A switching power supply design method based on forward converter

1 Introduction

After years of development, switching power supply technology has achieved great success, and its application is also very common and extensive. However, due to its complex structure, the large number of components involved, and the need to reduce costs and improve reliability, there are still some problems that need to be solved. For example: the design and production of power supplies require high technical support; the debugging of circuits requires practical experience and is also difficult. For the first problem, although the various switching power supplies are currently in various forms and structures, most of them are derived from several basic dc-dc converter topologies, or these basic circuit combinations. Therefore, several basic dc-dc converters can be analyzed and the existing circuit design formulas can be applied to the design of actual switching power supplies. For the second problem, with the development of computer hardware and software and the continuous improvement of simulation technology, people can use simulation technology to solve problems in the development and production of switching power supply products.

Based on the analysis of the basic buck converter circuit topology, this paper analyzes the related forward converter and two-switch forward converter, and finds that the parameter calculation formulas of the forward converter and two-switch forward converter circuits can be derived from the design formula of the buck converter circuit through equivalent transformation; in addition, the circuit simulation test is carried out using pspice simulation software, and the simulation results prove the correctness of the switching power supply circuit design.

2 Buck Transformer Topology and Parameter Design

The circuit topology of the basic buck converter is shown in Figure 1. It is composed of a voltage source vi, a series switch s, a freewheeling diode vd and a current load composed of lc. The size of l determines the output current ripple, while the output voltage ripple is determined by c. This is the most basic DC converter.

Figure 1 Basic buck converter

Reference [1] gives the circuit design formula of the buck converter. According to the output formula of the buck converter:

In the formula: ρ is the duty cycle, and: ρ=ton/t, then ρ=vo/vi.

The calculation formula of inductance l is:

Where: f is the switching frequency;

iomin is the minimum output current.

The calculation formula of capacitance c is:

Where: δvo is the output voltage ripple.

3 Derivation of the forward conversion formula

3.1 Topology and working mode

The main circuit topology of a single-tube forward converter is shown in Figure 2. Since the forward converter is based on the basic buck converter, it has an additional isolation transformer T1, a diode VD1, and a reset circuit consisting of a recovery winding N3 and a clamping diode VD3. Since the circuit form has changed, the parameter calculation formula of the above basic buck converter cannot be directly used during design. This paper analyzes the working mode of the forward converter and uses the equivalent transformation method to convert the forward converter into a basic buck conversion circuit. In this way, the parameter calculation formulas (2) and (3) of the basic buck conversion circuit can be extended to the parameter calculation of a type of forward converter, and a new design formula is established.

Figure 2 Main circuit structure of a single-tube forward converter

The working mode of the forward converter is:

(1) When v1 is turned on, diode vd1 is turned on, and the input grid transmits energy to the load through transformer coupling. At this time, the filter inductor l1 stores energy;

(2) When V1 is turned off, the diode VD1 is turned off, and the induced potential generated in the inductor L1 causes the freewheeling diode VD2 to turn on. The energy stored in the inductor L1 is released to the load through the diode VD2.

3.2 Equivalent transformation and parameter calculation

According to the analysis of the working mode of the forward converter, it can be found that the on-off of the diode vd1 is synchronized with the on-off of the switch tube v1. Therefore, the diode vd1 can be replaced by an equivalent switch tube v. If the conduction voltage drop of v1 can be ignored, the induced voltage of the secondary winding of the transformer is:

Where: k is the turns ratio of the transformer, and k=n1/n2.

If a voltage source of magnitude vi′ is used to replace the secondary winding of the transformer, the output side of the entire forward converter can be equivalently transformed into a basic buck converter. The equivalent circuit is shown in Figure 3, where the switch v replaces the switch tube v1 and the diode vd1 in the circuit of Figure 2. Therefore, the main circuit topology of the forward converter after equivalent transformation is consistent with the topology of the basic buck converter shown in Figure 1. In this way, the parameter calculation formulas (2) and (3) of the basic buck converter can be used to design the forward converter.

Figure 3 Equivalent buck converter

From Figure 3, formulas (2) and (3) can be generalized to obtain the equivalent forward converter parameter calculation:

(1) Calculation of duty cycle:

(2) Calculation of filter inductance:

(3) Calculation of filter capacitor:

Formulas (5), (6) and (7) are the parameter calculation formulas for the forward converter. From formula (5), it can be seen that the duty cycle is not only related to the input and output voltages, but also to the turns ratio of the transformer. Compared with formulas (2) and (3), the calculation of the filter capacitor and inductor also includes a transformer turns ratio parameter k.

