Design of a 15W Triple Output DC/DC Module Power Supply

Summary:By introducing the detailed design process of a UC3843 control power supply for low-power multi-output DC/DC modules, this paper focuses on discussing the differences between the power design for multi-output modules and that for single-output modules, and details the design of the peripheral circuit parameters of a new chip UC3843 commonly used in DC/DC module power supply. Detailed process of engineering design of transformer and coupling inductance in power supply of multiple output modules and various problems that should be noticed to satisfy each performance index are given.

Key word:DC/DC converter; Multiplex output; Coupling inductance

Introduction

DC/DC module power supply has been widely used in microwave communication, aeroelectronics, ground radar, fire control equipment, medical devices and many other fields. There are many applications that require multiple outputs. For example, in the single-chip smart controller, the single-chip computer power supply needs 5V, and the operational amplifier integrated circuit usually needs 12V. In the design of multi-output power supply, there are many differences from single-output power supply, so there are many problems to consider and difficulties to solve. For example, consider not only the restriction of transformer pins, the design of multiple side transformers, the implementation of voltage regulator circuits in each circuit, but also the load adjustment rate and the cross-regulation characteristics of loads in each light and full load. In this paper, the design features of multi-output power supply are explained in detail through a design example of 15W three-way module power supply to single-chip smart controller.

Figure 1

1 Design Indicators for Power Supply

The design specifications for 12V input, 5V/+12V three-way output module power supply are listed in Table 1.

Table 1 Design Indicators

2 Design principle of power supply

Fig. 1 is the schematic diagram of a multi-output switch power supply designed for power supply of single-chip computer motherboard.

In Fig. 1, the inductances L201, L202, L203 are coupling inductances, and L204 are offset windings, which are taken from the coupling inductance due to the restriction of the transformer pin.

The circuit uses a single-end forward converter circuit. When the converter is connected to the power supply, the input DC voltage is started after the voltage regulator and voltage reduction circuit composed of resistance R601 and 12V regulator D601 and triode V601 and V602. After entering normal operation, the power supply circuit of the bias winding L204 starts to work. The output of the bias winding is 12V voltage after diode D4 rectifier and C601 filter, which is higher than the self-supply voltage, making the diode D602? Biased, the startup circuit stops working. The offset winding provides the working voltage (12V) for UC3843 (IC301), and the converter goes into normal operation. Under PWM pulse width modulation, the output of each secondary winding passes through the diode rectifier and LC filter of each circuit, and then generates the DC output voltage of each circuit. + The 5V output voltage is compared with the 2.5V reference voltage in the programmable regulator TL431 (IC401) after the partial voltage of resistors R402 and R406, and then fed back to foot 2 of UC3843 through the optical coupler (IC101) to control the duty cycle of the pulse and stabilize the 5V output. Coupled inductance L202 and L203 achieve + 12V two-way voltage regulator. The overcurrent protection resistors R101 and R102 detect the overcurrent signal of the switch tube and feed it into foot 3 of UC3843 to block the output signal of UC3843 for overcurrent protection.

Figure 2

3 Design option

DC/DC module power mainly consists of small and medium power, mostly under 150 W. The circuit topology used is mainly flyback and forward converter, and sometimes push-pull converter. The power requirement is small, and all chip elements are used in the design.

3.1 Master Chip Selection

The main control chip uses a new type of pulse width modulation integrated circuit UC3843, which is a special chip for current mode control. Figure 2 is the schematic diagram of UC3843. It has undervoltage lock circuit, low static current (1mA), high current output, built-in gap reference voltage, 500kHz operating frequency, low R0 amplifier, and voltage adjustment rate of 0.01V, very close to the linear regulator power supply adjustment rate, low starting current of only 1mA, starting circuit is very simple and so on.

3.2 Voltage regulator mode selection

For single output, only a regulator feedback circuit can be added to the output, while for multiple output, it must depend on the requirements: if each output voltage is required to be high, then each circuit should design a separate closed-loop regulator circuit, which makes the design more difficult; If only one is an important load, the other is lighter, and the output voltage is not strictly required, only the circuit where the important load is located must be given a feedback control loop, and the other two open-loop circuits rely on the coupling inductor to achieve a constant voltage.

