Application of current-mode control chips

1 Introduction

As we all know, the current-mode control chip retains the output voltage feedback part of the voltage-mode chip and adds a feedback link: comparing the current signal VS with the output VE of the error amplifier and then controlling the latch, as shown in Figure 1.

Its working principle is: the constant frequency clock pulse sets the latch output pulse to drive the power tube to conduct, and the current pulse amplitude in the power supply circuit gradually increases; when the current signal amplitude reaches the VE level, the state of the pulse comparator flips, the latch is reset, the drive is removed, and the power tube is turned off. The circuit detects and adjusts the current pulses one by one in this way to achieve the purpose of controlling the output.

2 Introduction to UC3842

This is explained through a high-efficiency single-ended flyback DC/DC converter. The circuit uses UC3842, which is a high-performance fixed-frequency current-mode control chip. It can be well applied in the design of isolated single-ended switching power supplies and DC/DC converters. Its biggest advantage is that it has few external components, simple external circuit assembly, and low cost.

Figure 1 Principle of current-mode control chip

Figure 2 UC3842 functional block diagram

Its functional block diagram is shown in Figure 2, and the main pin functions are as follows:

Pin 1: Error amplifier output, can be connected to RC compensation network to compensate for the error

The inverting input of the difference amplifier determines the closed-loop gain and frequency response of the amplifier, making the chip work stably.

Pin 2: Error amplifier inverting input terminal, receiving external control signal.

Pin 3: Current signal acquisition terminal, used to collect the current of the primary coil

The current signal is generated by the sampling resistor, and the voltage generated by the sampling resistor is compared with the error voltage output by the error amplifier to generate a pulse signal with a modulated pulse width to control the primary peak current. Therefore, this type of chip is called a current type pulse width modulator.

Pin 4: External timing resistor RT and timing capacitor CT determine oscillation

Device operating frequency, f=1.8/RT.CT

Pin 5: GND ground pin.

Pin 6: Output drive pin, using totem pole output circuit, the output current can be

Up to 1A, it can directly drive bipolar transistors and MOSFET tubes.

Pin 7: Power input terminal, the maximum input voltage is 30V, the opening threshold is set at 16V, and the closing threshold is set at 10V. The difference between the two values ​​is 6V, so it can effectively prevent the circuit from oscillating when working near the threshold voltage.

Pin 8: +5V reference voltage source output pin, can provide a maximum current of 50mA.

UC3842 has a good linear regulation rate, which can reach 0.01%/V, because the change of input voltage Vi is immediately reflected as the change of inductor current. It can change the output pulse width in the comparator without passing through any error amplifier. Adding a level of output voltage Vo to the control of the error amplifier can make the linear regulation rate better; it can significantly improve the load regulation rate, because the error amplifier can be used specifically to control the output voltage change caused by load changes, especially to greatly reduce the voltage increase at light load. The external circuit compensation network of the error amplifier is simplified, the stability is improved and the frequency response is improved, with a larger gain-bandwidth product. The current limiting circuit is simplified. Since the peak inductor current is induced on the resistor Rs, a pulse-by-pulse limiting circuit can be naturally formed. As long as the level on Rs reaches 1V, the PWM will be immediately turned off, and this peak inductor current detection technology can sensitively limit the maximum output current.

3 Circuit Design

The circuit diagram is shown in Figure 3. The circuit works in the flyback mode, using a 0.2 ohm resistor R10 to sample the current signal, which is filtered by C9 and input to the current signal acquisition terminal 3 pin; and an optocoupler is used for feedback isolation. The feedback winding provides feedback voltage, and the reasonable turns ratio keeps the voltage provided by the feedback winding at about 18V, which does not change with the input voltage or the weight of the load, but is just greater than the threshold voltage of 16V of the chip 7 pin (power pin); R1 is the starting resistor, and C7 is the starting capacitor; it can be seen that the +12V output does not use a voltage regulator, but uses a resistor R21 to connect to the reference terminal of TL431 for +12V voltage regulation, which will inevitably have a certain impact on the accuracy of +5V. However, after testing, the variation range of +5V is 4.95V~5.08V, and the variation range of +12V is 11.6V~12.6V, which meets user requirements; the +12V output winding can also be stacked and wound on the +5V winding, which is more conducive to the stability of the +12V output voltage, but the +5V winding wire needs to be thickened; overvoltage protection is achieved by V9, R15, R16, C21, and V10, which is actually achieved by short-circuit protection caused by the conduction of the thyristor, but it works reliably and effectively.

4 Conclusion

When actually making it, please note:

(1) To improve efficiency, a horizontal magnetic core EC28 is used, which has a larger window. The primary and +5V output windings can be wound with Φ0.67 wires, two in parallel; the +12V output winding can be wound with a Φ0.67 wire; the feedback winding and -12V output winding can be wound with a Φ0.4 wire. Current signals can also be collected using current transformers, but they are slightly more complicated.

(2) The input voltage range is 18V to 35V; the function of V1 is to restore the action of fuse FU when the input voltage is connected in reverse without burning the power supply.

Figure 3 Circuit diagram