Non-isolated power supply working principle _ What are the advantages and disadvantages of non-isolated power supply

With the development of the electronics industry, the requirements for power supplies are getting higher and higher, the volume is smaller, and the reliability is higher. Power modules have emerged as integrated devices. The utility model has the advantages of isolation, strong anti-interference ability, self-protection function, and convenient system integration, and is widely used.

The non-isolated power supply is directly input to the electronic circuit with 220V, and the output is output through the electronic component, and the input and output are directly connected through the electronic component, so it is called a non-isolated power supply;

In LED lighting applications, due to the high efficiency of non-isolated switching power supplies, there are more customers favoring and occupying a part of the market share. Represented by SN3 910, there are a series of functional ICs on the market, such as BP2808, SMD802, AM850, and SN3910. . . Wait. In terms of price, some domestic manufacturers' ICs have dropped to more than two yuan. Therefore, for low-cost applications, it is a reasonable choice. Next, take SN3910 as an example to illustrate the application lines and ideas of such ICs.

1. When Q1 is turned on, the input current Iin passes through the load LED, inductor L1, Q1 to the input power supply negative. When the LED is illuminated, the current in the L1 inductor rises slowly and reaches a peak value until Q1 is turned off, and L1 stores energy.

2. When Q1 is disconnected, because the voltage across the capacitor cannot be abrupt, the current flowing through the inductor cannot be abrupt. The principle is that the current flowing through the L1 inductor passes through the freewheeling diode D1, and the load LED forms a loop. The current in the inductor drops from the peak value to a value (if the value is greater than zero, Q1 is on, then it works at CCM; if it is equal to zero, Q1 is turned on immediately, then it works at BCM; if it is equal to zero, Q1 is not turned on in time, then DCM), Until Q1 turns on.

Note: For most BUCK circuits, multiple designs work in CCM because of the following two benefits:

1. Working in CCM, the output ripple current is relatively small.

2, working in CCM, the output current is better control: Io=(ILpkh+ILpkl)/2

Here Io is the output current rms, ILpkh and ILpkl are the inductor current peak and valley, respectively.

It can be seen from the DATAsheet that the internally set VCS voltage is: 250mV. By setting the CS pin to ground resistance, the peak current through the load LED can be set. How does the purpose of achieving constant current?

From the formula Io=(ILpkh+ILpkl)/2, it can be seen that ILpkh is set by RCS, and if ILpkl can be set, the problem is solved.

A capacitor from the TOFF pin to ground, used to set TOFF (off time), has the same turn-off time in each duty cycle; from the discharge loop of the inductor, if the inductor current is passed during the TOFF time If it does not fall to 0 (CCM), then Io=(ILpkh+ILpkl)/2 can get a constant ILpkl, thus achieving the goal of constant current.

So we can consider how to set the inductance L1, RCS, COFF. Explain in a common example:

INPUT: 85~265Vac 50/60HZ

OUTPUT: 40V 0.35A (12LED)

First of all: set TOFF, the following formula can be found from the data sheet.

Assume: COFF=220, then: Toff=11.73uS.

Assuming an output current ripple factor of 0.8, then:

RCS=VCS/IPKH=0.25/(0.35+(0.35*0.8)/2)=0.51R

Calculate the inductance:

L is equal to 40*11.73/(0.35*0.8)=1.676mH

1. The safety requirements of the luminaire are high, and the part of the non-safety certification material that the person can touch and the circuit need to be reinforced.

2. Relative to the isolation type, the volume can be relatively small and the price is relatively low.

3. The efficiency is relatively high.

Non-isolated power supplies generally use inductors as the main components of constant current control. Basically, they are wound by a single coil. The processing is easy and the magnetic core is relatively small. The requirements are not high and it is easy to mass-produce.

The energy efficiency of non-isolated drives can generally be 0.93-0.96.

The non-isolated drive power supply is about 5% more energy efficient than the isolated drive power supply, which means that using a non-isolated drive power supply can save about 5% of the electricity cost compared to using an isolated drive power supply. Because the non-isolated drive power supply has high efficiency and low loss, the internal components generate less heat, and the temperature rise of the whole drive is much lower. The service life of the drive power depends mainly on the life of the electrolytic capacitor. The life and temperature of the electrolytic capacitor are directly The relationship between the electrolytic capacitors is reduced by a factor of 10 for every 10 degrees of temperature reduction, so that a non-isolated drive power source can easily achieve a longer life.

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