An Adjustable deadtime Control Circuit for LLC Resonance Control Circuit

This study introduces the principle of deadtime generation and gives a deadtime control circuit. The deadtime can be adjusted by the current generated by the MOS transistor in the subthreshold region to control the charging and discharging speed of the capacitor dynamically, and the whole circuit is simulated in the cadence simulation environment. The simulation results show that the designed circuit can manually adjust the dead time in the range of 70 ns-1, 650 ns depending on external settings, and greatly reduce the complexity of the circuit.


Introduction
In resonant converters, the half-bridge LLC resonant converter has better circuit characteristics than other resonant-driven converters and is more and more widely used in daily life.First, a half-bridge LLC resonant converter can realize the soft switch technology in full load range, which is beneficial to the high-frequency circuit.Secondly, the input voltage has a wide range of changes, and only a small frequency change is needed to control the output voltage to maintain the output voltage stability, and the input voltage and load have little influence on the output voltage.Then the resonant inductance can be replaced by the transformer's excitation inductance and leakage inductance, and the output part uses the capacitive filtering method, which makes the overall circuit of the converter smaller [1,2].
This paper introduces a deadtime control circuit.By adjusting the current generated by the use of a MOS tube in the subthreshold region [3], the speed of capacitor charging and discharging can be controlled dynamically, the complexity of the circuit can be reduced, the time range of the adjustable deadtime can be widened, and the efficiency of the power supply can be improved.

Principle of deadtime generation
Figure 1 shows a schematic diagram of the LLC resonant control circuit based on the PWM control mode [4], where the MP1 tube is a switch tube and MN1 is a synchronous rectifier tube.The control circuit generates PW_H and PW_L signals to control the alternating conduction of MP1 and MN1.Due to the on-off delay characteristic of the MOS tube, there is a state of simultaneous conduction of MP1 and MN1.The conduction resistance of the two tubes is very small, but the resulting conduction current will be very large.To improve the efficiency of the power supply and prevent the occurrence of a large instantaneous current from burning out the device due to the direct-going phenomenon of the switching power tube, a circuit with a deadtime between the switching signals of the two tubes is needed to prevent the situation [5].The resulting pilot signal is shown in Figure 2.
Fixed dead time is a common method with a simple structure.However, to ensure that the power tube and rectifier tube are not conducting at the same time in the whole working mode, the dead time should be long enough.During this time, the circuit may consume power through the MN1 tube's parasitic diode discharge.The power consumption produced by the conduction of a bulk diode is: where VF is the forward voltage drop of the bulk diode, TD is the deadtime, IO is the output current, and FS is the switching frequency.It can be seen from Formula 1 that the conduction loss of the bulk diode increases with the increase of current and switching frequency.

Overall structure
The function of the adjustable deadtime control circuit in the drive circuit is shown in Figure 3.The lowside power device waits for the high-side power device switch voltage to become high level, and the high-side power device waits for the low-side switch voltage to become low level before turning on.This avoids the extra loss caused by the body diode conduction and reduces the hard switch loss.Ensuring circuit safety also improves conversion efficiency [6].

Subthreshold zone
In the characteristics of MOS devices, the VGS of the MOS device is lower than VTH.When the MOS device is suddenly switched off, there is even a leakage current, which has an exponential relationship with VGS, i.e., the sub-threshold conductivity of the MOS tube.The relationship between MN2 leakage current and VGS can be expressed as: where ȗ is a non-ideal factor, VT =kT/q.Within a certain range, for every 80 mV increase in VGS, ID will go up by an order of magnitude, and we can adjust the dead time by VDT.

