Isolated single-ended flyback switching power supply based on current-mode PWM integrated controller UC3842/UC3843

Switching power supplies have been widely used for their high efficiency and small size. Traditional switching power supplies generally use voltage-type pulse width modulation (PWM) technology, while current-type PWM technology has developed rapidly in recent years. Compared with voltage-type PWM, current-type PWM has better voltage regulation and load regulation, and the stability and dynamic characteristics of the system are also significantly improved. In particular, its inherent current limiting capability and parallel current sharing capability make the control circuit simple and reliable.

The current-mode PWM integrated controller has been commercialized, which has greatly promoted the development and application of low-power switching power supplies. The current-mode PWM control of low-power power supplies has replaced the voltage-mode PWM control of low-power power supplies. The UC3842 series control chip launched by Unitrode is a typical representative of the current-mode PWM controller.

DC/DC Converters

The converter is one of the most important components of the switching power supply. There are five basic types: single-ended forward, single-ended flyback, push-pull, half-bridge and full-bridge converters. The following focuses on the analysis of the isolated single-ended flyback converter circuit, and the circuit structure diagram is shown in Figure 1.

Figure 1 Circuit structure diagram

The circuit works as follows: when M1 is turned on, it stores energy in the primary inductor of the transformer, and the diode VD connected to the secondary of the transformer is in a reverse bias state, so the diode VD is cut off, and no current flows through the secondary of the transformer, that is, no energy is transferred to the load; when M1 is cut off, the voltage polarity in the secondary inductor of the transformer is reversed, so that VD is turned on, charging the output capacitor C, and at the same time, current I flows through the load R. The equivalent topology of M1 on and off is shown in Figure 2.

Figure 2 Equivalent topology of M1 on and off

Current-mode PWM

Compared with voltage-type PWM, current-type PWM control not only retains the output voltage feedback control, but also adds an inductor current feedback link, and uses this current feedback as the ramp function required for PWM.

The following is an analysis of the operation of the current-mode PWM circuit under ideal no-load conditions (mutual inductance is not considered). The circuit is shown in Figure 3. Assuming V is turned on, we have

L·diL/dt = ui (1)

iL increases linearly with a slope of ui/L, where L is the primary inductance of T1. Ud=R1·iL is sampled by the non-inductive resistor R1 and sent to the pulse width comparator A2 for comparison with Ue. When Ud>Ue, A2 outputs a high level and sends it to the reset end of the RS latch. At this time, one of the two inputs of the NOR gate must be high, and the power switch tube V is turned off through the NOR gate output low level. When the clock output is high, the NOR gate output is always low, blocking PWM. This period of time is determined by the output pulse width of the clock oscillator OSC, that is, the dead time of the PWM signal. At the same time as the oscillator output pulse falls, both inputs of the NOR gate are low, and the NOR gate output is high, and V is turned on.

Figure 3 Ideal no-load current-mode PWM circuit

In short, the rising edge of the PWM signal is determined by the falling edge of the oscillator, and the falling edge of the PWM is determined by the inductor current limit signal and the error signal Ue. The falling edge of the maximum pulse width is controlled by the rising edge of the oscillator. Figure 4 is its working timing diagram.

Figure 4 Working sequence diagram

UC3842 Introduction

Unitrode's UC3842 is a high-performance fixed-frequency current-mode controller that includes an error amplifier, PWM comparator, PWM latch, oscillator, internal reference power supply, and undervoltage lockout units. Its structure diagram is shown in Figure 5.

Figure 5 UC3842 structure diagram

The functions of each pin are briefly described as follows.

Pin 1 COMP is the output of the internal error amplifier. Usually a feedback network is connected between this pin and pin 2 to determine the gain and frequency response of the error amplifier.

Pin 2 FEED BACK is the feedback voltage input terminal. This pin is compared with the reference voltage (usually +2.5V) at the same-direction input terminal of the internal error amplifier to generate a control voltage to control the width of the pulse.

ISENSE on pin 3 is the current sensing terminal. In the peripheral circuit, a small-resistance sampling resistor is connected in series to the source of the power switch tube (such as VMos tube) to convert the current of the pulse transformer into a voltage, which is sent to pin 3 to control the pulse width. In addition, when the power supply voltage is abnormal, the current of the power switch tube increases. When the voltage on the sampling resistor exceeds 1V, UC3842 stops outputting, effectively protecting the power switch tube.

Pin 4 RT/CT is the timing terminal. The sawtooth oscillator is connected to the common terminal of the timing capacitor C and the timing resistor R.

