I make switching power supplies. I used to make adapters, chargers, and
metal case switching power supplies. Later, I made LED power supplies. I
initially made some 1W and 3W high-power LED drivers, but later I made less. The
reason is simple, there is no market. The switching power supply I made has an
integrated MOS switching power supply chip and a transformer. The cost of these
two is placed there, and of course the performance is also placed there. But I
believe that eventually the low-power LED constant current driver will eliminate
the RC step-down power supply.
Because consumers will gradually become more rational, a lamp made from a
resistance-capacity step-down power supply has almost no practical value and can
only be used as a decoration and toy. If LED really enters the field of general
lighting, the resistance-capacitance reduction will Piezoelectric power sources
simply won't do the job. I can predict that in the future, as LED performance
improves and prices decrease, power supply costs will also become a very
important part of the cost of LED lamps. For real lamps, resistor-capacitor
voltage reduction is simply not up to the task. The popularity of RC step-down
power supplies is just a transition. In the end, constant current power supplies
are the authentic ones.
I am still optimistic about low-power LED lamps. For low-power LED lights,
the current main problem is that the light attenuation is too large and the
price is not ideal. But now it still has advantages over high power for general
lighting. I think it will be within five years for low-power LED lamps to enter
the general lighting field and compete with energy-saving lamps. The entry of
high-power LED into general lighting is definitely five years away. So now I
focus on the R&D and production of low-power LEDs.
I noticed that the current low-power LED lamps used in general lighting
mainly include LED desk lamps, LED honeycomb lamps, and LED fluorescent lamps.
Especially LED fluorescent lamps. Since the second half of 2007, many people
have begun to develop them. It can be said that they are extremely hot.
Basically, eight out of ten people who come to me now are doing this, so I also
started making power supplies for LED fluorescent lamps, and I have been working
on them for a while, so I would like to briefly talk about the development and
production of this kind of power supply. methods and principles. It's my
personal experience.
About the appearance of LED fluorescent lamp power supply
Nowadays, LED fluorescent lamp power supplies are generally required by
lamp manufacturers to be placed in a lamp tube, such as a T8 lamp tube. Very few
are external. Don't know why this is happening. In fact, it is difficult to
build a built-in power supply and the performance is not good. But I don’t know
why so many people still ask for it. Maybe it all fell with the wind. The
external power supply should be said to be more scientific and convenient. But I
also have to go with the flow and do whatever the customers want. But do it
inside
Setting up the power supply is quite difficult. Because of the external
power supply, there are basically no requirements on the shape. It doesn’t
matter how big or small you want to make it, or what shape you want to make it
into. There are only two types of built-in power supplies. One is the most
commonly used one, which means it is placed under the light board, with the
light board on top and the power supply below. This requires the power supply to
be very thin, otherwise it cannot be installed. Moreover, this will only knock
down the components, and the wiring on the power supply will only be lengthened.
I don't think this is a good idea. But everyone generally likes to do it this
way. I'll do it. Another way is to use less and put them at both ends, that is,
at both ends of the lamp. This is easier to make and the cost is lower. I've
done it before, basically these two built-in shapes.
Questions about the requirements and circuit structure of this kind of
power supply
My opinion is that because the power supply must be built into the lamp,
and heat is the biggest killer of LED light failure, the heat must be small,
that is, the efficiency must be high. Of course, a high-efficiency power supply
is required. For T8 lamps that are one meter long, it is best not to use one
power supply, but to use two, one at each end to disperse the heat. This
prevents the heat from being concentrated in one place. The efficiency of the
power supply mainly depends on the structure of the circuit and the components
used. Let’s talk about the circuit structure first. Some people also say that
the power supply needs to be isolated. I think it is absolutely unnecessary,
because this kind of thing is originally placed inside the lamp body and cannot
be touched at all. There is no need to isolate, because the efficiency of an
isolated power supply is lower than that of a non-isolated power supply.
Secondly, it is best to output high voltage and low current. Only such a power
supply can achieve high efficiency. What is commonly used now is the BUCK
circuit, which is a step-down circuit.
It is best to make the output voltage above 100 volts and set the current
at 100MA. For example, if you drive 120 units, it is best to drive three strings
of 40 units each. The voltage will be 130 volts and the current will be 60MA.
