The output of a switching power supply is a function of the DC input voltage, duty cycle, and load. In switching power supply design, the goal of the feedback system is to ensure that the output voltage remains within a specific range and exhibits good dynamic response performance regardless of changes in the input voltage, duty cycle, and load.
Current-mode switching power supplies have two operating modes: continuous current mode (CCM) and discontinuous current mode (DCM). In continuous current mode, due to the presence of a right-half-plane zero, the output voltage tends to decrease as the load current increases. It eventually corrects the output voltage after several cycles, potentially causing system instability. Therefore, special care must be taken to avoid the right-half-plane zero frequency when designing the feedback loop.
When a flyback switching power supply operates in continuous current mode, the frequency of the right-half-plane zero is lowest under the conditions of lowest input voltage and heaviest load, and the gain of the transfer function does not change significantly as the input voltage increases. When the switching power supply transitions from continuous mode to discontinuous mode due to an increase in input voltage or a decrease in load, the right-half-plane zero disappears, thus stabilizing the system. Therefore, under conditions of low input voltage and heavy output load, designing a feedback loop compensation to ensure sufficient phase and gain margins in the transfer function of the entire system allows the switching power supply to operate stably regardless of the mode.
1 Typical Design of Flyback Switching Power Supply
Figure 1 shows a typical circuit of a flyback switching power supply designed for a frequency converter. It mainly includes an AC input rectifier circuit, a flyback switching power supply power stage circuit (composed of a PWM controller, MOSFETs, transformer, and rectifier diodes), an RCD buffer circuit, and a feedback network. The PWM control chip used is the UC2844. The UC2844 is a current-mode controller with an internally adjustable oscillator (capable of precise duty cycle control), a temperature-compensated reference, a high-gain error amplifier, and a current sampling comparator.
The input parameters for the switching power supply design are as follows: Three-phase 380V industrial AC power is rectified and used as the input voltage Udc of the switching power supply, and the design is based on a minimum DC input voltage Udcmin of 250V; the operating frequency f of the switching power supply is 60kHz, and the output power Po is 60W.
When the system operates at its lowest input voltage, heaviest load, and maximum duty cycle, the switching power supply is designed to operate in continuous current mode (CCM) with a ripple factor of 0.4. The parameters of the designed switching power supply are as follows:
The transformer's primary inductance Lp = 4.2mH, and the number of primary turns Np = 138; 5V is the feedback output terminal, U5V = 5V, the load R5 = 5Ω, and the number of turns N5V = 4; the filter capacitor consists of two 2200μF/16V capacitors connected in parallel, with an equivalent series resistance Resr = 34mΩ; the 24V output load R24 = 24Ω, and the number of turns N24V = 17; the 15V output load R15 = 15Ω, and the number of turns N15V = 11; the -1.5V output load R-15V = 15Ω, and the number of turns N-15V = 11.
2. Transfer function of power stage circuit
The current-mode controller block diagram includes two feedback loops: an external voltage loop that feeds back voltage information, and an internal current loop that feeds back current information (as shown in Figure 2). The input to the current loop is the difference between the control voltage UC and the sampled inductor current US, and the output of the current loop is the duty cycle D. When US is less than UC, the PWM modulator (Fm) outputs a high level, and the power switch is turned on; when US is greater than UC, the PWM modulator outputs a low level, and the power switch is turned off. An RS flip-flop with a fixed-frequency clock signal is automatically set for the next cycle. In this way, the peak inductor current is precisely controlled by the control voltage.
In the control block diagram, Gvd(s) is the transfer function from the duty cycle control terminal of the power stage circuit to the output voltage, Gid(S) is the transfer function from the duty cycle control terminal to the inductor current, Fm(s) is the gain function of the PWM modulator, He(s) is the open-loop sampling gain of the current mode control, Rs is the current sampling resistor, and Uref is the reference voltage.
Based on the AC small-signal mathematical model of the flyback switching power supply established in the literature, the simplified average PWM switching model established by Voperian, and the current-mode mathematical model established by Ridley Engirleering, an equivalent circuit model of the flyback switching power supply is established, and the transfer function of each component can be obtained.
Both Gvd(s) and Gid(s) have two poles, but when the current loop gain is large enough, their two poles can cancel each other out, thus yielding the following approximate transfer function with a single pole:
Where: UI is the DC input voltage, Uo is the output voltage, RL is the equivalent load resistance, RS is the current sampling resistor, and n=Ns/Np is the transformer turns ratio.
The zeros and poles are calculated as follows:
Where D is the duty cycle, Lp is the primary inductance, RL is the load resistance, C is the output capacitor, and Resr is the equivalent series resistance of the output capacitor.
From equation (1), the transfer function from control to output of the switching power supply shown in Figure 1 can be obtained:
For multi-output loads RL, this refers to the equivalent load at the control output. Following the approach outlined in the literature, all other outputs are "mapped" to the 5V feedback output, thus...
The zero frequency of the right half-plane is frz = ωrz/2π = 12kHz, the zero frequency of the capacitor's equivalent series resistance (ESR) is fz = ωz/2π = 2.2kHz, and the load pole frequency is fp = ωp/2π = 125kHz.
flyback converter
The flyback converter is derived from the Buck-Boost converter. By replacing the inductor with an isolation transformer, the flyback converter shown in the figure below can be obtained.
