Design of Soft-Switching Converter Driving Circuit Based on Boost ZVT-PWM

In high-frequency PWM switching converters, a reliable gate drive circuit must be designed to ensure that the power MOSFET works at high frequency, high voltage, and high current. A good-performance drive circuit requires that the trigger pulse should have a sufficiently fast rise and fall speed, and the front and rear edges of the pulse should be steep; the internal resistance of the drive source should be small enough and the current should be large enough to increase the switching speed of the power MOSFET; in order to reliably trigger the power MOSFET to turn on, the gate drive voltage should be higher than the device's turn-on voltage; to prevent mis-turning on, it is best to provide a negative gate-source voltage when the power MOSFET is turned off. For soft-switching converters, when designing the drive circuit, the phase relationship between the main switch and auxiliary switch drive signals must also be considered.

Taking the boost ZVT-PWM converter as an example, this paper designs a soft-switching converter drive circuit that can meet the above requirements using the integrated chip MC34152 and CMOS logic devices.

MC34152

The MC34152 is a monolithic dual MOSFET high-speed integrated driver with two high-current output channels that are fully suitable for driving power MOSFETs. It has a low input current and is compatible with CMOS and LSTTL logic circuits.

Each channel of MC34152 consists of two parts: logic input stage and power output stage. The input stage is composed of Schmitt trigger, a logic circuit with the largest bandwidth, and uses diodes to achieve bidirectional input limiting protection. The output stage is designed as a totem pole circuit structure. The logic judgment of the comparator with a reference voltage of 5.7V and the output level of the Schmitt trigger determines the output state of the NAND gate (in-phase or inverted output), and then determines the "push" or "pull" working state of the two same-type output power tubes. This structure gives the chip a strong driving capability and low output impedance. Its output and absorption current capabilities can reach 1.5A, and the standard on-state resistance at 1.0A is 2.4W. It can quickly charge and discharge large capacitive loads; for a 1000pF load, the output rise and fall time is only 15ns, and the transmission delay from the logic input to the drive output (rising edge or falling edge) is only 55ns, so it can drive power MOSFETs at high speed. Each output stage also contains a built-in diode connected to VCC to clamp positive voltage transient changes, and the output end is connected to a 100KW dropper resistor to ensure that when VCC is lower than 1.4V, the MOSFET gate is kept at a low potential.

Design of drive circuit for soft switching converter

The boost ZVT-PWM converter is a zero voltage conversion soft switching converter, and its structure is shown in Figure 1. It consists of two parts: the main circuit and the control system. In the main circuit, S is the main switch, S1 is the auxiliary switch, and the control system includes a PWM signal generation circuit and a drive circuit.

Figure 1. Block diagram of ZVT-PWM converter

Index requirements

Converter: switching frequency fS=100KHz; input voltage Vi=12V; output voltage Vo=48V; output power Po=100W.

Driving circuit: output peak current Iom5V; driving pulse rise time tr and fall time tf are

Figure 2 ZVT-PWM converter drive circuit

As shown in Figure 2, the pulse modulation wave output by the PWM control chip is sent to the input terminal (pin 2) of MC34152 after being inverted and shaped by CD4069, and is output from pin 7 as the driving signal of the main switch. At the same time, the pulse modulation wave output from the PWM control chip is shaped by another inverter of CD4069 and input to the CLK terminal (pin 3) of CD4013 as the clock signal. The rising edge of the signal triggers the Q terminal (pin 1) to output a high level, and charges the capacitor C9 through the variable resistor RP. When the charging voltage reaches VCC/2, the reset terminal works to reset the D flip-flop, the potential of the Q terminal becomes a low level, and the capacitor C9 is quickly discharged to zero through the diode D2, ready to enter the next cycle. Therefore, through the monostable circuit, the pulse signal output from the Q terminal of CD4013 is once again inverted and shaped by CD4069 and sent to the other input terminal (pin 4) of MC34152, and is output from pin 5 as the driving signal of the auxiliary switch. Because the time constant of the monostable circuit RC network (composed of RP and C9) is RP·C9, the width of the output pulse can be changed by adjusting the size of the variable resistor RP, thereby solving the phase relationship between the auxiliary switch and the main switch, that is, the delay problem, ensuring that the main switch is turned on and off at the right time, achieving the best soft switching effect. At the same time, the use of the high-speed integrated driver MC34152 improves the quality of the entire control system.

Parameter calculation

In the ZVT-PWM converter, in order to achieve soft switching conversion, S1 is turned on before S is turned on to stimulate the auxiliary inductor Lr and the auxiliary capacitor Cr to produce resonance and create zero voltage turn-on conditions for S. It can be seen that the driving signals of the two switches must maintain a certain phase relationship, and their delay time TD should satisfy the following relationship:

(1)

After the main switch completes zero voltage switching, in order not to affect the operation of the main switch, the working time of the auxiliary switch cannot be too long, and is generally selected to be 1/10 of the switching period TS. Combining formula (1), we can get:

(2)

For the monostable circuit, during the charging period, the circuit equation is:



Let VC(0)=0

∴ Assume the charging time is τ, then:



∴ τ=-RCln0.5≈0.69RC (3)

Considering that the delay time is TD=1/10, and the charging time is the required delay time, then



C(C9)=150pF, from formula (3):



R(RP) is an adjustable resistor of 10KW"50KW. By adjusting R(RP), the requirement of TD in formula (1) can be met.

Diode D2 can use ordinary diode IN4148.

Since the core part of the drive circuit MC34152 is an integrated component, the peripheral circuit components are only RP, C9 and D2. Therefore, only the parameters of these three components need to be designed. The auxiliary inductor Lr and auxiliary capacitor Cr of the main circuit should be considered in combination with formula (2).

Simulation and experimental results

Based on the circuit in Figure 2, the author conducted simulation and experiments, and the results are shown in Figure 3 (a) and (b) respectively. The upper and lower parts of the figure are the driving signal waveforms of the main and auxiliary switch gates, respectively. The leading and trailing edges of the pulse are steep, and the rise and fall time and transmission delay meet the index requirements.

(a) Simulation waveform



(b) Experimental waveform

Figure 3 Signal waveform of ZVT-PWM converter driving circuit

It is worth pointing out that, according to the different requirements of the main and auxiliary switches on the phase relationship, it is only necessary to adjust the relevant parameters of the corresponding delay circuit, and the driving circuit structure shown in Figure 2 is also applicable to other types of soft switching converters.