Abstract: Introduced the low power general off-line type power source’s pulse-duration modulation current mode controller NCP1200A principle, and through develops the multi-channel isolation instead stir up the converter experiment prototype, and the test result further showed this controller has merit.
Key word: Electric current mode control; Counter-violent; Front blanking; Tendency from power supply
Introduction
Instead stirs up the converter to have the circuit structure to be simple, input output electricity isolation, voltage regulation scope wide, characteristics and so on easy multi-channel output, thus suitable to take in the electric power electronic installation’s auxiliary switch power source.
The current control pulse-duration modulation is one kind of novel control technology, it has overcome the traditional voltage control pulse-duration modulation technology shortcoming, enables the switching power supply system to have the fast transient response, the high stability, the overload and the short circuit protection simple and so on characteristics. NCP1200A is one kind of low power general off-line power source’s electric current pattern pulse-duration modulation controller, it has represented to the subminiature switching power supply direction significant leap, its interior including fixed time the part, feeds back the component, the low pass filter and from the power supply and so on. This article through regarding this chip function research, and designed successfully manufactures the multi-channel isolation outputs instead to stir up converter’s prototype.
1 current control PDM keyer NCP1200A synopsis
NCP1200A uses the standard electric current model system, the actuation turn-off time establishes a determination by the summit current. This control chip has the following characteristic:
–Does not need the accessory power supply winding;
–The interior has the output short circuit protection circuit;
–The idling waits for an opportunity the power loss to be extremely low;
–Electric current pattern belt jumping cycle function;
–The interior has the 250ns front blamking circuit;
–the 250mA peak value pulls/fills the electric current ability;
–The internal fixed frequency is 40/60/100kHz;
–Transmitted light coupler direct connection.
Its internal structure as shown in Figure 1, various pins function is as follows:
–The foot 1 uses for to adjust has the jumping cycle level;
–The foot 2 meet the light pair to this foot, adjusts a time side summit current setpoint according to the output demand;
–The foot 3 examine side electric current, and will examine the value through the front blamking circuit to send in the internal comparator;
–Foot 4 earths;
–The foot 5 outputs actuate exterior MOSFET switching valve;
–The foot 6 meet to the typical value for 10μF the exterior large capacity electric capacity;
–The foot 7 use for to guarantee that suitable leaks the electric current distance;
–The foot 8 meet to the high-pressured cocurrent skeleton line, this foot pours into the constant current the Vcc large capacity electric capacity.
2 design features and matters needing attention
2.1 tendencies from power supply (DSS)
The tendency causes from the power supply technology instead to stir up the converter which designs not to need the accessory power supply winding, may supply power directly by the high-pressured cocurrent skeleton line. DSS principle based on the Vcc large capacity electric capacity from a low level to high level charging and discharging. Its work process is as follows:
When the power source puts through, if Vcc<VccH, then current supply connection, non-pulse output; If Vcc drops, and is bigger than VccL, the current supply shuts off, the output is a pulsation; If Vcc rise, and is smaller than VccH, the current supply connection, the output is a pulsation; Typical value VccH=12V, VccL=10V, its work process schematic drawing as shown in Figure 2.
2.2 jumping cycle working pattern
Because the switching power supply has the good efficiency under the normal load condition, but when the output reduces, its efficiency will start to drop. In NCP1200A inlays the jumping cycle function, when the output demand reduces to the given value below, then automatically jumps over the switching period, this is realizes through surveillance foot FB. When the load demand reduces, when the internal ring circuit request small summit current, the setting value achieves the determination at this time the level, the integrated circuit will prevent the electric current to continue to reduce, and starts to cause the output pulse to present the blank, realizes the jumping cycle pattern. When FB jump threshold value (default value 1.2V), the summit current will not surpass 1V/Rsense. When the integrated circuit enters the jumping cycle pattern, the summit current will not be smaller than Vpin1/3.3, this time the user may through change the foot 1 voltage value to adjust the jumping cycle the summit current. Like this in the idling situation, NCP1200A always waits for an opportunity the efficiency to be possible very easy to achieve the design requirements.
