Author: Vasilyeva K.L.
The article describes the main characteristics of the miniature B82804A GDT transformers manufactured by Epcos-TDK. The company is the world leader in the production of electronic components and soft magnetic materials. Features of application of Gate Drive Transformers in MOSFET Gate Drive Circuits and prospects of using of power modules based on such components in powerful converters and other power equipment are shown.
The development of power semiconductor pulse technique and the increasing demand for modern hardware components every year led to the appearance of miniature devices and components with improved characteristics on the market . At present moment, among the key elements widely used in power supplies, amplifiers, powerful drives, analog and digital microcircuits one can mention main switching devices - MOSFET transistors.
Currently, the latest developments of companies - world leaders in the electronic industry are aimed at manufacturing and improving highly efficient and powerful MOS-transistors, as well as control devices. The main device types produced by MOSFET are presented in Figure 1. [1].
Firstly, the increased interest to such components is caused by the fact, that MOSFET output signal is controlled not by the current, but by the voltage through the gate, which is separated from the conducting channel by a dielectric layer (SiO 2) unlike a bipolar transistor [2-5]. At the same time, the current practically does not flow along the gate circuit excepting the moment of opening and closing. This technical design allows you to abandon the work on discrete elements and greatly simplify the circuit. In particular, one can use drivers which, in addition to control, also provide protection against overcurrent and short circuit. In addition, MOS transistors have sufficiently large input impedance values. They are characterized by increased heat resistance and low noise level, in particular, at low frequencies.
Nevertheless, despite a number of noted MOSFET advantages, the choice of a drive circuit for powerful transistors is not always an easy task. This problem is caused by the fact that the using of the driver implies its coordination with the power module in terms of control voltage and gate current [3-5]. In addition, when creating powerful converters, difficulties can also arise when creating topologies of floating-key circuits, while integrated circuits for controlling “upper arm” transistors do not provide the required level of current and isolation protection [4].
In this regard, the pulse transformers (Gate Drive Transformers) applicationis a simple and economical solution. Along with galvanic isolation, GDT transformers can control the gate of a powerful transistor in half-bridge and bridge circuits, as a result of which they are often installed in power supplies, frequency converters and DC / AC converters and other electrical circuits units.
Unlike optical drivers the modules with Gate Drive Transformers are fast and less sensitive to noise. In addition, GDT transformers are easy to operate, manufacture and have a low cost. When designing GDTs, developers typically use small-sized toroid ferrite cores (Figure 1). The choice of core's material is determined on the base of the operating frequency [6,7]. To reduce the influence of stray parameters, a uniform, distributed dense winding with a small diameter wire is provided. Winding methods and features of calculating of GDT based on ferrite rings are represented in the source [6]. However, the selection of core's material and size, the carrying out and verification of calculations, as well as the implementation of high-quality winding, require a certain amount of time and skills. In this regard, GDT transformers are widespread.
Among components produced in series by different companies the special attention is devoted to B82804A GDT series (Epcos) (Figure 2).
These transformers are characterized not only by improved electrical parameters, but also by miniature dimensions (8,1mm x 6,7mm x 5,4mm) (Figure 3). The basis of Gate Drive configuration is a EP5 ferrite core. Small size of the core allow to sufficiently decrease a board space even when using multiple transformers at once. The connection of units for a MOST gate's handling in a frequency range from 150 kHz up to several MHz. All types of B82804A components are optimized for isolation voltage 1500 V, environmental temperature 85°C and operating temperature 125°C. The configuration and wire bonding is realized in accordance with determined standards [8].
- L – Inductance, H;
- U – Isolation voltage, V;
- fрез –Resonant frequency, Hz;
- R(DC) – Primary and secondary winding resistance, Ohm;
- Turns number ratio
Technical characteristics of B82804A transformers produced by Epcos a re listed in the table 1. Despite the fact, that the application of pulse transformers is rather often followed by stray parameters appearance (such as leakage inductance and the product's own capacitance), the winding method developed by Epcos allows to decrease the influence. For instance, in case of B82804A components the values of stray inductance between primary and secondary windings can vary from 25 pF to 95 pF depending on chosen element[8].
