate by using DC28V.