Taking the BOOST circuit as an example, there are two algorithms to calculate the maximum average input current, that is, the inductor current.

小太阳yy

Taking the BOOST circuit as an example, there are two algorithms to calculate the maximum average input current, that is, the inductor current. [Copy link]

 

 

Taking the BOOST circuit as an example, there are two algorithms to calculate the maximum average input current, that is, the inductor current. The first is to calculate the output power, such as 30w output power, and calculate the efficiency as 0.8, and then calculate the input power, and divide it by the input voltage to get the maximum average input current, that is, the inductor circuit. There is another algorithm, for BOOST, IL=I out/(1-D) The maximum average current calculated by these two algorithms is different. Which one should be used as the standard? The first method calculates a larger value

maychang

"The result calculated by the first method is too high"

Your second approach does not take efficiency into consideration.

小太阳yy

maychang posted on 2022-10-3 14:56 "The result of the first method is too high." Your second method does not take efficiency into consideration.

Should we use the first method to calculate or the second method to calculate with a 1.2 times margin, so that the two results are similar in the end? When reading the calculation in the book, there is no mention of efficiency. . .

小太阳yy

maychang posted on 2022-10-3 14:56 "The result of the first method is too high." Your second method does not take efficiency into consideration.

Moreover, when I read the book about calculating the inductor current and inductance value, there was no mention of efficiency. So do I also need to consider efficiency when calculating the inductance value?

maychang

小太阳yy posted on 2022-10-3 16:20 Should we use the first method to calculate or the second method to calculate and leave a 1.2 times margin, and the final results of the two methods will be similar...

The calculation results of the two methods should be the same. In fact, the latter method is indeed the same as the first method when efficiency is taken into account.

maychang

小太阳yy posted on 2022-10-3 16:25 Moreover, when I read the book about calculating the inductor current and inductance value, there was no mention of efficiency. So I also need to consider it when calculating the inductance value...

"So do I need to consider efficiency when calculating the inductance?"

Of course, efficiency should also be considered when calculating the inductance value. However, the requirements for the inductance value are not high. Even if it deviates from the calculated value, the output voltage can be corrected by slightly changing the PWM duty cycle, so it is not a big problem not to consider efficiency when calculating the inductance value.

小太阳yy

maychang posted on 2022-10-3 17:09 "Do I need to consider efficiency when calculating the inductance value?" Of course, efficiency should also be considered when calculating the inductance value. However, the requirements for the inductance value are not high...

Now considering the efficiency, the calculated inductor peak current is about 4.1A. Calculated based on an efficiency of 75, can I choose an inductor of 4.2A? Is the margin too small? ... It is calculated based on the minimum input voltage and the maximum output voltage. In actual situations, the input voltage may drop by about 1V. In this case, will the inductor be able to withstand it? ... Now I still don’t understand what the inductance value affects... If I use different input voltages and output voltages to calculate, I get different inductance values. For example, if I use the lowest input and the highest output to calculate, I get a value of 15μH. When I slightly increase the input, the inductance value will increase again. So I finally chose a small inductance value. Is the inductance value enough when the input voltage is high?

maychang

小太阳yy posted on 2022-10-3 17:16 Now considering the efficiency, the peak current of the inductor calculated is about 4.1A. Calculated according to the efficiency of 75, can I choose a 4.2A inductor...

"I still don't understand what the inductance value affects."

This brings us to the title of this post. The title is "Take the BOOST circuit as an example..." The problem is that the calculation of inductance is different for different circuits. The calculation of the Boost circuit cannot be rigidly transferred to the Buck circuit.

As for the influence of inductance in Boost circuit, it is mentioned in the “Bidirectional DC Converter” I posted a few days ago.

maychang

小太阳yy posted on 2022-10-3 17:16 Now considering the efficiency, the peak current of the inductor calculated is about 4.1A. Calculated according to the efficiency of 75, can I choose a 4.2A inductor...

"If I use different input voltages and output voltages to calculate, I get different inductance values. For example, if I use the lowest input and the highest output, I get a value of 15μH. If I increase the input slightly, the inductance value will increase again."

I don't know how you calculated it, or I don't know what you based your calculation on.

小太阳yy

maychang posted on 2022-10-3 18:03 "If I use different input voltage and output voltage to calculate, I get different inductance values. For example, if I use the lowest input and the highest output to calculate, I get...

Then I will list the calculation process in detail. I hope you can correct me.

First, the input voltage is 9-16V, and the typical value is 13.5. Then the output voltage range is 32-50V and the output current value is 444mA.

The typical output voltage is 42V, so I calculated the duty cycle as 50%, 67%, and 81% according to (input 9V, output 50V), (input 13.5V, output 42V), input 16V, and (output 32V) respectively.

