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  1. #1
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    mosfet gate driver circuit

    The MOSFET gate drivers are used to ensure a complete on/off state for mosfets .

    Also, if the MOSFET is a high side switch the source will be considered as a floating point while it is not tied to the ground , so it is important to ensure that the voltage Between the gate and the source is more than 12 v to avoid the negative temperature coefficient region .


    In the IR2110 i can't understand how the under voltage block will be triggered if the voltage between the source and the gate is less than 10v .

    see the attachment plz .[/img]

    •   Alt7th February 2005, 13:15

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  2. #2
    VVV
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    what is undervoltage

    Actually the undervoltage detector senses the voltage on the bootstrap capacitor, or the Vcc voltage, depending on which driver you refer to. But this voltage is used by the corresponding output stage to drive the MOSFET gate, so sensing it makes perfect sense, because the driver will only lose a few hundred millivolts. So in good approximation this voltage can be considered the MOSFET drive voltage.

    Note that this is a DC voltage, and therefore, easier to sense. The MOSFET gate is driven at a certain frequency with a certain duty-cylce, so sensing it would be much more complicated.


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    •   Alt7th February 2005, 17:19

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    under voltage detector

    HI VVV

    thanks for reply, but the problem is that i am confused with the voltage refering in this circuit .

    i mean what is the reference of the UV circuit .

    see also , http://www.innovatia.com/Design_Cent...%20Drivers.htm
    he illustrated how he had solved the floating point of the load using the ir2110

    he said :

    [These are typically implemented by floating an RS flop off the high-side supply rail and using two ground-referenced (low-side) high-voltage transistors to pulse the R and S inputs. The RS-flop output is buffered by a high-current driver, with bootstrapping circuitry to provide gate-voltage drive (VGH) above the supply rail (Vg).]



    i can't understand how did he referenced the flip flop to the ground of the 2 ground-referenced (low-side) high-voltage transistors to pulse the R and S inputs.

    what is the meaning of "floating the RS flip flop "


    Also,referring to the data sheet of ir2110 & and its application note an-978.pdf

    [ The input commands for the high side channel have to be level-shifted from the level of COM to whatever potential the tub is floating at which can be as high as 500V. As shown in the Figure the on/off commands are transmitted in the form of narrow pulses at the rising and falling edges of the input command. They are latched by a set/reset flip-flop referenced to the floating potential. ]


    i can't understand how did he refrenced the flip flop to the source floating point and the low side?



    •   Alt9th February 2005, 07:10

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    meaning of floating supply voltage

    The floating point is the Vs terminal. The power supply for the high-side driver is the bootsrap capacitor, installed between Vb and Vs. This voltage is floating, that is its reference point, Vs moves with respect to COM. That happens when the high-side MOSFET you drive turns on; its drain is connected to a positive supply rail and when it turns on its source goes to that supply rail, so Vs has to follow the MOSFET source and Vb will be higher than Vs by (Vb-Vs) volts.

    There are two undervoltage circuits. The upper UV DETECT block is referred to Vs, the other to COM. The reason is that you drive one MOSFET with respect to Vs, the other with respect to COM, so you need to make sure that the drive voltages with respect to THESE points are correct, since the sources of the MOSFETS are connected to these points and the gates have to be more positive than these points.

    Now the upper driver can be considered separately powered by the boost capacitor, connected between Vb and Vs. Then Vs can be any potential with respect to COM. The trouble is, your drive signal is resferred to COM, so how do you tell this driver to turn on the MOSFET? They use two resistors, connected to Vb, as drain resistors for two MOSFETs, having their sources connected to COM. Since the sources are at COM potential, they can be driven by a signal referred to COM.
    The drain curents of these MOSFETs will develop voltages across those resistors. Let's assume, for the sake of the argument that the voltages developed across the two resistors are equal to (Vb-Vs) volts. Then the bottom of those resistors will swing between Vb and Vs (with respect to Vs). As far as the upper driver is concerned, that is all it needs to work, two voltages varying from Vs to Vb. For this driver, Vs is the "ground" and its drive voltages are with respect to this "ground".

    Try simulating or even breadboarding a simple circuit with a bipolar transistor. Measure the voltage across R2, it will be always constant, but with respect to Vb, no matter how you adjust V2 (within limits; this circuit is just to illustrate the point). In the original circuit, since Vb-Vs is constant (you have a capacitor between those pins), the voltage can also be said to be referred to Vs.
    Think of the capacitor between Vb and Vs as a battery.

    The circuit powered between Vb and Vs contains the flip-flop and the actual driver, its own UV block, etc., but now Vs can be at hundreds of volts above COM. The R-S flip flop is needed because the MOSFETS would dissipate too much if they conducted current all the time, so they just pulse the currents and use the flip-flop to "memorize" the last command and turn on or off the external MOSFET.

    In the circuit shown, if you connect/ disconnect V1 you will also generate pulses across R2. Try it and see what happens as you adjust V2.

    That's about it.



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    mosfet gate drive circuits

    Hi vvv

    thanks for reply , but when i tryed to use the ir2110 to drive the h bridge , connecting the Vs to the source of the high side MOSFET causes a problem .


    the voltage across the Vbs is 6.5 v which is under the rated uv circuit 9.4 v
    so the HO is allways on latched.

    noting that the voltage between vb & vs is 15v till powering the h bridge , at this moment the vbs drops to 6.5 v or to zero some times and the HO latches


    so , what is the problem?



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    under voltage detector bipolar

    HI vvv

    i think the solution of my ir2110 is in this page :
    http://www.e-insite.net/ednmag/archi...1795/17di5.htm

    "Precharged capacitor ensures high-side start-up "

    Unfortunately, this short period of unstable power conditions produces erratic driver behavior.

    [When you use a high-voltage bridge driver, such as the IR2110 or the newer IR2155, a complete driver circuit can exhibit erratic behavior when you apply power. The main problem arises from the bootstrap capacitor that discharges when the power appears. Thus, the floating supply is not ready to fulfill its primary function of driving the high-side MOSFET. The circuit needs a few commutations to rapidly charge the bootstrap capacitor to allow it to supply the upper driver.

    The main supply directly feeds the IR2155. However, you can extend the method to other topologies, such as a ballast supplied by a power-factor-corrected auxiliary supply. At power-up, R1 precharges CBOOT via the R2 resistive path to ground. Zener diode D1 limits the voltage across CBOOT to 16V. This limiting action protects the upper driver from damage by overvoltage. Thus, when the IR2155's VCC pin reaches the upper threshold level, CBOOT is already charged and the MOSFET bridge delivers square waves without exhibiting erratic behavior.]


    see the attached circuit



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    ir2155 application

    Hi VVV,
    I'm caring the upside UV detect circuit and how to implement it.
    Could you help me or supply some paper and circuits to a reference?
    Thanks a lot.



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