Multioutput Flyback DC-DC Converter for MIL Applications

This paperwork explains the design and development of isolated triple output dc-dc converter for military applications. Converter has designed with flyback topology with opto-coupler based feedback for regulated main output and regulators are used to provide another two outputs. It is realized with switching frequency of 190KHz (internal free run frequency) and can be able to operate up to 210KHz with external synchronization. LTM46xx micro-modules are used as buck regulators to provide required lower output voltages. Current mode pulse width modulation controller IC is used to drive the MOSFET switch. It has following features like inbuilt EMI filter, external inhibit function, external synchronization capability, input under voltage and over voltage protection, primary side over current, output over current and short circuit protection. Converter is designed to operate for wide input range from 18V to 36V with efficiency of more than 75% in full load conditions.


Introduction
Topology selection is the major activity while designing any dc-dc converter power supply. Converter topology can be selected based on power handling capability and efficiency. Optimized topology with less component count to meet smaller size is important. Hence flyback converter becomes the first choice. It has various advantages over other topologies as it does not require output inductor for operation [1].
Flyback converter main transformer acts as inductor which stores the energy during on period and transfers the stored energy to the secondary side during off period [1]. Converter is designed to develop one main regulated secondary voltage, and which is fed to buck regulators further to produce required another two output voltages. LTM46xx micro-modules are used as buck regulators to provide required lower output voltages, which can operate for input voltage of 2.2V to 5.5V and has efficiency of more than 95% [2].

Specification and operation of dc-dc converter
Below table 1 shows the specification of proposed dc-dc converter. Fig. 1 shows the operational block diagram of dcdc converter.
Opto-coupler based isolated feedback method [3] is used for the isolation between input and output stages. Sensed feedback voltage will be given to error amplifier of PWM IC [4] and output of EA will further be compared with ramp voltage which gives required duty to generate output voltage. Current mode control is implemented for accurate and faster loop stability. Primary inductor current through switch is sensed by small value resistor which generates ramp voltage.
Reverse polarity protection & Input Overvoltage Protection are mosfet based circuits which placed in input return path as shown in Fig.2.This circuit protects the converter against input over voltage and negative voltages. Converter has input under-voltage protection where converter goes in shutdown mode if input voltage goes below 16.5V and regain when input increases to 17.2V. Fig.  2a. Shows the operation of UVP protection and same is implemented and tested in hardware.
Short circuit and overload protection are implemented as shown in fig. 2b. Whenever short and overload condition occurs on output side (120% of output current), converter will go in hiccup protection mode. Transformer inductor current is sensed through small resistance value (Rsense), averaged value is then compared with constant reference. Rsense can be selected by considering the power dissipation across it.

Transformer design
Transformer is key part and plays very important role in any kind of dc-dc converter. Hence proper selection of core turns ratio, operating flux density, wire gauge of conductors become very crucial.
The power handling capacity of a transformer core can be determined by its area (WaAc) product method [6]. Area product (Ap) is given as the product of the core cross section (Ac) ae window area (Wa).
Where, After finalizing the number of turns, calculate the required wire gauge to carry the flowing current in respective windings. Use multiple wires to reduce the losses and heat.
Calculate the copper and core losses to know the temperature raise and should not reach beyond the maximum operating temperature.
Required ESR to meet ripple voltage can be calculated using equation (7) = ∆ _ (7) Multiple number of capacitors can be selected to meet required ESR.

Primary Mosfet selection
Main critical part of any dc-dc converter is selection of mosfet switch. It consists various losses like conduction loss, gate charge loss, switching losses and Coss losses.
Hence mosfet losses can be calculated by using equation Suitable mosfet can be chosen based on losses, drain to source voltage, and drain current. In higher losses, use the proper snubber to reduce the stresses on switch.

Output diode selection
The forward voltage drops (VFD) of diode directly impact on diode loss and it becomes critical in high current load applications.
Diode loss can be selected by using equation (9) Hence suitable diode can be chosen based on losses and output current and voltage stress ratings. Schottky diodes are preferred on output side.

Quality of components and analysis
Component grade for the military and defense application is critical as converter will expose to harsh environment. Hence MIL qualified components to be selected. Industrial components with extended temperature (-55°C to +155°C) can be selected in case of cost constraints.
All components should be well derated as per required MIL-STD to reduce the stress on components and increasing the life.
Following analysis like power dissipation, derating analysis, FMECA analysis, reliability analysis to be considered while designing rugged and reliable power supplies for military applications.
Power dissipation analysis is required to know the dissipation of each component and thermal raise.
Derating analysis is required to reduce the stress level of components and increasing the working life of components.
FMECA is failure mode analysis which is required to analyze the failure effect of each component on power supply performance and end system. Reliability analysis will tell the life of product. Higher the reliability longer the life. For this reliability is done as per MIL-STD-217F handbook [7].

Practical results and waveforms
Proposed converter is designed, implemented, and tested under all the required tests conditions. Converter meeting all the electrical specifications under temperature ranges from -55°C to +85°C. Converter will comply with MIL-STD461E [8], MIL-STD810D [9], MIL-STD1275 and MIL-STD704A&D [10] tests requirements.
Converter can deliver 35W power output with higher efficiency of more than 75% over an entire input voltage ranges from 18V to 36V at full-load condition.

Conclusion
It is always challenging for designer to design rugged and higher reliable dc-dc converter for military and defense applications.
Because such designs require less component count, proper thermal design to dissipate heat, good layout design to avoid noise interference.
These are achieved by selecting the optimize the topology and selection of circuits which requires minimum components. Using of multiple layers for thermal and mounting the higher dissipating parts on chassis for the thermal solution. Good layout is achieved by separating the EMI section and power section. In power section, again signal tracks are routed away from power tracks and sensitive signal tracks in feedback loop section are routed with guard rings.
The proposed converter is tested for wide input voltage range from 18V to 36V under all load conditions from -55°C to +85°C (base plate).