LED driver design: driving method, device selection, ripple suppression

LED driver design: driving method, device selection, ripple suppression缩略图

Mainstream several kinds of LED drive mode

1、Resistor-capacitor-buck

The use of capacitor impedance in AC to limit the input current, so as to obtain DC level to LED power supply. This driving method is simple in structure and low cost, but the input non-isolated scheme, there are security risks. And the conversion efficiency is very low, can not do constant current control.

LED driver design: driving method, device selection, ripple suppression插图

2, isolated flyback circuit

The use of flyback circuit, through the transformer on the secondary side of the DC level, and then through the optocoupler will be this level of ripple feedback back to the original side, so as to stabilise the self-excitation. This type of circuit meets the safety certification requirements, and the output constant current accuracy is better, higher conversion efficiency. However, due to the need for optocoupler and vice-side constant current control circuit, resulting in a complex system, large volume, high cost. Currently has been gradually replaced by the original edge of the programme.

LED driver design: driving method, device selection, ripple suppression插图2

3, the original edge of the programme

The primary side of the programme is completely in the AC primary side control output power and current, the most accurate constant current accuracy of 5% can be achieved, the secondary side of the simple output circuit can be. Primary side mainly rely on the auxiliary side of the feedback to control the output voltage, relying on the current limiting resistor on the primary side of the current control, while multiplying the turns ratio to control the accuracy of the output current. Primary side of the programme inherited the advantages of the isolated flyback circuit, while the architecture is simple, can be done in a small volume and low cost, has become the mainstream driver.

The problem of constant current accuracy of the primary side: due to the production accuracy of the transformer is difficult to control, resulting in the primary side of the scheme in the use of low-quality transformers, the output current drift is large. Therefore, the primary side of the programme through the improvement of the increase in the vice side of the constant current control circuit, so that although more complex than the ordinary primary side of the programme, but compared to the flyback programme, you can still eliminate the optocoupler, etc., the system has the highest cost-effective.

LED driver power supply design considerations

According to the power grid rules of electricity and LED drive power supply characteristics of the requirements, in the selection and design of LED drive power supply should take into account the following points:

1, high reliability

Especially like the LED street light drive power supply, installed in the high altitude, maintenance is not convenient, maintenance of the cost is also big.

2, high efficiency

LED is an energy-saving products, drive power supply efficiency should be high. For the power supply installed in the structure of the lamp, especially important. Because the luminous efficiency of the LED with the LED temperature increases and decreases, so the LED heat dissipation is very important. High efficiency of the power supply, its power loss is small, in the lamps and lanterns within the heat is small, also reduces the temperature rise of the lamps and lanterns. It is beneficial to slow down the light decay of LED.

3, high power factor

Power factor is the power grid requirements for the load. Generally 70 watts or less of electrical appliances, there is no mandatory indicator. Although the power factor of the individual appliances is not big low power factor on the grid is not very big, but at night we light, similar loads are too concentrated, will produce more serious pollution of the grid. For 30 watts to 40 watts of LED driver power supply, it is said that in the near future, maybe there will be a certain indicator requirements on the power factor.

4, drive mode

Now there are two kinds of pass: one is a constant voltage source for more than one constant current source, each constant current source individually to each road LED power supply. This way, the combination of flexible, all the way LED failure, does not affect the work of other LED, but the cost will be slightly higher. Another is direct constant current power supply, LED series or parallel operation. Its advantage is that the cost is a little lower, but the flexibility is poor, but also to solve the problem of a LED failure, does not affect the operation of other LEDs. These two forms, in a period of time co-exist. Multiple constant current output power supply method, in terms of cost and performance will be better. Maybe it is the mainstream direction in the future.

LED driver design: driving method, device selection, ripple suppression插图4

5, surge protection

LED surge resistance is relatively poor, especially the reverse voltage resistance. Strengthen this aspect of protection is also very important. Some LED lights installed in outdoor, such as LED street lights. Due to the grid load start dumping and lightning induction, from the grid system will invade a variety of surges, some surges will lead to LED damage. Therefore, the LED driver power supply should have the ability to inhibit the intrusion of surge, to protect the LED from being damaged.

