Harmonic Pole Drive of MOSFET

1. Introduction

In the past decade, high-power field-effect transistors (MOSFETs) have sparked a revolution in the power supply industry and greatly promoted the development of other areas of the electronics industry. Due to the faster switching speed of MOSFETs, the switching frequency of power supplies can be made higher, from 20kHz to 200kHz or even 400kHz to the current MHz. The size of switching power supplies has become smaller, resulting in a large number of new products using small power supplies. The increase in switching frequency has accelerated the transient response speed, reduced the size of components, and increased power density. However, it has also brought some problems, such as high switching frequency causing excessive switching losses, which reduces power supply efficiency. [1]

However, the drive loss of the power field-effect transistor (MOSFET) limits the efficiency of the power converter at high switching frequencies. The use of LC resonance technology can reduce this loss, and most of the energy is recovered during the charging and discharging process, effectively clamping the gate voltage without limiting the duty cycle.

2. MOSFET drive loss

Almost all current MOSFET power converters use traditional totem pole drive, and the total energy provided by the power supply VDD is

Figure 1 MOSFET drive circuit
(1) Where Ts is the switching period, Fs is the switching frequency, and iDD is the transient current following VDD. This current changes as the voltage Vgs of the main MOSFET changes from 0 to VDD. The total gate current is

Formula (3) describes the relationship between gate drive loss and switching frequency. More importantly, it reflects two problems:

1) For a specific application and a given VDD, if the frequency does not change, the loss coefficient cannot be changed.
2) Reducing the gate impedance RG (a lumped parameter that includes the MOSFET gate impedance, the driver device turn-on impedance and other wiring and package impedances) does not reduce the drive loss. A smaller RG reduces the charge and discharge time, but increases the current amplitude, and the drive loss does not change.

3. Resonant drive technology

In order to reduce the loss in (3), resonant drive technology has been valued, which uses an LC circuit to charge and discharge the VGS of the MOSFET. C is the inherent gate capacitance L is set according to the situation.
The following factors should be considered when using gate drive technology:

A. High-frequency PWM converters require faster gate drive speeds

For non-resonant converters, the switching loss of the PWM converter increases rapidly as its frequency increases. First, it is necessary to reduce the VGS rise and fall transition time to keep it at a stable loss level. In addition, the VGS transition time limits the maximum and minimum duty cycles. When the switching frequency becomes higher, the same duty cycle range requires a smaller transition time.

Most commercial power MOSFETs are enhancement mode devices (N-channel with Vth>0), and the discharge voltage VGS of some resonant drives will drop below 0V due to LC parallel resonance [2][3]. Excessive VGS oscillation delays the turn-on transition, reduces the available duty range, and has additional drive energy.

B. Preventing false turn-on in high-frequency switching states

When the switch tube operates in a high-frequency state causing the VGS transition time to increase, the voltage drop speed (dV/dt) of the switch point S of the PWM converter will accelerate. Point S is on the synchronous buck converter in Figure 2. When M1 is turned on, the voltage at point S drops rapidly, injecting a transient current (iDG CGD (dV/dt)) into the parasitic capacitance CGD of M2. If iDG is too high, a switching voltage VGS is generated, and M2 will be mis-turned on [4]. In order to prevent M2 from being mis-turned on, a low-impedance path must exist between its gate and source (as shown in Figure 3).

[page]4. A new resonant driver

To solve the above problems, we propose a new resonant driver [5], as shown in Figure 3. In this circuit, a complementary driver pair MDR1 and MDR2 are the same as the traditional driver. An inductor LR is inserted in the resonant element, and two diodes DDR1 and DDR2 are used to clamp VGS and recover the driving energy. The resonant current will only appear when the switch tube LR is turned on or off, and the change of duty cycle does not affect the circuit operation. Moreover, when the diode recovers the driving energy, it provides a corresponding low impedance path.

