Abstract
This paper presents a UC3843 control multiple small power DC / DC power module detailed design process, focusing on the design of multiple-output power modules and single output of the different details of the DC / DC power modules in the new common UC3843 chip design of the external circuit parameters are given multiple-output module power transformers and coupled inductors in the design process and meet the performance indicators should be noted that a variety of issues.
Keywords: DC / DC converters; multi-output; UC3843; coupled inductors
Introduction 
DC / DC module has been widely used in railway communications, microwave communications, industrial control, marine electronics, avionics, ground radar, fire-fighting equipment and medical equipment, teaching equipment and many other fields, many of which require multi-output applications, such as SCM intelligent controller, microcontroller power supply needs of 5V, while the op amp usually 12V. Multiple outputs in the design, there are many different places and single output, it is necessary to consider the transformer pin constraints, multi-secondary transformer design, and the brightest of the voltage regulator circuit, but also consider the light load and full load each channel to adjust the load rates, and cross-load regulation characteristics. This article by an intelligent controller to the microcontroller power supply power supply module 15W three-way design examples to elaborate multi-output power module design.

Figure 1 multi-output switching power supply schematic
The working principle of power modules
  This power supply for the microcontroller board designed for multi-output switching power supply shown in Figure 1, where the inductor L201, L202, L203 is coupled inductor, L204 is the bias winding, the transformer pin constraints, from the coupled inductor.
  Power supply works as follows: circuit using single-ended forward converter, when the power converter, the input DC voltage through the resistor, 12V regulator D601 and transistor Q601, Q602 Buck regulator circuit consisting of, start UC3843 into the normal work, the bias winding L204's power supply circuit to work, output by the A4 and the C601 12V rectifier and filtered output voltage, higher than the self-supply voltage, the diode reverse biased A4, start circuit to stop working. Converter into normal operation, the power based on the work of PWM pulse width modulation, the output of the secondary winding through the diode rectifier and capacitors and inductors of the LC type filter composed of filtered to produce the required output DC voltage from various quarters. +5 V output voltage from the resistor divider R402 and R406, the source and programmable voltage regulator TL431 (U3) of the 2.5V reference voltage comparator, and then back to the UC3843 through optocoupler U2 to 2 feet, total control pulse air ratio, stable 5V output. Coupled inductors L202 and L203 to achieve + /-12V two regulators. Over-current protection resistor R101, R102 detected over-current signal switch, into the 3843's 3 feet, blocking UC3843 output signal, over-current protection.
SCHEME
DC / DC power modules for small and medium-power-based, mostly in the 150W power under the circuit topology used in flyback and forward to the main circuit, and sometimes also used push-pull circuit, the power requirements of small size, the design of chip components used in all . The module's main control chip UC3843.
For multiple outputs, each output voltage if the high precision, each road should be closed-loop design their own independent regulator circuit. If only it is important to load all the way, other way the load lighter, and the output voltage accuracy is not very strict, you can simply add to the important feedback loop in which the load control loop. The module's three-way output, 5V output is more important because the load, the maximum output current (2A), 12V is the op amp power supply, allowing the voltage changes in the 1 ~ 2V range, current is small (0.25A), so where only the main road in the 5V voltage regulator circuit increases, + /-12V regulator with the way the performance achieved by the coupled inductors.
Single output, the filter inductance inductance using an independent, but the multi-output, the output filter inductance can not be used independent of the inductor, while the use of coupled inductors, the three-way around the output filter inductor in a core on the common. This is because the main road controlled only 5V, 5V output voltage output characteristic makes good, and + /-12V two poor, if the independent inductors, will have the following questions: First, cross-regulation problem, that is, or attached to the main road Road load changes should be attached to the road from the output voltage fluctuations with the way the problem; followed each output has its own filter, resonance is different from the way only a controlled converter, the resonant frequency of the high-impedance, causing closed-loop decrease loop gain and phase shift of current-mode control loop were particularly serious. A coupled inductor, which reduces manufacturing costs, reduce power volume, to meet the volume requirements.
UC3843 peripheral circuit design
Switching Frequency Selection
Switching frequency of the power settings 250kHz, UC3843 operating frequency up to 500kHz, 4 feet is the RT / CT sawtooth oscillator timing resistor and capacitor in common, for the UC3843, kHz.
Start Circuit
DC voltage by the resistance, 12V regulator D601 and transistor Q601, Q602, after the composition of the buck regulator circuit, the UC3843 offers start voltage. Start voltage higher than 8.4V to work when the UC3843, if the starting voltage is low, you can adjust the R601 and R602 through the resistance to achieve the start voltage 9.6V.
Over-current protection circuit design
R101, R102 for the over-current sense resistor, the resistance small, to reduce the resistance of the loss. UC3843 detection voltage into the 3 feet higher than 1V, the overcurrent protection circuit, so 6 feet to stop the output square wave, the circuit to stop working. In addition can also use the 1 foot UC3843 voltage diode drop below the two properties when the UC3843 is close to the power supply with remote side.