6, protection function

Power supply in addition to the conventional protection function, it is best to increase the LED temperature negative feedback in the constant current output, to prevent the LED temperature is too high.

7, imitation protection

Lamps and lanterns outside the installation type, the power supply structure should be waterproof, moisture-proof, the shell should be sunlight-resistant.

8, drive the life of the power supply to be compatible with the life of the LED

9, to meet the requirements of safety regulations and electromagnetic compatibility

With the increasing application of LED, LED driver power supply performance will be more and more suitable for the requirements of LED.

LED driver device selection considerations

Here are some tips from recolux.

1, do not use bipolar power devices

Some designers in order to reduce the cost of LED drive and the use of bipolar power devices, which will seriously affect the reliability of the circuit.

Because with the LED driver circuit board temperature, the effective working range of the bipolar device will be rapidly reduced, which will lead to device failure at rising temperatures and thus affecting the reliability of LED lamps, the correct approach is to use MOSFET devices, MOSFET devices, MOSFET devices to be much longer than the service life of the bipolar device.

2, try not to use electrolytic capacitors

LED drive circuit in the end to use electrolytic capacitors? Currently there are supporters and opponents, supporters believe that if the circuit board temperature can be controlled, in turn, to achieve the purpose of extending the life of electrolytic capacitors.

For example, the choice of 105 degrees life of 8000 hours of high-temperature electrolytic capacitors, according to the prevailing formula for estimating the life of electrolytic capacitors, "the temperature is reduced by 10 degrees, the life expectancy doubled," then it is in the 95-degree environment life of 16,000 hours, in the 85-degree environment life expectancy of 32,000 hours, in the 75-degree environment life expectancy of 64,000 hours. The working life is 64,000 hours at 95 degrees, 32,000 hours at 85 degrees, and 64,000 hours at 75 degrees. If the actual working temperature is lower, the life span will be longer! From this point of view, as long as the choice of high-quality electrolytic capacitors to drive the life of the power supply is no effect!

There are also supporters of the high ripple current brought about by the electrolytic capacitors and the resulting low-frequency flicker will cause physiological discomfort to some people's eyes, the amplitude of the low-frequency ripple will also lead to some digital camera equipment, the appearance of poor-frequency flicker of the bright and dark grids. Therefore, high quality light source lamps and lanterns still need electrolytic capacitors. However, opponents believe that the electrolytic capacitor will naturally age, in addition, the temperature of the LED lamps and lanterns is extremely difficult to control, so the life of the electrolytic capacitor is bound to decrease, thus affecting the life of the LED lamps and lanterns.

In this recolux believes that in the LED driver circuit input part can consider not to use electrolytic capacitors, in fact, using PI's LinkSwitch-PH can save electrolytic capacitors, PI's single-stage PFC/constant-current design allows designers to save large-capacity capacitors, in the output circuit, can be used to replace the electrolytic capacitors with high-voltage withstand ceramic capacitors to enhance the reliability of the capacitors.

"Some people design two-stage circuits with a 400V electrolytic capacitor at the output, which will seriously affect the reliability of the circuit, and it is recommended to use a single-stage circuit with ceramic capacitors will be sufficient." He emphasised. "For less concerned about the dimming function, high temperature environment and the need for high reliability of industrial applications, I strongly recommend not to use electrolytic capacitors for design."

3, MOSFET withstand voltage should not be less than 700V

Withstand voltage 600V MOSFET is cheaper, many think that the input voltage of LED lamps and lanterns is generally 220V, so the withstand voltage 600V is enough, but many times the circuit voltage will be up to 340V, in the case of a surge, 600V MOSFET is easy to be pierced, thus affecting the life of the LED lamps and lanterns, in fact, the choice of 600VMOSFET may save Some of the cost but pay the price of the whole circuit board, so, "do not choose 600V withstand voltage MOSFET, it is best to use more than 700V withstand voltage MOSFET.