We explain this circuit based on the waveform of Figure 3b. At the beginning, VGS=0 (t

CG_M1 is an equal gate capacitance M1, ZO is the characteristic impedance of the resonant circuit, and WO is the resonant frequency. During the time from t2 to t3, VGS_M1 is clamped to VDD by DDR1 and iLR. At t3, MDR1 is turned off and the energy recovery process is initialized: the inductor current conducts the body diode MDR2, and the current path is MDR2-LR-DDR1-VDD. When the steady-state voltage VDD passes through LR, the reduction of the inductor current is linear, and the recovery time trec (=t4-t3) can be simply expressed as

From time t1 to t2, the energy transferred from the DC power supply VDD to the resonant inductor is

, the energy of the gate capacitor is

The energy will be returned to the power supply VDD at time t3~t4. Because of the principle of energy feedback, the energy loss of the circuit in Figure 3 is smaller than that of the traditional gate drive. During the period from t5 to t6, resonance occurs and the capacitor energy

Convert to inductor

, t6 to t7 is just the energy flow, and finally from t7 to t8 the inductor energy returns to the power supply.

Compared with the traditional drive circuit, this circuit has the following advantages:

the drive energy can be recovered during the charge and discharge conversion process. This problem has been mentioned above, and this can be explained through a more detailed calculation. RG is the resistance value, including the MDR on-resistance and the parasitic resistance of LR, the gate impedance of the main MOSFET M1 and the resistance of other wiring. The transient inductor current iLR during the charging resonance process is

The VGS clamp provides fast startup and optimized overdrive voltage. Diodes DDR1 and DDR2 not only play the role of energy recovery, but also clamp VGS at 0 or VDD to prevent overdrive. For a given power MOSFET, the drive speed in Figure 3 is mainly determined by the resonant inductor LR. Selecting a small LR can increase the switching speed and increase energy loss. For most high-frequency applications, the MOSFET rise/fall time is determined by the maximum rise time. In this case, the selection of LR must meet the following requirements

[page]5. Experimental results

According to Figure 4 (a), the M2 driver tube uses ZETEX's ZXMD63C02X, whose N-channel source-drain on-state impedance is 0.13 ohms and P-channel source-drain on-state impedance is 0.27 ohms. The M1 main switch tube uses Vishay Sliconix's Si7442DP, with a source-drain on-state impedance of 2.6 milliohms and a gate impedance of 1.2 ohms. When LR=470nH, the experimental waveform is shown in Figure 4 (b). The first waveform D is the duty cycle driving waveform of the power MOS tube, the second is the waveform of the resonant inductor LR, and the third is the voltage of the source and gate of the MOS tube. These waveforms are basically the same as those in Figure 3. The last waveform is the driving loss, and the result is compared by selecting different inductors. Selecting a suitable resonant inductor can effectively reduce the driving loss.

5 Conclusion
With the increase of PWM switching frequency, a very small inductor can be used in the current resonant circuit, making it possible to make the resonant drive circuit into an integrated circuit. The development of soft switching technology has further reduced the switching loss of the main switch tube. In a low-power power supply, the drive loss of the switch tube will become one of the losses that cannot be ignored. To further improve the power supply efficiency, the drive loss cannot be ignored. It is believed that the resonance technology will be widely used in the driver chip.

References
[1] Yang Yong, “How to reduce MOSFET switching loss” Journal of Electrical Engineering, 1995 (4) 31-33.
[2] SH Weinberg, “A novel lossless resonant MOSFET driver,” in Proc.Power Electron. Spec. Conf., 1992, pp. 1003–1010.
[3] ID de Vries, “A resonant power MOSFET/IGBT gate driver,” in Proc.Appl. Power Electron. Conf., 2002, pp. 179–185.
[4]B. Razavi, Design of Analog CMOS Integrated Circuits. New York:McGraw-Hill, 2001, pp. 166–169.
[5] I Yuhui Chen “A Resonant MOSFET Gate Driver With Efficient Energy Recovery” IEEE TRANSACTIONS ON POWER ELECTRONICS. 2004 2(3) 473-476