Latching solenoid driver (H-Bridge)

Discussion in 'The Projects Forum' started by johsey, Sep 29, 2010.

  1. johsey

    Thread Starter New Member

    Sep 29, 2010
    Hi all,

    I'm currently designing a circuit which has to drive a latching solenoid. This solenoid is part of a machine and will "lock" a pluggable external device. This circuit must remain low cost but robust too.
    The circuit works as following:
    At the start of the machine, a test is done in order to define if the solenoid has been damaged (open-circuit behavior) and must be replaced. During this test, the solenoid is powered with a voltage < 6V to prevent it to activate and the current through it is measured with a sensing resistor.

    After this integrity test, the user can plug the external device into the machine: the solenoid act as a lock under 24V (latching).

    The user can unplug the device after its use or if an error occured: the solenoid is unlatched under 24V (reverse voltage).



    • ESR = 14.11Ω
    • U < 6V (during integrity check)
    • U = 24V (latch/unlatch)
    • Pulse Ton >= 1ms (single shot)

    The circuit is a basic MOSFET H-Bridge. When ON, The MOSFET QM bypass R3 which limits the current through the solenoid during the integrity test. A MCU will drive the MOSFETs with optcouplers (ideally simulated with switches).

    Please note that the load is only resistive because I haven't done any investigation to calculate the inductance of the solenoid yet (not specified in datasheets). The gate resistors values can be better defined in order to improve the trade-off between resistor power dissipation and MOSFET turn on/off time. Actual MOSFETs can (and will) be replaced by better suited and cheaper MOSFETs.

    The plot shows the MOSFETs gate voltage, the current through the load and the H-Bridge voltage during the three steps:

    1. 1ms-2ms: pulse for integrity check
    2. 3ms-10ms: R3 bypassing
    3. 4ms: solenoid latching
    4. 7ms: solenoid unlatching
    What is the source of the little "spikes" on the gates voltages of the unused H-Bridge side when the other is switching and what is their impact on the circuit?

    Are they caused by a charge current flowing via Ciss and Crss thus generating a voltage drop between source and gate? How to overcome them (capacitor in parallel of source resistor) ?

    Any circuit design improvement ideas or other solutions are welcome.

    Thank you.