4, try to use a single-stage architecture circuit

Doug said that some LED circuits use a two-stage architecture, that is, "PFC (power factor correction) + isolated DC / DC converter" architecture, such a design will reduce the efficiency of the circuit. For example, if the efficiency of the PFC is 95% and the efficiency of the DC/DC part is 88%, the efficiency of the whole circuit will be reduced to 83.6%! "

PI's LinkSwitch-PH device simultaneously integrates the PFC/CC controller, a 725VMOSFET and MOSFET driver into a single package, increasing the efficiency of the drive circuit to 87%!" Doug noted, "Such a device greatly simplifies board layout design by eliminating up to 25 components used in traditional isolated flyback designs! Eliminated components include high voltage bulk electrolytic capacitors and optocouplers." Doug said the LED two-stage architecture is suitable for older drivers where a second constant current driver circuit must be used to keep the PFC-driven LEDs at a constant current. These designs are obsolete and no longer cost-effective, so in most cases it is better to use a single-stage design.

5, try to use MOSFET devices

If the design of the LED lamps and lanterns power is not very high, Doug suggests the use of integrated MOSFET LED driver products, because the benefits of doing so is the integrated MOSFET on resistance is less, the heat generated by the score is less than the discrete, in addition, is the integration of the MOSFET is the controller and the FET in the together, generally have overheating shutdown function, in the MOSFET will be overheating automatic Shut down the circuit to protect the LED lamps and lanterns, which is very important for LED lamps and lanterns, because LED lamps and lanterns are generally very small and difficult to air cooling.

LED driver design to suppress ripple 5 methods

LED driver design, for ripple, theoretically and practically are certain to exist, usually inhibit or reduce it has five practices.

Output ripple is the AC component superimposed on the DC stable quantity when the DC power supply is output. Using an oscilloscope to look at it, you will see the voltage fluctuating slightly up and down, just like water ripples, so it is called ripple. Usually inhibit or reduce it has the following practices:

1, increase the inductance and output capacitance filtering

According to the formula of the LED driver power supply, the size of the current fluctuations within the inductor and inductance value is inversely proportional to the output ripple and output capacitance value is inversely proportional to the value. So increase the value of inductance and output capacitance value can reduce the ripple.

Output ripple and output capacitance relationship: vripple = Imax / (Co × f). It can be seen that increasing the output capacitance value can reduce the ripple.

It is common practice, for output capacitance, to use aluminium electrolytic capacitors to achieve high capacity. But the electrolytic capacitor is not very effective in suppressing high-frequency noise, and the ESR is also relatively large, so it will be connected in parallel with a ceramic capacitor next to it to make up for the lack of aluminium electrolytic capacitors.

At the same time, the LED driver power supply work, the input voltage Vin is unchanged, but the current is changing with the switch. At this time, the input power supply will not be very good to provide current, usually near the current input (BucK type, for example, is near SWITcH), parallel capacitors to provide current.

2, the second level of filtering, is to add another level of LC filters

LC filter on the noise ripple suppression effect is more obvious, according to the ripple frequency to be removed to select the appropriate inductance and capacitance of the filter circuit, generally can be very good to reduce the ripple. However, this case needs to consider the sampling point of the feedback comparison voltage.

Sampling point selected before the LC filter (Pa), the output voltage will be reduced. Because any inductor has a DC resistance, when there is a current output, there will be a voltage drop across the inductor generated, resulting in a lower output voltage from the power supply. And this voltage drop varies with the output current.

3, LED driver power supply output, connected to the LDO filtering

This is the most effective way to reduce ripple and noise, the output voltage is constant, do not need to change the original feedback system, but also the highest cost, the highest power consumption. Any LDO has an index: noise rejection ratio.

After the LDO, the ripple is generally below 10mV.

4, in the diode and capacitance C or RC

Diode high-speed conduction cut-off, to consider the parasitic parameters. During the reverse recovery of the diode, the equivalent inductance and equivalent capacitance become an RC oscillator, generating high-frequency oscillations. In order to suppress this high-frequency oscillation, the diode needs to be connected in parallel at both ends of the capacitor C or RC buffer network. Resistance is generally taken 10Ω-100Ω, capacitance is taken 4.7pf-2.2nf.

In parallel with the diode capacitance C or RC, the value should be repeated tests to determine. If the selection is not appropriate, but will cause more serious oscillation.

5, the diode connected to the inductor (EMI filter)

This is also commonly used to suppress high-frequency noise. For the frequency of noise, select the appropriate inductive components, also can effectively suppress noise. It should be noted that the rated current of the inductor to meet the actual requirements

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