.This kind of power supply is used a lot. I just think it is a little bad. If
the switch tube is out of control, the LED will be exhausted. LEDs are so
expensive now. I am more optimistic about the boost circuit. The advantages of
this type of circuit, as I have repeatedly said, are that the efficiency is
higher than that of the buck circuit, and the second is that if the power supply
is broken, the LED light will not break. This ensures nothing goes wrong. If a
power supply is burned out, you will only lose a few dollars. If you burn out an
LED fluorescent lamp, you will lose hundreds of dollars in cost.
So I always recommend a boost power supply. Also, it is easy to make the PF
value high for the boost circuit, but it is more troublesome for the buck
circuit. I absolutely believe that the benefits of using a boost circuit for LED
fluorescent lamps are overwhelmingly stronger than that of a buck circuit. The
only disadvantage is that under the condition of 220V mains input, the load
range is relatively narrow, generally only applicable to 100 to 140 LEDs in one
or two strings. For those with less than this number, or those sandwiched in the
middle, it is Inconvenient to use. However, those who make LED fluorescent lamps
now generally use 100 to 140 for the 60cm long ones, and 200 to 260 for the 1.2
meter long ones, which are still OK to use. Therefore, now LED fluorescent lamps
generally use non-isolated buck circuits and non-isolated boost circuits. This
kind of circuit is used in LED fluorescent lamps, which should be regarded as my
first creation.
Opinions on high PF LED fluorescent lamp power supplies and high current
LED fluorescent lamp power supplies:
Personally, I think that many of these practices are just a waste of time.
Now let me first ask what are the advantages of LEDs compared to traditional
lamps. First, energy saving, secondly longevity, and thirdly, they are not
afraid of switching on and off, right? However, the current high PF methods use
passive valley-filling PF circuits. The original driving method is changed from
48 series and 6 parallel to 24 series and 12 parallel. In this case, the
efficiency will decrease under 220V. About five percentage points, so the LED
fluorescent lamp power supply generates higher heat, and the lamp beads will
also be affected a little.
Another problem is that the method of 24 in series and 12 in parallel will
make the wiring of LED fluorescent lamp beads very uncomfortable and difficult
to wire. In my opinion, the best way is the 48-in-a-string method, which is
mainly due to its high efficiency, low heat generation, and easy and
uncomplicated wiring. What's more, some people are now proposing 24 parallel and
12 series. This method is only suitable for isolated power supplies. It is not
suitable for non-isolated power supplies. Some people who don't know much about
power supplies think that their non-isolated power supply with a constant
current output of 600MA is awesome. In fact, they have not carefully put it in a
lamp and tried it. It would be strange if it doesn't heat up like this.
Therefore, the current low-voltage and high-current power supply for LED
fluorescent lamps is really a waste of time. Encouraged, let’s continue
talking.
Two constant current control methods
What I want to talk about next is the two constant current control modes of
the switching power supply, resulting in two approaches. These two approaches
are quite different in terms of principles, device applications, and
performance.
1. Represented by current constant current LED dedicated ICs
The first type is represented by current constant-current LED dedicated
ICs, mainly such as 9910 series and AMC7150. All current brands of LED
constant-current driver ICs are basically of this type. Let’s call them
constant-current IC types. But I think the effect of this so-called constant
current IC is not very good when it comes to constant current. The control
principle is relatively simple. It is to set a current threshold in the primary
circuit of the power supply. When the primary MOS is turned on, the inductor
current rises linearly. When it rises to a certain value, When this threshold is
reached, the current is turned off, and the trigger circuit triggers conduction
in the next cycle. In fact, this kind of constant current should be a kind of
current limiting. We know that when the inductance is different, the shape of
the primary current is different. Although there is the same peak value, the
average current value is different. Therefore, when this kind of power supply is
generally produced in batches, the consistency of the constant current is not
easy to control. Another feature of this type of power supply is that the output
current is generally trapezoidal, that is, fluctuating current. The output is
generally smoothed without electrolysis. This is also a problem. If the current
peak value is too large, it will affect the LED. If the output stage of the
power supply does not have the kind of power supply that uses electrolysis to
smooth the current, it basically falls into this category. That is, to determine
whether it is this control method, it depends on whether its output is coupled
with electrolytic filtering. I used to call this kind of constant current false
constant current, because its essence is a kind of current limit, and it is not
a constant current value obtained by comparing it with an op amp.
