The resistor in the gate circuit or how to do it right

    Good day to all!

    This short article will probably become a cheat sheet for beginning developers who want to design reliable and effective power semiconductor switch control circuits, update and refresh the old knowledge of experienced specialists, or maybe scratch the memory bins of readers at least somewhere.

    I will be very glad to any of these cases.

    In this article I will try to describe the most common questions of the choice of gate resistors for power electronic devices. It is based on the knowledge gleaned by me from various literature, apnotes from TOSHIBA, Infineon, Texas Instruments, as well as from modest practice. It is worth noting that this information does not provide directly universal recommendations for each power key. Nevertheless, it is possible to analyze what assumptions can be important and what influence they can have on the choice of gate resistors for discrete power transistors, as well as for power modules.

    The basics

    The gate resistor is located in the circuit between the driver of the power transistor and the gate of the transistor itself, as shown in the image in the header of the article.

    Open or closed field key (IGBT / MOSFET) depends on the voltage applied to the gate. A change in this voltage charges or discharges the gate capacitance of the power device, which consists of the gate-collector capacities$ C_ {gc} $ and shutter-emitter $ C_ {ge} $and a small capacity of the shutter itself. The charge of the input capacities of the key will turn it on (current$ I_ {g.chrg} $), and the discharge will turn off (current $ I_ {g.dischrg} $)

    The resistor in this circuit limits the charge / discharge current of the input capacities, in addition, a correctly selected resistor will not allow the key to open spontaneously, which can sometimes happen, due to a rapid change in voltage at the power terminals of the key, for example, this can happen when the adjacent bridge has half-bridge topology the key opens. In this case, the capacity$ C_ {ge} $The current flowing through the gate resistor is recharged and causes a voltage drop on it, which can open the key. In addition, the key opening threshold often drops significantly with increasing temperature of the semiconductor crystal.

    What you need to know and how to choose the “right” resistor

    1. Maximum charge / discharge current of the driver output

    Any driver microcircuit has such a parameter as the maximum output current. If the gate current when opening / closing the key exceeds the maximum output current, the driver may fail, therefore, in this case, the gate resistor will limit the output current of the driver.

    You can make an equivalent circuit model by which to calculate the required value of the resistor:

    Following simple inferences, we can obtain formulas for calculating the driver current, and select the gate resistor so as not to exceed the maximum allowable driver parameters:

    $ I_ {g.chrg} = \ frac {V_ {CC} -V_ {EE}} {R_ {drv.ON} + R_g}, $

    $ I_ {g.dischrg} = \ frac {V_ {CC} -V_ {EE}} {R_ {drv.OFF} + R_g}. $

    2. Power Dissipation

    Also one of the important functions of the gate resistor is to dissipate the power of the output stage of the driver microcircuit. According to the model above, power dissipation can be calculated using the following formulas:

    $ P_ {g.chrg} = \ frac {Q_g} {2} \ cdot (V_ {CC} -V_ {EE}) \ cdot f_ {sw} \ cdot \ frac {R_ {drv.ON}} {R_ { drv.ON} + R_g}, $

    $ P_ {g.dischrg} = \ frac {Q_g} {2} \ cdot (V_ {CC} -V_ {EE}) \ cdot f_ {sw} \ cdot \ frac {R_ {drv.OFF}} {R_ { drv.OFF} + R_g}. $

    Here $ Q_g $ - the shutter charge of the key, and $ f_ {sw} $- switching frequency.
    After calculating and selecting a resistor, it is important to observe the following condition:

    $ P_ {g.chrg} + P_ {g.dischrg} + P_ {drv} <P_ {drv.MAX}, $

    Where $ P_ {drv} $- own driver consumption.

    There is still a small note, in most datasheets the keys indicate the shutter charge under certain conditions, for example, with a shutter control voltage of + 15V ... -15V, if your circuit has a different control voltage, for example + 15V ... 0V, or +15 ... -8V, it is enough to accurately determine the shutter charge will help the following relationships:

    $ Q_ {g (0 ... 15V)} = 0.6 \ cdot Q_g, $

    $ Q_ {g (-8V ... 15V)} = 0.75 \ cdot Q_g. $

    3. Turn-on speed and electromagnetic compatibility

    Let us consider switching losses as a function of the resistance of the gate resistor. I will take the key that I recently used in my small project - IKW40N120 from my favorite Infineon:

    As you can see, with increasing shutter resistance, the switching speed decreases and switching losses increase. Accordingly, this will affect the efficiency of the system as a whole. On the contrary, if you apply a lower shutter resistance, the switching will become faster and losses will be reduced, but the noise caused by the rapid increase in current and voltage will increase, which can be critical when you need to meet the requirements of electromagnetic compatibility, therefore, you must choose the shutter resistance very carefully .

    4. The same “spurious” inclusion

    In the beginning, when I wrote about the functions of the gate resistor, I mentioned the possibility of the key turning on spontaneously. To prevent this from happening, you can calculate the voltage that may appear on the gate of the transistor, look at the image below and write down two small formulas:

    $ I_ {dischrg} = C_ {gc} \ cdot \ frac {\ mathrm {d}} {\ mathrm {d} t} V_ {ce}, $

    $ U_g = I_ {dischrg} \ cdot (R_g + R_ {drv.OFF}). $

    And do not forget that the opening voltage of the key strongly depends on the temperature of the crystal, and this also needs to be taken into account.


    Now we have formulas for optimal (to some extent) selection at first glance of such a simple element of the power circuit as a gate resistor.

    It is possible you have not found anything new here, but I hope that at least someone this note will be useful.

    Also, in order to broaden my horizons, including in the area of ​​power key management, I strongly advise you to devote an hour or two a week to reading all sorts of articles and apnotes from eminent manufacturers of power electronics, especially about the use of driver chips. I am sure you will find a lot of interesting things there. To start, and to delve deeper into this topic, I suggest this one .

    Thanks for reading!

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