Switching Power Supply Control Chip M51995 and Its Application
Abstract: m51995a is a primary PWM control chip of off-line switching power supply specially designed for AC / DC conversion launched by Mitsubishi company. This paper describes its working principle in detail and gives typical applications.Abstract: M51995A is a offï¼ line SMPS's primary PWM control chip of specific design for ACï¼ DCConversion by MITSUBISHI ï¼ This paper described in detail its work principle and showed typical applicaTIons.Key words: oscillation PWM comparison latch current limit intermittentKeywords:OscillaTIonï¼ PWM comparison lockï¼ Current limitï¼ DisconTInuity1 IntroductionM51995a is a primary PWM control chip of off-line switching power supply specially designed for AC / DC conversion. The chip has built-in high-capacity Tuteng column circuit, which can directly drive MOSFET. M51995a not only has the ability of high-frequency oscillation and fast output, but also has the function of fast response current limit. It is also characterized by intermittent operation during overcurrent. The main features of the chip are as follows:? 500KHz working frequency;? The output current reaches Â± 2a and the output rise time is 60 Î¼ s. Descent time 40 Î¼ sï¼? The starting current is small, and the typical value is 90 Î¼ Aï¼? The voltage difference between starting voltage and closing voltage is large: the starting voltage is 16V and the closing voltage is 10V;? Improved totem pole output method, small penetrating current;? The overcurrent protection works in intermittent mode;? The pulse by pulse method is used to limit the current quickly;? Undervoltage and overvoltage latch circuit.2 pin arrangement and descriptionSee Figure 1 for pin arrangement.Each pin is defined as follows:Collector: totem pole output collectorVout: totem pole outputEmitter: totem pole output emitterVF: VF control terminalOn / off: working enable endOVP: overvoltage protection endDet: detection endF / B: voltage feedback terminalT ï¼ on: timing resistance on terminalCF: timing capacitor terminalT-off: off terminal of timing resistanceCT: detection capacitor terminal in intermittent modeGND: chip groundCLM - negative pressure overcurrent detection endCLM : positive pressure overcurrent detection endFigure 1m51995ap pin arrangementFigure schematic block diagram of 2m51995aVCC: chip power supply terminal3 working principleThe principle block diagram of m51995a is shown in Figure 2. It is mainly composed of oscillator, feedback voltage detection and conversion, PWM comparison, PWM latch, overvoltage latch, undervoltage latch, intermittent working circuit, intermittent mode and oscillation control circuit, drive output and internal reference voltage.(1) OscillatorThe equivalent circuit of the oscillation circuit is shown in Figure 3. The CF voltage presents a triangular wave due to the charge and discharge of the constant current source. During normal operationFigure 3 oscillator equivalent circuitThe charging current is I1 = ut ï¼ on / RonThe discharge current is I2 = ut ï¼ off / Roff ut ï¼ on / 16ronOscillation period isT=(UOSCHï¼UOSCL)CF/(I1ï¼I2)Where (uosch uoscl) is the peak value of triangular wave, uosch â 4.4V, uoscl â 2.0V, ut on â 4.5V, ut off â 3.5V. The maximum pulse width of the chip output is the rise time of the triangular wave, and the fall time of the triangular wave is the dead time. When overcurrent occurs, the intermittent mode and oscillation control circuit begin to work. At this time, the voltage of t-off terminal depends on the voltage of VF terminal, and the charging current of the oscillator is the same as that in normal operation,The charging current is I1 = ut ï¼ on / RonDischarge current isI2'=(UVFï¼UVFO)/Roffï¼UTï¼on/16RonOscillation period isT=(UOSCHï¼UOSCL)CF/ï¼I1ï¼I2ï¼Where UVF is VF terminal voltage, uvfo â 0.4V,Fig. 4 Application of VF terminal in forward converterIf UVF ï¼ uvfo If UVF ï¼ uvfo > ut ï¼ off â 3.5V, then UVF ï¼ uvfo = ut ï¼ off. Therefore, when UVF > 3.5V, the operation of the oscillator is the same as that without overcurrent. Usually, the voltage at the VF terminal is proportional to the output voltage of the converter. In this way, when the output voltage becomes low due to overcurrent, the dead time becomes longer accordingly, so as to further reduce the duty cycle. Fig. 4 shows the application of VF terminal in forward converter, where RC constitutes a low-pass filter; In the flyback converter, the bias winding voltage can be divided and connected to the VF terminal, because the bias winding voltage is directly proportional to the output voltage of the converter.(2) PWM comparison latch sectionFig. 