Analysis of common MOSFET and IGBT insulated gate isolation drive technology
MOSFET and IGBT insulated gate bipolar high-power tube and other devices between the source and the gate are insulated silicon dioxide structure, DC power can not pass, so the low-frequency state of the driving power is close to zero. However, a gate capacitance Cgs is formed between the gate and the source, so that a certain dynamic driving power is required at the high frequency alternate turn-on and when the turn-off is required. The Cgs of small power MOSFETs are typically within 10-100 pF, and for high power insulated gate power devices, the gate capacitance Cgs is large. Typically between 1-100 nF, a large dynamic drive power is required. Moreover, due to the drain-to-gate Miller capacitance Cdg, the gate drive power is often not negligible.
Because the IGBT has a current tailing effect, it requires better anti-interference when it is turned off, and requires a negative voltage drive. The MOSFET is faster, and there is no negative voltage when it is turned off, but when the interference is heavy, the negative voltage shutdown is very beneficial for improving reliability.
Isolation drive technology
In order to reliably drive insulated gate devices, there are many mature circuits. 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.
1. Optocoupler isolated driver
The advantages of the optocoupler are small size, the disadvantages are: A, slow response, and therefore a large delay time (high-speed optocouplers are generally greater than 300ns); B, the output stage of the optocoupler needs isolated auxiliary power supply .
2, passive transformer drive
There are three ways to isolate an insulated gate power device with a pulse transformer: passive, active, and self-powered. The passive method is to drive the IGBT device with the output DC of the transformer secondary. This method is simple and does not require a separate drive power supply. The disadvantage is that the output mode distortion is large because the gate-source capacitance Cgs of the insulated gate power device is generally large. The way to reduce the distortion is to change the primary input signal 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 changes greatly, the positive and negative amplitudes of the output drive pulse change too much, which may cause the operation to be abnormal, so it is only suitable for occasions where the duty cycle does not change much.
3, active transformer drive
The transformer in the active method only provides an isolated signal, and a shaping amplifier circuit is used in the secondary to drive the insulated gate power device. Of course, the driving waveform is better, but a separate auxiliary power supply amplifier is required. The auxiliary power supply may introduce parasitic interference if it is not handled properly.
4. Modulated self-contained power transformer isolation driver
The use of self-contained power technology, using only one transformer, not only saves the auxiliary power, but also gets faster speed, of course, is a good method. At present, the generation of self-contained power sources has two methods of modulation and time division.
Modulation technology is a relatively classic method, that is, high-frequency (several MHZ) modulation of the PWM drive signal, and the modulation signal is applied to the primary of the isolated pulse transformer, and the self-supply power is obtained by direct rectification in the secondary, and the original PWM modulation signal It needs to be demodulated. Obviously, this method is not simple. Another disadvantage of the modulation is that the demodulation of the PWM increases the delay of the signal, and the modulation is adapted to deliver a lower frequency PWM signal.
5. Transformer isolation driver for time-sharing self-supplied power supply
Time-sharing technology is a relatively new technology. The principle is that the signal and energy are transmitted separately, that is, the information is transmitted on the rising and falling edges of the input PWM signal of the transformer, and the drive is transmitted in the flat-top stage of the input signal. The energy needed. Since only the signal is transmitted on the rising and falling edges of the PWM signal, there is substantially no energy transfer, and thus the output PWM pulse has a small delay and distortion, and a steep drive output pulse can be obtained. The shortcoming of the time-sharing self-contained power supply driver is that the transformer has a large volume at low frequencies, and it is difficult to drive an IGBT exceeding 300 A/1200 V due to the limitation of self-sufficient energy.