2. The second constant current method should be called switching power
supply.
This control method is similar to the constant voltage control method of
switching power supply. Everyone knows that TL431 is used for constant voltage,
because it has an internal reference of 2.5 volts, and then uses resistors to
divide the voltage. When the output voltage is higher or lower, a comparison
voltage is generated, which is amplified to control the PWM signal, so this
control method can control the voltage very accurately. This control method
requires a benchmark and an op amp. If the benchmark is accurate enough and the
amplification of the op amp is large enough, then the setting is very accurate.
Similarly, to make constant current, you need a constant current reference and
an op amp, which uses resistor overcurrent detection as a signal, and then uses
this signal to amplify to control PWM. Unfortunately, it is not easy to find a
very accurate reference signal now. Commonly used triodes are transistors, which
have a large temperature drift as a benchmark. You can also use the conduction
value of a diode of about 1V as a benchmark. This is also possible, but it is
not high. The best is to use an op amp plus a TL431 as a benchmark, but The
circuit is complex. However, the constant current accuracy of such a constant
current power supply is much easier to control. As for the constant current
controlled by this mode, the output must be electrolytic filtered, so the output
power supply is smooth DC, not pulsating. If it is pulsating, sampling will not
be possible. So to determine which one it is, just look at whether there is
electrolysis in its output.
The two constant current control modes determine the use of two different
types of devices. First, they determine that the two circuit devices are used
differently, have different performances, and have different costs.
The LED power supply made of constant current control IC represented by the
9910 series is actually current limiting and the control is relatively simple.
Strictly speaking, it does not belong to the mainstream mode of switching power
supply control. The mainstream mode of switching power supply control must have
reference and op amp. But this kind of IC can only be used for LEDs when it
comes out, and it is difficult to be used for other things, just because LEDs
have extremely low ripple requirements. But because it is only used for LED, the
price is higher now. Basically, it is made using 9910 plus MOS tube, and the
output does not have electrolysis. Generally, I see that many people use
I-shaped inductors as power conversion inductors. This kind of power supply is
generally shown in the chip information of the manufacturer, and it is basically
a step-down type. I won’t say much more, there are many more people who are good
at this than me.
The second one, represented by me, is the constant current driver in
switching power supply control mode. This kind of chip uses an ordinary
switching power supply chip as the core conversion device. There are many such
chips, such as PI's TNY series, TOP series, ST's VIPER12, VIPER22, Fairchild's
FSD200, etc., or even only use triodes or MOS tubes. RCC, etc., can be done. The
advantage is low cost and good reliability. Because ordinary switching power
supply chips are not only cheap, but also classic products that have been
extensively used. ICs like this generally integrate MOS tubes, which is more
convenient than adding a MOS to the 9910, but the control method is more
complicated and requires an external constant current control device. You can
use a triode or an op amp. Magnetic components can use I-shaped inductors or
high-frequency transformers with air gaps.
I like to use transformers, because although the cost of inductors is very
low, I feel that their load capacity is not good, and it is not flexible to
adjust the inductance. So I think a better device choice is an ordinary
integrated MOS switching power supply chip plus a high-frequency transformer. It
is the most ideal choice in terms of performance and cost. There is no need to
use a constant current IC or something like that. It’s not easy to use and
expensive.
Finally, one of the most important ways to distinguish these two power
supplies is to see whether their output is filtered by electrolytic
capacitors.
Regarding the power supply issue - whether it is a current-limiting
constant current control power supply or an op amp controlled constant current
power supply, the power supply problem must be solved. That is, when the
switching power supply chip works, it needs a relatively stable DC voltage to
power the chip. The working current of the chip ranges from one MA to several
MA. There are chips like FSD200, NCP1012, and HV9910. This kind of chip is
high-voltage self-feeding, which is convenient to use. However, high-voltage
feeding causes the heat of the IC to rise, because the IC has to withstand about
300V DC. As long as there is a little current , even an MA has a damage
consumption of 0.3 watts. Generally, LED power supplies are only about ten
watts, and a loss of a few tenths of a watt can lower the efficiency of the
power supply by several points. There is also a typical example like QX9910,
which uses a resistor to pull down the power. In this way, the loss is on the
resistor, which probably costs a few tenths of a watt. There is also magnetic
coupling, which uses a transformer to add a winding to the main power coil, just
like the auxiliary winding of a flyback power supply, so as to avoid losing a
few tenths of a watt of power. This is one of the reasons why I use a
transformer instead of isolating the power supply. It is to avoid losing a few
tenths of a watt of power and improve the efficiency by a few points.
