Hi,

I am currently working on a voltage regulator design for my project at university. We have not learn anything about transistor and I am somewhat no sure about my idea, so I try to find some help here.

Specifications for the regulator are listed below:
Input power source: A lab power source of 36V and 4A max, I need a voltage input around >=28V

Output is desired to be 15V(adjustable through the adjustable voltage regulator) and can hold up to 3.5A.

Use university parts are preferred since they are free and may get bonus for using only school-provided parts.

Parts used and parts that are available at school:
The motor is a dc motor. We used it for the previous project at 7.5V and >1A. However, we try to double the speed by boosting the motor voltage to 15V. The motor's stopping current is around 3.2A.

Q1 is a PNP. Transistors that are available at school are:
2N3906BU
2N3904BU
2N6426G

The trimmer(variable resistor) is PV36Y103C01B00, and we have PV36Y102C01B00,PV36Y104C01B00 at school.

The adjustable voltage regulator is LM317.

Other parts that our school can provide can be found in the following link:https://docs.google.com/spreadsheet/ccc?key=0AjpsGmmb_spEdHhnMFJqQTNzc3lSQUR3V0ZtUzlKMVE

Problem occurred:
The transistors at school can not hold more than 200mA, which means I need to parallel the transistors or come up with a new design.

I want to know which is more feasible, using parallel transistors(if so, is there anything I need to pay attention to, cause some threads talked about resistors for the parallel transistors), or come up with another design.

Note:
If you find anything on the schematic that can potentially be a problem, please feel free to say it.
You help is very much appreciated and thank you for taking time to read this long text.

This design is not originally my idea, I combined two circuits found in the LM317 and 78XX voltage regulator's datasheet, and thanks to those guys' work.


Best wishes
Eric

Enclosure: Schematic of the voltage regulator

 

The circuit you posted is a linear regulator with 28V in and 15V out @ 3.5A. That puts the power dissipated in the PNP transistor at 46W...... which would melt it or destroy it by a kind of failure mode called "safe operating area" which every transistor has. The design as shown is not feasible.

I would recommend a switcher design such as one using a LM2576 or similar device to avoid the massive power dissipation problem.

 

I read through some threads on internet and found that some people proposed this idea. I do not know if I am right or not.

Note: resisters are 1 ohm and current flow through each transistor should be less than 1/3 of its max value(Ic). The beta value of transistors should be the same with an error less than 5%.

 

This is why I sometimes cry for the generation who are being taught using simulations and never build circuits. The circuit you posted is a linear regulator with 28V in and 15V out @ 3.5A. That puts the power dissipated in the PNP transistor at 46W...... which would melt it or destroy it by a kind of failure mode called "safe operating area" which every transistor has. The design is not feasible.

Apologize for my poor English.

Yes, I can understand your point and that is exactly what I am worrying about.

The transistor we have been provided can only hold 200mA max and I need something to hold 3.5A. And the solution I was imagining is to use parallel transistors which I hope could work the same way as resistors sharing current.

Also, I think that I need to worry about the branch connecting the resistor and adjustable volt. regulator, since I believe some current(I do not exactly how large; the Imax for the regulator is 1A) will flow through there and may cause the resistors to burn. The solution to this is to connect many resistors in series instead of using just one resistors.

About the solution you recommended, I thought about it and think it can be the backup solution for the project. Because we are encouraged to use free stuff provided by school and it could be costly and time consuming to buy a volt. regulator.

By the way, we do simulation before bread-boarding and only after the successful test on breadboard, we can then convert the design to PCB. So, no worry about safety

Thanks for replying.

Eric

 

Your main problem is not current, it is power dissipation. This design is not feasible to be done with a linear regulator such as your schematic shows. I would use a buck switching converter such as LM2576.

 

Thanks bountyhunter.

I understand the power part now, you mean the transistor can not dissipate that much of power and suggesting to use something that is capable of larger(suitable) power.

Power depends on I^2*R, not just current.

The situation is buying parts from elsewhere is really inconvenience compare to order parts from school. Since the project was given to us last week and I have to come up with a design and test it on the breadboard and convert it to BCD design on Altium by this Friday. So, I really appreciate to find some designs that use only the "little" parts that are in stock, no matter how many the design needs to use. Ordering parts online or looking for domestic shops would be time consuming and uncertain.

 

Power in a linear regulator is not I(squared) x R. That is only in a resistor. Power in a linear regulator is:

(Vin - Vout) x I LOAD

plus other smaller power from bias current.

The linear regulator design you have chosen is not feasible. The transistors you list are all in tiny TO-92 package which means their thermal resistance is about 125 degrees C/Watt. You would need about 100 of them in parallel to handle the 46W of power dissipation.

 

Thanks for the clarification, it is more clear now.

As you pointed out, I decided to use another design given in the LM317 datasheet named as "Adjustable 4A Regulator".
This design connects three LM317 in parallel and used a opamp and a transistor as the feedback system for balancing the three volt regulators. I did my best calculating the power and current and seems this design is very likely to work. The control system dissipates small power and most of the power is at the three LM317.

I believe by adding more LM317 in parallel based on the design, the temperature of the LM317 can be further decreased and the stability can increase(since one LM317 breaks down will not affect other LM317)

Schematic is on page 22 on the datasheet( www.national.com/ds/LM/LM117.pdf ）.