How to design a PWM switch mode DC-DC converter

Discussion in 'General Electronics Chat' started by odinaustin, Aug 24, 2012.

  1. odinaustin

    Thread Starter New Member

    Feb 2, 2011
    4
    0
    Hello everyone,

    This is my first time to post help here. I am not electronics guy and this is my first electronic project. Please give me some suggestion or help to my problem. Thank you very much!

    The project is about building up a furnace with a k-type thermocouple for temperature sensing and a PWM switch DC/DC converter as a power supply controller. Basically the temperature read from the thermocouple will determine how much the duty cycle of the PWM will be for the voltage supply to the furnace resistive heaters. I attached a schematic of the furnace system control here for your further understanding.(Fig.1)

    [​IMG]

    For the thermocouple, I am using AD595 as signal conditioner for the k-Type thermocouple. The analog signal from AD595 will be sent to a LabJack data acquisition card that I can read from the LabView on computer.

    For the PWM switch mode DC/DC supply, I am using a MIC5021 (MICREL Inc.) as an n-channel MOSFET driver. The operation frequency is about 100KHz. The power supply is +24V voltage supply. The voltage of heaters I want to apply is about 20V and the maximum current I want to achieve is about 3A. I attached the schematic here. I placed low pass filters at both input and output to secure that I have clean voltage supplying to the load. (Fig.2)

    My questions are:

    1. How to design the inductance and capacitance of the input LC filter?
    2. What kinds of n-channel MOSFET should I choose?
    3. The sensing resistor is about 0.01 Ohm. What kinds of special resistor should I choose?
    4. In the buck converter part, how do I calculate the inductance and the capacitance?
    5. What kinds of diode should I choose for the buck converter part?
    6. At the output filter, how do I calculate the capacitance and inductance?
    7. Will the capacitor at the output filter kill the sensing resistor if the duty cycle increases too fast?
    8. Is there any flaw in the schematic design?


    [​IMG]
    Thank you very much for your reading and answer!
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,013
    3,233
    1. The input filter calculations depend upon what EMI standards you are trying to meet.

    2. You want an N-MOSFET that has at least a 50V rating, with low on-resistance. The on-resistance affects the efficiency and how much heat sinking the transistor requires.

    3. You can buy low value resistors especially made for measuring current. Its resistive value is the only important parameter.

    4. Here's a calculator to determine the values for a buck converter, as you have.

    5. Use a Schottky diode with about a 6A/40V or greater rating.

    6. Unless your load is particularly sensitive to noise (and a resistive load isn't) then you only need one output capacitor and don't need the additional Output filter inductor and capacitor.

    7. No. The output filter has no significant effect on the sense resistor voltage or current.

    8. Looks ok to me. Make sure you use a proper circuit layout with a good ground plane and short current paths between all high currents components such as the input capacitor, transistor, diode, and output inductor.
     
  3. odinaustin

    Thread Starter New Member

    Feb 2, 2011
    4
    0
    Hi Zapper,

    Thanks for your detailed answers.Do you have any link about introduction of the EMI standard or the details about the calculation?

    What kinds of load will be sensitive to the power supply? I searched online but did not get information about this point? Sorry, i am quite fresh to electronics and maybe my question is quite stupid.

    Do you think the input filter will be potentially harmful to the sensing resistor? Especially when the MOSFET is on, the energy from the input filter will be a big pulse to the sensing resistor?

    Have a good day!

    Bo
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,013
    3,233
    Answers in blue.
     
Loading...