Key Design of High-Power Adjustable Switching Power Supply

Abstract: This paper presents a new design of high-power adjustable switching power supply. Using Buck switching power supply topology, MC33060 with single-channel PWM output and current and voltage feedback detection as the control IC, and dual-channel output IR2110 driver chip, an adjustable high-voltage and high-power switching power supply is designed, which effectively solves the problem that the output voltage and power of ordinary switching power supplies cannot reach a very high limit under non-isolated topology structure, and has overcurrent protection circuits. Based on the application of MC33060, this paper introduces the design method of adjustable switching power supply, and then explains in detail the composition of this system and the role of each part. Finally, the article summarizes the characteristics of the system.

1. Introduction

Switching power supplies have emerged as a substitute for linear regulated power supplies, and their applications and implementations are becoming increasingly mature. Integrated technology has enabled electronic devices to develop in the direction of miniaturization and intelligence. New electronic devices require switching power supplies to have smaller volumes and lower noise interference in order to achieve integration. For small and medium power switching power supplies, it is to achieve monolithic integration, but in the field of high-power applications, due to its excessive power loss, it is difficult to achieve monolithic integration, and the system volume has to be minimized while ensuring various power supply parameters based on its topological structure.

2. Typical switching power supply design

A switching power supply is generally composed of a pulse width modulation (PWM) control IC (Integrated Circuit) and power devices (power MOSFET or IGBT), and meets three conditions: switching (the device operates in a nonlinear switching state), high frequency (the device operates at a high frequency rather than a low frequency close to the upper frequency), and DC (the power supply output is DC rather than AC).

2.1 Control IC

Take MC33060 as an example to introduce the control IC.

MC33060 is a high-performance voltage-driven pulse width modulation device produced by ON Semiconductor. It uses a fixed-frequency single-ended output and can operate at -40°C to 85°C. Its internal structure is shown in Figure 1 [1]. Its main features are as follows:

1) Integrates all pulse width modulation circuits;

2) Built-in linear sawtooth oscillator, external components only one resistor and one capacitor;

3) Built-in error amplifier;

4) Built-in 5V reference voltage, 1.5% accuracy;

5) Adjustable dead zone control;

6) Built-in transistor provides 200mA driving capability;

7) Undervoltage lockout protection;

Figure 1 MC33060 internal structure

Brief description of its working principle: MC33060 is a fixed-frequency pulse width modulation circuit with a built-in linear sawtooth oscillator. The oscillation frequency can be adjusted by an external resistor and a capacitor. The oscillation frequency is as shown in formula (2-1):

The width of the output pulse is achieved by comparing the positive sawtooth voltage on capacitor CT with the other two control signals. The output of power tube Q1 is controlled by the NOR gate, that is, the output is valid only when the sawtooth voltage is greater than the control signal.

When the control signal increases, the width of the output pulse will decrease. The specific timing is shown in Figure 2 below.

Figure 2 MC33060 timing diagram

The control signal is input from the outside of the integrated circuit, one way to the dead time comparator, and one way to the input of the error amplifier. The dead time comparator has an input offset voltage of 120mV, which limits the minimum output dead time to approximately 4% of the sawtooth wave period, that is, the maximum duty cycle of the output drive is 96%. When the dead time control input is connected to a fixed voltage (range 0-3.3V), additional dead time can be generated on the output pulse. The pulse width modulation comparator provides a means for the error amplifier to adjust the output pulse width: when the feedback voltage changes from 0.5V to 3.5V, the output pulse width decreases from the maximum conduction percentage time determined by the dead zone to zero. The two error amplifiers have a common mode input range from -0.3V to (Vcc-2.0), which can be detected from the output voltage and current of the power supply. The output of the error amplifier is often at a high level, and it is "OR" operated with the inverting input of the pulse width modulator. It is this circuit structure that the amplifier only needs a minimum output to dominate the control loop.

