The power converter has high-voltage and high-current switching waveforms that generate electromagnetic interference (EMI) in the form of both conducted and radiated disturbances. To reduce these high-frequency emissions, there are classical solutions consisting of filtering and shielding. However, another approach can be applied to minimize the EMI generated by power transistors.
A new control technique for insulated gate bipolar transistors (IGBTs) acts on the turn-on transition waveforms, just as an integrated adjustable snubber would. This control method, which can be applied to MOSFETs or IGBTs, reduces di/dt during turn-on commutation. In an elementary commutation cell, such as a boost converter, this method allows sensible reduction of overcurrent and high-frequency oscillations associated with diode reverse recovery. To measure the effectiveness of gate-voltage control in reducing EMI and switching losses, this technique may be compared with the classical control method, which consists of increasing the gate resistance of the transistor.
Source of EMI in Power Converter
The use of high-frequency switching devices in a power converter induces high-current and high-voltage variations (di/dt and dv/dt) that excite parasitic elements in the power circuit, leading to conducted emissions at high frequencies.
Investigating the boost converter in terms of its frequency behavior, the pulse-width modulation (PWM) process introduces electromagnetic interference (EMI) at high frequencies.
As shown in Fig. 1a, the PWM pulses are trapezoidally shaped and can be characterized by the rise time (with the rise time tr equal to fall time tf) and pulse duration d. The frequency spectrum of a trapezoidal pulse (Fig. 1b) shows that, for such fast devices, the generated frequencies range from hundreds of kilohertz to several megahertz.
Gate-Voltage Control Technique
Often insulated-gate power transistors (MOSFET or IGBT) are driven by a voltage-pulse generator that usually feeds the gate circuit (VGS for a MOSFET and VGE for an IGBT) with a zero or negative voltage while in the off state, and with a positive voltage close to 15 V while in the on state. The proposed method makes it possible to act on the commutation behavior of insulated gate transistors at turn-on by introducing an intermediate gate-voltage level. The turn-on control consists of applying gate-voltage control to an IGBT, as represented in Fig. 3. The control signal is composed of an intermediate gate-voltage level VINT, which is maintained during time TINT (Fig. 2).
The voltage level VINT is the principal control parameter and must be slightly over the threshold voltage VTH of the insulated gate transistor. The gate-voltage control is applied to a 20-A 600-V IGBT embedded in the boost-converter structure shown in Fig. 3.
Using the gate-voltage control, the experimental results (Fig. 4) show how the collector current rate-of-rise at turn-on may be controlled by varying the intermediate voltage level VINT. The gate-voltage control technique greatly reduces the amplitude of the diode reverse-recovery current when the current rate-of-rise is slowed down.
When the current has reached its final value (Fig. 4), it's no longer necessary to slow down the switching operation because it would uselessly increase switching losses. Thus, the ideal value for TINT corresponds to a gate signal that jumps up to 15 V as soon as the diode reverse-recovery phase is over. For a shorter intermediate time, the current would rise with its natural slope as soon as the control voltage would be set to 15 V (Fig. 5). Of course, this ideal time TINT depends on the switched current value, whereas the switched voltage value has no influence.
Another area where the gate-voltage level control provides an advantage is the total energy dissipated during switching. You can see in Fig. 6 that switching losses are more important for gate-resistance control than with the gate-voltage control technique. Also, you can note that this difference increases when the current rate-of-rise is reduced, because the gate-resistance value is much smaller in the gate-voltage control method (Fig. 6).
The turn-on gate-voltage control circuit is equipped with overcurrent and short-circuit protection for IGBTs (Fig. 7).
Reduction of EMI
Power-electronic converters usually produce conducted and radiated emissions. EMI generated by switching processes can be reduced at the source using the gate-voltage control technique to control the switching operation at turn-on.
Fig. 8 shows the current and voltage across the IGBT during turn-on transition when gate-resistance control (with Rg = 12 Ω) is used. In Fig. 9, the same parameters are shown for the case where gate-voltage control (VINT = 5.3 V) is applied. Notice in Fig. 8 the appearance of the high-frequency oscillations of the current and voltage at the end of the commutation.
The common mode represents the major part of conducted emissions generated by the power converter. These emissions are caused by currents flowing between input phases and the system ground. In a power converter, they are principally caused by the combination of fast-switching power devices and various stray parasitic elements, primarily capacitance, in the power converter. In Fig. 10, this total parasitic-coupling capacitance, Cp, is the sum of the heatsink's capacitance and other stray capacitance.
In this experiment, the test circuit is a boost converter in which an IGBT is embedded. The power supply is provided through a line impedance stabilizing network (LISN), which provides a defined impedance for any high-frequency current generated by the converter shown in Fig. 10. The LISN also prevents noise from the supply from entering the system under test. Conducted-emission measurements are obtained by observing the voltage developed across the LISN impedance.
Conducted emissions were measured over the 9-kHz to 30-MHz range with gate-voltage and gate-resistance control. According to the results shown in Fig. 11, gate-voltage control reduces emissions in the 7-MHz to 18-MHz range and in the 22-MHz to 27-MHz range.
The power transistor's commutations are responsible for radiated perturbations generated by a converter at frequencies higher than 30 MHz. Radiated emissions for the test circuit were measured in an anechoic shielding room using the two techniques for gate control (Fig. 12).
As with conducted emissions, it may be seen that radiated emissions for the converter under test are lowered across a wide 30-MHz to 100-MHz range when the proposed control technique is employed. Beyond this frequency range, the attenuation of emissions is less.
S.J. Underwood, “DC-DC Converters Supress EMI: Minimizing EMI at its Source.” Power Electronics Technology, December 2002.
H. Sawezyn, J.J. Franchaud, N. Idir, and R. Bausière, “New Closed-Loop Voltage Control for Insulated Gate Power Transistors,” IASTED 2001: Power and Energy Systems, July 3-6, 2001, pp. 365-370, Rhodes, Greece.
V. John, B.S. Suh and T. Lipo, “High-Performance Active Gate Drive for High-Power IGBTs,” 1999, IEEE Industry Applications, 35, 1108-1117.
N. Idir, J. Franchaud, and R. Bausière, “Process and Control of the Power Transistors Commutation,” 1988, Patents n BF98/04251 and n BF99/911887.
For more information on this article, CIRCLE 342 on Reader Service Card