A Gate Drive Circuit for Dual N-type H-bridge Power Transistors

A new grid driver of a high-side N-type switch tube is designed, which is easy to integrate. The circuit has the characteristics of low cost and fast speed and solves the problem of the unstable output voltage of a high-side switch tube when a low-side switch tube is not working in a traditional bootstrap circuit. The power supply voltage of the circuit is 30 V, including the voltage bias module, a high-level voltage shift circuit module, and a charge pump module. It can meet the driving requirements of a large size and high-side N-type switch tube. In this study, the 0.18 μm MGN process was used for design and simulation. The simulation results show that the grid driver has good driving ability.


The architecture selection of the H-bridge circuit structure
According to the different types of power tubes, the H-bridge structure can be divided into two types, among which one is a PMOS-NMOS circuit and the other is a DUAL-NMOS circuit.The main difference between the two circuit structures is that the high-end power tube uses an NMOS tube or PMOS tube [1] .Because p n

2.5P P |
, therefore, the area used by the PMOS tube is much larger than that used by the NMOS tube when the output current is the same.From the perspective of the power tube grid driver circuit, the grid driver circuit of the PMOS-NMOS structure is simpler.For the DUAL-NMOS structure, the driving circuit is relatively complex.
According to the on-off condition of the PMOS tube, the gate-source voltage Vgs must meet its value greater than the threshold voltage Vthp, which is generally 5 V. Therefore, the high-end drive circuit needs to generate a grid voltage less than VIN-Vthp.Similarly, the gate-source voltage Vgs must be greater than the threshold voltage Vthn if the NMOS tube is to be switched on.Therefore, the highend drive circuit must provide a voltage greater than VIN+Vthn, which can be generated by both the charge pump circuit and the bootstrap circuit.To save the chip area, the design adopts the DUAL-NMOS structure with a smaller power tube area when the output current is the same [2] .

Characteristics analysis of high-voltage DMOS tube
Considering that the DMOS power tube is applied in this design as a power-switching tube, its switching characteristics are very important [3] . Figure 1 shows the circuit structure of the driving power MOS tube, where RG is the grid input resistance and RO is the output resistance of the driving control circuit.
In Formula (1), U is the voltage amplitude of the supply, Tc is the time when VGS rises from 0 to U, and VDS is the drain-source voltage of the MOS tube.MOS tube discharge current Isink is: In Formula (2), TF is the time for VGS to descend from U to 0. As can be seen from Formula (3), to reduce the on-resistance of the MOS tube, the gate drive circuit is usually required to provide a certain driving capacity and sufficient driving voltage.When VGS increases, the on-resistance Ron decreases [4] .In the actual design, the drawing and filling current of the power tube can be estimated according to the charging and discharging current.

Traditional high-side N-type switch tube drive scheme
A bootstrap circuit, also known as a booster circuit, is the use of a bootstrap diode, bootstrap capacitor, and other electronic components so that the capacitor discharge voltage and power supply voltage are superposed to increase the voltage.However, the bootstrap circuit has an important limitation: it requires periodic charging of the external bootstrap capacitor for the circuit to work properly.The bootstrap circuit is composed of a fast recovery diode DBOOT and capacitor CBOOT.It is analyzed by one half-bridge drive.The high side input and low side input of the circuit are logically opposite.Under the action of the PWM input signal, the M1 tube and M2 tube alternately open.When the circuit starts to work, the M2 tube is turned on and the M1 tube is turned off first, the capacitor CBOOT negative terminal is 0 V, power supply voltage VDD charges capacitor CBOOT through diode DBOOT, and finally, the voltage difference between positive and negative terminals of CBOOT is VDD.At this time, the PWM signal reverses, the M1 tube is on, and the M2 tube is off.The negative voltage of the capacitor CBOOT rises to VBB.Since the voltage at both ends of the capacitor cannot change abruptly, VCC reaches VBB+VDD, and VG increases with VCC [5] .This bootstrap circuit drive scheme has the advantages of a simple structure and fast switching response and is widely used in half-bridge drivers and switching power supply systems.However, in this scheme, CBOOT generally requires several hundred nF, which is difficult to integrate.In addition, the level switching circuit and high voltage driving circuit need to consume power when the N-type switch tube is driven at the high side to open.A long driving time will inevitably cause the voltage reduction of the high-side drive power supply.Therefore, after the switch tube on the high side is opened for some time, the switch tube on the low side should be opened to charge the CBOOT.In this way, the logic of the control algorithm cannot make the high-side N-type switch tube open at 100% duty cycle [6] .