[page] 2.2 DC/DC Power Supply Topology

DC/DC power supply topologies are generally divided into three categories: buck, boost, and buck-boost. Here we introduce the buck topology, and the simplified effect diagram is shown in Figure 3. The output has the same polarity as the input, the input current pulsation is large, the output current pulsation is small, and the structure is simple.

Figure 3 Bulk step-down chopper circuit

During the switch-on time ton, the input power supplies power to the load and the inductor; during the switch-off period toff, the energy stored in the inductor forms a freewheeling circuit through the diode to ensure the continuity of the output. The load voltage satisfies the following relationship (2-2):

2.3 Typical circuit and parameter design

The typical circuit is shown in Figure 4 below.

Figure 4 MC33060 buck chopper circuit

MC33060 is the main control chip that controls the on and off of the switch tube. From its internal structure and function, we can see that there is a +5V reference voltage inside the MC33060, which is usually used as the inverting reference voltage of the two comparators. In the design, the comparators at pins 1 and 2 are used as output voltage feedback, and the comparators at pins 13 and 14 are used to detect whether the current of the switch tube is overcurrent. In the circuit, pin 2 is connected to the reference voltage through an inverting circuit, and the buck output feedback is connected to pin 1 of MC33060 through a common phase circuit. When the circuit is in working state, the voltages of pins 1 and 2 will be compared with each other, and the output waveform pulse width will be adjusted according to the difference between the two to achieve the purpose of controlling and stabilizing the output.

The overcurrent protection in the circuit uses a 0.1 ohm power resistor with a rated power of 1W as a sampling resistor. At the current overcurrent point, the voltage on the sampling resistor is 0.1V. Pin 14 is used as a sampling point, so the reference voltage of pin 13 is set to 0.15V by the Vref voltage divider, which leaves a certain margin compared to 0.1V. When the sampling voltage is higher than the set value, MC33060 will automatically protect and turn off the PWM output. The protection point is also related to the control signal of pin 3. According to the functional analysis of this pin, the integral feedback circuit is selected so that the voltage of the Comp pin of the buck circuit is always within the normal range (0.5V-3.5V) when the buck circuit is unloaded or fully loaded.

The frequency of the output PWM waveform is determined by the capacitor of pin 5 and the resistor value of pin 6. The step-down circuit adopts a waveform frequency of 25KHz. The capacitor with CT value of 1nF and the ordinary resistor with RT of 47K meet the design requirements.

3. Design of this system

This design uses a buck topology in a DC (Direct Current)/DC conversion circuit. The input is 220VAC and 0-10V adjustable DC voltage, the output is 0-180V adjustable, and the maximum output current can reach 8A. The system block diagram is shown in Figure 5. In the design of high-power switching power supplies, a soft start circuit is often used to prevent high surge current shocks at startup, which is not the focus of this design.

Figure 5 System composition block diagram

3.1 Rectification and filtering circuit

A full-bridge rectifier circuit is used, as shown in Figure 6 below. The maximum output current requirement is 8A. Considering power loss and a certain margin, a 10A square bridge KBPC3510 and a 10A fuse are selected. The rectified voltage reaches 310V, and two 250V/100uF capacitors are used for filtering. In the figure, the switch S1 and the resistor R1 are connected in parallel as the "soft start" part, which is not explained in detail here. For detailed soft start design, see the soft start design of various switching power supplies.

Figure 6 Rectifier circuit.

[page] 3.2 Control IC and Input Circuit

The MC33060 control circuit and input adjustment circuit are shown in Figure 7 and Figure 8 respectively. MC33060 is selected as the control IC. The selection of its peripheral devices is not repeated here. Please refer to the parameter selection part in the typical circuit design. Comparator 1 is used for voltage sampling and comparator 2 is used for current sampling. The input adjustable voltage is sent to the negative end of the comparator as a reference voltage to control the output size of the power supply after voltage division.

