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Analysis of PICKit2 Half-Bridge Circuit

Discussion in 'General Electronics Chat' started by rjburke377, Feb 18, 2009.

  1. rjburke377

    Thread Starter New Member

    Feb 17, 2009
    I've purchased a Microchip PICKit2 Development Programmer and am trying to understand the electronics on the board. One area of the board has me a bit puzzled. There is a half-bridge circuit that I assume allows the Programmer to set the output voltage to the target part being programmed (See attached picture).

    Is it true that this circuit can vary the output voltage (VDD_TGT_OUTPUT) between ~0V and +V_TGT (usually +5V)? I shouldn't expect it to use the capacitors to pump up the voltage above +V_TGT right? My simulation doesn’t protect against shoot-through current so it may invalidate my analysis. I don't know.

    Using multisym (See attached picture) I found the following:


    Duty Cycle Pk-Pk Min Max
    10% 372uV 4.83V 4.83V
    20% 561uV 4.82V 4.82V
    30% 754uV 4.82V 4.82V
    40% 1.83mV 4.82V 4.82V
    50% 2.29mV 4.82V 4.82V
    60% 24.2mV 4.78V 4.8V
    61% 41.1mV 4.76V 4.8V
    62% 54.2mV 4.75V 4.8V
    63% 180mV 4.63V 4.78V
    64% 524mV 4.21V 4.77V stable
    65% 51.5mV 3.24V 3.29V stable
    66% V V deflates
    90% 27mV -17.5mV 10.2mV

    My multsim circuit doesn’t use feedback to automatically adjust the frequency or duty cycle.

    Is there a way to calculate an optimal Frequency for VDD_TGT_N and VDD_TGT_P? I chose 5MHz at random. Is there a procedure for "tuning" the frequency to get optimal performance in terms of efficiency and stability?

    The voltage will increase to 4.83V with a duty cycle of ~62% or greater. Remain at whatever level at ~64%. Decrease at ~66%. Is this an expected behavior for this type of circuit? Or does the lack of shoot through current protection make this behavior wrong?

    Sorry for all the questions but I can't find any reference materials that I can relate to. If there is an authoritative source then please let me know.

    Last edited: Feb 18, 2009