Then the output power is calculated under these three conditions: 14W, 18W, 23W, and the efficiency is calculated as 0.8, and the input power is 17W, 23W, and 28w respectively. Then according to 9V, 13.5V, and 16v, the three power

The average input current, that is, the average inductor current, is calculated to be 1.06A, 1.71A, and 3.2A respectively. Then, according to the formula I_L peak_MAX=IL AVE (boost) + ΔIL (Boost) / 2, ΔIL (Boost) = inductor r * input current (that is, average inductor current)

The inductor r I obtained is 0.4, and the maximum PEAK current of the calculated inductor is 1.3A, 2.05A, and 3.85A. Then the calculated inductor value is based on the formula L (Boost) = VIN x (D_Duty Cycle/ Fsw) / ΔIL (Boost)

That is, the inductance values under the three conditions of 9V input and 50V output, 13.5v input and 42v output, and 16v input and 32V output are 15μH, 36μH, and 51μH respectively. Only three cases are calculated. The rest of the cases are calculated when the input and output voltages change.

In this case, the calculated inductance value will change, so which one should I choose? The book recommends choosing parameters under the worst conditions, that is, when the input is 9 and the output is 50, then the inductance value is 15μH. The key is to calculate those large

What about the inductance? If I choose 15μH, then wouldn't the inductance be insufficient when other conditions are met? I'd like to add that the switching frequency is set to 366KHZ

Also, the maximum peak current calculated is 3.85A. However, the saturation current of the inductor I want to choose is only 3.8A, 47μH, and there is no larger one to choose. Is this not feasible?

小太阳yy

maychang posted on 2022-10-3 18:02 "It is still unclear what the inductance value affects." This is the title of this post. The title is "Taking BOOST Circuit as an Example...

Go study your information again immediately

maychang

Xiaoyangyy posted on 2022-10-3 18:51 maychang posted on 2022-10-3 18:03 "If I use different input voltages and output voltages to calculate, I get different inductance values, for example...

I don't quite understand this sentence: "The output voltage range is 32-50V and the output current value is 444mA".

Is it possible that the output voltage changes but the output current does not change? Are you making a switching power supply with a constant output current?

The output voltage of a common switching power supply is constant. For such a switching power supply, the output current will also change when the load changes, but the output voltage will not change much. The design of such a switching power supply must take into account the load change (output current change). For some switching power supplies with special requirements, the output voltage is adjustable. For such a switching power supply, the load may change greatly, and of course the output current will change even more. The design difficulty of this switching power supply with adjustable output is much greater than that of a switching power supply with constant voltage output (load change), and sometimes it cannot even be realized, so another solution must be found.

maychang

Xiaoyangyy posted on 2022-10-3 18:51 maychang posted on 2022-10-3 18:03 "If I use different input voltages and output voltages to calculate, I get different inductance values, for example...

"The output voltage range is 32-50V and the output current value is 444mA"

Your subsequent calculation "then the output power is calculated to be 14W, 18W, and 23W under these three conditions" is obviously calculated based on a constant output current of 444mA. Are you going to make a Boost power supply with a constant current output?

小太阳yy

maychang posted on 2022-10-3 19:15 "The output voltage range is 32-50V and the output current value is 444mA." Your subsequent calculation "and then calculate the output power is also these three types...

Yes, it is DCDC constant current control using a chip

小太阳yy

maychang posted on 2022-10-3 19:08 I don’t quite understand this sentence: “The output voltage range is 32-50V and the output current value is 444mA”. Does the output voltage change, ...

The voltage change of the load is because the driver is an LED. The actual voltage will be different when the current is the same.

maychang

Xiao Taiyangyy posted on 2022-10-3 19:40 Yes, it is DCDC constant current control made by chip

"Yes, it is DCDC constant current control done by chip."

Why didn't you say it earlier? Constant current output and constant voltage output require very different considerations. Your calculations may have been in vain.

maychang

Xiao Taiyangyy posted on 2022-10-3 19:40 Yes, it is DCDC constant current control made by chip

"Yes, it is DCDC constant current control done by chip."

Use a chip? What model? A chip designed for constant voltage output may not be suitable for constant current output.

maychang

Xiao Taiyangyy posted on 2022-10-3 19:40 Yes, it is DCDC constant current control made by chip

"Yes, it is DCDC constant current control done by chip."

Since constant current output is required, you must design output overvoltage protection. This is because when the load current changes, the current negative feedback will cause the output voltage to change. When the load is open (which is unavoidable), the feedback will cause the output voltage to rise to a large value. Excessive output voltage will break down capacitors, diodes, power switch tubes, and even chips.

maychang

Xiao Taiyangyy posted on 2022-10-3 19:40 Yes, it is DCDC constant current control made by chip

"Yes, it is DCDC constant current control done by chip."

Please post your chip model and the electrical schematic diagram you designed. Otherwise, we cannot continue the discussion.

小太阳yy

maychang posted on 2022-10-3 20:40 "Yes, the DCDC constant current control is done with a chip." Post your chip model and the electrical schematic you designed. Otherwise, I can't continue...

I haven't started drawing the circuit diagram yet, but I've been calculating the parameters. I'm using this chip. I have another question. If I configure it as a SEPIC circuit, is the calculation method the same as BOOST? For example, if the inductance is worth calculating, and if I use a coupled inductor, can the calculated inductance value be divided by 2 to use?