NiMH No.7 battery.What is the role of resistance in a sinusoidal alternating current circuit?

　　What is the role of resistors in sinusoidal alternating current circuits provided to you by electronic enthusiasts? The selection rules and working principle of the gate resistor in the IGBT circuit. Some drivers have only one output terminal, such as Luomuyuan TX-K841L and TX-KA962F. This requires a series network of resistors and diodes in parallel with the original Rg. Use To adjust the driving speed in 2 directions. What is the role of resistance in a sinusoidal alternating current circuit?

　　The relationship between the voltage across a resistive element and the current flowing through it is governed by Ohm's law. When the sinusoidal current flows through the resistor R, as shown in the figure, the selected voltage and the reference direction of the current are consistent, then according to Ohm's law:

　　If the current i is selected as the reference sinusoidal quantity, then, substitute it into the above formula:

　　The current and voltage waveforms are shown in the figure. It can be seen from the above that when flowing through

　　When the current in the resistor is a sinusoidal function, the voltage on the resistor is a sinusoidal quantity with the same frequency as the current. Current and voltage are in phase, and their effective values also obey Ohm's law, which is:

　　If expressed in phasor form, then we have

　　The above equation is the complex expression of Ohm's law. This formula also expresses the phase relationship and effective value relationship between the sinusoidal voltage and current on the resistive element. According to the formula, the phasor diagram of voltage and current can be drawn, as shown in the figure.

　　Selection rules and working principle of gate resistor in IGBT circuit

　　1. The role of gate resistor Rg

　　1. Eliminate gate oscillation

　　The gate-emitter (or gate-source) electrode of an insulated gate device (IGBT, MOSFET) is a capacitive structure, and the parasitic inductance of the gate loop is inevitable. If there is no gate resistance, the gate loop will drive pulses in the driver. A strong oscillation will be generated under the excitation, so a resistor must be connected in series to attenuate it quickly.

　　2. Power loss of transfer driver

　　Capacitors and inductors are reactive components. If there is no gate resistor, most of the driving power will be consumed on the output tube inside the driver, causing its temperature to rise a lot.

　　3. Adjust the switching speed of the power switching device

　　If the gate resistance is small, the switching device will turn on and off quickly and the switching loss will be small; otherwise, it will be slow and the switching loss will be large. However, if the driving speed is too fast, the voltage and current change rate of the switching device will be greatly increased, resulting in greater interference. In severe cases, the entire device will be unable to work, so an overall plan must be taken.

　　2. Selection of gate resistor

　　1. Determination of the gate resistor value

　　Under various considerations, the selection of the gate resistor will vary greatly. For the initial test, you can choose from the following:

　　Different brands of IGBT modules may have their own specific requirements, which can be debugged near the recommended values in their parameter manuals.

　　2. Determination of gate resistor power

　　The power of the gate resistor is determined by the power of the IGBT gate drive. Generally speaking, the total power of the gate resistor should be at least twice the gate drive power.

　　IGBT gate drive power p=FUQ, where:

　　F is the operating frequency;

　　U is the peak-to-peak value of the drive output voltage;

　　Q is the gate charge, please refer to the IGBT module parameter manual.

　　For example, a common IGBT driver (such as TX-KA101) outputs a positive voltage of 15V and a negative voltage of -9V, then U=24V,

　　Assume F=10KHz, Q=2.8uC

　　It can be calculated that p=0.67w, the gate resistor should be a 2W resistor, or two 1W resistors in parallel.

　　3. Other precautions when setting the gate resistor

　　1. Minimize the inductive impedance of the gate circuit. Specific measures include:

　　a) The driver is close to the IGBT to reduce the lead length;

　　b) The driver gate-emitter leads are twisted, and do not use too thick wires;

　　c) The distance between the two drive lines on the circuit board is as close as possible;

　　d) The gate resistor uses a non-inductive resistor;

　　e) If it is an inductive resistor, several can be connected in parallel to reduce the inductance.

　　2. Select different gate resistors for IGBT turn-on and turn-off.

　　Usually, in order to achieve better driving effect, IGBT can be turned on and turned off at different driving speeds. It is often necessary to select Rgon and Rgoff (also called Rg+ and Rg-) respectively.

　　Some IGBT drivers have separate output controls for turning on and off, such as Luo Muyuan TX-KA101, TX-KA102, etc. You only need to connect Rgon and Rgoff respectively.

　　Some drivers have only one output terminal, such as Luomuyuan TX-K841L and TX-KA962F. This requires a series network of resistors and diodes to be connected in parallel to the original Rg to adjust the driving speed in two directions.

　　3. Connect an Rge=10-100K resistor directly to the gate-emitter of the IGBT to prevent the IGBT from being burned through the Miller capacitor by accidentally adding high voltage to the main power when the drive lead is not connected. Common models of Luomuyuan driver boards (such as TX-DA962Dx, TX-DA102Dx) already have Rge, but considering the above factors, users are best to install Rge between the gate-emitter of IGBT or the gate-source of MOSFET.

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