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Control design for PWM converter

  1. Feb 18, 2015 #1
    Hi Everyone,

    here is Hubert again. Here attached is my design and the problems I am facing.

    Can anyone be of help?

    Thanks in advance.

    Best regards
    Urbain
     

    Attached Files:

  2. jcsd
  3. Feb 18, 2015 #2

    Svein

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    The main difference is the removal of the reference voltage.

    If VLED<1.5V, no current flows through the LED, the output of the INA210 is 0V - and that is what you use as a reference voltage.
     
  4. Feb 18, 2015 #3
    Hi Svein,
    thanks for your fast reply, so how can i add the INA210 in my circuit into for the circuit to be functional as required?
     
  5. Feb 18, 2015 #4

    Svein

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    Start with your first circuit. This has VLED as a feedback - when it increases, it is regulated down. If you look at the output of INA210, you have the same polarity - when the output increases, you want VLED to go down. Thus - sum VLED and the output of INA210 using a resistor in series with each. Experiment with the relation between the resistors. Or - insert a 10k potentiometer with VLED and the output of INA210 at each end and the slider to the op-amp input.
     
  6. Feb 18, 2015 #5
    Hi Svein, thank but i don't get it.
    please can you just give me a rough sketch of what you mean?
     
  7. Feb 18, 2015 #6

    Svein

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    upload_2015-2-18_17-28-44.png
    Like this. Use your first circuit, add the INA210 and connect the output as shown here.
     
  8. Feb 18, 2015 #7
    Hi Svein,
    thank you for your response, but i believe adding a 10K in the circuit also increase power loss thereby reducing the overall efficiency of my design (correct me if I am wrong).
    I wish to have a design with high efficiency.
     
  9. Feb 18, 2015 #8

    Svein

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    1. The regulator system tends to minimize the voltage across the potentiometer.
    2. Even with a voltage of 1V across the potentiometer, the current through it is negligible compared to the power draw of the rest of your circuit.
    If you are really into high efficiency, read the application notes from Linear Technology - for example: http://cds.linear.com/docs/en/design-note/dn310f.pdf
     
  10. Feb 19, 2015 #9
    Hi Svein,
    Thanks. I will look at the document and get back to you.

    Best regards
    Hubert
     
  11. Feb 20, 2015 #10
    @Svein,
    thank you so much, your idea works perfectly and the problem is solved.

    My second problem is still without a solution. Can anyone help?
    how to use another current monitor to measure the LED current and use it as feedback to the temperature measurement? The Goal is to fix a maximum temperature for the LED.

    Best regards
    Hubert
     
  12. Feb 22, 2015 #11

    Baluncore

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    As I understand it, you intend to use the LED current and LED forward voltage to identify the LED die temperature.
    Will you then back-off the current to limit LED temperature rise?

    What make and model LED are you using?
    Can you please post your latest schematic model.
     
  13. Feb 22, 2015 #12
    Hi Baluncore,

    Thank you once more for your time.
    Yes you understood what my plans are. Indeed, i will then back off the current to limit LED temperature rise.
    I am using an OSRAM LUW HWQP LED "3.25V and 1A "

    Here attached is my lastest schematic. I can also feed VLED directly to the Error amp. Vref is my precison output voltage.

    Thanks in Advance.

    Best regards
    Hubert
     

    Attached Files:

  14. Feb 22, 2015 #13

    Baluncore

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    Re: OSRAM LUW HWQP data sheet;
    http://www.osram-os.com/Graphics/XPic4/00151034_0.pdf/LUW HWQP - OSLON Black Flat.pdf

    With Ifwd = 1A and Vfwd = 3.35V the LED will have 3.35 W power dissipation.
    Knowing from the table on page 4 that the thermal resistance is 5.5°C / W.
    We can expect a temperature rise of 5.5°C * 3.35W = 18.5°C
    To limit junction temperature to 125°C requires the environment remain below about 105°C.

    Relative Forward Voltage versus temperature is shown at the top left of page 10.
    It shows that relative to 25°C, at the maximum operating temperature of 125°C, the voltage will have fallen by only 0.18V
    This is less than the manufacturing tolerance grades, 8F, 8G & 8H listed at the bottom of page 5.
    The Characteristics table on page 4, shows a 0.75 volt manufacturing spread.

    It will therefore not be possible to determine temperature from Vfwd without a calibration process.
    You will probably need to calibrate every LED with short "on" pulses to measure Vfwd at 25°C.


