Testing and Analysis Skills for LLC Application of Switching Power Supply System

EMC has become an important part of reliability in Electronic products/equipment; more and more attention will be paid! Especially for our industrial & consumer products to meet their corresponding certification and export requirements, the corresponding national policies are constantly improving; at the same time, the deepening of international trade; EMC technology has become a rigid indicator for electronic products/equipment! As the power supply systems of electronic products/equipment have begun to use a large number of high-frequency switching power supplies and are becoming more and more high-end; therefore, the requirements for the power supply environment are getting higher and higher; EMC will be more and more important!

EMC has become an important part of reliability in electronic products/equipment; more and more attention will be paid! Especially for our industrial & consumer products to meet their corresponding certification and export requirements, the corresponding national policies are constantly improving; at the same time, the deepening of international trade; EMC technology has become a rigid indicator for electronic products/equipment! As the power supply systems of electronic products/equipment have begun to use a large number of high-frequency switching power supplies and are becoming more and more high-end; therefore, the requirements for the power supply environment are getting higher and higher; EMC will be more and more important!

Electronic products/equipment we often encounter EMI problems; my lectures and my public account all have solutions and summaries for analyzing EMI-conduction design! Electronic designers who have read my article and listened to my class; gave me feedback results; currently encountering EMI problems, according to your EMI filter design rules, we have indeed solved our EMI-conduction problem; so many People benefit!

Electronic product/equipment EMI-radiation problem; most designers have no barriers to entry! For example, our electronic products/equipment often have EMI-radiation problems of 30MHZ-50MHZ, especially around 30MHZ; sometimes both! I also have a theoretical analysis; my “Switching Power Supply: EMC Analysis and Design”

Switching power supply PFC; FLY; LLC framework EMI problem I talk about actual combat methods!

The application of LLC in switching power supply systems >75W with PFC design is now more and more extensive; at the same time, its superior efficiency and EMI performance are basically the first choice for medium power power supply solutions; there are LLC aspects on the network and various platforms The detailed information introduction; I will use the basic test analysis skills in the application of LLC to be an analysis reference for electronic designers!

The basic principle and waveform data of the switching power supply system-LLC are as follows: At the same time, the EMI characteristics can also be understood through the waveform in the time domain.

Testing and Analysis Skills for LLC Application of Switching Power Supply System

According to the circuit principle, series or parallel inductance and capacitance can form a resonant circuit, so that when the power supply is a DC power supply, the current in the circuit changes according to the sinusoidal law. Since the current or voltage changes according to the sinusoidal law, there is a zero-crossing point. If the switching device is turned on or off at this time, the loss generated is zero.

The switching power supply system-LLC principle scheme is designed as follows:

Testing and Analysis Skills for LLC Application of Switching Power Supply System

General design guidelines

Criterion 1: The converter is designed to work at normal input voltage;

Criterion 2: The converter must be able to adjust automatically, when the input voltage is maximum and there is zero load;

Rule 3: The converter must always work in the ZVS area;

Switching power supply system LLC main working principle: The DC characteristic of LLC resonant conversion is divided into zero voltage working area and zero current working area. This transformation has two resonance frequencies.

One is the resonance point of Lr and Cr, and the other resonance point is determined by Lm, Cr and load conditions.

If the load is heavier, the resonance frequency will increase.

In consideration of improving efficiency as much as possible, the operating frequency needs to be set near fr when designing the circuit.

Among them, fr is the resonant frequency of Cr and Lr series resonant cavity.

When the input voltage drops, you can reduce the operating frequency-to get a larger gain. By selecting the appropriate resonance parameters, the LLC resonance conversion can work in the zero-voltage operating area regardless of load changes or input voltage changes.

The above-mentioned working mechanism of LLC-resonant converter is a basic knowledge point for engineers who have done switching power supply design; if LLC can master simple methods in actual testing and fault mechanism judgment, it will be very useful for debugging and application; I provide my practical methods for your reference!

The LLC constant voltage power supply shown in the above figure and the following LED backlight constant current design and application

Testing and Analysis Skills for LLC Application of Switching Power Supply System

1. LLC half-bridge resonant converter, our typical application on the primary side (high voltage common ground) and secondary side (output terminal common ground), the test analysis is basically the same; to confirm whether the IC can work normally (the back stage of the IC does not work) )!