New high-side N-type switch tube drive scheme
The grid driving scheme of the new high-side N-type switch tube designed in this paper is shown in Figure 3.The working principle that the charge pump can provide continuous power is used to maintain the conduction of the high-side N-type switch tube at 100% duty cycle.When IN1 input is high-level and IN2 input is low-level, IN1 signal passes through high-voltage level conversion module 2, M10 tube is closed, M7 and M8 tubes are opened, and the power supply voltage passes through M7 and M8 tubes to charge the grid capacitor of MNpowerH.When the grid capacitance is filled: where VD2 is the positive pilot voltage drop of diode D2.At this time, VD2>VG, MNpowerH works in the saturated region, so the output voltage VSW is: . ( When the current of the M8 tube is charged to capacitor C1.The positive plate voltage of C1 is: The negative voltage of C1 is: While IN1 remains high, the CLK signal controls the M9 tube.When the CLK signal is high-level, the M9 tube opens.At this time, the positive plate voltage of C1 remains unchanged, and the negative plate voltage of C1 becomes: When CLK changes from high-level to low-level, the M9 tube shuts down and C1's negative plate voltage returns to VSW.Since the voltage of the positive and negative stages of the capacitor cannot be mutated, the positive plate voltage of C1 increases with the increase of the negative plate voltage.At this time, the positive plate voltage of C1 becomes: The voltage at both ends of diode D3 is VC1+-VG>VD3, the D3 tube is on, and C1 starts discharging.All the charge released by C1 is transferred to the grid capacitance of MNpowerH, and the VG is gradually raised.
After several CLK cycles of charge and discharge by the charge pump, VG gradually rises until it is higher than VBB.At this time, MNpowerH moves from the saturated region to the linear region, and finally, the high-side N-type tube MNpowerH is fully switched on, and VSW approaches the power supply voltage.
In the end, VG will remain unchanged, as will VGS and MNpowerH, respectively: The on-resistance impedance formula of the linear region of the Mos tube is: According to Formula (12), to reduce the on-off resistance of the switching tube, in addition to increasing the width-to-length ratio of the switching tube, MNpowerH can also be pushed to the deep linear region by increasing the voltage VGS.According to Formula (11), under certain conditions, VBB, VD1, and VD3 remain unchanged, and VGS can be adjusted by adjusting the ratio of R1 and R2.
When the input of IN1 is low and the input of IN2 is high, IN1 uses the high-voltage level conversion module 1 to shut down the M7 and M8 tubes and cut off the power supply to the VG.At the same time, IN1 turns off CLK's control over the M9 tube and stops the charge and discharge of the charge pump.Finally, IN1 opens the M10 tube through the high-voltage level conversion module 2, shorting the gate and source level of the MNpowerH tube, and MNpowerH is shut off.
According to the comparison between the traditional circuit and the new circuit, we can see that the new high-side N-type switch tube circuit designed in this paper has the following advantages: (1)  There is no need to use external bootstrap capacitor CBOOT, saving the use cost of the system.(2) For the grid driver chip, the VCC pin is saved and the packaging cost is reduced [7] .

Simulation of output stage circuit
The output signal VSW of the driving circuit is connected to the power supply voltage VBB and ground VSS in series with a resistance value of 8 ȍ.The current flowing through the resistance R is measured, that is, the pull and fill current of the power tube.The pull-down current reflects the driving capacity of the output circuit.During simulation, VBB is 30 V [8] .IN and CLK are clock signals with an amplitude of 5 V and a frequency of 10 KHz.When the simulation temperature is 27 ႏ, the current flowing through the resistance R is measured.The drawing and filling current are shown in Figure [9] .As can be seen from the figure above, the pulling current of the grid circuit of the H-bridge is 107 mA and the filling current is 124 mA.
We simulate the output VSW of the drive circuit and the results are shown in Figure 5.As can be seen from Figure 5, during the pull-up phase of the gate drive circuit, i.e., MNpowerH is turned off, MNpowerN is turned on, and MNpowerN is turned off.It can be seen that when the power transistor is pulled up, the circuit dead time is 296 ns.In the gate drive circuit pull-down phase, on the contrary, when the power transistor is pulled down, the circuit dead time is 211 ns.The setting of the dead time can prevent one bridge arm from being completely turned off, while the other bridge arm is already in a conductive state [10] .

Conclusion
In this paper, a new type of high-side N-type switch gate driver is designed, which uses a charge pump circuit instead of the traditional bootstrap drive circuit.The output signal VSW range is from 0 to 30 V. The pulling and pouring currents of the power tube are 107 m and 124 mA respectively, so the circuit has a good driving ability.The dead time when the power transistor is pulled up is 296 ns, and when it is pulled down, it is 211 ns.The design of the dead time avoids the simultaneous conduction of two power transistors.The new circuit does not require the use of external bootstrap capacitors, saves usage costs, and has features such as easy integration and strong driving ability.

Figure 1 .
Figure 1.The circuit structure of driving power MOS tube

Figure 2 .
Figure 2. Composition of drive scheme based on the bootstrap circuit

Figure 3 .Figure 3
Figure 3. Driving scheme composition based on charge pump circuit

Figure 4 .
Figure 4. Simulation of grid drive circuit pull-filling current results

Figure 5 .
Figure 5. Pull-down simulation on the grid drive circuit