Important waveforms of flyback
When the switching transistor is turned on, the current in the inductor rises. It can be seen that its current graph is very similar to that of a BUCK-BOOSK circuit. The difference is the turns ratio of the primary and secondary sides. Here, it can also be seen that the transformer is essentially an inductor.
Single-ended flyback switching power supply
The single-ended flyback switching power supply is shown in the figure. In this circuit, "single-ended" means that the high-frequency transformer's core operates only on one side of the hysteresis loop. "Flyback" refers to the fact that when the switching transistor is on, the induced voltage in the primary winding of the high-frequency transformer T is positive at the top and negative at the bottom, and the rectifier diode D1 is in the off state, storing energy in the primary winding. When the switching transistor is off, the energy stored in the primary winding of transformer T is rectified by the secondary winding VD1 and filtered by capacitor C before being output to the load.
Single-ended flyback switching power supplies are the lowest-cost power supply circuits, with an output power of 20-100W. They can output different voltages simultaneously and have good voltage regulation. The only drawback is the relatively large output ripple voltage and poor external characteristics, making them suitable for relatively fixed loads.
The maximum reverse voltage that the switching transistor VT1 used in a single-ended flyback switching power supply can withstand is twice the circuit's operating voltage, and the operating frequency is between 20-200kHz.
Principle Analysis
EMI circuit (transient filter circuit)
After the mains power is connected to the PC switching power supply, it first enters the transient filter circuit.
EMI, or Electromagnetic Interference, is typically addressed using common-mode filters, which include common-mode capacitors, unbalanced transformers, or common-mode inductors. Common-mode capacitors bypass the common-mode current of the two input lines to ground, while common-mode inductors present a balanced impedance, meaning the impedances in the power and ground lines are equal. This impedance exhibits impedance characteristics against common-mode noise.
The function of a common-mode filter is to eliminate the "switching interference" unique to switching power supplies, so as to ensure that the equipment itself and other equipment in the power grid are free from interference.
F1: Fuse, used to protect the circuit when the current is too high.
R1 R2: Discharge resistors, used for filtering and discharging this part. Multiple resistors are used to distribute the power of the discharge.
C11: X capacitor, which filters differential-mode interference, i.e., the two ends of the input.
L1: Common-mode inductor, which dampens common-mode current.
rectifier filter circuit
Alternating current, after being rectified by the rectifier bridge, is filtered by C2 to obtain a relatively pure DC voltage. If the capacitance of C2 decreases, the output AC ripple will increase.
NTC thermistor: A negative temperature coefficient thermistor is connected in series at the input of the circuit to increase the impedance of the line, which can effectively suppress the surge current generated by the surge voltage when the power is turned on.
When the circuit enters steady-state operation, the NTC heats up due to the continuous operating current in the circuit, causing the resistance value of the resistor to become very small, and the effect on the circuit can be completely ignored.
Chip startup circuit
The CR6842 has two boot modes:
(1) Traditional startup method: When using VDD as the startup pin, the chip supports startup before rectification and startup after rectification and filtering. The startup circuit is as follows.
(2) Start-up mode with OCP compensation function: When using pin 3 VIN as the start-up pin, the chip has OCP compensation function, but only supports the start-up mode after rectification and filtering.
On the left, when the input voltage of the system changes, the current flowing through the Vin terminal through the start-up resistor also changes. The chip automatically compensates by detecting the change value at this port, so that the system can achieve constant power output.
On the right side, when the power supply is turned on, the capacitor C1 at the VDD terminal is charged through the startup resistor R11 until the voltage at the VDD port reaches the chip's startup voltage Vth(ON) (typical value 16.5V). Only then is the chip activated and drives the entire power supply system to work normally.
Switch ON path and current detection (current limiting protection)
The main application scenarios for single-ended flyback switching power supplies include the following:
Communication equipment: Communication equipment such as routers, switches, and wireless base stations require a stable power supply, while also having high requirements for size and efficiency. Self-excited single-ended flyback switching power supplies can meet these requirements.
Industrial automation equipment: In the field of industrial automation, equipment such as PLCs (Programmable Logic Controllers), sensors, and industrial computers typically require efficient and reliable power supplies. Single-ended flyback switching power supplies can meet the power requirements of these devices.
Home appliances: Home appliances such as televisions, stereos, computer monitors, and chargers require a stable DC power supply to operate. Single-ended flyback switching power supplies can provide efficient and stable power conversion, ensuring the normal operation of these devices.
LED Lighting: In the field of LED lighting, single-ended flyback switching power supplies power large outdoor LED displays and commercial lighting systems, ensuring uniform brightness and accurate color.
Automotive electronics: In the field of automotive electronics, single-ended flyback switching power supplies have also been widely used, such as in car audio systems and car dashboards.
In summary, single-ended flyback switching power supplies have a wide range of applications, especially excelling in fields requiring efficient, stable, and reliable power supply. With continuous technological development and advancements, the performance of single-ended flyback switching power supplies will be further improved, providing more reliable and efficient power solutions for a wider range of electronic devices and systems.
