Research on the standby function of PWM chip in switching power supply

1 Introduction

Standby refers to the state when the product is connected to the power supply but is not running its main function. The purpose of standby is to reduce the loss of the power supply when it is unloaded or lightly loaded. With the popularization of electrical appliances and network products, the power consumption of electronic products in standby state has attracted more and more attention from international energy conservation, environmental protection organizations and relevant countries. Standby energy consumption not only wastes electricity, but also creates huge environmental pressure. We know that the switching loss is proportional to the operating frequency of the power supply. Therefore, we can try to reduce the operating frequency of the power supply when the output power of the power supply becomes smaller or even enters the standby state. This can be achieved through many control function chips, such as the integrated chip L5991. At present, many PWM chips do not have the standby function of variable frequency. Therefore, we can learn from this functional circuit of the L5991 chip to realize the standby function of other PWM chips.

2 Introduction to the standby function circuit of the L5991 chip

The L5991 chip is developed from the BCD60II technology and is designed to use a fixed-frequency current mode control to achieve offline DC/DC power supply applications. The L5991 is a standard current-mode PWM controller that has programmable soft start, output/input synchronization, latching (for overvoltage protection and power management), precise limit duty cycle control, pulse current limiting, overcurrent protection with soft start, and a standby function that reduces the oscillator frequency when no load or light load.

Figure 1 shows the basic internal circuit of the chip's standby function. Pin 2 is connected to two resistors (RA and RB) and a capacitor (CT), which are connected as shown in Figure 1 to set the operating frequency (fosc) of the oscillator in normal operation and the operating frequency (fsb) in standby mode respectively. In fact, as long as the standby signal is high, the pin can be internally connected to the reference voltage Vref through an N-channel FET, so the timing capacitor CT is discharged through RA and RB. When the standby signal becomes low, the N-channel FET is turned off and the pin is left floating, and CT is only discharged through RA, so the oscillator frequency will become lower. VCT is controlled by Vref through RA and RB in normal operation, and is regulated by RA in standby mode. When the voltage on CT reaches 3V, the capacitor will discharge quickly internally. When the voltage drops to 1V, it starts to charge again.

Figure 1 Basic circuit of the standby function of the L5991 chip

In normal operation, RT will be equal to RA//RB, and its frequency formula is

fosc≌ (1)

In standby mode, RT=RA, and the frequency formula is:

fSB≌ (2)

Where: KT=

L5991 detects the feedback voltage associated with the load, automatically reduces the oscillator frequency when the load drops to a defined value (controlled by the parameters of the components in the circuit), and restores its normal operating frequency when the load increases and exceeds the second limit value. In this way, the system controlled by the L5991 operating frequency can rely on its standby function to achieve frequency conversion when the system is in standby and working. When the system is in standby, the frequency is reduced, and the standby frequency can be made very low by setting the circuit parameters, thereby reducing switching losses.

As a current mode controller, the output voltage Vcomp of the error amplifier of L5991 is proportional to the peak value of the main current except for the offset. Therefore, the load condition of the power supply can be inferred by monitoring Vcomp.

If, due to a reduced load, the main current peak decreases and Vcomp drops to a fixed limit (VT1), the oscillator frequency will be set to a lower value (fsb). If, the main current peak increases and Vcomp exceeds VT2, the oscillator frequency will be reset to the normal value (fosc). The change in frequency causes a change in Vcomp and in the opposite direction for energy balance reasons, so providing an appropriate hysteresis prevents the oscillator frequency from varying between fsb and fosc.

3 Implementation of the standby function of the flyback switching power supply

Based on the standby principle of the L5991 chip mentioned above, we can imagine adding a standby function to the flyback switching power supply composed of UC3842. By detecting the feedback voltage associated with the load and using the output value of the error amplifier inside the chip, the frequency is changed.

Pin 1 of the UC3842 chip is the error amplifier output. Figure 2 shows the basic circuit of the chip's standby function.

Figure 2 Basic circuit of chip standby function

The main principle of this circuit is to detect the output value of the feedback voltage after passing through the error amplifier, and drive the switch tube to be turned on or off through a hysteresis comparator (Schmitt trigger) to achieve the change of RT, thereby changing the oscillation frequency of the power supply.

We can see that the working state of the power supply (normal operation or standby) depends on the setting of the threshold of the hysteresis comparator, and the threshold depends on the output value of the error amplifier during standby and normal operation of the power supply.

In the actual designed circuit, when the power supply circuit is unloaded, the output is about 1.6V, and when it is lightly loaded, it is above 1.8V. Therefore, we set the threshold of the hysteresis comparator according to this value. The hysteresis comparator is composed of a 555 chip plus external resistors. The circuit diagram of the comparator is shown in Figure 3.

Figure 3 Hysteresis comparator circuit

In Figure 3, the reference power supply VDD of the 555 chip is +5V, which is given by the reference voltage output from pin 8 of UC3842. The upper and lower thresholds of the hysteresis comparator are calculated as follows:

VTH=VDD    （3）

VTL=    （4）

According to the threshold value determined above, the resistance value of each resistor is determined.

When the load of the power supply circuit changes, the power supply operates at a corresponding frequency according to the threshold of the hysteresis comparator.

4 Test results

Based on the above principle, a single-ended flyback switching power supply circuit controlled by the UC3842 chip was constructed [1][2][3], and a standby circuit was added, in which CT=4700μF and RA=RB=20kΩ were taken to verify the above principle.

Figure 4 shows the frequency change when switching from no-load to 5W load, the frequency changes from 20kHz to 40kHz, and when switching back to no-load, the frequency changes from 40kHz back to 20kHz, as shown in Figure 5.

Figure 4 Frequency conversion when switching from no-load to load

Figure 5 Frequency change when load switches to no-load

Figures 6 and 7 show the oscillation pulses of the power supply circuit and the driving signal waveforms of the power MOSFET device in the normal working state and the standby working state. The working frequency of the power supply can be clearly seen from the figures.

Figure 6 Oscillation pulse and drive waveform during normal operation

Figure 7 Oscillation pulse and drive waveform in standby mode

5 Conclusion

The switching power supply composed of UC3842 can completely add a standby function circuit to realize the standby function, reduce the switching frequency when no-load, and effectively reduce the switching loss. In addition, similar detection control circuits can also be added to other PWM chips to realize the standby function.