The design of IGBT driver circuits includes selecting the insulation levels of the upper and lower bridges, determining the drive voltage level, determining the drive power of the driver chip, and short-circuit protection circuits, among other things. Today, we will focus on determining the drive current and power, that is, how to determine whether the current capability of a driver chip can drive a specific type of IGBT, and if not, how to enhance the drive output capability.
01 Calculation of peak current of driver chip
When selecting an IGBT driver chip, a crucial step is to calculate the maximum drive current required by the IGBT. Without considering the addition of a Cge capacitor at the gate, the IGBT drive circuit can be simplified into an RLC circuit, as shown in the shaded area of the figure below.
Solving this circuit yields the following equation for the peak circuit:
Ipeak: The maximum current that the drive circuit can output.
ΔUge: Maximum value of gate power supply minus minimum value
RG,ext: External gate resistance value; RG,int: Internal resistance value of the device.
As can be seen from the formula above, the maximum drive current depends on the gate voltage level and the gate resistance value. Once these two parameters are determined, the required maximum drive current is basically determined. Of course, different turn-on and turn-off resistors may be used in some designs, so it is necessary to calculate the turn-on and turn-off currents separately. Based on the calculated turn-on and turn-off current values, the chip's drive current can be initially selected. The peak value given in the chip datasheet should not be less than the calculated current value, and appropriate engineering margins should be considered.
02 Push-pull circuit amplifier circuit increases drive current
If the output current of the driver chip cannot drive a specific IGBT, a relatively simple method is to use a push-pull circuit to further enhance the peak current output capability of the driver chip. Using transistor amplification is a common approach, and the calculation steps are as follows:
(1) Calculate the required maximum peak current Ipeak based on the selected drive voltage level and gate resistance.
(2) Select PNP/NPN transistors with appropriate voltage ratings to form a push-pull circuit.
(3) Calculate the base current of the transistor by referring to the current transfer factor hFE in the datasheet of the selected transistor.
(4) Calculate the output resistance of the driver chip's output terminal.
The above steps outline the general procedures for using BJTs in push-pull amplifier circuits. Important considerations include the BJT's voltage rating and the matching of the base resistor. Because push-pull amplifier designs using BJTs are relatively simple, they are widely used. Commonly used BJT models in high-power applications include MJD44/45H11 (80V).
It should be noted that MOSFETs have their own advantages over BJTs in push-pull circuit design, mainly in terms of higher power density. BJTs are usually packaged in D-PAK, while MOSFETs are usually packaged in SO8. Additionally, MOSFETs require lower control current and have faster switching speeds, making them more suitable for FPGA digital control and multi-level soft turn-off. However, when using MOSFETs in push-pull designs, attention must be paid to the matching of the gate voltage of the lower-bridge n-channel MOSFET with the supply voltage. Therefore, a Zener diode needs to be added to the gate. In high-power applications, the MOSFET IRF7343 (-55V/+60V) is a commonly used device with a voltage rating and performance close to those of BJTs.
03 Calculation of Average Drive Power
In the design of the drive circuit, besides the peak drive current, the effective value of the current is also an important parameter to consider. The former determines whether a specific IGBT model can be effectively driven, while the latter determines whether its heat generation or temperature rise meets the design requirements. The following figure shows the test waveforms of the gate current and voltage of the FF1200R17KE3. The test configuration is as follows:
Rg,on = 1.3 ohm
Rg,off = 1.4 ohm
Vge=+/-15v
Based on the above formula, Ipeak = 7.66A can be calculated, and the test value is basically close to the calculated value.
According to the datasheet for device FF1200R17KE3, Qg=14uc and Rgint=1.6ohm.
If we approximate the gate current waveform as a triangular wave, the duration of the triangular wave can be simplified and calculated using the following formula.
Assuming the switching frequency of the device is 2.5kHz and the time of one switching cycle is T=400us, the effective value of the drive current can be calculated using the following formula.
Based on this current value, the characteristic curve of the push-pull output transistor is consulted to obtain the transistor's losses, which are then used to calculate whether the transistor's temperature rise meets the operating requirements.
04 Summary
This article briefly introduces the calculation of the gate peak current in the design of IGBT driver circuit and the general calculation process of using a transistor push-pull output circuit. Finally, it considers the heat generation and temperature rise of the transistor in combination with practical applications.
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