Delay circuit
The adjustable deadtime control function controls the delay circuit by controlling the speed at which the VDT voltage is converted into a current for capacitor charging and discharging, thus achieving the effect of controlling the deadtime through the VDT voltage [7].The delay circuit controlled by the core circuit is shown in Figure 4.
When the input voltage changes from a high level to a low level, the inverter charges through M4 to capacitor C, and the charging time is as follows: ln 2 0.69 When the input voltage changes from low to high, the charge on capacitor C is discharged to the ground through the M3 tube in the inverter, where the M2 tube is equivalent to a current source and clamps the discharging current.The reverse threshold voltage of the Schmidt trigger is VT, and the current of M2 is i, so the delay time is: It is known that the discharge time of the capacitor voltage is proportional to the size of the capacitor and inversely proportional to the size of the current.The discharge time of the capacitor can be increased by increasing the size of the capacitor C or decreasing the current of M2.The delay circuit validation simulation is shown in Figure 5.As shown in Figure 6, after logic processing, only when PW_H is high, can Ctrl_L be allowed to be high, which is reflected in the drive circuit that the low-side switch power tube is allowed to turn on after the high-side switch power tube is disconnected.In the same way, PW_L is logically processed so that PW_L and Ctrl_H are allowed to be at a high level.The embodiment in the drive circuit is that the high-side switch power tube is allowed to turn on after the low-side switch power tube is disconnected, realizing the function of adaptive dead time of the drive circuit.

Adjustable deadtime circuit
According to Formula 5, the influence of current on time is linear, that is, the current needs to increase to 10 times as much as the delay time decreases to 1/10 of the original value.This paper needs to realize the adjustable dead time of 70 ns-1, 650 ns.As shown in Figure 7, the MOS transistor current working in the sub-threshold region has an exponential relationship with VGS.Only a small voltage variable can achieve a wider range of adjustable dead time.First of all, it is processed by the source follower composed of MN1, R1, and R2, where R1 and R2 play the role of the voltage divider.If MN1 source voltage is V1, the relationship between VDT and V1 is: It can be seen that the small signal gain is: In Formula 6, we can see that when VDT>VTH, the gain increases monotonically from zero and increases with the increase of gm, Av is approximately equal to 1, the output voltage changes approximately with the input voltage, and the voltage translation of both is VGS.The source follower here acts as a voltage buffer.By separating the post-stage circuit from the pre-stage circuit and performing voltage splitting processing, voltage V2 on R2 can be reasonably set to make MN2 work in the subthreshold area.As shown in Figure 8, the abscissa is the voltage VDT.After passing through the source follower, the output voltage V2 is 220 mV when VDT is less than 600 mV, and the deadtime is 1, 650 ns at this time.After that, the voltage value of V2 rises approximately linearly with the VDT, and the deadtime is about 70 ns when the VDT voltage is 3 V.Compared with using linear and negative quadratic V-I characteristics to control the deadtime, the exponential V-I characteristics designed in this paper have a wider range of deadtime adjustment and lower power consumption.The dead time will not only affect the straight-through of the power tube but also affect the function of the soft switch [8].Therefore, the size of the deadtime should be reasonably set according to the resonant capacitance and resonant inductance [9].

Simulation result
This paper uses nuvoton 0.35 ȝM 40 V BCD process.As shown in Figure 9, we set the power supply voltage to 15 V, the switching frequency to 500 kHz, the temperature to 25 ႏ, the VDT to 2.5 V, and observe the waveforms of the grid drive signals PW_H and PW_L of the two power tubes.
Comparing the basic waveforms of the three, we can see that the rise time of PW_W and PW_L is 223 ns along the deadtime, the decrease time is 228 ns along the deadtime, and the two dead times are approximately equal, within the error range of the deadtime.Deadtime was simulated at -40 ႏ, 25 ႏ, and 125 ႏ with VDT of 2 V, 2.5 V, and 3 V.The following table 1 were obtained:

Conclusion
The adjustable deadtime circuit designed in this paper can effectively reduce the conduction loss of the body diode, and the control method is simple.The designed circuit can realize the adjustment of dead time between 60 ns and 1, 500 ns, and improve the reliability and intelligence of the drive converter [10] as well as efficiency and circuit stability.

Figure 3 .
Figure 3. Drive circuit frame diagram Figure 4. Delay circuit block diagram of current control

Figure 5 .
Figure 5. Verification simulation diagram of the delay circuit

Figure 8 .Figure 9 .
Figure 8. Simulation diagram of the relationship between adjustable deadtime range and VDT