Low-Loss Rectifier by Self-Driven MOSFET with Gate Drive Voltage Control Circuit Y

Low-Loss Rectifier by Self-Driven MOSFET with Gate Drive Voltage Control Circuit Y. Kagawa, A. Furukawa, M. Takeshita, A. Iwata, I. Suga, S. Yamakawa and M. Inoue Mitsubishi Electric Corporation/ 8-1-1 Tsukaguchi-Honmachi, Amagasaki, Hyogo 661-8661/Japan E-mail: [email protected]

Abstract — The low-loss self-driven rectifier we developed requires no external power supply and uses a novel CMOS control circuit that generates the power MOSFET drive signal by boosting the intrinsic body diode voltage drop. The rectifier significantly improved conduction loss — a 47% decrease from intrinsic-body-diode-based conduction loss — during half-wave rectification. It can replace with a common diode for a rectifier.

I. INTRODUCTION Rectifiers with lower conduction loss make electronic power equipment more efficient. A synchronous rectifier is a switched-power MOSFET with low on-state resistance, which replaces a common diode [1]. The rectifier decreases conduction loss, but a power MOSFET requires an external power supply for driving. A self-driven synchronous rectifier Figure 1. Low-Loss Rectifier concept. is driven by the output voltage of a DC/DC converter [2], but cannot be used when the output voltage exceeds gate oxide breakdown voltage. The low-loss rectifier we propose does not require an external power supply and contains a control circuit that generates gate drive voltage from the small voltage drop of the diode between body and drain (intrinsic body diode) via a charge pump. It is a promising candidate for two- terminal rectifier due to its low power consumption and high voltage application.

II. DESIGN AND SIMULATION A. Low-loss rectifier The rectifier has a power MOSFET driven by a control

circuit (Fig. 1). The power MOSFET’s intrinsic body diode yields a voltage drop (Vf) when a current (If) flows forward, Figure 2. Simulated I-V of intrinsic body diode and on-state MOSFET. and Vf is supplied to a charge pump in the control circuit as Gray line: intrinsic body diode. Black line: MOSFET. input voltage. A charge pump boosts the output voltage (Vout) of the control circuit to the gate drive voltage from Vf. The B. Low-loss rectifier circuit on/off control circuit switches the power MOSFET based on the direction of the rectifier’s current. In simulation results for Fig. 3 shows the low-loss rectifier circuit diagram. It the I-V characteristics of the on-state power MOSFET with 5 consists of a control circuit, a power MOSFET (MOS1) for V gate drive voltage and that of the intrinsic body diode (off- rectification and an input circuit connected parallel to MOS1. state power MOSFET), the voltage drop of the on-state power The input circuit has a power MOSFET (MOS2) and an input MOSFET is lower than that of the intrinsic body diode alone resistor (R1). This configuration resembles that of a current- when the same current flows (Fig. 2). The rectifier of the sensing MOSFET. Note that the threshold voltage of MOS2 intrinsic body diode corresponds to the rectifier of a (Vth2) is smaller than that of MOS1 (Vth). MOS2 and an conventional diode, so conduction loss is reduced by the optional power diode (D1) provide supply current to the rectifier compared to a conventional diode rectifier. control circuit and protect it from high voltage. The R1 voltage drop is the input voltage (Vin) of the control circuit.

(a) Mode I: If > 0, Vout < Vth2

Figure 3. Low-Loss Rectifier circuit.

The control circuit has three terminals — IN, GND, and OUT. IN is connected to MOS2 side of the R1, GND to the anode, and OUT to MOS1 and MOS2 gates. The control circuit includes a charge pump and an on/off control circuit.