3.3 Selection of Inductance Winding Mode for Multiplex Output Filtering

Of the three outputs in this example, 5V (Uo1) is an important load, the output current (2A), 12V is the power supply of the operational amplifier, the voltage is allowed to change in the range of 1 to 2V and the current is small (0.25A). Therefore, only 5V main circuit plus feedback control circuit, and the voltage stabilization performance of + (+) 12V auxiliary circuit is achieved by coupling inductor. For the specific situation of this example, the output filter inductance should not be independent inductance, but coupling inductance. That is, the output filter inductance of three channels should be winded on a magnetic core. Only 5V main circuit is controlled, and the output characteristics are good, while (+) 12V two channels have little influence.

4 Power supply design process

4.1 UC3843 Peripheral Circuit Design

4.1.1 Switch Frequency Selection

The operating frequency of secondary power supply products is generally between 100 kHz and 400 kHz. In this case, the switching frequency is set to 250kHz, the operating frequency of UC3843 can reach 500 kHz, and foot 4 is the common end of the timing resistance and capacitance of Rt/Ct serrated oscillator. For UC3843,

In formula: R is R 304 in figure 1, with a value of 6.8 k;;;

C is C302 in Figure 1 with a value of 1nF.

4.1.2 Overcurrent Protection Circuit Design

In Fig. 1, R101 and R102 are overcurrent detection resistors. R101 and R102 are designed according to ISMAX_1.0V/RS. This resistance should be set very small to reduce the loss on the resistance. In Fig. 1, two 10_resistors are designed in parallel. The detection voltage is fed to foot 3 of UC3843.

When the voltage of foot 3 is higher than 1V, the overcurrent protection circuit operates, causing foot 6 to stop outputting rectangular waves and the circuit to stop working. Foot 3 is also connected with an RC filter to suppress the spike current of the switch, which is composed of R103 and C306 in Fig. 1.

Design of 4.1.3 Feedback Error Amplifier

R302, R303 and C305 are integral regulators. The proportional relationship between resistance R302 and R303 affects the dynamic characteristics of the system. The ratio of R302 to R303 can change the magnification of UC3843 Voltage Error Amplifier. For a certain feedback voltage, the output pulse width of the PWM regulator can be different, thus affecting the output voltage adjustment amplitude, that is, the dynamic response adjustment amplitude in the index. The size of the integrator's capacitance C305 affects the adjusting speed of the system, that is, the dynamic response time of the output in the index.

Selection of 4.2 Power Device

The power MOSFET is used in the switching devices of the converter. Based on the empirical formula for calculating the voltage of the single-tube converter, the power MOSFET is obtained.

Formula: Udmax is the voltage of the drain source;

D is the duty cycle.

Therefore, the reverse voltage of the power MOSFET should be greater than 144V, and the current is determined by the current of the high frequency transformer winding. In Fig. 1, the IRF630 with voltage 200V and current 9A is selected for V101.

Design of 4.3 High Frequency Transformer

Selection of 4.3.1 Magnetic Core

Multiplex output transformers generally require a larger window area. DC/DC module power supply can use FEY, FEE, EUI type magnetic cores. For forward converters, the primary transformer must theoretically have reset winding Nr. Considering the problem of transformer foot, select a magnetic material with high saturated magnetic induction strength, remove the reset winding, so that each core works at the lower part of the magnetization curve, avoiding the magnetic core saturation.

First determine the magnetic induction strength Bm to calculate and select the core type.

1) Considering high temperature, the saturated magnetic induction intensity B will decrease, and in order to reduce core loss in high frequency operation, the working magnetic induction strength is generally selected from 0.2 to 0.25T. The magnetic material RM2.2KD with high saturated magnetic induction intensity is selected, and its Bs is 0.44T.

