“In recent years, resonant converters have become more and more popular and are widely used in various application scenarios such as computer servers, telecommunications equipment, lamps, and consumer electronics. The resonant converter can easily achieve high energy efficiency, and its inherent wide soft switching range can easily achieve high-frequency switching, which is a key attractive feature. This article focuses on introducing a 300W power supply featuring half-bridge LCC resonant conversion digital control and synchronous rectification.
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In recent years, resonant converters have become more and more popular and are widely used in various application scenarios such as computer servers, telecommunications equipment, lamps, and consumer electronics. The resonant converter can easily achieve high energy efficiency, and its inherent wide soft switching range can easily achieve high-frequency switching, which is a key attractive feature. This article focuses on introducing a 300W power supply featuring half-bridge LCC resonant conversion digital control and synchronous rectification.
The STEVAL-LLL009V1 shown in Figure 1 is a digitally controlled 300W power supply. The primary side components include PFC stage and DC-DC power stage (half-bridge LCC resonant converter), and the secondary side components include synchronous rectifier circuit and STM32F334 microcontroller. Converter) and output synchronous rectification are digitally controlled, while the power factor correction (PFC) stage is based on the L6562ATD critical mode PFC controller.
The working mode of the evaluation kit can be set to constant voltage (CV) mode or constant current (CC) as required. The on-board fast protection circuit provides all necessary protection functions and has high reliability. The evaluation kit has been evaluated for performance within the range of 270-480V AC input and the entire load. The test results prove that the power quality parameters are within the acceptable range of the IEC 61000-3-2 general AC power harmonic standard.
Preface
The solution proposed in this paper adopts digital conversion control method, rather than standard design based on analog IC. The main advantage of the numerical control method is that it is flexible to set up, parameters and operating points can be adjusted instantly under any given conditions, without any hardware changes, and analog control can only be adjusted within a specific range. Digital control method can realize advanced functions such as dimming method (analog or digital), dimming control (0-10V, wireless communication), dimming resolution, temperature monitoring, various protections, communication connections, etc., with only one chip. Therefore, the system cost is more cost-effective, and it is easier to implement than the simulation method. In addition, in high-noise working conditions, the numerical control method can ensure higher stability of the power supply: the numerical control power supply is not susceptible to component tolerances, temperature changes, voltage drift and other factors.
Figure 1: STEVAL-LLL009V1 evaluation kit
System Overview
The STEVAL-LLL009V1 evaluation kit has two modes: constant voltage (CV) and constant current (CC). The constant voltage mode (CV) can convert 270V-480V AC input to 48 V constant voltage and a maximum current of 6.25 A DC output; constant The current mode (CC) can output 6.25 A DC current of 36V-48V. By flipping the switch SW1 on the main power board, the evaluation kit can be set to CV mode or CC mode.
The DC-DC power stage is called the primary side power layer, and the microcontroller stage is called the secondary side power layer. The microcontroller sends a control signal to the electrically isolated half-bridge gate driver STGAP2DM to drive the DC-DC power stage MOSFET switch.
Figure 2 is a block diagram of the STEVAL-LLL009V1 evaluation kit, which embeds the topological circuits and components required by the primary and secondary sides.
The evaluation board provides a 0-10V input to control the brightness of the LED. The dimming control 0-10V input is only applicable when the evaluation kit is running in constant current (CC) mode. The STEVAL-LLL009V1 evaluation kit implements an analog dimming method with a current resolution of 1%.
A daughter board with an isolation amplifier is also plugged into the evaluation board to detect the output voltage of the PFC, which is also the input voltage of the DC-DC power stage.
The PFC stage is based on MDmeshTM K5 power MOSFET; in order to achieve high energy efficiency, the half bridge of the LCC converter uses MDmeshTM DK5 power MOSFET. The secondary side synchronous rectification (SR) circuit uses STripFETTM F7 power MOSFET to reduce on-state loss.
The evaluation kit is equipped with complete safety protection functions, such as open circuit protection, short circuit protection, resonance current protection, DC-DC power stage input undervoltage protection and overvoltage protection.
The offline flyback converter based on VIPer267KDTR supplies power to the primary and secondary circuit, including the control board, gate driver IC and signal conditioning circuit.
Experimental results show that under wide input voltage and wide load conditions, the evaluation board has achieved higher power efficiency, power factor close to unity, and lower THD% distortion rate. This is due to the excellent performance of STMicroelectronics' power devices. And the control strategy implemented using STM32F334 32-bit microcontroller.
Figure 2: Block diagram of the STEVAL-LLL009V1 evaluation kit
Resonant converter
The DC-DC power stage changes the PFC output voltage to the required output voltage. There are many topologies available for DC-DC power conversion stages, such as LLC resonant converters. Each topology has its own advantages and disadvantages. Applications such as chargers and LED lighting may require an electrically isolated DC-DC power stage to handle a wide input or output voltage. Taking these requirements into account, a half-bridge LCC resonant topology is implemented in the DC-DC power stage of STEVAL-LLL009V1, as shown in Figure 3.
Figure 3: Half-bridge LCC resonant conversion stage with synchronous rectification function
In STEVAL-LLL009V1, the parallel capacitor Cp is connected to the secondary side of the transformer. Therefore, the parasitic capacitance of synchronous rectification and the leakage inductance of the transformer become part of the resonant circuit.
