ECRI Microelectronics

Design of 20W Magnetic Isolated Feedback space DC DC converter

Design of 20W Magnetic Isolated Feedback space DC DC converter

Abstract: This paper describes the design proposal of a 20 W flyback magnetic isolated feedback space DC DC converter, the design procedure of power stage and the isolated feedback circuit are presented, the simulation and experiment results are also given.
Key Words: magnetic isolated feedback, single -ended flyback,space DC DC converter

1 Overview
In the common isolated space DC DC converter, the power level isolation is usually realized by transformer, and the feedback isolation part is usually realized by optocoupler. Optocoupler isolation feedback has the advantages of few devices, simple circuit and strong anti-interference ability, but its current transfer ratio (CTR) will change significantly under irradiation, which will directly affect the performance of DCDC converter. In order to solve this problem, magnetic isolation feedback which is not sensitive to irradiation is usually used instead of optocoupler isolation in the field of space anti-irradiation and long-term high reliability applications.
The simplest structure of the magnetic isolation feedback is connected to the primary PWM feedback terminal directly by the transformer or the output inductor winding direct sampling, using the least devices, but the adjustment rate and dynamic performance of DCDC converter is poor.In space DC DC converter with strict requirements, in general, the error signal is amplitude modulated in the secondary using IC or discrete devices, using a small transformer to transmit the signal to the primary, and then through the demodulation circuit feedback to PWM.This kind of amplitude modulation can provide good electrical performance index, but it is easy to produce low-frequency noise when the carrier frequency is not synchronized with the switching frequency.
The main specifications of the space DC DC converter designed in this paper refer to the DVHF2815D space DC DC converter of VPT Company, with rated output power of 20W and output voltage of ± 15V. In order to meet the requirements of electrical performance and anti-radiation specifications, a magnetic isolation feedback scheme with synchronous carrier frequency and switching frequency is adopted.

2 Power Level Circuit Design
2.1 Main Topology Selection
In isolated space DC DC converters, the common topologies are forward, flyback, push-pull and half-bridge.Push-pull and half-bridge structure uses two MOSFETs, which have high core utilization, but many devices and complex circuits, and have the common risk of two MOSFETs under irradiation, forming a low resistance path from bus to input ground terminal, resulting in serious consequences of DCDC converter burning out, so this kind of structure is generally not used in anti-irradiation space DC DC converter.Both single-ended forward and single-ended flyback structures use a MOSFET, which is suitable for anti-radiation design, the difference is that the transformer works in different ways, the forward structure generally has one more output inductor and one freewheeling diode than the flyback structure.
2.2 Main transformer design
The flyback transformer has many functions such as energy storage, electrical isolation, current limiting inductance, and often supports DC current component, so it is necessary to select the magnetic core with air gap.
To avoid core saturation, the number of turns in the primary winding needs to satisfy the following formula:

(1) N is the number of turns of the primary winding;
V is the primary DC voltage, unit V;
T is the turn-on time of the switch tube, unit? S;
B is the peak-to-peak value of the change of AC magnetic flux density, unit T;
A is the effective area of the central column of the magnetic core, in units.
In this design, the minimum input voltage V = 18V, the maximum switching cycle duty cycle D = 0.45, the switching frequency FS = 300kHz, the maximum on-time T = D/FS = 1.5μs, the maximum change in flux density B = 0.14T, and the effective area of the core column A = 16mm2.

(2) Np is the number of turns of the primary winding;
Ns is the number of turns of the secondary winding;
Vo is the output voltage in units of V.
VD is the conduction voltage drop of the rectifier diode, unit V;
Vin (min). Lowest input DC voltage in V
D is the duty cycle.
In this design, the output voltage Vo = 15V, the turn-on voltage drop V = 1V, the minimum input DC voltage Vin (min) = 18V, and the maximum switching cycle duty cycle D = 0.45.
Design of 2.3 Output Rectifier and Filter Circuit
Figure 1 is the output rectifier filter circuit, for flyback space DC DC converter, the output adopts half-wave rectifier mode, the minimum reverse voltage V of rectifier tube needs to meet the following formula:

(3) where Ns is the number of turns of the secondary winding
Np is the number of primary winding turns;
Vin (Max) is the highest input DC voltage, unit V;
Vo is the output voltage in units of V.
In this design scheme, the output voltage Vo = 15V, the winding ratio of transformer secondary and primary windings is 13:12, the maximum input DC voltage is 50V, from which V > 69.2V is obtained. Considering the requirements of peak voltage, derating design and output current, 10CmQ150 Schottky diode of IR company is selected as rectifier.
In order to take into account the output ripple voltage index and the size requirements of space DC DC converter, the output using open filter circuit, including large capacitance C1, C2 for 22F, secondary filter inductance L1 for 1uH, capacitance C1, C2 for 1pF, thus effectively suppress output ripple voltage.


