Drive circuit for gate turn-off thyristor and power MOSFET

Drive circuit for gate turn-off thyristor and power MOSFET

Gate turn-off thyristor (GTO) driver circuit

Designing and selecting a gate drive circuit with excellent performance is very important to ensure the normal use and performance optimization of the GTO. In particular, special attention should be paid to the gate turn-off technology, which is the key to the correct use of the GTO.

1. The waveform of the ideal gate signal

GTO is a current-driven device, and Figure 1 shows the ideal voltage and current signal waveforms of its gate.

Drive circuit for gate turn-off thyristor and power MOSFET
Figure 1 – GTO ideal gate voltage and current signal waveforms

(1) Basic requirements for opening the signal

The leading edge of the turn-on pulse is required to have a high amplitude and a sufficient pulse width. The leading edge of the pulse charges the junction capacitance, and the leading edge is steep, the charging is fast, and the forward current will be established quickly, which is conducive to the rapid turn-on of the GTO thyristor. The gate very strong trigger is generally about 3 times larger than the rated trigger current. If GTO is activated quickly, this value can be set larger. Strong triggering can shorten turn-on time, reduce turn-on loss, and reduce tube voltage drop. The width of the trigger current is used to ensure the reliable establishment of the anode current, and the trailing edge should be as slow as possible to avoid oscillation of the anode current of the GTO thyristor.

(2) Basic requirements for gate turn-off signal

The gate turn-off pulse must have a sufficient width, not only to ensure that the carriers can be extracted during the falling time, but also to ensure that the remaining carriers need a certain time to recombine. The magnitude of the turn-off current generally takes (1/5~1/3) Im (turn off the main current), which is determined by the turn-off gain.

(3) Basic requirements of reverse bias circuit

After the GTO is turned off, a gate reverse voltage can still be applied, and its duration can be tens of microseconds or the entire blocking state time. The higher the gate reverse bias voltage, the greater the anode current that can be turned off. The higher the reverse bias voltage, the greater the anode du/dt tolerance.

2. GTO drive circuit example

The drive of the GTO gate can be divided into three parts: turn-on drive, turn-off drive and gate reverse bias. Figure 2 is a direct-coupled GTO drive circuit, and VT is the driven GTO. The power supply of this circuit is provided by the high frequency power supply after being rectified by diode, diode VD1 and capacitor C1 provide +5V voltage, VD2, VD3, C2, C3 constitute a voltage doubler rectifier circuit to provide +15V voltage, VD4 and capacitor C4 provide -15V voltage. When the field effect transistor VF1 is turned on, it outputs a positive strong pulse;

When VF2 is turned on, it outputs a flat top part of a positive pulse; VF2 is turned off and VF3 is turned on, and a negative pulse is output: after VF3 is turned off, resistors R3 and R4 provide gate negative bias.

Drive circuit for gate turn-off thyristor and power MOSFET
Figure 2 – Direct coupled GTO driver circuit

Power MOSFET driver circuit

Power MOSFETs are voltage-controlled devices. Unlike current-controlled devices such as GTOs and GTRs, the control poles have gates, high input impedance, and gate-to-source capacitances of thousands of picofarads.

1. Requirements for power MOSFET to drive circuit

(1) The gate-emitter capacitor is charged with a low-resistance loop when it is turned on, and the gate-emitter capacitor is provided for discharge when it is turned off. The larger the inter-electrode capacitance, the greater the required drive current.

(2) In order to reliably trigger the conduction of the power MOSFET, the trigger pulse voltage should be higher than its turn-on voltage, generally 10~15V; a negative driving voltage of a certain magnitude is applied during turn-off, generally -15~-5V, thereby reducing the turn-off time and turn-off loss.

(3) The front and rear edges of the trigger pulse are required to be steep to improve the switching speed of the power MOSFET.

2. Example of power MOSFET drive circuit

In order to meet the driving requirements of MOSFET, dual power supply is usually used, and the connection between the driving circuit and the gate can be directly driven or isolated. A low-value resistor RG is placed in series with the gate to reduce parasitic oscillations. The resistance value should decrease correspondingly with the increase of drive current.

Figure 3 is a drive circuit using opto-isolation, where VF is the driven power MOSFET. The circuit consists of an optical isolator and a signal amplifying part. When the input signal ui is 0, the optocoupler is turned off, the high-speed operational amplifier A outputs a low level, the transistor V3 is turned on, and the driving circuit outputs a negative driving voltage to turn off VF. When the input signal ui is positive, the optocoupler is turned on, the high-speed operational amplifier A outputs a high level, the transistor V2 is turned on, and the drive circuit outputs a positive drive voltage to turn on VF.

Drive circuit for gate turn-off thyristor and power MOSFET
Figure 3 – Opto-isolated driver circuit

At present, there are many application-specific integrated circuits used to drive power MOSFETs, and the more commonly used ones are the IR2110, IR2115 and IR2130 chips of the American International Rectifier Company.