2) There are two ways to select the type of magnetic core, one is based on the formula (3)

Formula: Ae is the cross-section area of the magnetic core;

Aw is the window area of the core.

FS is the switching frequency;

Δ B is the range of magnetic flux density allowed by the magnetic material.

DC is the current density of the transformer winding conductor;

KC is the filling factor of the winding in the magnetic window.

Second, according to the manufacturer's Manual of magnetic core materials, the relationship between output power and size of magnetic core is given. The second method is to use FEY15.3 core, which has an effective cross-sectional area of 18.7 mm2.

4.3.2 Calculate the turn ratio

Uo=Uo1+UD=5.0+0.5=5.5V(4)

In: Uo1 is 5V main output voltage;

UD is rectifier MBR1545 positive pressure drop, take 0.5V.

Formula: N12 is the ratio of primary side to secondary side;

Ui=UminDmax=36 × 0.48 = 17.28V (where Umin is the power input voltage and Dmax is 0.48);

Uo is N2 output voltage.

The actual selection is n12=4:1.

4.3.3 Calculate and adjust the number of main and secondary side ramps

Δ Bm is the flux increment. Δ Bm=0.44-0.065=0.375T;

Ae is the cross-section area of the core. For FEY15.3 core, Ae=0.187cm2.

Actual N2 = 4 turns.

4.3.4 Calculate the number of original edge turns

N1=N2 × N12=4 × 4=16 turns (7)

4.3.5 Calculate the remaining two minor side ramps

In formula: Uo2 is + 12V auxiliary output voltage;

UD'is the positive pressure drop of rectifier SK3B, also 0.5V.

Actual N3=N4=10 turns.

Due to the large current of the original side and the secondary side of the main road, two-wire winding and three-wire winding are used to reduce the leakage.

4.4 Output Rectifier Design

To reduce power consumption and improve power efficiency, a Schottky rectifier diode is selected. The nominal current (IF) value of the output rectifier should be more than three times the Io value, i.e. IF1> 3Io; The reverse voltage withstanding UR of the rectifier is greater than or equal to 1.25 PIVs (PIVs=Uo+UMAX, UMAX=2UACMAX, UACMAX is the output ripple voltage amplitude). According to this principle, the Uo1 rectifier uses MBR1545, reverse voltage withstanding 45V and forward current 15A. Uo2 and Uo3 use SK3B, reverse voltage withstanding 100V, forward current 3A. The high reverse voltage selection here helps to reduce the loss on the rectifier tube.

4.5 Output Coupled Inductance Design

In order to keep the output voltage of the auxiliary circuit within the stable range of voltage 1V, the inductance of the main output and each inductance need to work in the continuous state of inductance current when multiple outputs are used. The conversion from high-voltage branch to low-voltage branch is carried out at design time, and the core, cross-section area of total traverse, traverse size and number of turns are selected by single coil according to the total output current. That is, the output filter inductance value is determined by circuit design first. In order to make the inductance current continuous to maintain the filter effect, the output filter inductance must be designed in a continuous state. The current flowing through the inductor should be greater than the minimum IOMIN of the load current, and the inductance value should be greater than the corresponding inductance value when the inductance value is greater than IOMIN.

Formula: n is the transformer turn ratio;

Uimax is the input power voltage;

Δ I is the allowable ripple current value of inductance current.

After determining the filter inductance value, the energy storage value based on the inductance is 0.5 × L × I2, select the magnetic model according to formula (10),

In formula: IMAX is the effective value of inductive current;

ISP is the peak of inductive current.

BMAX is the magnetic circuit flux density value.

Then the number of inductance turns is determined according to formula (11).

The coil turns and sizes are obtained from the actual current and secondary turn ratio of each branch.

Current continuous mode inductance cores can be selected as a material which is slightly worse than transformer cores, but in practical applications, if the prices of the two materials are not very different, the same material as transformer is often used. The inductance core is still FEY15.3 in Fig.1.

