Design and application of a novel single-ended flyback isolation switch regulated power supply

    Abstract: This article introduces the design principle of a novel single-ended flyback isolation switching power supply, and applies this switching power supply to the control, drive and protection circuit of a 15k high-power switching power supply. Tests have proven that the power supply is simple and practical, works stably and has reliable performance.

    Keywords: switching power supply, slope compensation, pulse width modulation

1 Introduction

In high-power switching power supplies, several sets of isolated power supplies are often required to provide separate power supply to the gate drive and protection circuits of the control and switching tubes. Simple, practical, and reliable isolated voltage-stabilized power supply will be the guarantee for the reliable operation and excellent performance of the switching power supply. Because of this, we chose the UC3842PWM controller as the core device of the isolated switching regulated power supply, and strived to make the power supply simple, practical and reliable in performance.

2 Introduction to UC3842PWM controller

    UC3842 is a high-performance fixed-frequency current-mode controller with single-ended output that can directly drive bipolar transistors and OSFETs. It has a small number of pins, simple peripheral circuits, easy installation and debugging, and excellent performance. , low price and other advantages. It can be isolated from the power grid through a high-frequency transformer and is suitable for forming a (20-50) W low-power switching power supply without a power frequency transformer. Due to the ingenious design of the device, very few components are required to form the circuit, which is very consistent with the principle of "applicable, sufficient and easy to use". In some situations where only DC voltage power supply is available, it plays an indispensable role and has good application prospects.

Its internal circuit mainly has the following performance:

●The maximum switching frequency can reach 500kHz.

●Using a totem pole output circuit, it can provide large current output, the output current can reach 1A, and can directly drive bipolar transistors and MOSFETs.

●There is a high-stability reference voltage source inside, with a typical value of 5.0V and a deviation of ±0.1V allowed. Temperature coefficient is 0.2mV/℃

●Good voltage stabilization performance. Its voltage regulation rate can reach 0.01%/V, which is comparable to the second generation linear integrated voltage regulator. The starting current is less than 1mA, and the normal operating current is 15mA.

●Latched PWM, capable of pulse-by-pulse current limiting.

●With internal adjustable reference power supply, undervoltage lockout is possible.

Figure 1 is the internal structural block diagram of the UC3842PWM controller. Its internal reference circuit generates a +5V reference voltage as the internal power supply of UC3842, and the attenuated 2.5V voltage is used as the error amplifier reference, and can be used as a circuit output 5V/50mA power supply. The oscillator produces square wave oscillation. The oscillation frequency depends on the external timing component. The resistor R connected between pin 4 and pin 8 and the capacitor C connected between pin 4 and ground jointly determine the oscillation frequency of the oscillator, f= 1.8/RC. The feedback voltage is connected to the inverting terminal of the error amplifier from pin 2. Pin 1 is connected to an external RC network to change the closed-loop gain and frequency characteristics of the error amplifier; pin 6 outputs a square wave that drives the switching tube as a totem pole output. Pin 3 is the current detection terminal, used to detect the current of the switch tube. When the voltage of pin 3 is ≥ 1V, UC3842 will turn off the output pulse to protect the switch tube from overcurrent damage. The UC3842PWM controller is equipped with an undervoltage lockout circuit with a turn-on threshold of 16V and a turn-off threshold of 10V. Because of this, oscillation when the circuit is operating near the threshold voltage can be effectively prevented.

3 Principle and design of single-ended flyback isolation switch regulated power supply

Figure 2 is the schematic diagram of the single-ended flyback isolation switching power supply circuit. When the switch tube V is turned on, the voltage on the primary side of the transformer is approximately the input voltage, but no current flows on the secondary side due to the reverse bias of the rectifier tube, and the transformer stores magnetic field energy. When the switch tube V is turned off, the voltage of each coil is reversed, the rectifier tube is forward-conducting, and the energy stored in the transformer is released to the load through the rectifier tube. One condition that a single-ended transformer must meet is to ensure that the magnetic flux is reset, that is, the working magnetic flux of the transformer must return to its original position in each cycle. Otherwise, the magnetic core will gradually reach saturation after multiple cycles and burn the switch tube.

Figure 3 is the actual circuit schematic diagram of a single-ended flyback isolation switching power supply. Since the high-power power supply has a DC voltage of +24V, the input voltage of this power supply is 24V. 7815 starts the UC3842 after raising the potential through a light-emitting diode. The voltage of the feedback coil 12 is rectified and divided by the sampling resistor and sent back to the reverse end of the error amplifier of the UC3842 to adjust the driving pulse width, thereby changing the output voltage. According to the actual given timing component values, the operating frequency of UC3842 is 166.7kHz. R4=0.5Ω is the current detection resistor. The detection signal on it is added to the slope compensation signal after a low-pass filter and sent to pin 3 of the UC3842. When the current flowing through the switch tube exceeds 2A, the UC3842 is turned off to protect the The switch tube will not be damaged. With slope compensation, the circuit becomes stable at any duty cycle. The capacitor C4 connected in parallel with the switching tube is to eliminate the peak voltage of the switching tube. The transformer uses a printed circuit board transformer, which does not require manual winding, which simplifies the manufacturing process of the transformer. At the same time, it reduces the volume of the entire power supply and enables complete coupling between coils (see Figure 4). It can be easily added or added as needed. Reduce the number of output groups. The transformer ratio is 1, the primary coil and the secondary coil are stacked in pairs (for better coupling), plus a set of feedback coils. This ensures the output of multiple sets of isolated 24V stable voltages.

4 Experimental analysis and conclusion

Figure 5 shows the 4-pin triangular wave oscillation waveform (Ch1) and the 6-pin square wave driving the switch tube (Ch2) of UC3842. Figure 6 shows the current waveform of the detection resistor (R4). Figure 4 shows the voltage waveforms of the primary and secondary windings. This is the waveform under light load conditions. The oscillation is caused by the resonance of the high-frequency transformer and C4. After loading, the oscillation amplitude will be significantly reduced without affecting the performance of the entire system. These waveforms and their test values ​​are basically consistent with the theoretical waveforms and calculated values. This switching power supply has been used in a 15kW DC regulated switching power supply, and tests have proven that it fully meets the predetermined performance index requirements.