Figure 7 MC33060 control circuit

Figure 8 Input regulation circuit

3.3 Inverse delay drive circuit

The reverse delay driving circuit is shown in Figure 8. The driving chip in the circuit adopts IR2110 from International Rectifier (IR) of the United States. It not only includes the basic switch unit and driving circuit, but also has the protection control function combined with the external circuit. The design of its suspended channel enables it to drive the switch tube working at a bus voltage not higher than 600V. It has an undervoltage protection function inside. Combined with the external circuit, it can easily design overcurrent and overvoltage protection. Therefore, no additional overvoltage, undervoltage, overcurrent and other protection circuits are required, which simplifies the circuit design.

Figure 8 Inverting delay drive circuit

The chip is a high-voltage gate driver with 14-pin dual in-line plug-in. The drive signal delay is in the ns level, and the switching frequency can range from tens of Hz to hundreds of kilohertz. IR2110 has two input signals and two output signals, one of which has a level conversion function and can directly drive the power device on the high-voltage side. The driver can run with the main circuit and only needs one control power supply, which overcomes the disadvantage of conventional drivers requiring multiple isolated power supplies and greatly simplifies the hardware design. The simple truth value diagram of IR2110 is shown in Figure 9 below.

Figure 9: IR2110 simple truth diagram.

IR2110 has two output drivers, whose signals are taken from the input signal generator. The generator provides two outputs. The low-side drive signal is directly taken from the signal generator LO, while the high-side drive signal HO must be converted to a level before it can be used for the high-side output driver. In this system, one IR2110 is enough to drive the dual tubes.

Because the two tubes are driven and cannot be turned on at the same time, the control IC outputs only one signal. Therefore, an inverting delay circuit needs to be added between the control IC output and the driver. After the PWM output of the control IC passes through the same-phase and inverting comparators, the pull-up of resistors R29 and R30 respectively charges capacitors C12 and C13 to generate a delay, so that the two PWMs have symmetrical complementarity and a certain dead zone interval, ensuring that the two switch tubes in the main circuit will not be turned on at the same time. The waveforms obtained at the HIN and LIN labeled ends in the circuit are shown in Figure 10 below.

Figure 10 Driving waveform after inversion

3.4 Main circuit and output sampling

The main circuit is shown in Figure 11, which uses a half-bridge switching circuit.

Figure 11 Main circuit

According to the rectified voltage and input current parameters, IRF840 is selected as the high-frequency switch tube, with a maximum withstand voltage VDS of 500V and a maximum on-current ID of 8A, which meets the design requirements. The freewheeling diode working in the high-frequency working state generally uses a fast recovery diode. Here, HFA25TB60 is selected, which can withstand a reverse voltage drop of 600V, a maximum on-current of 25A, and a recovery time of only 35ns. The output part is divided by two resistors to the voltage sampling circuit, as shown in Figure 12 below.

Figure 12 Voltage sampling circuit

3.5 Overcurrent protection circuit

The overcurrent protection circuit is shown in Figure 13 below.

Figure 13 Overcurrent detection circuit.

A 0.33 ohm 10W power resistor is connected in series at the upper end of the main circuit as a sampling resistor. When the current is too large, the photosensitive transistor in the optocoupler is turned on, and the detection circuit outputs a high level to the SD end of IR2110. Since SD is a low-level effective and high-level shutdown point, it can protect the circuit well when the current is too large. And as mentioned above, IR2110 itself has various protection circuits, so the peripheral current and voltage protection circuit can be greatly simplified.

4. Conclusion

This design provides a method for designing a high-power switching power supply in a non-isolated topology with a simple circuit structure. A half-bridge circuit is used in the main circuit to replace the traditional single-tube switching circuit. When the upper tube is turned off, the opening of the lower tube can better ensure the stability of the output freewheeling and the output of power. The calculation method of the inductance is not given in the article because it is not the focus of discussion. It can be calculated based on parameters such as the output current, voltage and RDS (MOSFET drain and source on-resistance) of the switch tube in the circuit. In practice, a certain margin value should be left. The system operation is basically stable and can be considered for application in industrial power supply design.