5 is the circuit diagram of PWM comparison and latch part. It can be seen from the diagram that the potential of point a isUA=5.8ï¼15.2k&TImes; (500Â·IF/B/3k)The point a potential is latched after comparison with the oscillating triangular wave, and output after logic combination with the control signal output from the oscillator. The waveform of each point is shown in Figure 6. Therefore, the logical relationship of points B, C, D and E isB = D Â· e, C = B Â· e = D Â· e (3) output circuitThe output of the chip is a totem pole circuit to drive the MOS transistor. The traditional totem pole circuit has the disadvantage of high penetration current, up to 1a, which will cause large ICC current and inevitable IC heating and noise voltage in high-frequency applications. M51995a uses an improved totem pole circuit, and the penetration current is about 100mA without deteriorating the performance.(4) Current limiting circuitIn Fig. 6, if the voltage at the current limiting terminal CLM or CLM - exceeds the threshold value ( 200mV / - 200mV) before the intersection of the rising edge of the point a waveform and the triangular wave, the overcurrent signal will cut off the output and this cut-off state will continue to the next cycle. In fact, this signal controlsFigure 5 PWM comparison and latchFig. 6 waveform of each point in PWM comparison and latch partThe state of is reset in the next dead time, so the current limiting circuit can work in each cycle, which is called "pulse by pulse current limiting". In order to eliminate the influence of noise voltage caused by parasitic capacitance, a low-pass filter composed of RC needs to be used, as shown in Fig. 7.(a) Case of CLM (b) case of CLM ï¼Figure 7 connection of CLM / CLM -Fig. 8 intermittent mode and oscillation control circuit timingFig. 9 intermittent working circuit diagramWhen the internal current limiting circuit works, the intermittent mode and oscillation control circuit start to work, that is, output high level. Fig. 8 is a timing diagram. When the output of the intermittent mode and oscillation control circuit is high and the VF terminal voltage drops below the critical value of about 3V, the intermittent mode circuit starts to work. Fig. 9 is a schematic diagram of an intermittent mode circuit. When the voltage at VF terminal is higher than uthtime, transistor V is turned on and the potential at CT terminal is close to GND; When the voltage at VF terminal is lower than uthtime, transistor V is cut off and CT will be charged and discharged. When SWA is closed and SWB is open, CT is 120 Î¼ When the SWB is closed and the SBA is open, the CT is charged by 15 Î¼ The current of a is discharged, so the CT end presents a triangular wave. The output pulse will be generated only during the voltage rise period of CT terminal. Obviously, the triangular wave frequency at the CT end is much lower than the switching oscillation frequency. In this way, the components in the power circuit, including the secondary rectifier diode, can be effectively protected to prevent overheating caused by overcurrent. When the intermittent mode is not used, it is recommended that the CT terminal be grounded.(5) Auxiliary function partThe det terminal can be used to control the output voltage. The circuit between the det terminal and the F / b terminal is very similar to the parallel adjustable voltage reference chip 431. When the det terminal voltage is higher than 2.5V, the operational amplifier has the ability to absorb current, and when the det terminal voltage is lower than 2.5V, the output is high resistance. Det terminal and F / b terminal have phase reversal characteristics, so it is recommended to connect resistance and capacitance in series between them for phase compensation.OVP and on / off terminals can be conveniently used to realize overvoltage protection and switch chip operation. Both have hysteresis characteristics. Under overvoltage protection and off state, the working current of the chip is provided by the starting circuit. The chip only works when the on / off terminal is at low level, and the threshold voltage is 2.4V. When the OVP terminal is higher than the threshold voltage of 750mv, the chip enters the overvoltage protection state (OVP). In order to reset the OVP state, the OVP terminal voltage must be lower than the threshold voltage or the VCC must be lower than the OVP reset supply voltage (typical value is 9V).Fig. 10m51995ap application in forward converter4 typical applicationsFigure 10 and Figure 11 show m51995ap in forward and flyback mode respectivelyFig. 11m51995ap application in flyback converterApplication of in converter. In the forward converter, the AC input is switched after full wave rectification and smoothing filtering. The secondary output is multi group output, and the voltage stabilizing control is carried out on the main output. 431 reference chip and optocoupler are used for sampling and error amplification to improve output accuracy and isolate primary and secondary voltages. Overcurrent detection uses a current detection transformer. The power supply can be switched by an external signal. Ron is recommended to be 10K to 75K, Roff is recommended to be 2K to 30K; The recommended power supply voltage is 12V to 17V; The starting current through R1 starting resistance is recommended to be 300 Î¼ Above a to start stably.