The charge pump boosts Vout to the gate drive voltage of MOS1 from Vin. The charge pump boosts over 0.3 V, because it consists of a MOSFET with low threshold voltage. It is (b) Mode II: If > 0, Vth2 ≤ Vout ≤ Vth operated by a square wave pulse generated by a ring oscillator. The ring oscillator consists of CMOS inverters and capacitors that regulate pulse frequency. The amplitude of the square pulse wave equals the magnitude of Vin. The maximum voltage of the square wave pulse is 0 V and the minimum is Vin, which is negative. The design frequency of the square wave pulse is 3.5 MHz. The on/off control circuit uses a MOSFET switching the rectifier by detecting the voltage polarity of Vin based on the direction of the current in the rectifier. When Vin is negative, the switch is turned off. When Vin is positive, it is turned on and connects the OUT and GND terminals. The gate drive voltage becomes 0 V because the gate charge of MOS1 and MOS2 is discharged by the on/off control circuit. (c) Mode III: If > 0, Vout > Vth Half-wave rectification uses four modes (Fig. 4):

1) Mode I: If > 0, Vout < Vth2: (Fig.4 (a)) When the current flows from the anode to the cathode, it flows through both the MOS1’s intrinsic body diode and the input circuit. The current in the MOS2’s intrinsic body diode and D1 flows through the control circuit. Vf is generated at MOS1’s intrinsic body diode and that for the voltage drop of MOS2 (Vf2) is generated, so a negative voltage (Vin = − (Vf −Vf2)) generated at the cathode of R1 is input to the control circuit. The control circuit boosts Vout to the gate drive voltage from Vin. 2) Mode II: I > 0, V ≤ V ≤ V : (Fig.4 (b)) f th2 out th (d) Mode IV: If < 0, Vout =0 V When Vout reaches Vth2, MOS2 is turned on. Vin rises to −Vf because Vf2 becomes 0 V with MOS2’s on-state Figure 4. Low-Loss Rectifier operation for half-wave rectification. Dot resistance. Vout rises more quickly via the charge pump due to arrows: current passes. high Vin.

70 3) Mode III: If > 0, Vout > Vth: (Fig.4 (c)) Fig. 6 shows a microphotograph of the control circuit chip. × When Vout reaches Vth, MOS1 is turned on. Vf decreases The chip is 2.0 mm 1.3mm in size and is fabricated using a due to lower on-state resistance. Vin also decreases, but Vin 0.18 μm CMOS process. The charge pump topology does not become smaller than the operating voltage of the resembles that of a boosted-gate charge pump circuit [3]. It charge pump because Vf2 is similar to 0 V, so Vout is kept at consists of the low threshold voltage N/P-MOSFET, on the the MOS1 drive voltage. order of 0.3 V, and the metal-insulator-metal (MIM) capacitor. The charge pump generates gate drive voltage from low 4) Mode IV: If < 0, Vout =0 V: (Fig.4 (d)) voltage. The on/off circuit consists of a three-stack When current reverses, the on/off control circuit detects configuration for the depletion NMOSFET. the voltage polarity of Vin and discharges the gate electron charge of MOS1 and MOS2. MOS1 and MOS2 are turned off, IN and the intrinsic body diodes of MOS1 and MOS2 block reverse current.

