The LLC topology is quite popular in 200 W-600 W Power applications, especially in a high-energy switching power supply. LLC solutions with traditional voltage control working modes in the market are different, and the NCP1399 of the Anshen Semiconductor is the industry's first LLC AC-DC control IC using current control mode. The device applies 600 V door driver to simplify the layout and reduce the number of external components, and use the hop cycle mode to improve light load energy efficiency, and integrate a series of protection features to improve system reliability, for large screen TV, integrated computers, industrial and medical care Equal high-power power supply system applications can significantly achieve high energy efficiency and ultra-low standby energy consumption at light load and full load.
Limitations of traditional voltage mode LLC
Traditional voltage mode LLC controllers are changing the voltage controlled oscillator (VCO) frequency through the secondary regulator, thereby achieving regulation. Since there is no direct connection to the primary end current, additional circuit systems need to be added to provide overload and short circuit protection. When the output generates transient changes (such as transferred from no load to full load), the transient response will be slower. Moreover, the LLC starts the behavior of the components of the resonant circuit, the actual resonant capacitor voltage, the output voltage of the start-up phase, starts large current voltage and initial conduction time, etc., avoid hard switch under all conditions is not easy. . When applied into standby mode, in order to reduce standby energy consumption, the system will turn off the PFC and LLC, but it is also necessary to have a secondary power supply in the system to maintain the minimum operating conditions when the MCU and the surrounding component standby, adding system costs.
Advantages of current mode LLC
Under the same conditions, the current mode LLC provides much fo higher than the traditional voltage mode LLC, which can be seen from this baud chart.
Figure 1. Porter
Therefore, in terms of dynamic load response, current mode LLC is provided with lower voltage drop, lower overshoot and faster response; in terms of line-shaped ripple suppression, current mode LLC control suppression linear ribbed ratio VCO LLC is 5 times, and its VOUT ripples related to VBULK can even be ignored; in linear transient, the current mode LLC control provides a small 10-fold voltage drop of 10 times higher than the VCO LLC.
NCP1399 Application Principle and Current Mode Control Algorithm
NCP1399 has two versions of Active Off-off mode (NCP1399AX) and Active ON shutdown mode (NCP1399BX). As shown in Figure 2, both versions use the industry's first 6-pin PFC controller NCP1602, with NCP1399 to become a complete LLC program. The PFC operation is shared by NCP1399 via VCC control, and the resistance partial pressure of PFC FB and LLC BO is shared. This share is also turned off. NCP1399AX uses the "SKIP" pin to enter the load state of the hop cycle mode, and when the detection of the FB voltage is lower than VB_REMOTE_OFF; NCP1399BX uses the internal setting "hop cycle mode" threshold level (IC can be programmed) ), And enter the shutdown mode with separate light coupling REM pins.
Figure 2. Typical application schematic of NCP1399 (previous: A version; next: b version)
Since the resonant capacitor integrates the primary current (iPrimary), the VCS voltage is proportional to the primary current, and during shutdown, the VCS voltage is positive or negative, and the VCS voltage at the time of shutdown is almost linearly dependent on the load current. However, the VCS turn-off voltage curve changes with VBULK, so that the current mode LLC control can be achieved by the relationship between VCS and the primary current and VCS voltage. The specific control algorithm is: 1) obtain positive resonance capacitance slope by VCS voltage dividing signal; 2) Offset to VCS voltage pressure signals to avoid light load time-optically coupling; 3) According to the FB pin voltage and VCS voltage slope (reflect Through the primary current line and load conditions) automatically adjust the MUPPER conduction time; 4) The same on time is then copied for the MLOWER MOSFET to ensure perfect symmetrical DC.
Figure 3. Current mode control principle
How does NCP1399 achieve high energy efficiency and ultra-low standby energy consumption?
Automatic adjustment dead time (DEADTIME, DT)
The DT period is to avoid cross-conducting of power MOSFETs to prevent overcurrent from damage to the system. The fixed DT period is often used to enable resonant converters, but due to the change of the excitation current, the fixed DT phase cannot be ensured by the fixed DT period. NCP1399 uses a proprietary manner when a zero voltage switch (ZVS) conversion is detected, and the DV / DT sensor monitors the HB pin slope, and provides logic signals to optimize the adjustment DT according to the resonant loop parameter.
