a lot of switching mode power supply design questions

If my symbols aren't failing me, I would say those are non enhanced MosFets, which means they are on and need to be reversed biased to be off. In this case, pulling it common I don't think is going to cut it. Are we using the correct components? Maybe an Enhanced NMOS is what you want?

--
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Where do you get "non-enhanced" mode MOSFET's in the discrete world? They are even rare in my integrated circuit world.

...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Are you sure that the MOSFET is really doing what you think it is. The slow rise in voltage of the drain of the MOSFET could be caused by a large capacitance on the drain.

The circuit looks to me to be basically this:

D1 ----------! load is a photodiode with voltage drop of 2V / 10A.

What sort of photo diode is that???????

Who knows. Can you ask the guy who did it.

The drawing isn't totally clear but one schottky appears to be across the load to prevent reverse voltage on it. During the power down or power up of the system, the LC circuit could ring and make a reverse voltage on the load. This may be bad.

You need a low ESR and low ESL and high ripple current. Pick them based one those numbers more than the capacitance.

well, that's the common symbol used late time I checked.

and after looking, I would say if this information is correct, he's using a P-channel which also could be an issue here.

But then again, maybe incorrect footprints are being used.

"

Depletion Mode MOSFET Symbol...

Newsgroups: alt.binaries.schematics.electronic Subject: Re: a lot of switching mode power supply design questions - DepletionModeMOSFET.pdf Message-ID:

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
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this circuits has minor problems but basically it can turn on-off mosfet. my deadbug doesn't work.

sorry for the wrong symbol, this is a n-chan mosfet IRL3803 with 1v Vgth. "D" connected to high voltage thus it's n channel.

vcc is 12 v coz i want to charge Cg quickly.

load photo diode is a high power led.

I am still confused about the feedback mechanism here, is there other way to do a close loop for smps?

It seems that the MOSFET is turning on and off properly, and the drain waveform is exactly as would be expected, depending on the actual frequency and the values of the inductors.

When Q1 turns ON, the drain voltage goes to zero, as it should, and the inductors are storing energy. If you look at the curent waveform, you should see the current rise linearly. If the current (or the MOSFET voltage) starts to rise sharply, the inductors are saturating, but this does not appear to be the case.

When Q1 turns OFF, the inductors will release their stored energy into the load, through the diode to Vcc. This will be seen as an exponential waveform on the drain as you show in your sketch. The current will be decreasing during this time, and may reach zero, at which point the Vd will reach Vcc as you seem to expect.

This appears to be a PWM LED brightness control, with inductance to reduce the power dissipation that would otherwise occur with a limiting resistor. The circuit appears to be needlessly complicated. There are many LED driver ICs that should meet your needs with fewer components.

The usual means for wide range brightness control is to set up a high frequency (~100kHz-1MHz) switching circuit to provide the maximum current through the LED, and then use PWM at a lower frequency (~100-200 Hz) to achieve brightness modulation with a range of up to 1000:1.

Look at devices made by

and

Here are more links:

Paul

Consult whoever drew the schematic.

Assuming he meant NMOS, what part did you actually throw in there? This could be a very low-current curcuit, in which case anything in a to220 body may be overkill. Not possible to comment without frequency and component value info (unreadable).

Can't make out what's on the other end of the load, either, from the drawing text resolution; some kind of Maxim part.

RL

Well tbat at least suggests the Maxim has really shipped a part. Maybe it doesn't really work as the OP thinks it does.

maxim one is high-side current sensing with 20x gain. it does carry 10A but I list my questions above. thanks.

That clears that up.

You still haven't really stated the goal of the circuit. It is, I assume, to make a constant current in the high powered LED.

I will start with an over simplified design. If you want to go further I will come back with more. [LOAD] below refers to a circuit with the LED in it.

A bit of ASCII art in two parts:

D1 -------!---+-+-----------!+\\ ! ! ! 0.1V--!+/ ! ! >--+----+ +---Drive C2 === Vcc/2--!-/ ! ! 100p ! ! !/ GND ----! PNP 2N4403 !\\ Q3 ! GND

When the current in the R1 tries to pass 10A, U1A discharges C2 to ground. U1B sees the voltage on C2 go below Vcc/2 and yanks its output down which via Q3 turns off the MOSFET.

The time it takes for the MOSFET to get turned off is very short but not so short that U1A stops pulling down on C2 before it gets down to ground.

