Introduction to isolated driving of MOS devices in 6 types of IGBTs

Due to the rise of uninterruptible power supply, IGBT technology has been developed rapidly.

IGBT is characterized by current trailing effect, so the performance requirements for anti-interference at the moment of shutdown are very strict, and negative voltage drive is required for assistance. When MOSFETs act in the circuit, the MOSFET speed is relatively fast, so the shutdown process does not generate negative voltage, but it is worth mentioning that this phenomenon is helpful to improve reliability in the case of heavy interference. This article will provide a general introduction to the isolated drive technology for IGBT as well as MOSFET devices to help you understand.

MOSFETs and IGBT insulated gate bipolar high-power tubes and other devices between the source and gate is an insulated silicon dioxide structure, DC can not pass, and thus the low frequency of the surface state drive power is close to zero.

However, the gate and the source constitute a gate capacitance Cgs, and thus require a certain dynamic drive power at high frequencies when the alternate turn-on and need to turn-off. Cgs for low-power MOSFETs is generally within 10-100pF, and for high-power insulated gate power devices, the gate capacitance Cgs is larger due to the gate capacitance. Generally between 1-100nF, thus requiring a larger dynamic drive power. More due to the drain to gate miller capacitance Cdg, the gate drive power is often not negligible.

Because IGBT has current trailing effect, in the shutdown requires better anti-interference, need negative voltage drive. mosfet speed is faster, shutdown can be no negative voltage, but in the heavy interference, negative voltage shutdown for improving reliability has great benefits.

Isolated drive technology

To reliably drive insulated gate devices, there are many mature circuits available. When the drive signal and the power device do not need to be isolated, the design of the drive circuit is relatively simple, and there are many excellent drive integrated circuits available.

Optocoupler Isolated Drivers

The advantage of the photocoupler is the compact size, the disadvantage is that the response is slow, and thus has a large delay time (high-speed type optocoupler is also generally greater than 300ns); the output stage of the photocoupler requires an isolated auxiliary power supply power supply.

Passive transformer drive

There are three methods to drive insulated gate power devices with pulse transformer isolation: passive, active and self-powered drive. The passive method is to use the output DC of the transformer secondary to drive the insulated gate device, this method is simple and does not require a separate drive power supply. The disadvantage is that the output waveform distortion is large, because the gate capacitance Cgs of insulated gate power devices is generally large. The way to reduce the distortion is to change the input signal of the primary to a large signal with a certain power, and the corresponding pulse transformer should also take a larger volume, but at high power, it is still generally unsatisfactory. Another disadvantage is that when the duty cycle varies greatly, the positive and negative amplitude of the output drive pulse changes too much, which may lead to abnormal operation, so it is only suitable for applications where the duty cycle does not vary much.

Active transformer drive

The transformer in the active method only provides an isolated signal, and there is another shaping and amplifying circuit in the secondary to drive the insulated gate power device, which of course has a better driving waveform, but requires a separate auxiliary power supply to the amplifier. The auxiliary power supply, if not handled properly, may introduce parasitic interference.

Modulated self-powered transformer-isolated driver

It is certainly a good way to use the self-feeding power supply technique, using only one transformer, which eliminates the need for an auxiliary power supply and gets a faster speed. Currently there are two methods of generating self-feeding power supplies: modulated and from time-sharing.

Modulation technology is the more classic method, that is, the PWM drive signal for high frequency (more than a few MHZ) modulation, and the modulated signal added to the isolated pulse transformer primary, in the secondary through direct rectification to get the self-generated power, while the original PWM modulated signal is required to be demodulated to obtain, obviously, this method is not simple. Another disadvantage of the modulation type is that the demodulation of PWM to increase the delay of the signal, the modulation method is suitable for the transmission of lower frequency PWM signal.

Time-sharing type self-contained power transformer isolation driver

Time-sharing technology is a relatively new technology, the principle of which is that the signal and energy transfer is taken separately, that is, the information is passed on the rising and falling edges of the transformer input PWM signal, and the energy required to drive is passed on the flat top phase of the input signal. Since only the signal is transmitted at the rising and falling edges of the PWM signal and basically no energy is transmitted, the output PWM pulse has minimal delay and distortion, and a steep drive output pulse is obtained. The shortcoming of the time-sharing type self-feeding power driver is the large size of the transformer when used at low frequencies, in addition to the difficulty of driving IGBTs over 300A/1200V due to the limitation of self-feeding energy.

As can be seen above, these different MOS device isolation drives have a good performance in IGBT applications, and each drive method can bring different functional support for designers. Of course, the content in this article is not all, but only a preliminary introduction to these drivers, in order to give you a preliminary understanding of this expertise.