The PFC output voltage charges the bulk capacitors to generate a stable DC-BUS current. The half-bridge configuration MOSFET switch generates a square wave voltage waveform between GND and DC-BUS, and is applied to the LCC resonant circuit composed of capacitor Cr, capacitor Cp (on the secondary side), Inductor Lr and isolation transformer.
Drive the half-bridge high-voltage MOSFET switch of the LCC resonant converter with a 50% PWM duty cycle and appropriate dead time. Because the energy storage current of approximately sinusoidal resonance always lags behind the voltage waveform (inductance area), the MOSFET output capacitor has time to discharge during the dead time before the next turn-on, and achieve zero voltage switching (ZVS) operation, as shown in Figure 4. Show. The PWM switching frequency controller is used to adjust the voltage increase amplitude of the resonant circuit and keep the voltage of the converter in the inductance area, so that the switch tube maintains the ZVS operation in the entire working range and reduces the switching loss.
Figure 4: HB-LCC level waveform at 100% load
Table 1: Comparison of LCC and LLC resonant converters
We used the fundamental harmonic analysis (FHA) method to analyze the gain of the half-bridge LCC resonant converter of the evaluation kit.
According to the gain calculation formula obtained using the FHA method and the LCC parameters selected for the half-bridge LCC resonant converter of the STEVAL-LLL009V1 evaluation kit, we obtain the relationship curve between the gain and the normalized frequency, as shown in Figure 5.
Figure 5: HB LCC converter-gain and normalized frequency
Synchronous rectification (SR)
On the secondary side of the transformer shown in Figure 3, the input voltage waveform is rectified by a synchronous rectifier in a full-bridge configuration, and interference signals are filtered out by the output capacitor to smooth the waveform. The synchronous rectification stage is digitally controlled by the STM32F334 microcontroller.
Driving the synchronous rectification MOSFET switch tube needs to detect the synchronous rectification (SR) terminal voltage (VDS_SR1 and VDS_SR2). The detection and control algorithm of MOSFET VDS (drain-source voltage) is discussed below.
The drain-source voltage detection network is composed of a fast diode and a pull-up resistor. The pull-up resistor is connected to the power supply voltage of the microcontroller (MCU), as shown in Figure 6. When the drain voltage of the SR MOSFET is higher than the MCU Vcc, a reverse bias voltage is applied to the diode, and the detection voltage is pulled up to Vcc. When the drain voltage is lower than Vcc, a forward bias voltage is applied to the diode, and the detection voltage is equal to the sum of this voltage and the voltage drop of the forward conducting diode. The pull-up resistor limits the current during positive bias.
Figure 6: Synchronous rectification VDS detection method
First, the body diode of the synchronous rectification MOSFET starts to conduct, and the VDS detection circuit measures the VDS drain-source voltage value. If the drain-source voltage (VDS) is lower than the set threshold (Vthreshold_ON C OFF set by the MCU DAC peripheral), the comparator The output (falling edge) triggers the non-repeatable single pulse mode of the MCU TIMER peripheral, as shown in Figure 7.
The MCU TIMER peripheral sends a pulse signal with a minimum duration of TON min to the corresponding synchronous rectification gate driver.
When the drain-to-source voltage (VDS) is higher than the set threshold (Vthreshold_ON C OFF set by the MCU DAC peripheral), the comparator output (rising edge) resets the MCU TIMER peripheral and stops to the corresponding synchronous rectification gate driver Send the pulse as shown in the figure. Figure 7.
The MCU continuously monitors the DC-DC power stage (HB-LCC) frequency and output current. If the frequency is higher than the set threshold and hysteresis value or the output current is lower than the set threshold and hysteresis value, the microcontroller (MCU) turns off the gate driver of the synchronous rectification stage. At this stage, the body diode of the MOSFET rectifies. When the frequency is lower than the set threshold and hysteresis value or the output current is higher than the set threshold and hysteresis value, the microcontroller (MCU) turns on the synchronous rectification stage gate driver.
According to the operating frequency of the DC-DC power stage (HB-LCC), the threshold (Vthreshold_ON C OFF) can be adjusted in the lookup table in the MCU.
Figure 7: Synchronous rectification digital control algorithm
Experimental result
We calculated the total energy efficiency, power factor (PF) and total harmonic distortion (THD) of STEVAL-LLL009V1 under different loads. When the load is 100%, the energy efficiency is higher than 93.5%. Figures 8, 9, 10, and 11 describe the performance of the evaluation kit in constant voltage (CV) and constant current (CC) modes, respectively.
Figure 8: Constant voltage configuration: the relationship between input voltage and energy efficiency under different loads
Figure 9: Constant voltage configuration: the relationship between input voltage and power factor under different loads
Figure 10: Constant voltage configuration: the relationship between input voltage and total harmonic distortion under different loads
Figure 11: Constant current configuration: the relationship between input voltage and energy efficiency under different LED voltage drops
The numerical control power supply proposed in this article can provide 300W output power in both constant voltage (CV) and constant current (CC) modes. Experimental results show that under wide input voltage and wide load conditions, the evaluation board has achieved high power efficiency, power factor close to unity, and low THD% distortion rate. This is due to the excellent performance of STMicroelectronics power devices, and A control strategy implemented using STM32F334 32-bit microcontroller.
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