3.3 Calculation formula verification

Now we use pspice simulation to verify the correctness of the formula. Design a forward converter, requiring its input voltage to be 48VDC, output voltage to be 12VDC, output current to be 5A, output voltage ripple component δvo to be 1V, and switching frequency f to be 50kHz. First, select ρ=0.4, that is, ton=8μs, and then calculate the transformer turns ratio of 1.6, l1=15μh, c1=24μf, and rl=vo/io=2.4ω from equations (5), (6) and (7).

Draw the electrical schematic diagram under pspice and perform transient time domain analysis on it. The simulation time is set to 1ms.

The simulated output voltage waveform is shown in Figure 4. It can be seen that the output voltage has stabilized at the required 12V after 0.2ms, and its output ripple also fully meets the requirements, thus proving that the formula deduced above is correct.

Figure 4 Main circuit of dual-switch forward converter

4 Switching Power Supply Design Example

Now use the above method to design a dual-tube forward switching power supply, requiring the input voltage to be 48VDC, the input variation range to be ±5%, the output voltage to be 12VDC, the output voltage ripple range to be 1V, the output current to be 5A, and the switching frequency to be 50kHz.

(1) Calculation of main circuit parameters

The dual-tube forward circuit is selected as the main circuit of the switching regulated power supply, as shown in Figure 5. Its working principle is the same as that of the single-tube forward converter, except that the two switches here are turned on and off at the same time, and because of vda and vdb, no additional reset circuit is required. The control circuit adopts a simple voltage control mode.

Figure 5 Main circuit of dual-switch forward converter

Here we can directly use the formula of the forward converter to calculate its parameters. From the design requirements, we know that t=1/f=20μs, r=vo/io=2.4ω, iomin=11.5v/2.4ω=4.79a. Since the maximum duty cycle of the dual-tube forward circuit can only be 0.5, we can choose a duty cycle of 0.45 when the input is 45.6v (minimum input voltage), and then calculate the transformer ratio of 7/4 from formula (5), calculate the inductance l=13μh from formula (6), and solve the capacitance c=25μf according to formula (7).

(2) Calculation of control circuit parameters

The switching power supply adopts duty cycle control, which can be divided into two categories: voltage mode control and current mode control. Voltage mode control has only one voltage control loop, while current mode control also has a current inner loop. Voltage mode control is adopted here, as shown in Figure 6, where operational amplifier U1 is a voltage controller and operational amplifier U2 is a comparator. The control principle is that the feedback voltage uf sampled from the output voltage is compared with the given voltage V3, and after the proportional integral link, the output voltage is compared with the sawtooth wave V1 to generate a PWM wave to drive the switch tube.

Figure 6 Control circuit of switching regulated power supply

The control circuit uses a PI voltage regulator. The parameters that need to be determined are C2, R3 and R2, as well as the output sampling resistor. The sampling voltage is selected as 1/6 of the output voltage, and the value of the sampling resistor is finally determined based on the debugging results. According to the output voltage required by different requirements, the variable resistor R6 is adjusted to obtain the corresponding given voltage. The cutoff frequency is taken as 1/20 of the switching frequency, that is, τ=0.0004s. Take R2=10kω, first take the amplification factor kp=10, then R3=100kω, C2=τ/R3=40pf.

5 Simulation test

The designed switching power supply is simulated by pspice. First, the electrical schematic is drawn under pspice. The simulation circuit is shown in Figure 7. Then the parameters are designed according to the above steps. Finally, simulation tests and circuit debugging are carried out. Since the main purpose of the simulation test is to determine and debug the parameters, the role of the protection circuit can be temporarily ignored for the sake of simplicity. The optimal values ​​of each parameter after debugging are shown in Figure 7.

Figure 7 Schematic diagram of switching regulated power supply circuit

The switching power supply control circuit shown in Figure 7 is simulated using pspice, and transient time domain analysis is performed, with the simulation time set to 2ms. The simulation output waveform is shown in Figure 8:

Figure 8 PWM waveform, output voltage and sampling voltage waveform when vi=45v

Figure 9 PWM waveform and output voltage when vi=48v

It can be seen from Figure 8 that both the output voltage and the feedback voltage meet the design requirements.

It can be seen from Figures 9 and 10 that although the input voltage has changed, the system can automatically adjust the duty cycle so that its output voltage is stabilized at the required 12V, and the output ripple and stabilization time both meet the design requirements.

Figure 10 Output voltage waveform when vi=51v

6 Conclusion

This paper generalizes the parameter calculation formula of the basic buck converter to the parameter design of a type of forward converter circuit by performing an equivalent transformation on the forward converter circuit, and uses the pspice simulation software to perform simulation tests on the forward converter. The simulation results show that the derived formula is correct. Then, the switching power supply is simulated and debugged using pspice, proving that the derived forward converter parameter calculation formula is applicable to all isolated buck converter circuits.