C2 starts charging back up towards Vcc as soon as Q1 is turned off. Follows the charging curve of:

-t/(100p*100K) V = Vcc * (1 - e )

and hits 1/2 Vcc after 0.69 time constants or

0.69 * 100p * 100K =6.9us

When it gets to 1/2 Vcc, U1B uses Q2 to pull the gate of Q1 up again turning it back on and the cycle repeats.

The peak current and not the average current in L1 is what is being regulated but that should not be a big issue if the Vcc is fairly well known because the ripple will have a known amplitude.

The LOAD circuit may contain a second inductor and a capacitor if you want to have no ripple current in the LED.

the goal is 0-10A current source for led driving max chip: high side current sensing, 20x gain ucc chip: pwm, 400khz, width varies with input voltage ixdd chip: mosfet driver opamp: current amplifier

it.

You will have problems using a proto-board at 400 kHz. And the IRL3803, with 230 nSec rise time, will very likely overheat due to switching losses. The main reason for using high frequencies is to reduce the size of the inductive and capacitive components. If you have room, you would do much better to run this at about 50 kHz and use something like a 100 uH inductor. At that frequency, you might be able to use a breadboard.

Paul

I do use lower frequency but ripple is way too large. I think high frequency can lower ripple?

If the LED is used for lighting purposes, the human eye will integrate PWM above about 100 Hz. If there is some reason you need to keep LED current constant with low ripple, you can use a larger inductor, or add more filtering to get it as low as you wish. But that is probably not necessary. If you see any flicker, it is because there is some other instability in the circuit, probably a poorly compensated feedback loop. This is one reason why most brightness modulators run the LED at full brightness when it is ON, and use a fixed PWM to adjust the on/off ratio in a 100-200 Hz fixed frequency, as I posted previously.

Paul

This design aims to keep current adjustable yet constant with low ripple. I couldn't use low freq. dimm. adjustment. I wonder if I use a much bigger inductor, e.g. a 1000 uH compared to a theoretical 330 uH, may I get much cleaner current? Large inductor means large ESR, ESC. I'm using a homemade toroid inductor. 1000 uH with 176 ohm and 26 uF @1k Hz.

Trying to figure out what you're doing? wouldn't a photo feed back work better like used in laser diode systems to maintain a desired level of luminance? current monitoring may not be such the idea.

"

These figures don't make much sense, and the exact intent and purpose of the circuit, and its complete design specifications, are still a mystery. A toroidal inductor with 176 ohms ESR is nearly impossible unless it is wound with Nichrome, and I don't know what ESC is (maybe equivalent series capacitance?) You could use a 26 uF capacitor as part of a filter for the output, but I don't know where your 1 kHz comes from. And also how do you come up with a 330 uH theoretical inductance?

I found this Maxim circuit that can drive 30 amps of LED load with 5000:1 dimming, but it still uses PWM and will have high ripple:

If this is a laser diode, there are many factors to consider, and I don't think it is possible to adjust output over more than a limited range with a low ripple DC current. This seems helpful:

Here is a very extensive overview of laser diodes, from a Wiki link:

I really don't think this is a laser diode, as it would be about 20 watts (2V at 10 amps), which is generally only possible for extremely fast pulses or very high-end (and expensive) industrial lasers. And if it is, it must be controlled with photo feedback, and the output light power varies from

0-100% over a narrow range of current, and is highly variable with respect to temperature and composition.

If this is a high power visible LED, the highest currents I know of for single units is about 1 ampere continuous. If this is a cluster of ten or more such LEDs in parallel, then the mismatch in Vf and variations in temperature will result in current hogging and destruction of several of the devices. Standard practice is to put the LEDs in series and generate a higher voltage to drive the entire string.

Paul

On May 27, 11:35 am, snipped-for-privacy@gmail.com wrote: Top posting fixed. Please put answers after the questions not above.

That much is part of the goal. How much ripple can you have on the LED? What is the nominal forward drop of the LED? What is the range of input voltage?

Those are all your attempted method to get to the goal. Why do you want the high side current sensing? Why do you think a gain of 20x is a good idea? Why did you select 400KHz?

The high side current sensing doesn't really have any advantage that I can see. The gain of 20x seems on the low side if you are bothering to have gain in the current sense part at all. 400KHz forces you to use high frequency cores which have low mu values and hence low inductance per unit size. It also means that the MOSFET spends a lot of its time somewhere between on and off.

this

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LM324

it.

you