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34248Re: [beam] H_Bridge

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  • Wilf Rigter
    Mar 3, 2003
      Here is the simple pre-cursor design for the HyBridge schematic that helps explain the basic anti-shoot through design. 
      The detail of interest is the way the PN2222 and PN22907 transistors are  used to "break before make"  the source and sink power output devices during input transitions. 
      This h-bridge is a hydrid MOSFET / Darlington design for 12V-18V supplies.  The nominal continuous current is 4 Amp.  The IRFZ44 is a rugged n-channel MOSFET withVds=60V  Rds= 0.024 ohm,  ID=50A and is designed 10V gate drive signals. . The cheaper  IRFZ48 (Digikey US$1) with Rds= 0.01 ohm and Id=70A is an even better choice.  The TIP 125 is a 5 A 60V device costing about $0.50. 
      This circuit is interesting in itself because it is capable of forward, reverse, brake and freewheel. It is designed to be driven with rail to rail input signals (ie 0V and 12V).  The The freewheel and chopping  require that the input signals are tri-stated or driven by a 6V signal. Alternatively the four 10K resistors can each  be driven independently.   With both inputs grounded the active brake mode is on and the circuit draws just a few ma.   
      With just two added resistors the basic circuit can be modfied to include a simple free wheeling mode when both inputs are at 12V as shown here:
      Next follows a new low supply voltage, low saturation, current limited  HyBridge2 circuit
      HyBridge2  is a novel H-Bridge design that includes several interesting features
      1. Forward, reverse, brake and freewheel using two input pins
      2. Low (5V) supply voltage operation
      3. Lossless current limit sensing 
      4. Low sourcing saturation voltage drop
      5. Highly efficient proportional drive current.  
      6. Minimum through shoot current
      7. Low cost
      The circuit as shown is suitable for 3 - 5 A motors at 5V.  The circuit can be scaled with component changes to higher current and voltages. 
      The operating modes are selected by switching the inputs as follows:
      0V    0V        brake
      5V    0V        forward
      0V    5V        reverse
      5V    5V        freewheel  
      The saturation control circuit dynamically adjusts base current to the output device depending on load conditions to achieve a low output saturation voltage. The two series diodes is in parrallel with the compound PNP base emitters and shunts input current when the collector- emitter voltage is saturated (<400mV).  Since the TIP42 saturation voltage is proportional to the ratio of Ib/Ic,  a low Ic shunts base current from the PN2907 and reduces the PN2907 emitter /TIP42 base current accordingly.  Since the TIP2 base current is now proportional to the collector current it can be used for lossless current sensing for the current limit circuit.     
      The lossless current sensing may not be very accurate but it is very simple indeed.  The 10 ohm sensing resistors may be adjusted to suit.
      With the two 10K resistors acting as a voltage divider at the PN2222 base, the current limit occurs at about 1V across the 10ohm resistor. That corresponds to 100ma of TIP42 base current and is equal to 2A of motor current.  The current limiting controls the coresponding MOSFET gate voltage and that device must have a large enough heatsink to dissipate the IR heating .
      Saturated bipolar drive is notoriously inefficient at low load currents. The proportional base drive makes this circuit very efficient in variable load applications.   The TIP42 base current can be as high as 100mA near the short circuit limit and with a lightly loaded motor that base current drops to 20ma.   
      The IRLZ44 is a rugged n-channel MOSFET withVds=60V  Rds= 0.02 ohm,  ID=50A and is designed logic level gate drive signals. It may seem like overkill but at US$2 each think of it as cheap insurance.     
      Finally here are two implementations of an all MOSFET H-Bridge with forward, reverse brake and freewheel using the anti shoot through design.  MOSBridge1 is meant for 5V or 6V supply applications.
      and my favorite version of all: the general  purpose  >>> MOSBridge2 <<< design.
      Use logic level MOSFETs for applications requiring up to 9V motor supply. Use normal MOSFETs for motor supplies  in the 10V - 18V. Higher motor supply applications will require some protection of the MOSFET gates.  
      High logic level input signals can be anything between 2V to 24V. If necesary add a base emitter resistor to the PN2222 to increase noise immunity for the low level logic signal.    
      ----- Original Message -----
      ----- Original Message -----
      Sent: Monday, March 03, 2003 12:52 PM
      Subject: Re: [beam] H_Bridge

      At 09:40 PM 3/1/03 -0800, Wilf Rigter wrote:
      >Hello Dwayne,
      >Sounds interesting.  Do you have some links to the various motor
      >controllers that have evolved during discussions on the Open Source Motor
      >Controller (osmc) list?


      >Here is my contribution to the (beam) H_Bridge discussion:

      I like this.  The only drawback is that the TIP125's are darlington devices
      - I'd expect to see between 1-2V drop across them under heavy load.  And,
      of course, if you did not need the free-wheel mode, you could eliminate the
      extra components needed to create that mode.


      Dwayne Reid   <dwayner@...>
      Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
      (780) 489-3199 voice          (780) 487-6397 fax

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