Power supply design: Power adapter solution based on SinoWise SH79F081

Preface

At present, notebooks have changed from high-end consumer goods to general consumer goods, and their shipments have exceeded those of desktop computers. Netbooks have now increased the number of portable computers. Regardless of whether they are notebooks or netbooks, these computers need a power adapter to provide power. Although they are equipped with one when they are purchased. However, some people need one at home and one at work because of portability, and some people need to buy a new adapter because the original adapter is damaged. All of these have led to a strong demand for adapters. The current original adapters all output fixed voltages according to the power of the paired computer. One notebook needs a corresponding adapter. In this way, the adapter cannot be universal.

At present, there are power adapters on the market that can use a toggle switch to select several output voltages. By selecting the toggle switch, you can output a matching voltage that is suitable for most computers on the market. Although this adapter provides a certain degree of versatility compared to the original adapter, it still has many defects.

1: The output voltage range is narrow. It can only output a few voltages set by the toggle switch, which cannot match most portable products.

2: The output voltage fluctuates greatly.

3. The output voltage is unsafe. Because it uses a toggle switch, anyone may accidentally toggle the output voltage to mismatch and burn out the portable product.

4: The output load condition is not intuitive. It cannot reflect the current output voltage, current and other parameters.

In view of the above, we launched the LCD display power adapter designed by our company SH79F081, which makes up for the defects of the toggle switch power adapter.

1: The output voltage range is wide. It can be output from 12V to 24.5V in 0.5V steps.

2: The output voltage fluctuation is small. The output voltage is set below 0.3V.

3: Automatic output protection. When there is load, the adjustment cannot be performed. The set value in no-load adjustment is automatically saved.

4: The output voltage is displayed synchronously on the LCD. At the same time, the USB voltage and output current are also displayed synchronously.

5: With overload protection and recovery function.

H79F081 Introduction

SH79F081 is a high-speed and efficient enhanced 8051 compatible microcontroller. It retains most of the 8051 microcontroller features while being expanded and enhanced. The specific features are as follows:

FLASH ROM: 8k; RAM: 256 internal bytes

Oscillation 400K-12M, internal integrated RC oscillation 12M

25 CMOS bidirectional IOs, 4 selectable open-drain, integrated pull-up resistors

3 timers

1 set of 12-bit complementary PWM, 2 sets of 8-bit complementary PWM

8-channel 10-bit ADC, built-in comparison function

Enhanced UART

SPI master/slave mode interface

Low voltage reset, WDT

28pin SOP

The chip packaging is as follows:

Solution Design

The first version of the power adapter is implemented with the SH79F081+3843/7575 control solution. In the case of DC input, 3843 is responsible for boost PWM control, and SH79F081 provides feedback to 3843 based on the sampled voltage. In the case of 220v AC, 7575 is responsible for buck PWM control, and SH79F081 also provides 7575 feedback control based on the sampled voltage. The USB end uses a fixed 34063 output 5v control, which does not require MCU control, but only needs to collect the USB output voltage and display it. The display part uses chip IO to simulate LCD display.

The following circuit modules are analyzed:

DC Section

The DC part controls the input DC 12V through 3843 PWM and boosts the output to the set voltage value. The control principle is to control the voltage feedback terminal of 3843 to let 3843 automatically adjust PWM. The MCU part controls the feedback value fed back to the voltage feedback terminal of 3843 based on the comparison between the current output voltage value and the set value.

AC Section

The AC part is actually a switching power supply, and the principle is also to control the output voltage through the voltage feedback terminal of 7575.

USB Part

The USB part uses a 34063 chip. Through the resistor voltage division on pin 5, a fixed output of 5V can be set.

MCU controls the PWM part

SH79F081 collects the current output voltage AD value, compares it with the set value, and then the PWM port outputs a PWM waveform with a certain duty cycle, which is filtered to form a DC waveform with a certain voltage and provided to the feedback end of the PWM control chip for control. The PWM output control part is divided into two paths for control. One path is output to the 3843 voltage feedback end of the DC part to realize DC boost control; the other path is output to the TL431 part of the AC part to realize closed-loop control and adjust PWM by changing the current of the optocoupler.

LCD Display

The LCD display uses IO simulation LCD driver. At the same time, the key part shares IO with the LCD. It should be noted that the above resistance value needs to be adjusted according to the corresponding LCD. If it is too large, it may cause the drive waveform to deform and the display effect to be poor.

Current acquisition

Because the chip has a 10-bit high-precision AD, we used a small resistor to directly collect the current. In order to prevent consumption and to accurately collect the current, we used a 25 milliohm power resistor.

MCU peripheral settings

SH79F081 uses 3 AD channels to collect the current output voltage, output current, and USB output voltage. It performs corresponding control based on the current AD value. It also has backlight control, PWM output control, etc.

Follow-up

The PWM control of the first version of the power adapter above uses an external PWM chip instead of the PWM module of SH79F081, so our second version of the power adapter uses a chip built-in PWM module. Only the AC part still uses 7575 control. This can save a DC PWM control 3843 and corresponding peripheral devices, and also save a USB PWM control 34063 and corresponding peripheral devices. After this upgrade, BOM devices can be reduced, PCB size can be reduced, power consumption can be reduced, and conversion efficiency can be improved.