Increasing 1V ac to 3V ac

Discussion in 'The Projects Forum' started by IvyK, Apr 7, 2012.

  1. IvyK

    Thread Starter New Member

    Apr 7, 2012
    15
    1
    Hi all,

    I'm currently designing a power supply for a powerline robot. I have an input varying ac voltage of 1Vrms - 12Vrms. My aim is to produce a constant 20Vdc output. The system needs to be very efficient. I can get the system to work with an input voltage of 3-12Vrms. This is the route i want to go:

    Input voltage--->MOSFET rectifier--->boost converter--->regulator

    The input voltage is a varying 1-12Vrms ac supply at 50Hz,

    The MOSFET rectifier rectifies the AC to a unipolar waveform,

    The boost converter will boost the voltage to above 20V, for the control of the boost converter I want to use a 555 timer. This removes the PWM control circuit that behaves strangely when the input to the boost converter varies to quickly. I think this is because of the settling time it needs.

    The regulator will regulate the voltage to 20Vdc.

    The problem I'm having is when the input voltage is 1 and 2Vrms. I can't boost it to the required voltage level of 20Vdc. I have looked at many routes to take i.e. switched capacitors or charge pump capacitors but from the readings this is suitable for applications in the mA range. I need at least a few amps out after the boost converter. This includes voltage multiplier circuits such as voltage doublers. I've looked at cascaded boost converters but this doesn't work wen I simulate is on multisim 11 and its inefficient.

    I want to implement a voltage transformer that could be switched in when the voltage is 1 to 2 (or 3) Vrms using 1:4 transformer ratio. I know the bigger the transformer the greater the efficiency. This is a pretty small transformer. Is it not practical? :confused: Are there any other suggections?

    Thanks in advance
     
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    Is your MOSFET rectifier maintaining good efficiency down at the lowest input voltages? As well as requiring to have a potentially low Rds(on), the MOSFETs will need to continue to have adequate gate drive to realise this, even when the AC input voltage is small.

    Similarly, the device drive levels in the converter circuits need to remain adequate. Some form of auxiliary step-up supply will probably be in order. My apologies if these are simplistic points, but these sorts of things are sometimes overlooked.
     
    Last edited: Apr 7, 2012
  3. wayneh

    Expert

    Sep 9, 2010
    12,090
    3,027
    If "a few" means 5A, then you're talking 100 watts of output. For that to be provided by a 1Vrms AC supply, you're talking at least 120A on the input. That's NOT going to be a small transformer. :eek:
     
  4. IvyK

    Thread Starter New Member

    Apr 7, 2012
    15
    1
    Hey there.

    @adjuster
    The MOSFET bridge does have a low Rds and is running. For my simulation i haven't designed proper drivers (will do so at a later stage) but the devices are being turned on to perform the rectification at low voltages. Don't worry about being too simplistic... simpler is better:)

    Right now i'm just trying to get this to work.

    @wayneh
    I see, lets brainstrom for a minute and say the current at the output wasn't an issue. Let me also explain the input senario in more detail.
    This robot runs along the powerlines and uses a current transformer to generate voltages. The current in the line changes all the time (100 to 1000s of amps) relating to the 1-12Vrms change in my system. I have to run a battery charger circuit at the output of the regulator thats why i need the constant voltage. The ICs require round about 20Vdc. I can't change that input current transformer so i need to make a plan with this 1 and 2Vrms.

    Is it practical to design a transformer to step up 1V to 3V or 4V?
    Do you even get such transformers?
     
  5. wayneh

    Expert

    Sep 9, 2010
    12,090
    3,027
    I'm no transformer expert but a 3:1 or 4:1 ratio should be no problem at all. In addition to the windings ratio, you'll need to specify the current in the primary and secondary coils.
     
  6. IvyK

    Thread Starter New Member

    Apr 7, 2012
    15
    1
    ok. You mean 1:4 or 1:3 ratio. Thanks :)
     
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