The rectifier thus achieves rectification by repeating the GND above operation. Fig. 5 shows simulation results for half-wave rectification. In simulation results, we used the voltage drop of the intrinsic OUT body diode (Vf) for comparison. When Vout is smaller than Vth2, Vin is -0.3 V because Vf is 0.6 V and Vf2 is 0.3 V. After Figure 6. Microphotograph of control circuit chip. MOS2 is turned on, Vin is 0.7 V similar to Vf. When Vout reaches to Vth, MOS1 is turned on. Vf the drops to 0.4 V, as does Vin, so the rectifier reduces conduction loss for half-wave III. MESUREMENT AND RESULT rectification. When current reverses and the polarity of voltage We tested the half-wave rectification of the proposed of Vin is positive, both MOS1 and MOS2 are turned off. The rectifier, connecting an AC power supply to the anode, and a × conduction loss is calculated by the sum of If Vf. In load resistor (R ) to the cathode. Current flowing through simulation, the conduction loss of the rectifier is calculated at load the rectifier is regulated by Rload, which was 25 Ω due to the 50% less than that of the intrinsic body diode. maximum current being limited to 5.6 A for the 100 VAC signal supply. The experiment used a discrete MOSFET and diode produced by International Rectifier Corp. (IR). MOS1 (IRFP260) had a threshold voltage of 3 V. MOS2 (IRL620S) had a threshold voltage of 1.5 V. The D1 was BYW80-200. R1 was 10 kΩ. The measurement condition was 100 VAC, and the AC frequency was from 60 Hz to 500 Hz. The time domain waveform was measured using an oscilloscope. Fig. 7 and 8 show experimental results of the rectifier current flow and voltage drop during half-wave rectification. Forward current flows in the rectifier and the intrinsic body diode, but reverse current does not flow (Fig. 7). Currents have the same value. When only the intrinsic body diode is used, Vf is 0.75 V at maximum. In the rectifier, current flows (a) Input voltage in the intrinsic body diode immediately after the AC signal is

applied, and Vf is similar to that using the intrinsic body diode alone. Vf drops to 0.4 V when the control circuit turns on MOS1 via the gate drive voltage generated from Vf of the intrinsic body diode (Fig. 8). As a result, conduction loss of the rectifier is 47% less than that of the intrinsic body diode alone during half-wave rectification. In AC frequency dependence of the reduction ratio between conduction losses of the intrinsic body diode and the proposed rectifier (Fig. 9), simulation results are similar to those measured, so the rectifier is effective below 1 kHz. The loss reduction ratio increases with decreasing frequency. When DC voltage is supplied, the loss reduction ratio exceeds 50%. (b) Voltage drops. Gray line: intrinsic body diode. Black line: Low-Loss Rectifier.

Figure 5. Simulation result for half-wave rectification.

71 IV. CONCLUSION The low-loss MOSFET-based rectifier we developed can replace a conventional diode. Our rectifier contains a control circuit generating the gate drive voltage of the MOSFET and thus needs no external power supply. The rectifier showed a 47% smaller conduction loss than the rectifier of a conventional diode during half-wave rectification.

ACKNOWLEDGMENT We would like to acknowledge Tatsuo Oomori and Hiroshi Fukumoto for their ongoing encouragement. We also thank Jun Tomisawa and Kenichi Morokuma for their valuable discussions and Takuya Sakai and Takahiro Ono for Figure 7. Experimental results for rectifier current. Gray line: intrinsic body support in experiments. diode. Black line: Low-Loss Rectifier.

REFERENCES [1] C. Blake, D. Kinzer and P. Wood, “Synchronous Rectifiers versus Schottky Diodes: A comparison of the losses of a synchronous rectifier versus the losses of a Schottky diode rectifier,” Applied Power Electronics Conference and Exposition, Conference Proceedings, vol. 1, pp. 17-23, 1994. [2] J. A. Cobos and J.Uceda, “Low output voltage DC/DC conversion,” Industrial Electronics, Control and Instrumentation, International Conference on, vol. 3, pp. 1676-1681, 1994. [3] A. Umezawa, S. Atsumi, M. Kuriyama, H. Banba, K. Nauke, S. Yamada, E. Obi, M. Oshikiri, T. Suzuki and S. Tanaka, “A 5-V-Only Operation 0.6-μm Flash EEPROM with Row Decorder Scheme in Triple-Well Structure,” Solid-State Circuits, IEEE Journal of, vol. 27, issue 11, pp. 1540-1546, 1992.

Figure 8. Low-Loss Rectifier vs. intrinsic body diode voltage drop. Gray line: intrinsic body diode. Black line: Low-Loss Rectifier.

Figure 9. Frequency dependency of reduction ratio between intrinsic body diode and Low-Loss Rectifier conduction loss. Dots: measurement. Lines: simulation.