Jump cycle mode
Due to the ZCS (zero current switch) of the primary MOSFET ZCS (zero current switch), the LLC resonant converter can achieve the highest energy efficiency at moderate load and full load. However, under light load and no-load conditions, it will produce uncommon energy consumption if normal frequency modulation control techniques. NCP1399 uses proprietary hop cycle mode technology to increase light load energy efficiency and reduce noise as much as possible. It performs a hop cycle mode based on the FB pin voltage level, and the hop cycle comparator compares the FB pin voltage and pre-selection values. When VFB is lowered below the pre-selected hop cycle level, the IC enters the hop cycle mode via a dedicated shutdown sequence. In this mode of operation, the IC power consumption is lowered, and the PFC segment and the BO / PFC FB high-voltage divider voltage divider using the PFC mode pin can be disabled. Since NCP1399 always enters the hop cycle mode in a specific manner, this is always When the MLOWER drive pulse ends 3/2 (or 3/2 of the last MUPPER pulse) of FMAX (or 3/2 of the last MUPPER pulse), the resonant capacitor voltage is kept to vBulk * 1/4, thereby reducing the switching loss. When the pre-selected lag is added inside the FB pin voltage, the IC returns to normal mode, the first pulse of MLower is extended, and the bootstrap) capacitance is recharged, and the resonant circuit is excited to prepare ZVS conditions for subsequent MUPPER. The turning process, the next MLower pulse is extended based on the conduction time CMP signal.
Figure 4. Enter the hop cycle mode and return from the hop cycle mode to normal mode
PFC conduction / shutdown function
The PFC conduction / shutdown function is based on FB pin voltage monitoring. The PFC preset timer (Timer) is used to overcome the problems during transients, and the three-state output control PFC FB / LLC Brown-Out (BO) voltage divider and PFC VCC. This PFC conduction / shutdown feature reduces light load energy during hopping cycle mode.
NCP1399 proprietary control provides strong protection
Proprietary start sequence
Hard switch boots may occur in the resonant switching power supply application. NCP1399 uses a specific boot scheme to ensure
The stable, no hard switch under any start condition is started. NCP1399 uses a soft start counter and a D / A converter to implement digital nonlinear soft boot sequences.
2. Overload protection
Overload protection is implemented by FB pin voltage detection. The on-time comparator defines a comparison value to control the on time.
When the secondary short circuit occurs using NCP1399, the FB pin voltage rises. When the FB pin voltage reaches the maximum, the fault timer / counter is started, the controller disables the drive pulse and enters the protection mode.
3. Built-in programmable function
NCP1399 has a built-in multi-programmable function, including: empty service clamping and fault option, TSD threshold, adjustable
The threshold of the section Select Remote and shutdown mode, VCC on / off threshold, CS bias, and slope compensation gain, providing CS Leb, hopping period mode, internal and external threshold, fault timer / counter, and its duration - including auto recovery Timer, accumulated or non-cumulative selection, OTP / OVP latch or automatic recovery selection, BO selection (IBO, VBO, hop period function), maximum conduction time fault detection (on-time and latch / automatic recovery), The first MUPPER and MLOWER pulse width of the start sequence, the soft start increment affects the soft start duration, the activation and lag of the P on / OFF function, and timer function, etc., the parameters can be appropriately adjusted according to the specific situation in different applications. And optimize, provide design flexibility and speeding up product listing.
Evaluation board energy efficiency test
The figure below shows that when the input voltage is 110 V and 230 V, the output current is in the range of 0-20 a, and the energy efficiency of NCP1399 is more than 90%.
Figure 5. Evaluation board energy efficiency test
Conclusion
The LLC AC-DC controller launched by Anshen Mei semiconductor NCP1399 uses current mode control and breaks through the voltage mode control, with more transient dynamic reactions, providing higher light load and average energy efficiency, and can be achieved without auxiliary power The ultra-low standby energy consumption of standby mode is reduced, which uses a reliable boot start mode, integrated overload protection, overcurrent protection to avoid hard switch, and undervoltage detection, open label detection, automatic DT adjustment, overvoltage A series of protection features such as protective and over temperature protection to ensure system reliability. The designer can also use the NCP1399 built-in programmable function, adjust and optimize the parameters according to the specific application, and design high-energy, ultra-low standby power applications for high-power, ultra-low standby energy consumption systems.
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