This paper mainly expounds the key parameters characterizing the driving ability in the driving chip: the relationship between driving current and driving time, and explains how to correctly understand the performance of these parameters in practical application through experiments.
Overview of driver chip
Power devices such as MOSFET and IGBT need the cooperation of driving circuit to work normally. Figure 1 shows the circuit in which a driver chip drives a power MOSFET. When M1 is turned on and M2 is turned off, the power supply VCC charges CGS and CGD through M1 and RG, so as to turn on the MOSFET. The charging simplified circuit is shown in Figure 2. When M1 is turned off and M2 is turned on, CGS discharges through RG and M2, so as to turn off the MOSFET. The discharge simplified circuit is shown in Figure 3.
The main indicators to measure the driving ability: driving current and driving speed
There are two main indicators to measure the driving ability of a driving chip: driving current and driving rise and fall time. These two parameters are marked in the general driver chip specification. In practical application, engineers often only pay attention to the driving current and ignore the parameter of rise and fall time. In fact, the rising and falling time of driving is also important, sometimes even more important than the driving current. Because the rising and falling time of the drive directly affects the opening and closing speed of the power device.
Figure 4 shows a simplified timing diagram of gate driving voltage and driving current when a MOSFET is turned on. The period from T1 to T2 is the establishment time of the source current (IO +) driven by the gate from zero to the peak current. At time T3, the gate voltage reaches the Miller platform and the source current begins to charge the Miller capacitor of MOSFET. At T4, the Miller capacitor is charged, the source current continues to charge the input capacitor of the MOSFET, and the gate voltage rises until it reaches the power supply voltage Vcc of the gate drive. At the same time, from T4 to T5, the source current also drops from the peak current to zero.
Here is a very important stage: the establishment time of the source current from T1 to T2. Different driver chips have different current establishment time, which will affect the speed of driving.
The following describes the influence of driving current establishment time on driving speed by measuring the performance of two chips slm2184s and ir2184s.
Table 1 compares the tests of slm2184s and ir2184s. Although the peak source current [IO +] and peak perfusion current [IO -] of slm2184s are smaller than the test value of ir2184s, the current establishment time of slm2184s is much shorter than that of ir2184s.
Table 1: comparison of driving current and driving time between slm2184s and ir2184s
Actual measurement: slm2184s vs ir2184s drive test comparison
✔ Figure 5 ~ Figure 16: waveforms of driving current and driving time of measured slm2184s.
✔ Figure 17 ~ figure 28: waveforms of driving current and driving time of measured ir2184s
Slm2184s drive test waveform
Ir2184s drive test waveform
From the above experimental tests, it can be seen that the driving speed of the driving chip depends not only on the driving current, but also on factors such as the establishment time of the driving current and the input capacitance of the MOSFET. Although the driving current of some driving chips is relatively large, because its current rising and falling speed is very slow, it does not play the role of large driving current well. Even in most applications, the driving speed (TR and TF) is not as good as that of driving chips with small driving current. Therefore, when selecting the driving chip, we should not only pay attention to the size of the driving current, but also pay attention to the rise and fall time under a certain load capacitance. Of course, the most appropriate way is to measure the waveform of the driving end according to the actually selected power tube, so as to judge whether a suitable driving chip is selected.