Fig. 1 Schematic diagram of output rectifier and filter circuit

Design of isolated feedback circuit
3.1 Sampling Comparison Amplifier Circuit Design
In the design of the secondary sampling comparison amplifier circuit, the basic working principle is to get the error signal by comparing the divider resistance with the reference, to set a suitable compensation network to meet the requirements of space DC DC converter stability, adjustment rate and dynamic index, and then to form a modulation signal after a certain proportion of amplification and transfer it to the primary feedback closed-loop.
Fig. 2 is a schematic diagram of a sub-sampling comparison amplifier circuit, in which a double operational amplifier LM158 is used as an error amplifier and an inverse amplifier, an error signal is obtained by comparing an output sampling signal with a reference voltage, and then the signal voltage is amplified in a certain proportion.A 5V reference source is connected to the in-phase input end of the error amplifier and the inverting amplifier, the full-range sampling resistors R2 and R3 are connected between the output Vo + and Vo-, whose ratio is 11, the two resistor voltage dividers are connected to the inverting input end of the error amplifier, the NPN triode Ⅵ 1 is operated in a switching state, the base is connected to a synchronous square wave signal to form a carrier wave, the error signal is converted into a pulse signal after amplitude modulation, and is transmitted to the primary through an isolation transformer T2.Then the demodulation circuit is converted into DC signal and connected to the PWM feedback terminal, so that the loop is controlled.

Fig. 2 Schematic diagram of sampling comparison amplifier circuit
3.2 Oscillating Circuit Design
In order to obtain the carrier signal synchronized with the switching frequency, a simple idea is to take the primary switching tube operation waveform through the transformer winding, or directly take the secondary rectifier tube operation waveform, after clipping and shaping, to get the appropriate square wave signal.For the space DC DC converter in this design scheme, it is difficult to take the rectifier tube operation waveform for dual output, so the transformer winding is used as the carrier signal.

Fig. 3 Oscillating circuit schematic diagram
Fig. 3 is a schematic diagram of an oscillation circuit, wherein winding p4-m4 takes an autonomous transformer and provides a synchronous oscillation signal with an output negative terminal Vo- as a reference potential. As the duty ratio of the signal changes with the change of a load and an input voltage, the pulse width of the flyback connection method does not change obviously compared with that of the forward connection method, and the output of the space DC DC converter is not easily out of control when the forward connection method is not in use or when the input is at a high end.
3.3 Demodulation Circuit Design
Fig. 4 is a schematic diagram of the demodulation circuit, wherein the VREF is the reference terminal of the primary PWM and provides a DC bias voltage for the transformer T2; the secondary error signal Vea is modulated and transmitted to the primary through the isolation transformer T2; the diode 1, the resistor R1, and the capacitor C1 constitute a detector to convert the AC signal into a DC error signal; and then the resistors R2 and R3 are connected to the feedback terminal VFB of the PWM by voltage division to form a closed loop of the system.

Fig. 4 Schematic diagram of demodulation circuit
4 Simulation and test results
For each part of the above design scheme, the electrical performance is verified by Saber software simulation and simulation test.
4.1 Simulated Circuit Waveform
Using Saber software to simulate in time domain and setting the simulation time to 15ms, the waveform of DCDC converter from startup to output stabilization can be obtained accurately, as shown in Figs. 5 ~ 10.

Figure 5 MOSFET Drain Simulation Waveform

Fig. 6 Simulation waveform of rectifier anode

Fig. 7 Simulation waveform of magnetic isolation drive

Figure 8 Startup Overshoot/Delay Simulation Waveform

Figure 9 Load Step Simulation Waveform (100% -50%)

Figure 10 Load Step Simulation Waveform (50% -100%)
4.2 Simulated test results
Based on the simulation circuit, a simulation test board is set up, the technical state of the circuit is determined by debugging and parameter optimization, and the waveforms of various points in the simulation test of space DC DC converter are measured by using a digital storage oscilloscope, as shown in Fig. 11-16. By comparison, the test waveforms are consistent with the simulation waveforms, and the rationality of the circuit operation is verified.

Figure 11 MOSFET Drain Test Waveform

Fig. 12 Anode test waveform of rectifier tube

Fig. 13 Magnetic isolation drive test waveform

Figure 14 Initiation Overshoot/Delay Test Waveform

Figure 15 Load Step Test Waveform (100% -50%)

Figure 16 Load Step Test Waveform (50% -100%)
After testing, the main technical indicators of the simulation test board meet the requirements of the agreement, as shown in Table 1.

5 Concluding remarks
This paper introduces the design and implementation of a 20W flyback magnetic isolation feedback circuit, and verifies the principle and performance of the circuit by simulation software and test board.

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