Selecting the number of turns of inductance must first satisfy that the number of turns ratio of inductance is equal to the number of turns ratio of the output winding of the main transformer, because if the number of turns ratio of coupling inductance L201 and L202 cannot be guaranteed to be equal to that of the transformer, there will be additional current flow between Uo1 and Uo2, resulting in a large ripple in their output. Secondly, the inductance turns of each circuit are multiplied by 2 or 3 for each corresponding transformer turn. Fig. 1 Selects three times the number of transformer turns, which can be double-wired and wrap around to fill the window width. The number of Uo1 output inductance turns is NL201=3N2=3. × 4 = 12 turns.

Uo2 output inductance turns are NL02=3N3=3 × 10=30 turns, the number of Uo3 output inductance turns is NL203=NL02=30 turns.

The offset winding NL204 provides 12V operating voltage for UC3843, and its output voltage is Uo2, so the number of turns NL04=NL202=30 turns.

Mutual inductance must be well coupled in order to satisfy the load adjustment rate. Therefore, when winding each winding, the width of the whole skeleton should be covered, and several wires of the same diameter should be winded side by side to ensure coupling across the width of the whole skeleton.

5 Other considerations

1) When debugging multi-output power supply, the secondary circuit should be disconnected first, and the main circuit should be well adjusted to ensure that the main circuit works normally, then the secondary circuit should be adjusted to reduce the debugging difficulty.

Figures 5 and 6

2) In order to meet the design criteria, in addition to the load adjustment rate at full load, the load adjustment rate at light load should also be considered. In order to prevent the output rectifier from being damaged by too high output voltage during no load, a false load must be added to each output. In Fig. 1, R5, R6, R7 are all false loads of the corresponding circuit. The false load value should not be too large, and the size can be determined by experiments. In addition, the false load between the main circuit and the secondary circuit should be adjusted to meet the voltage range of the secondary circuit within the index.

3) The no-load output voltage of the main circuit can be determined by the piezoresistor of TL431. When the no-load output voltage of the main circuit is low, R406 can be reduced to ensure the 2.5V reference of TL431. Table 2 is a set of data for measuring the no-load output voltage of each circuit when the resistance value of R406 changes. When the main no-load output voltage and false load are determined, if the secondary no-load output voltage exceeds the index range, the parameters of the rectifier can be changed appropriately. For example, when the secondary 12V output is 12.7V beyond the index 12.5V, the rectifier with a positive pressure drop of 0.7V can be used instead of a rectifier with a positive pressure drop of 0.5V. Output false load can also adjust the no-load output voltage.

Table 2 Changes in output voltage of each circuit when the resistance value of R406 changes

4) When laying out the wiring, each component should be placed in sequence according to the schematic diagram, the connection between the leak of the switch tube and the original side of the transformer should be as short as possible, and all peripheral components of UC3843 should be as close to the integrated circuit as possible, especially decoupling capacitance and bypass capacitance, which must be near the corresponding pins. The bottom layer is connected with the heat sink pad through multiple holes near heating elements such as switch tube to improve the heat sink effect.

6. Measured data and waveforms

Figure 3 shows the serrated output of UC3843 foot 4, which can be used to determine if UC3843 is working properly.

Figure 4 shows the output rectangular wave of UC3843 foot 6 when it is idle.

Figure 5 shows the leak-source waveform of the switch tube when the main circuit is loaded with 25 and the secondary circuit is unloaded. It can be seen that at this time D is about 0.35 and the pulse width has started to adjust.

Table 3 Voltage fluctuation of output when load changes

Fig. 6 shows the leak-source waveform of the switch tube when the main and auxiliary circuits are fully loaded. It can be seen that D is about 0.5 and the pulse width is adjusted to.

Table 3 shows the measured output voltage fluctuations of the three output power sources when the loads of the main and auxiliary circuits change.

7 Conclusion

The design of multiple output is more complex than that of single output. It is necessary to design the parameters of UC3843 peripheral components, multi-sided high frequency transformers, coupling inductances correctly according to the design criteria, and adjust the feedback loop in order to ensure the indexes in an all-round way. The power supply designed in this paper has been used in the data collection and control circuit of single-chip computer.