This paper mainly elaborates on the key parameters of characterizing the driving capability in the driver chip: the relationship between the drive current and the driving time, and explains how to correctly understand the performance of these parameters in practical applications through experiments.
Driver CHIP overview
Power devices such as MOSFETs and IGBTs require the coordination of the drive circuit to function properly. Figure 1 shows a driver chip driving a power MOSFET circuit. When M1 is turned on and M2 is turned off, the power supply VCC charges Cgs and Cgd through M1 and Rg, so that the MOSFET is turned on, and its charging simplified circuit is shown in Figure 2. When M1 is turned off and M2 is turned on, Cgs are discharged through Rg and M2, so that the MOSFET is turned off, and its discharge simplified circuit is shown in Figure 3.
The main indicators of driving ability are: drive current and drive speed
There are two main indicators to measure the driving ability of a driver chip: the drive current and the rise and fall time of the drive. These two parameters are noted in the general driver chip specification. In practical applications, engineers often focus only on the drive current and ignore the parameters of rise and fall time. In fact, the indicator of the rise and fall time of the drive is equally important, sometimes even more important than the indicator of the drive current. Because the rise and fall time of the drive directly affects the opening and shutdown speed of the power device.
Figure 4 shows a simplified timing diagram of the gate drive voltage and drive current when a MOSFET is turned on. The period from t1 to t2 is the settling time of the source current (IO+) driven by the gate from zero to the peak current. At t3, the gate voltage reaches the Miller platform and the source current begins to charge the MOSFET's Miller capacitor. At t4 moment, the Miller capacitor is charged, the source current continues to charge the input capacitance of the MOSFET, and the gate voltage rises until it reaches the gate-driven supply voltage, VCC. At the same time, during the period from t4 to t5, the source current also drops from peak current to zero.
There is an important stage here: the settling time of the source current from t1 to t2. Different driver chips have different current settling times, which affect the speed of the drive.
Test the comparison
The following is to illustrate the influence of the drive current settling time on the drive speed by measuring the performance of the two chips SLM2184S and IR2184S.
Table 1 compares the tests for the SLM2184S and IR2184S. Although the peak source current [IO+] and peak sink [IO-] of the SLM2184S are smaller than the test values of the IR2184S, the current settling time of the SLM2184S is much shorter than the settling time of the IR2184S.
Table 1: Comparison of drive current and drive time for SLM2184S and IR2184S
Actual measurement: SLM2184Svs IR2184S driver test comparison
Fig. 5~Figure 16: Measured waveform of the drive current and drive time of the SLM2184S.
Figures 17 to 28: The waveform of the drive current and drive time of the IR2184S was measured
The SLM2184S drives test waveforms
Figure 5. SLM2184S drive source current, load capacitance 100nFCH1: drive loser; CH2: Drive output; CH4: Drive source current
Figure 6. SLM2184S drive source current rise speed, load capacitance 100nFCH1: drive loser; CH2: Drive output; CH4: Drive source current
Figure 7. SLM2184S drive sink current, load capacitance 100nFCH1: drive feeder; CH2: Drive output; CH4: Drive sink current
Figure 8. SLM2184S drive sink current rise rate, load capacitance 100nFCH1: drive loser; CH2: Drive output; CH4: Drive sink current
Figure 9. SLM2184S drive rise speed, load capacitance 1nFCH1: drive losers; CH2: Drive output; CH4: Drive source current
Figure 10. SLM2184S drive up speed, load capacitance 1nFCH2: Drive output
Figure 11. SLM2184S drive descent speed, load capacitance 1nFCH1: drive losers; CH2: Drive output; CH4: Drive sink current
Figure 12. SLM2184S drive down speed, load capacitance 1nFCH2: Drive output
Figure 13. SLM2184S drive rise speed, load capacitance 2.2nFCH2: Drive output
Figure 14. SLM2184S drive up speed, load capacitance 3.3nFCH2: Drive output
Figure 15. SLM2184S drive down speed, load capacitance 2.2nFCH2: Drive output
Figure 16. SLM2184S drive down speed, load capacitance 3.3nFCH2: Drive output
IR2184S drives test waveforms
Figure 17. Drive source current of IR2184S, load capacitance 100nFCH1: drive loser; CH2: Drive output; CH4: Drive source current
Figure 18. DRIVE source current rise rate of IR2184S, load capacitance 100nFCH1: drive loser; CH2: Drive output; CH4: Drive source current
Figure 19. Drive sink current of IR2184S, load capacitance 100nFCH1: drive feeder; CH2: Drive output; CH4: Drive sink current
Figure 20. IR2184S drive sink current rise rate, load capacitance 100nFCH1: drive the loser; CH2: Drive output; CH4: Drive sink current
Figure 21. IR2184S drive rise speed, load capacitance 1nFCH1: drive losers; CH2: Drive output; CH4: Drive source current
Figure 22. Drive rise speed of IR2184S, load capacitance 1nFCH2: Drive output
Figure 23. IR2184S drive descent speed, load capacitance 1nFCH1: drive losers; CH2: Drive output; CH4: Drive sink current
Figure 24. Drive descent speed of IR2184S, load capacitance 1nFCH2: Drive output
Figure 25. Drive rise speed of IR2184S, load capacitance 2.2nFCH2: Drive output
Figure 26. Drive rise speed of IR2184S, load capacitance 3.3nFCH2: Drive output
Figure 27. Drive descent speed of IR2184S, load capacitance 2.2nFCH2: Drive output
Figure 28. Drive descent speed of IR2184S, load capacitance 3.3nFCH2: Drive output
Test summary
From the above experimental tests, it can be seen that the driving speed of the drive chip not only depends on the size of the drive current, but also affects factors such as the settling time of the drive current and the input capacitance of the MOSFET. Although the drive current of some driver chips is relatively large, because its current rise and fall speed is very slow, it does not play a good role in large drive current, and even in most applications, the driving speed (tr and tf) is not as good as the drive chip with small driving current. Therefore, when selecting a driver chip, it is necessary not only to pay attention to the size of the drive current, but also to 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 drive end according to the actual selected power tube, so as to determine whether the appropriate driver chip is selected. Test summary
Resources
1. Schematic HVIC Gate Driver Application Manual
SLM2184S Datasheet: 600V Half-Bridge Driver