Isolation of bridge drive circuits

乱世煮酒论天下

Isolation of bridge drive circuits [Copy link]

1. Floating phenomenon of driving level

1) Taking the single-phase full-bridge NMOS tube circuit as an example, the four switch tubes are 1423 from left to right and from top to bottom. Since the ground potential of the lower tube 23 is constant, only the drive circuit needs to be added to the drive;

2) To drive the upper bridge 14, a driving voltage must be added between the gate of the upper bridge and the corresponding ground; assuming that none of the four tubes are conducting, the ground of the upper tube is half of the bus voltage, assuming it is 50V, which means that the drive voltage of the upper bridge must be higher than 50V;

3) When tube 1 is turned on, the ground of tube 1 is the bus voltage, which is 100V. At this time, the driving voltage of tube 1 needs to be higher than 100V. Since the ground potential of the upper bridge floats, a floating driving circuit design is required;

4) There are several solutions for this floating drive circuit. For high-voltage and high-power power supplies, pulse transformers or optocoupler isolation can be used. Optocoupler isolation requires an isolated power supply; low-power uses bootstrap boost drive and P-type tube drive.

2. Bootstrap boost drive. When the two switches of the power supply form the upper and lower bridge arms and are turned on alternately, the bootstrap boost drive can be used. The key lies in the bootstrap capacitor and reverse high-voltage diode on the driver chip.

1) When the lower tube is turned on, the ground potential of the upper tube drops to the negative pole of the DC bus, and the bootstrap capacitor is charged and boosted;

2) When the lower tube is turned off, the upper tube is turned on, and the ground of the upper tube rises accordingly. Due to the presence of the reverse high-voltage diode, the voltage on the bootstrap capacitor will not leak;

3) Since the energy storage on the bootstrap capacitor is limited, it is necessary to periodically turn on the upper and lower bridges of the same bridge arm, and the lower bridge must be turned on first when the circuit is just started.

3. P-type tube driver

1) For low-power circuits that do not meet the bootstrap boost driving principle, that is, the upper and lower bridges are not turned on alternately, such as the BUCK circuit, the gate of the switch tube is floating. A simple solution is to use a P-type electronic switch to solve the voltage floating problem and connect the high-voltage side bridge arm switch of the bridge circuit to a PMOS tube;

2) For the bridge circuit, if the gate voltage of the two switch tubes on the upper bridge is lower than the positive voltage of the DC bus, the switch is turned on; when the gate potential is equal to the DC bus potential, the switch is turned off; the control voltage required by the gate is no longer floating.

The most commonly used switching device is IGBT, which is usually driven by bootstrap. If I want to drive it with a P-type tube, is it possible? IGBT is a combination of an N-channel field effect tube and a transistor. If I replace it with a P-channel field effect tube, is it possible?

maychang

[For low-power circuits that do not meet the bootstrap boost driving principle, that is, the upper and lower bridges are not alternately turned on, such as the BUCK circuit, the gate of the switch tube is floating]

The Buck circuit also has the upper and lower tubes turned on alternately, so an N-channel MOS tube can be used for bootstrap drive without the need for a P-channel MOS tube.

maychang

[IGBT is a combination of an N-channel field effect transistor and a transistor. Is it possible to replace it with a P-channel field effect transistor? ]

Yes, you can, but you need to ask the IGBT manufacturer whether they are willing to produce it for you.

maychang

By the way: The title is "Isolation of Bridge Drive Circuit", using pulse transformer or optocoupler drive is isolation, but bootstrap drive is not isolated.

盛世辉煌电子科学

This is indeed a more profound discussion. Driver isolation is not easy.

振动试验仪器

The DC power supplies of the four-way output drive circuits must be isolated from each other, and the input signal enters the drive circuit through optocoupler isolation, so the four-way drive circuits are insulated from each other.