2.3 short circuit protection function
Through monitors the feedback loop continually, NCP1200A can examine presents the short circuit the situation, exists side by side soon the output to reduce, by carries on the protection to the entire electric circuit. Once short circuit vanishing, the controller then restores to the normal work condition. Therefore, regarding the firm output power power source which assigns, may separate this protection function very conveniently.
2.4 soft start realizations
The Vcc voltage fell the time which from 12V needed to 10V had decided whether the system did start normally, caused the corresponding transient state breakdown duration from the output capacity charge to be smaller than the Vcc voltage falls from 12V to the 10V time, otherwise the feedback surveillance return route will work as short circuit situation processing, will enable the power source to start normally. On the Vcc voltage and foot 6 external connection capacitance value C has the relations. The supposition system achieves the time which the rated load needs is 6ms, therefore the Vcc fall time must be bigger than 6ms, here hypothesis is 10ms. Is including the MOSFET actuation the integrated circuit current sinking is 1.5mA, by type C=i · δt/ΔV, may result in needs the capacitance value the size. May know according to this according to the quarter hypothesis’s condition C=7.5μF, uses in the actual electric circuit C=10μF, with the aim of realizing the soft start function.
2.5 chip base pin against negative voltage peak function
This chip may through attach the peripheral circuit to prevent the sensitive base pin to sustain the voltage peak damage. Negative signal has the very big damage to only then the bias’s control chip, enables it the steady work. Because if designs improper, causes the transformer primary side inductance and the foot 6 external connection electric capacities has the resonance, on 6 will produce negative pulse in the foot, will create the chip damage. To prevent this situation effectively the occurrence, may use two kind of electric circuits which shown in Figure 3 to protect the chip.
in 2.6 inlays the front to disappear (LEB) the function
Generally the control chip, before a side’s electric current examination input, must to the current waveform which examines carry on low-pass filtering processing, prevents, because switching valve’s switch peak causes transient state overflow phenomenon. The NCP1200A interior has the 250ns front blamking circuit, thus does not need the sur- low-pass filtering network to carry on processing to the examination electric current, causes the circuit design to be simpler.
3 test results
This article through experiments the prototype to further introduce NCP1200The principle of work as well as in actual design simple. This test equipment is the 48V cocurrent input, three group isolation outputs instead stir up the converter. Each parameter establishment is: Vin=48V, Vcc=Vcc1=Vcc2=15V, VEE=-15V, turn-on frequency f=40kHz, the transformer primary side inductance value L=330μH, specified output P=14W, dutyfactor D=0.4 (to prevent D>0.5, but enables electric circuit steady work, and does not need to add pitch compensation). Electric circuit’s structure principle as shown in Figure 4, uses the RCD passive absorption network to prevent MOSFET to burn down.
Figure 4 electric circuit’s structure principle
Develops experimental prototype as shown in Figure 5, the concrete size for 105mm×30mm×23mm, uses stands puts in-line type structure, is advantageous with the main body device attachment.
Full-scale test profile like chart 6~ shown in Figure which by develops the experimental prototype which obtains 8. Figure 6 is on the Vcc electric capacity’s charging and discharging profile, has realized the chip tendency well from the power supply, simultaneously has guaranteed the installment soft start.
Figure 7(a) and (b) respectively be electric circuit work, in the electric current continual condition (CCM) and makes and breaks the condition (DCM) on a side sampling resistance’s current waveform and MOSFET leaks between the source the voltage waveform, experiments electric current which and voltage burr in the profile exists, is because a side transformer’s leakage inductance, the MOSFET internal parasite electric capacity as well as two side’s diode restores reverse creates. Because NCP1200A has the 250ns LEB function, therefore in the examination electric current’s radio impulse will not affect electric circuit’s stability.
Figure 8 is the prototypical work in the specified output and the underloading situation actuation profile contrast, from this may see the chip to jump the cycle active status, thus reduces the power loss effectively, achieves the high efficiency the request.
4 conclusions
NCP1200A is one performance fine current control PDM keyer, suits in the subminiature switching power supply design. It has the tendency from functions and so on power supply, soft start, jumping cycle low loss work, short circuit protection. Does not need the periphery separation component nearly, can realize the multi-channel isolation to output the high efficiency small instead to stir up the converter, thus, this chip widely applies in the exchange - cocurrent adapter, the off-line battery charger, the accessory power supply and so on.