Introduction to non-isolated step-down power supply design methods
The non-isolated step-down type is a commonly used power supply structure
now, accounting for almost 90% of fluorescent lamp power supplies. Many people
think that the only type of non-isolated power supply is the step-down type.
When talking about non-isolation, they think of the step-down type, which is not
safe for the lamp - it refers to the damaged power supply. In fact, there is
only one type of buck-type, and there are two basic structures, namely boost and
buck-boost, that is, BOOST AND BUCK-BOOST. Even if the latter two power supplies
are damaged. It will not affect the LED, which has this advantage. The step-down
power supply also has its advantages. The first point is that it is suitable for
220V, but not suitable for 110V, because the voltage of 110V is already low, and
it will be even lower when it is reduced. In that case, the output current will
be large, the voltage will be low, and the efficiency will not be improved. too
high.
Step-down 220V AC, after rectification and filtering, it is about 300
volts. After the step-down circuit, the voltage is generally reduced to about
150V DC. In this way, high-voltage and low-current output can be achieved, and
the efficiency can be increased. Generally, MOS is used as a switch tube and a
power supply of this specification. My experience is that it can be almost 90%
achieved, and it is difficult to go higher. The reason is very simple.
Generally, the self-loss of the chip will be 0.5W to 1W, while the fluorescent
tube power supply is only about 10W. So it was impossible to go any higher.
Nowadays, power efficiency is a very fictitious thing. Many people only boast
about it, but in reality it cannot be achieved at all.It is common for some
people to say that the efficiency of a 3W power supply is 85%, and it is also
isolated.
Let me tell you, even if it is in frequency hopping mode, the no-load power
consumption is the smallest, which is 0.3W. It can output 3W low voltage, which
can reach 85%. In fact, 70% is considered very good. Anyway, now Many people
boast that they don't do drafts and can fool laymen, but now there are not many
people who make LEDs who understand power supplies.
I have said that to be efficient, first of all, it must be non-isolated,
and then the output specifications should be high voltage and low current, which
can save the conduction loss of power components. Therefore, the main losses of
this kind of LED power supply are the chip's own Loss, this loss generally
ranges from a few tenths of a watt to one watt. Another one is the switching
loss. Using MOS as a switching tube can significantly reduce this loss, while
using a triode switching loss is much greater. So try not to use triodes.
Another thing is to make a small power supply. It is best not to be too
economical and not to use RCC, because the manufacturers of ordinary RCC
circuits cannot do the quality at all. In fact, chips are also cheap now.
Ordinary switching power supply chips with integrated MOS tubes cost only two
yuan at most. There is no need to save a small amount of money. RCC only saves
some material costs. In fact, the cost of processing, repairs, etc. is higher.
In the end, the gain outweighs the loss.
The basic structure of a buck power supply is to connect the inductor and
load in series to a high voltage of 300V. When the switch tube is turned on and
off, the load realizes a voltage lower than 300V. There are many specific
circuits, and there are many on the Internet. I will not draw a diagram to
explain. Nowadays, 9910 and other constant current ICs on the market are
basically implemented using this kind of circuit. But in this kind of circuit,
when the switch tube breaks down, the entire LED light panel is destroyed. This
should be considered the worst part. Because when the switch tube breaks down, a
full 300V voltage is applied to the light panel. Originally, the light panel
could only withstand a voltage of more than 100 volts, but now it has become 300
volts. Whenever this happens. The LED will definitely burn out. So many people
say that non-isolated ones are unsafe. In fact, they mean step-down ones. Just
because most of the non-isolated ones are step-down ones, they think that damage
to non-isolated ones must damage the LED. In fact, there are two other basic
non-isolated ones. Structure and power supply damage will not affect the
LED.
The buck power supply must be designed with high voltage and low current to
achieve high efficiency. To explain in detail, why? Because high voltage and low
current can make the pulse width of the switching tube current larger, so that
the peak current is smaller. In addition, it has a negative impact on the
inductor. The loss is also smaller. You can know from the circuit structure that
the circuit is not convenient to draw.
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