Feedback error amplifier design
R302 and R303 constitute integral regulator, a proportional relationship between the dynamic characteristics of the system. Changes in their ratio can change the UC3843 error amplifier voltage magnification, for a certain amount of feedback voltage, can adjust the output PWM pulse width different, thus affecting the magnitude of the output voltage regulation, the impact indicators of the dynamic response of the output amplitude of accommodation. Integrator capacitor C305 of the size of the system to adjust the speed of impact indicators that the output of the dynamic response time.
Design of High Frequency Transformer
High-frequency transformer selection criteria for high magnetic saturation induction, low residual magnetism. Multiple-output transformers generally require a large window area, DC / DC power modules can be used FEY-based, FEE-type, EUI-type, etc., for the forward circuit, in theory, the need for reset transformer primary winding Nr, taking into account the transformer foot here bit of the problem, select the high saturation magnetic flux density, and remove the reset winding, so that each magnetic magnetization curve in the lower part of the work, to avoid core saturation.
Consider the high temperature saturation magnetic induction Bs will decline, while working to reduce high-frequency core loss, the maximum magnetic induction in the general work of choice for the 2000 ~ 2500Gs. Here select the high saturation magnetic flux density magnetic RM2.2KD, saturation magnetic induction intensity 440mT.
Core material according to the manufacturer's manual gives the output given the relationship between power and core size. This selected FEY15.3 core, the effective cross-sectional area of 182mm2.
The calculation, winding turns are as follows: 16 primary turns N1, 5V transformer secondary main road N2 for 4 turns, +12 V transformer with secondary side road 10 turns N3,-12V secondary transformer with Road N4 is 10 turns.
When the primary winding, secondary side current of the main road a larger order to reduce the leakage inductance, respectively, and around the law by two-and three-wire and around the law.
Output Coupled Inductor Design
Controlled by the way in the other two rely on the control mode coupled inductor regulator, in order to maintain the auxiliary output voltage regulation stability in the 1V range, multi-output, the main output inductor and the inductance requirements of each road work in the inductor continuous state.
Coupled inductor design is different from the single output inductor, and its design method is to slip into the low-voltage high-pressure branch conversion, according to the total output current coil selected by a single core, get the total wire cross-sectional area and the size of the coil wire , turns. Last redistribution to each branch, according to various quarters and secondary turns ratio of the actual current by the coil turns and size.
Continuous current mode of the module and the transformer using the same inductor core material, still FEY15.3.
Choice of inductor turns first to meet the inductor turns ratio is equal to the output of the main transformer winding turns ratio transformer in each group with the corresponding number of turns multiplied by 2 or 3 times to design the inductor turns. Here's 3 times the transformer turns election, just to fill the lane and around the window width. V01 output inductor is 12 turns NL201 turn, V02 output inductor turns NL02 30 turns, V03 NL203 output inductor turns 30 turns. NL204 bias winding to provide 12V operating voltage for the UC3843, because its output voltage is equal to V02, so the number of turns for 30 turns.
In order to meet the load regulation, mutual inductance must be well coupled. Therefore, the winding of each group should use the same diameter of a few wires wound side by side and cover the width of the entire skeleton.
Other Considerations
(1) In the case of the output need to be isolated, should provide a level for driving the displacement current leakage from a specific power output back into the primary circuit of low resistance. This is usually a low impedance circuit connected to the output ground to the main ground on the secondary capacitor (insulated from the needs of a particular output voltage to determine the appropriate value), the output capacitor ground terminal Y through a secure connection with the primary ground, such as Figure 1 capacitor C001 connected to the isolation of communication with the main 3.4V output ground between the power output. If not for the isolated output channels to provide this low impedance capacitor, the transformer winding capacitance displacement current common mode transmission will be through any alternative routes to return to their primary source in the transformer, wire transfer through the displacement of the AC input current may be sufficient to result in the transfer exceeds the adjusted target. This type of circuit the need for an additional transformer winding capacitance between the capacitor according to the decision, if the isolation between the output capacitor and the primary is small enough, you can not have this additional capacitor. However, this must be verified by testing the need for additional components. In addition, without this component, the switch DS waveform to oscillate.
(2) In addition to full time to pay attention to adjust the output rate, but also take into account the light-load or no-load output regulation. In order to prevent no-load output voltage is too high and damage to the output rectifier, must be added to each output are dead load, dead load value of the experimental methods can be used to determine the size of the module to the Lord Luke 430W, with Luke 1.5KW . Dead load plus too big, it will reduce efficiency. Also attached to the main road and the road to tie between the dead load adjusted to meet the voltage range with the road.
(3) TL431 output load voltage divider resistors can be determined, when the no-load output voltage is low, can reduce the R406, to ensure that the 2.5V reference TL431, by adjusting the voltage divider R402, R406 adjustable ratio of output load voltage . Can not meet the load when the load-adjusted rate may be appropriate to change the rectifier parameters.
(4) layout, the various components of the order followed in accordance with the schematic display, the switch transformer primary drain connection as short as possible, UC3843 all the external components as close as possible to its own, especially to decoupling capacitor and bypass capacitor must be distributed in the vicinity of the corresponding pin to be in the transformer, switch and other heating devices through multiple vias near the ground is connected with the thermal pad to improve heat dissipation.