CXAC85271 adopts a discontinuous forward conversion topology structure, combined with resonant switching technology, which not only achieves high-efficiency operation of the power system, but also significantly reduces electromagnetic interference (EMI) noise. This chip is suitable for replacing traditional linear transformers and has multiple advantages such as small size, low cost, and high integration.
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[ CXAC85271 ]"
In the context of the increasing popularity of electronic devices today, efficient, stable, and compact power management solutions have become a must-have in the market. CXAC85271, as a high-performance non continuous forward resonant switching power supply control IC, is widely used in various AC-DC conversion scenarios, such as mobile phone chargers, music players, portable tool power supplies, etc. It has won the favor of engineers and manufacturers with its excellent efficiency and reliability. This article will provide an in-depth analysis of the structure, characteristics, working mode, and application recommendations of CXAC85271, helping readers to fully understand the advantages and applicable scenarios of this chip.
1、 CXAC85271 Overview and Product Features
CXAC85271 adopts a discontinuous forward conversion topology structure, combined with resonant switching technology, which not only achieves high-efficiency operation of the power system, but also significantly reduces electromagnetic interference (EMI) noise. This chip is suitable for replacing traditional linear transformers and has multiple advantages such as small size, low cost, and high integration.
Its main features include:
• Highly integrated CMOS control ICLess system components and simple design;
• Support driving low-cost bipolar power transistorsReduce BOM costs;
• Resonant switch technologyImprove efficiency while reducing EMI, and pass EMI certification without the need for additional filtering components;
• Built in multiple protection mechanisms, including overload, over temperature, and under voltage protection;
• Low standby power consumptionComply with energy-saving standards;
The packaging form is SOT23-6Suitable for compact design.
2、 Internal structure and pin function
CXAC85271 integrates key modules such as control logic, driver modules, and protection circuits internally. The pin configuration is as follows:
• BAS (Pin 1)Drive external power switch tubes, control their conduction and cutoff, and optimize switch losses;
• GND (pin 2)The system reference ground requires a low impedance connection;
• COL (Pin 3)Detecting the collector voltage of the power transistor through coupling capacitance and controlling the switching timing;
• CS (Pin 4)Detect switch current, support overcurrent protection and mode switching;
• VDD (pin 5)Chip power supply pin, internal regulator stable at 3.3V;
• AUX (pin 6)Provide current for the base drive module and can be limited by an external resistor.
3、 Working conditions and electrical characteristics
CXAC85271 operates stably within a wide voltage range, with a recommended operating voltage VDD of 3.1V~3.5V and a junction temperature range of -25 ℃ to 100 ℃. The typical internal clock frequency of the chip is 12.1MHz, supporting multiple switching frequency modes to meet different load requirements.
The key electrical parameters include:
• VDD voltage stabilization range: 3.1V~3.5V;
• Static current (sleep state):≤ 8μ A；
• Overcurrent protection threshold: -235mV (typical);
• Thermal shutdown temperature: 115 ℃ (typical), with a hysteresis of 35 ℃.
3.1. Extreme working conditions
(1) If the chip operates for a long time under the above extreme parameter conditions, it may cause a decrease in device reliability or permanent damage
When any parameter reaches or exceeds these limit values in actual use.
(2) All voltage values are tested relative to the system ground;
3.2. Recommended working conditions
3.3. Electrical characteristics;
Unless otherwise specified:
(1) The minimum and maximum electrical parameters are applied under abnormal working conditions.
(2) Typical electrical parameters applied to TJ= TJTYPAnd VDD= VDDTYP
(3) When working beyond the recommended normal working conditions, both functionality and performance are unpredictable, and reliability will decrease.
(4) For parameters that depend on F, calculations should be based on F.
4、 Typical Application Circuits and Performance
CXAC85271 is suitable for AC-DC power systems with a single voltage input, such as 12V/1A charger solutions. In typical applications, its efficiency exceeds 80%, no-load loss is less than 150mW, and it supports a maximum output power of 60W (230V input).
Figure 3 shows its typical RDFC (resonant discontinuous forward) application circuit, which achieves complete power management function through a small number of peripheral components and is suitable for scenarios such as mobile device chargers and embedded power supplies for household appliances.
5、 Detailed explanation of working mode
CXAC85271 has multiple operating modes to meet energy efficiency and protection requirements under different load conditions:
5.1. standby modeReduce switching frequency and duty cycle during light load to reduce power consumption;
5.2. Normal ModeWhen the load is running between 20% and 100%, the switch tube works in a resonant state with optimal efficiency;
5.3. Overload modeLimit the conduction time when the output current exceeds the limit to protect power devices;
5.4. Follow and Power Burst modesPeriodically limit output during severe overload or short circuit to prevent system damage.
5.5. Start/Stop Sequence
The controller of CXAC85271 is composed of VDDPin power supply, high resistance starting resistors (Rht1 and Rht2 in Figure 3) input voltage after rectification
Obtain a small amount of current (IDDSLEEP ）When VDDThe voltage on the pin reaches VOVDTHRThe controller is awakened, but still requires a larger current (IDDWAKE）To enter the startup phase, the controller will maintain it for a period of time until all internal circuit modules are started, and then switch to normal operating mode. In both startup and normal operation states, the controller uses an internal parallel regulator to stabilize VDDThe parallel regulator does not work when the power supply voltage is in sleep mode. Adjustment voltage during startup phase (V)DDREG(S) ）It will be higher than the normal operating voltage (VDDREG(R) ）To
High, this helps to provide sufficient V before the voltage provided by the auxiliary winding of the transformer risesDDVoltage.
