DC/DC buck converter power efficiency (voltage mode)

Discussion in 'The Projects Forum' started by naeim29, Jul 29, 2011.

  1. naeim29

    Thread Starter New Member

    Jul 29, 2011
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    I'm designing a circuit of DC/DC synchronous buck converter for low power application(voltage mode) in 65nm by Cadence tools,I finished the circuit design and it is able to step down 1.1 V to 400 mV(Reference voltage),but its power efficiency is not good(around 35%)I read so many papers to find how i can optimize the power efficiency and I did so many changes for example sizing of switch transistors,changing frequency,changing driver structure...but still its power efficiency is too low!!!in one paper i found that if difference between input voltage and reference voltage is high the efficiency will reduce but I'm not sure how much it will reduce!I will be so appreciated if someone can help me to how I can increase efficiency!thank you so much!
    here is my results so far:

    Average Input Current : 103.7 uA
    Input Voltage: 1.1 v
    Average Output Voltage: 399.8 mv
    Average Output Current: 99.9 uA
    Switching Frequency: 5 MHz
    Settling time: 20 usec
    voltage ripple: 3.97 mv (0.99%)
    current ripple(peak to peak): 993.4 nA (0.99%)
    Power efficiency ([Pout/Pin]%): 35%
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Where is your schematic?

    Why are you switching at such a high frequency? Can't you get that down to 500kHz?

    [eta]
    You're going to go through the design piece by piece and determine where the power is being dissipated; then work on those individual pieces.
     
    Last edited: Jul 30, 2011
  3. naeim29

    Thread Starter New Member

    Jul 29, 2011
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    thank you so much for replying me!I have changed several times the switching frequency but it does not have any effect on efficiency as you can see here, my PWM frequency is about 50 KHz but still efficiency is too low!
    but when I tried my circuit for step-downing input voltage from 1.1v to 800mv its efficiency was about 70% due to increasing output power. i do not know why when I'm trying to step-down 1.1v to 400mv my efficiency drops too much!!!
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    It looks like you are confused as to which is the noninverting and which is the inverting input to your error amp, as you have conflicting labels on them. I cannot tell by your schematic which is which, either.

    Almost nothing has values assigned in the way of resistance or capacitance.

    It is not obvious what should be the Vdd or GND supply pins.
    You are using more than 1 voltage source. This makes it more difficult to determine what your actual efficiency is.

    Many DC-DC converters are much less efficient at partial capacity than at nearly 100% capacity, but 35% is truly poor. You need to turn on the power indicators for all of your components to find out where the power is being dissipated.
     
  5. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    Average Input Current : 103.7 uA
    ...
    Average Output Current: 99.9 uA

    Is it even oscillating?? :eek:

    It's going to be very hard to get any good efficiency at incredibly low currents like 100uA Iin, unless your entire control circuit and all switching losses total less than 10uA or so which is extremely unlikely.

    What is your desired Vout and Iout for the finished converter??
     
  6. naeim29

    Thread Starter New Member

    Jul 29, 2011
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    thanks for you replying,this is only labeling conflict and i'm not confused with that.I had only one voltage source(input voltage as vdd) which i used it as vdd for each components like amplifier,comparator...but i thought if i have separated power supply maybe i can increase efficiency but it has not any effect so again i use one voltage supply(Vin) for all my components.
    one of my concerns is about this value of input and output currents(about 100uA) is it reasonable for converting 1.1v to 400mv?isn't too low for load of 4kΩ(Rout=4kΩ)!!!should not be some mA!?I asked it because the output current is not explicit in my specifications.here is loop stability and also total simulations result,as you can see the Phase Margin frequency is about 10.36 MHz for Amplifier loop(output of amplifier IPRB0 probe).also I reduced the switching frequency[PWM](from 5MHz to about 48KHz) for reducing switching power loss but it has not any effect on power efficiency!!!but i should mention that this frequency is not complete fixed!
    I'll be more appreciated for more helps!
     
  7. naeim29

    Thread Starter New Member

    Jul 29, 2011
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    0
    thanks for your replying.yes as you can see it is oscillating but not that much(about some nA).Vin=1.1v and vout should be 400mv,but the output current is not explicit in my specifications,I am not sure this value of output current for load resistance Rload=4kΩ is reasonable to reach high power efficiency?!and i'm confused that how i can increase output current to for example some mA!!!:confused: thanks for your help!
     
  8. steveb

    Senior Member

    Jul 3, 2008
    2,433
    469
    You're getting good advice. Isn't it strange that you get the same efficiecy at 5 MHz as at 50 kHz, and actually 5 MHz isn't very practical? Also, isn't it strange that your efficiency matches that of a linear regulator (35 % is close to 400 mV/1100 mV and 70% is close to 800 mV/1100 mV)?

    Why are you driving the MOSFETs the way you are, and what is your value of Vdd? If Vdd is too low, you can't turn on either transistor fully and if Vdd is too high, then you will have both on at the same time and get a short through two on transistors.

    It would be helpful if you drew out a block diagram of the basic functional stages and try to make it clear exactly what you are trying to do. You have many functions built into the OPAMP compensator (or so it seems) signal filtering, proportional gain, derivative gain, integral gain etc. all are possible, depending on the component values. And a top down design specification is helpful for you to know what you are doing, and also helpful for us to understand so we can help you.
     
    naeim29 likes this.
  9. naeim29

    Thread Starter New Member

    Jul 29, 2011
    7
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    thank you so much for your help!you right my circuit was working like simple regulator and i found that the problem is with the switching output voltage which was not correct due to wrong value of L & C (LC filter),now is better and the efficiency improved till 72% but still i should work on it to improve it more.but still i have so many ripples at output,it should be 400 mv but it's fluctuation is between 249mv to 458mv!
     
    Last edited: Aug 6, 2011
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