A. Circuit design, welding and debugging or power system failure; we can evaluate the work of IC by testing its driving waveform;

B. Test and analysis skills; power-on test: IC test the voltage and drive waveform of the basic power supply pin!

Note that when the oscilloscope test is in the trigger mode, the single trigger baseline is within a few uS/tens of uS. Disconnecting the drive load unit of the IC can confirm whether the IC can meet the conditions for normal operation; the test data is as follows;

Testing and Analysis Skills for LLC Application of Switching Power Supply System

Testing and Analysis Skills for LLC Application of Switching Power Supply System

CH2: DRV1 (IC) CH4: DRV2 (IC)

When the IC and the control circuit are normal, the IC-drive circuit will send out a series of drive pulses of about 50% each. At the same time, the IC setting or the fixed dead time Td can be checked.

1. The typical test analysis of LLC half-bridge resonant converter on the primary side (high voltage common ground) and secondary side (output common ground) is basically the same; confirm whether the system can work normally when loaded!

A. Pay attention to using the oscilloscope’s trigger method, which can help us solve complex problems! ! !

B. Several issues that need to be considered when the system is powered on! !

Power supply system: Need to consider the impulse voltage & impulse current of power-on

IC detection: need to consider the power-on sequence & power-on speed (transient response judgment mechanism of the detection circuit)

C.If there is a problem when the system is working under load when the power is on, the following issues need to be paid attention to

① Power-on test triggers over-current protection

② The open loop triggers over-voltage (OVP) protection when the system is powered on

③ System detection circuit works abnormally or device abnormality when powering on

④ When powering on, if the control IC has multiple protection function mechanisms, it is necessary to confirm the status of the power-on protection action of the protection function first

When tested by oscilloscope in trigger mode, the single trigger baseline starts waveform data when a few uS/tens of uS is powered on and the test system is working under load; the test data is as follows; the power-on working status of the LLC system is detected:

Testing and Analysis Skills for LLC Application of Switching Power Supply System

CH2: Switch MOS lower tube-DRV2 (drive) CH3: Switch MOS upper tube-DRV1 (drive)

When the system control detection circuit is powered on, the LLC-drive will change from light load to full load; when power is on, the upper limit frequency of 50% PWM operation can be checked (IC setting or fixed)

Testing and Analysis Skills for LLC Application of Switching Power Supply System

CH2: Switch MOS lower tube-DRV2 (drive) CH3: Switch MOS upper tube-DRV1 (drive)

When the system control detection circuit is powered on, the LLC-drive will change from light load to full load; when the power load increases, the operating frequency of LLC will gradually decrease (test 151.5KHZ as shown in the figure above)

Testing and Analysis Skills for LLC Application of Switching Power Supply System

CH2: Switch MOS lower tube-DRV2 (drive) CH3: Switch MOS upper tube-DRV1 (drive)

When the system control detection circuit is powered on, the LLC-drive will change from light load to full load; when the power load increases, the operating frequency of LLC will gradually decrease (as shown in the figure above, test 100KHZ). If the test system appears to trigger the IC protection status When the system drive is abnormal or shuts down; at this time, it is necessary to check the status of the protection function pin at the same time to judge the fault! !

Testing and Analysis Skills for LLC Application of Switching Power Supply System

CH2: Switch MOS lower tube-DRV2 (drive) CH3: Switch MOS upper tube-DRV1 (drive)

When the system control detection circuit is powered on, the LLC-drive will change from light load to full load; when the power load increases, the operating frequency of LLC will gradually decrease (as shown in the figure above, test 88.65KHZ). If the test system has the function of triggering the IC In the protection state; the system drive will be abnormal or shut down; at this time, it is necessary to check whether the input front-end PFC is working normally; when the PFC work is not started or the load is abnormally overloaded; the working frequency of the LLC will run to the lower limit frequency set by the IC work Protection function; you can check the voltage and sequence of PFC power-on work, reduce the load to judge the fault! ! !

The Links:   CM200DY-34A NL8060AC26-11