If VDDThe voltage of the pin drops to VUVDTHRAfterwards, the controller will return to sleep mode to reduce the demand for power supply current. The controller will restart after the power is restored to obtain a smoother power on curve, VDDThe storage capacitor should be large enough to maintain the supply voltage at V after the controller is startedUVDTHRThe above.
5.6. Switching between startup and normal working power modes
In both startup and normal operating modes, CXAC85271 has multiple modes for controlling voltage conversion to achieve maximum conversion efficiency and under load
When the current changes greatly, the current output is limited. The description of the relevant modes is shown in the table below.
When the control IC enters the startup state from sleep mode, the system will operate in standby mode. Usually, the converter operates at a lower output power at this time
Pressure, so the current during the initial few switches will be very high, which will quickly switch the operating mode to normal mode or over
Load mode.
5.7. RDFC voltage output I/V characteristics;
Figure 5 illustrates the output characteristics of RDFC power supply under different operating modes. INOMWith VNOMIt is based on the different load rate conditions in the application circuit;
The output current is plotted against the voltage.
The correct mode switching point will depend on the specifications of the application and the internal oscillation frequency (F) of the control ICCLK）And the alignment of CS feet (VOCPH and
VOCPL is different. These parameters and their impact will be explained later.
5.8. Switching waveform
The waveform of the collector voltage and current of the power transistor Q1 on the primary side is shown in Figure 6. TRES is the transition transformer, and the cutoff is resonant and continuous
It is worth noting that in overload mode, when the current exceeds the current protection threshold OCPH (detected through the CS pin), the primary
The switching transistor Q1 will turn off.
5.9. Resonance control;
The natural resonance period of transformers and their peripheral parasitic components is determined by detecting the current flowing into and out of the COL pin through coupling capacitors
Ding.
The COL pin is used to control the saturation state of the primary switching crystal (Q1 in Figure 3) when the primary switching component is conducting (see details)
See“ Optimized Base Drive” ）When the switch component is turned off, the COL pin (which generates a low impedance path internally) will be detected;
The current flowing in and out determines the resonance time; As mentioned above, the rate of voltage change on the transformer causes the flow in and out of the COL pin
The current determines the TRES period and the optimal conduction time thereafter, and this resonance period is also used to determine the maximum conduction time of the primary switching crystal
Between: TON = 0.75 x TRES
Therefore, the maximum duty cycle (DNORMAL）About 43%. The conduction time of the switching component controls the output power, so it is suitable for light and overload loads
In this situation, the conduction time will be reduced. The minimum conduction time under overload conditions is determined by the internal CS window period (T)CSBLANK）Defined.
When the primary switch is turned on, the collector voltage may rapidly decrease in an instant, causing a large coupling current to flow into or through the coupling capacitor
Flow out COL feet. Inside the IC, an internal ACTICLAMP control circuit controls a low impedance clamp transistor to ground. This clamp transistor
The TACT time before the primary switching transistor is turned on and after the primary switching switch is turned off briefly conducts. This clamp transistor will be used in other applications
Keep the time in the OFF state. In some application circuits, the current generated by the coupling capacitor is sufficient to conduct the internal ESD
The voltage of the protective diode is across the clamp transistor, which limits ICOLThe current of this specification is within the absolute maximum voltage limit specified in the specification sheet
It is marked. When the actual capacitive coupling current exceeds this specification, protective diodes and current limiting resistors must be added to the circuit (Figure
Dcol1, Dcol2, and Rcol in the third class.
5.10. Optimize the base drive of power transistors;
In order to reduce the losses of primary power switching transistors, their base current needs to be carefully controlled. In order to reduce the loss of conductivity. In the effort
When the transistor is initially turned on, the base current will be at the maximum current IBASMAXDriving crystal TFONThe time (force on or“); FON” Sudden wave),
During the remaining conduction time, the base current will decrease to a lower current to operate the collector voltage and maintain it at the default voltage
This will reduce the conduction time and its losses. Operation during this period TPBD It is called proportional collector driving current IBASPBD 。 
6、 Comprehensive analysis of protection functions
CXAC85271 integrates multiple protection mechanisms to ensure system security and reliability:
• Collector Overvoltage Protection (COVP)To prevent breakdown of the switch tube;
• Over temperature protection (OTP)Stop output when the chip overheats to prevent thermal damage;
• Overcurrent protection (OCP)Detect and limit the primary current through the CS pin;
• Undervoltage lockout (UVLO)When the VDD voltage is insufficient, it enters sleep mode and automatically restarts after the voltage is restored.
7、 Precautions for packaging and welding
CXAC85271 adopts SOT23-6 packaging, with a compact size suitable for high-density PCB layout. When welding, attention should be paid to electrostatic discharge (ESD) protection. It is recommended to use reflow soldering process with a peak temperature not exceeding 260 ℃ (duration≤ 10 seconds) to prevent device damage.
8、 Conclusion
CXAC85271 has become a highly competitive control chip solution in AC-DC power supply design due to its high efficiency, low noise, high integration, and comprehensive protection functions. Whether used for external adapters or embedded power systems, its excellent performance and reliability can meet the stringent requirements of modern electronic devices for power supply.
For more technical information, sample requests, or procurement information about CXAC85271, please visitJTM-IC official websiteGet the latest data manual and support services.
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