| Table 1 – Technical characteristics of B82804A GDT produced by Epcos [9] | ||||||
| L, µH | Figure | Turns number ratio | R(DC)primary, Ohm | R(DC)secondary,Ohm | fresonant, MHz | Part number |
| 300 | Figure 3 А | 2,5:1:1 | 1,8 | 0,3 | 2,6 | B82804A0304A225 |
| 317 | Figure 3 А | 2:1:1 | 1,6 | 0,45 | 2 | B82804A0324A220 |
| 264 | Figure 3 А | 1:1:1 | 1,5 | 1,5 | 2,9 | B82804A0264A210 |
| 350 | Figure 3 В | 1:1 | 1 | 0,65 | 1,2 | B82804A0354A110 |
| 690 | Figure 3 В | 1:5:1 | 1,65 | 0,86 | 0,7 | B82804A0694A115 |
| 473 | Figure 3 В | 2:5:1 | 1,5 | 0,3 | 1,7 | B82804A0474A125 |
In practice,the direct connection of a miniature GDT transformer can be realized at the output of the micro's driver through an isolation capacitor. The secondary winding is connected directly to the gate of the MOS transistor, while the resistors should be located at the side of the primary winding of the transformer. Protective diodes are installed at the driver output and must be connected even in the absence of problems with the reactive component of the current in the transformer [4].
A general scheme for MOSFET driver with a pulse transformer is shown at the figure 4. According to the scheme shown in the figure 4, the isolation capacitor is installed in series with the primary winding of the pulse transformer to relieve voltage during magnetization. Zener diode (VD) is used in in order to reduce voltage surges on the gate. The resistor R g is installed to avoid the appearance of a ripple current on the gate. The studied driver is a fairly simple device which can be easily integrated into the circuit unit. Nevertheless, it should be taken into account that the output voltage decreases along with an increase of pulse duty factor [10].
At high values of pulse duty factor one can use the scheme shown at the figure 5[10].
Figure 5 shows the pulse transformer TX1, resistors R3 and R4 connected to the primary and secondary windings, respectively, inductors LI and L2, as well as isolation capacitors C1, C5, C6. In addition, a series of components has been added to this circuit, in particular, energy storage capacitors C7, C4. The push-pull cascade is implemented due to transistors Q1 and Q2, a Zener diode D3 and a conventional diode D1 [10]. The implementation of such a driver is able to increase the switching speed and reduce the signal delay time. Moreover, the application of the circuit shown in Figure 4 increases the noise immunity and reliability due to the supply of negative voltage to the gate in the closed position.
Figure 6 shows the gate-drive circuits (MOSFET). The signal enters the primary winding of a pulse transformer. Secondary windings control the gate of the transistor's half-bridge. The application of GDT component in such a circuit is a good solution, since it does not require the using of isolated power sources to control the MOSFET circuit connected to the secondary winding of the transformer [11].
Summing up what has been said , this article shows the features of MOSFET gate driving, typical schemes of drivers using miniature pulse transformers in the units of such schemes. More over, the article shows general characteristics of GDT transformers produced by Epcos and advantages of such miniature components made in manual way.Also high reliability, switching speed and easy operation of GTD components when MOSFET gate driving are noted
comparing to optical drivers, that allows to realize highly effective and economical devices for power electronics.
Литература:
1. Balogh L. Design and Application Guide For High Speed MOSFET Gate Drive Circuits / Laszlo Balogh. – p.1–37.
2. А. Колпаков. Особенности теплового расчета импульсных силовых каскадов. Компоненты и технологии. 2002 - №1.
3. Пасынков В. В., Чиркин Л. К. Полупроводниковые приборы: Учебник для вузов. — 8-е издание, исправленное. — М.: Лань, 2006. — 480 с.
4. Dr. Ray Ridley. Gate Drive Design Tips. Power Systems Design Europe. 2006
5. B. Maurice, L. Wuidart. Drive circuits for power MOSFETs and IGBTs. Application Note. http://www.st.com
6. http://bsvi. ru/raschet-i-primenenie-gdt/
7. Е. Москатов. Методика и программа расчета импульсного трансформатора двухтактного преобразователя. Радио.2006- №6.
8. Miniaturized series of gate-drive transformers. Articles. Products and Technologies. 2012. Epcos. http://en.tdk.eu/
9. SMT gate drive transformers. EP5 series.B82804A. Datasheet. Epcos
10. D. Wang, H. Dai1, Z. Sun. Design and Simulation of Gate Driver Circuit Using Pulse transformer/ International Journal of Computer Science. 2013 – Vol.10, Issue 2, №2 – P. 305-310.
11. B. Kennedy. Implementing an Isolated Half-Bridge Gate Driver. Analog Dialogue. 2012. - P. 1-3.
