A power transistor is a type of bipolar junction transistor (BJT) that can withstand high voltages and large currents, sometimes also referred to as a Power BJT. However, it has a complex driving circuit and requires significant driving power. The working principle of GTRs and conventional BJTs is the same.
A GTR is a current-controlled bipolar double-junction high-power, high reverse voltage power electronic device with self-turn-off capability, developed in the 1970s. Its ratings have reached 1800V/800A/2kHz, 1400V/600A/5kHz, and 600V/3A/100kHz.
It combines the inherent characteristics of transistors, such as low saturation voltage drop, short switching time, and a wide safe operating area, with increased power capacity. Hence, circuits composed of it are flexible, mature, with low switching losses and short switching times, widely used in power supplies, motor control, general inverters, and other medium-capacity, medium-frequency circuits.
The drawbacks of GTRs include large driving current, poor surge current tolerance, and susceptibility to secondary breakdown damage. In switch-mode power supplies and UPS systems, GTRs are gradually being replaced by power MOSFETs and IGBTs. Its symbol is the same as that of a standard NPN transistor.
Transistor Structure
The power transistor (Giant Transistor), also known as GTR or BJT (Bipolar Junction Transistor), has a structure and working principle very similar to that of low-power transistors.
The GTR consists of three layers of semiconductor material and two PN junctions. Like low-power transistors, it comes in PNP and NPN types, with the NPN structure being more common in GTRs.
Working Principle
In power electronics, GTRs mainly operate in switching states. They conduct large currents when forward-biased (Ib>0) and are off when reverse-biased (Ib<0). Therefore, applying a sufficiently large pulse driving signal to the base of a GTR switches it between conduction and cutoff states.
BJT Characteristics
Output Voltage
The output voltage can be modulated by pulse width, resulting in a pulse sequence with amplitude equal to the DC voltage.
Carrier Frequency
Due to the longer on and off times of power transistors, the allowable carrier frequency is relatively low, with most frequency converters having an upper limit of around 1.2～1.5kHz.
Current Waveform
The lower carrier frequency results in significant high-frequency harmonic components in the current. These harmonics create eddy currents in the silicon steel, causing electromagnetic vibration and noise. Since the carrier frequency falls within the human ear's sensitive range, the electromagnetic noise of motors is pronounced.
Output Torque
The significant high-frequency harmonic components in the current slightly reduce the output torque at 50Hz compared to operation at utility frequency.
Basic Characteristics
(1) Static Characteristics
In the common emitter configuration, there are three operating regions:
a. Cutoff region. In this region, iB≤0, uBE≤0, uBC<0, and only leakage current flows through the collector.
b. Active region. iB >0, uBE>0, uBC<0, iC =βiB.
c. Saturation region. iB >Ics/β, uBE>0, uBC>0, where iCS is the collector saturation current determined by the external circuit.
Conclusion: Both PN junctions being forward-biased is a characteristic of saturation. In saturation, the voltage drop uCE between the collector and emitter is small, akin to a closed switch. Even with large currents, the loss is not significant. GTRs enter critical saturation just at the onset of saturation. Increasing iB further leads to deep saturation, which is desirable for switch operation to reduce uCE and conduction losses.
(2) Dynamic Characteristics
Output characteristics of GTR in common emitter configuration:
The leakage current of GTRs is minimal when off, and the saturation voltage drop is small when on. Thus, GTRs have low losses in both on and off states. However, during the switching process, both current and voltage are high, leading to significant switching losses. When the switching frequency is high, switching losses become a major part of the total losses. Therefore, shortening the on and off times is significant for reducing losses, improving efficiency, and enhancing operational reliability.
Driving Protection
GTR Base Drive Circuit
(1) Requirements for the base drive circuit:
a. Achieve electrical isolation between the main circuit and the control circuit.
b. During conduction, the base should receive a forward drive current with a steep front edge and a certain magnitude of forced current to accelerate the turn-on process and minimize turn-on loss.
c. During GTR conduction, the base current should keep the GTR in critical saturation, reducing on-state saturation voltage drop and shortening turn-off time.
d. To turn off the GTR, a sufficiently large reverse base current should be provided to hasten the turn-off speed and reduce turn-off losses.
e. The circuit should have strong anti-interference capabilities and certain protective functions.
(2) Base Drive Circuit
Integrated Driving
Integrated drive circuits overcome the drawbacks of general circuits with many components, complex designs, poor stability, and inconvenience. They also add protective functions.
GTR Protection Circuit
For high switching frequencies, using fast-acting fuses is ineffective. Buffer circuits are generally used. These include RC buffer circuits, charging/discharging type R, C, VD buffer circuits, and discharge-preventing type R, C, VD buffer circuits.
Circuit Analysis
A three-phase bridge PWM inverter circuit, with GTR as the power switching device and an inductive load. From the circuit structure, the three-phase bridge PWM frequency converter can only use bipolar control. Its working principle is as follows:
The three-phase modulation signals urU, urV, and urW are sinusoidal waves phased 120° apart. The three-phase carrier signal is a common triangular wave, uc, changing in both positive and negative directions.
The control method for the self-turn-off switching devices in phases U, V, and W is the same. Taking phase U as an example: during intervals where urU > uc, the upper bridge arm power transistor V1 is given a conduction drive signal, while the lower bridge arm V4 is given an off signal. Consequently, the output voltage of phase U relative to the neutral point N’ of the DC supply Ud is uUN’ = Ud/2.