    Relative spectral emission, at the top of page 8, shows a peak at 440nm in the blue.
    Knowing that " Energy = Plank's constant * frequency " we can compute Vfwd = 1240 / 440nm = 2.818V.
    Ignoring the Gaussian thermal energy spreading expected, that should be the intercept of the Ifwd curve with Vfwd axis.

    Looking at the graph of Ifwd versus Vfwd, (top left of page 9), we see it passes through the point 3.4V at 1.4A
    The series resistance of the LED will be Rs = (3.4V - 2.818V) / 1.4A = 0.416 ohms
    We can predict Vfwd at Ifwd = 1A as being 2.818V + ( 1.0A * 0.416R) = 3.234

    The graph shows that it is a close enough model.
    So the LED can be modelled as a 1.818V bandgap in series with a series resistor of about 0.416 ohms.
     
  15. Feb 23, 2015 #14
    Hi Baluncore,

    Thank you for your time and effort put together to help me. But I must admit, i do not get it.
    Are you saying my actual schematic is enough? should I just change the series Resitor to 0.416 ohms and the work is done?

    Or is there anything to do next?

    Best regards
    Hubert
     
  16. Feb 23, 2015 #15

    Baluncore

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    No.A model of the LED you are using looks like a voltage step in series with a resistor having a value of about 0.416 ohms.
    I am saying that it will be very difficult for you to implement LED die temperature estimation by monitoring LED Vfwd and Ifwd.
    I would not take it on without a micro-controller to first calibrate the LED model and then monitor the model on the fly.
     
  17. Feb 24, 2015 #16
    Hi Baluncore,

    thanks for your reply. Permit me to ask a foolish way. Do you think there is any other way to implement LED die temperature on the model of the LED i am using?
    I am still trying to figure that out.
    Thanks in advance
     
  18. Feb 24, 2015 #17

    Baluncore

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    By independently monitoring LED heatsink or substrate temperature, you can reduce the duty cycle, the LED current and so LED power dissipation as the environmental temperature approaches the limit of 105°C. If the change begins only at 95°C and takes place so that current falls linearly to zero at 105°C, then the LED will be over-temperature protected. That might be achieved by reducing the current reference voltage at high temperatures.
     
  19. Feb 24, 2015 #18

    Baluncore

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    @ Hubert Tchio. Looking at your circuit diagram, you have X2, a high-side P-channel MOSFET switch. N-channel MOSFETS have lower on-resistance, less drive requirements and switch faster for the same price. I believe you should invert your design to use the lower cost and more efficient switch.

    You drive the gate of X2 with a voltage comparator, U39, hopefully it has some feed-forward to speed up the transition. But does it have the drive needed, (about 100mA), to switch X2 quickly through the inefficient transition?
     
  20. Mar 2, 2015 #19
    @Baluncore . Thanks for your feedback. I will try the circuit with a high-side N-channel MOSFET switch to see what i get.
    So far the ciruit is ok. The voltage comparator compares my control voltage and my ramp input to provide a PWM signal at the output for driving the X2.

    My main problem now is still on how to implement a temperature measurement. I have decided to go for an LM335 IC temperature sensor or themistor, I have built a thermal design, it seems ok but I still don't know how to integrate this thermal design to my current LED driver. It would have been easy if my thermal design switches off (input to ground) my LED driver when the temperature is too high but rather the thermal design should reduce the forward current instead.

    Still looking for help.

    Best regards
    Hubert
     
  21. Mar 2, 2015 #20

    Baluncore

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    To taper the LED current from a maximum at 95°C to zero at 105°C you can use a couple of LM334 Current Sources. They are available in 3 terminal TO-92 package. Get the data sheet from; http://www.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=lm334&fileType=pdf

    Set up two in series, between the supply rails. (See the attached sketch). One configured as a temperature sensor, see page 6, fig 13; set it to 1uA per °K. The other as a zero tempco source, see page 8, fig 15; set it to (95°C+273°K) = 368uA.

    Now place the two together in thermal contact with the LED mounting. A difference current is available at the junction node of the two. Take that current through a signal diode to a resistor connected to the current sensor output. Once the temperature reaches 95°C the diode will conduct and add voltage to the current sense voltage. The current will be reduced as a result. Select the resistor to give zero LED current when 10uA flows. For 1A LED current, (giving a 2 volt signal), use R = 2V / 10uA = 200k.
     

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