Pin 5 GND is ground.

Pin 6 OUT is the output terminal, which is a totem pole output with a driving capacity of ±1 A. This totem pole structure is beneficial to the shutdown of the driven power tube, because when the transistor VT1 is turned off, VT2 is turned on, providing a low-impedance reverse current extraction loop when the power tube is turned off, accelerating the shutdown of the power tube.

Pin 7 Vcc is the power supply. When the supply voltage is lower than +16V, UC3824 does not work, and the power consumption is less than 1mA. The input voltage can be obtained by stepping down the high voltage through a large resistance resistor. After the chip is working, the input voltage can fluctuate between +10 and +30V, and it stops working when it is lower than +10V. The power consumption during operation is about 15mA, which can be provided by the feedback resistor.

The 8-pin VREF is the reference voltage output, which can output an accurate +5V reference voltage with a current of up to 50

The voltage regulation rate of UV3842 can reach 0.01%, the operating frequency is 500kHz, the starting current is less than 1mA, the input voltage is 10~30V, the reference voltage is 4.9~5.1V, the operating temperature is 0~70℃, and the output current is 1A.

Switching Power Supply

The switching power supply circuit composed of UC3842 is shown in Figure 6, where T is a high-frequency transformer. When the machine is just turned on, the 220V AC power first passes through PNF to filter out the radio frequency interference, then passes through rectification and filtering to obtain a DC voltage of about +300V, and then is stepped down by R2 to provide a +16V starting voltage to UC3842. R1 is a current limiting resistor, and C1 is a filter capacitor. After normal operation, the high-frequency voltage on the self-feeding coil N2 is rectified and filtered by VD1 and C1, and then used as the normal working voltage of UC3842. R5 and C4 are used to improve the frequency response of the internal error amplifier, and R1 is a slope compensation resistor. Switching frequency. C5 is a noise elimination capacitor, R10 is an overcurrent detection resistor, and R7 is a gate current limiting resistor of the VMOS switching power tube. C8, VD1, R11, VD2, and C9 form a two-stage absorption circuit for absorbing peak voltage. VD1 and VD3 use recovery diodes FR305. VD4 is the rectifier tube of the output stage, which adopts Schottky diode to meet the needs of high frequency and large current rectification.

Figure 6 Switching power supply circuit composed of UC3842

When the NMOS tube is turned on, the current of the primary coil N1 increases linearly, the magnetic field is enhanced, VD4 in the secondary coil is turned off, and capacitor C10 supplies power to the load; at this time, VD2 in the primary circuit of the pulse transformer is also turned off, and N1 plays the role of storing energy. When the NMOS tube is turned off, the current of the primary coil decreases, the magnetic field weakens, VD4 in the secondary coil circuit is turned on, and energy is released to the load through VD4 and C10, outputting a DC voltage, and part of the energy is released from VD2 to resistor R12 and capacitor C9.

In order to ensure that the DC voltage output by the switching power supply is not disturbed, a voltage stabilization circuit is provided in the circuit. First, the source of the NMOS tube is connected in series with the resistor R9 to convert the current signal into a voltage signal, which is sent to UC3842 as a comparison voltage to control the duty cycle of the excitation pulse to achieve the purpose of voltage stabilization. Second, the coil N2 in the transformer T is indirectly sampled to play a role in voltage feedback. After indirect sampling by N2, it is rectified by VD1 and C3 and sampled on C3. On the one hand, the voltage is sent to the 2 pin of UC3842 through the voltage divider R3 and R4 and added to the inverting input terminal of the error amplifier A3. On the other hand, it is directly sent to the 7 pin of UC3842 as the chip power supply voltage. When the circuit is just started, the input voltage is rectified and filtered to power the chip. After working, it is powered by the feedback voltage. Therefore, the power supply voltage of UC3842 reflects the change of the output voltage, plays a feedback role, and stabilizes the output voltage. Third, in UC3842, the slope of the sawtooth wave output by the sawtooth generator is also related to the input voltage. When the input voltage increases, the slope of the sawtooth wave increases, which reduces the duty cycle of the output excitation pulse, thereby maintaining the output voltage stable, and vice versa. This is actually equivalent to feedback control.

Summarize

UC3842 is a popular current-type PWM signal generator with the advantages of high precision, stable voltage, simple peripheral circuit and low price. It is widely used in small power switching power supplies with an output voltage range of 4.9~5.1V and a power of 20~60W.