Buck Converter: A problem with the output stage of TIP122

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Ganeya
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Buck Converter: A problem with the output stage of TIP122

Post by Ganeya »

Hi guys,

I'm building a Buck Converter to step down a 12V input to a 5V output using the TIP122 darlington pair as the switch.

I'm having a problem with the output stage. The tip switches smoothly with just VCC and signal to it. Once the output stage is connected. i.e the diode, the inductor and capacitor, the output reads 12V >> seems like it isn't switching anymore.

Here is a circuit diagram:
Please note the inductor value used is actually 220uH, and capacitor 22uF. The diode is a Fast Recovery diode. Currently I'm testing at open loop, no load conditions. i.e no feedback to the micro, no voltage divider and no load.

https://forum.allaboutcircuits.com/attachments/untitled1-png.104033/

Please Help. I've tried using a variety of different Inductor and Cap sizes and this didn't change anything.
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Re: Buck Converter: A problem with the output stage of TIP12

Post by AndreyB »

You've referecend an image at allaboutcircuits while allaboutcircuits requires users to be signed-in to see attachment.

I've fixed this one.
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kb1gtt
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Re: Buck Converter: A problem with the output stage of TIP12

Post by kb1gtt »

Can the TIP be high side drive like this? I suspect that once you reach about bit over 5V on your battery that the TIP fails to switch at all. This has nothing to do with the cap or inductor. I think your not driving the TIP, however I also don't know the transistor very well.

Can you find this chip or equivalent in a simulation program like QUCS, or TI's TINA? Simulating it is really helpful for seeing things you might not normally expect.

Oh, also why not use a more common buck converter like what we have on Frankenso? What features are you adding by using this chip? Better performance, lower cost, other?
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Re: Buck Converter: A problem with the output stage of TIP12

Post by Rhinoman »

The 4427 may not have enough current drive - its is a Mosfet pre-driver, is there any specific reason why you are not using a MOSFET? You should probably have a current limiting resistor in the base as well to protect the driver from over-current. What are your resistor values? do you have the switching logic in the code correct? have you tried just reading the feedback voltage and sending it out via the serial port to ensure that you are reading it correctly?
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Re: Buck Converter: A problem with the output stage of TIP12

Post by kb1gtt »

The 4427 in the schematic is not connected to VCC or GND, so it's not going to drive anything. If it is connected as we might guess it is, the 1.5A should be more than enough to drive the transistor gate. However with it's internal resistance and the gate voltage drop, it might not conduct anything close to 1.5A.
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Re: Buck Converter: A problem with the output stage of TIP12

Post by Rhinoman »

The 1.5A current rating of the 4427 is only an 'instantaneous rating' its only intended to provide the initial charge for the gate capacitance on a MOSFET for a period of nano-seconds, the continuous current rating is something like 200mA, I don't remember if they have over-temperature protection but Vds will increase rapidly as it heats. I would guess that the ATMEL chip is running a relatively slow PWM, given that its driving a darlington it would have to be, they aren't very fast switching. That could be an issue if the software isn't allowing long enough for the darlington to turn off, its quite common to use a 'booster circuit' on the base to speed that up.
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Re: Buck Converter: A problem with the output stage of TIP12

Post by kb1gtt »

A couple days now with no follow up, and I see that @ has only 1 post. This makes me wonder if this was SPAM. At minimum we need more information about the schematic.

Any how, I would expect the 1.5A should allow for the darlinton to do something. It sounds like it doesn't do anything.
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Re: Buck Converter: A problem with the output stage of TIP12

Post by matt »

i use this schematic on my frankenstein, and test with 2 hour driving. everything looks ok.
now it is daily driving car.
credit to @ for the circuit.

**my frankenso also using same circuit since got something wrong with Q1001.
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Re: Buck Converter: A problem with the output stage of TIP12

Post by Rhinoman »

You don't really need D15, what is the value/part number of D220?
I would increase the rating of C16 to 50V, the regulator is 45V silicon and that is the minimum voltage you will need to withstand to meet the lowest load dump pulse rating (clamped alternator).
What is D16 supposed to do? if it is meant to trap negative going spikes from the inductive circuits then it needs to be located with those drivers.
C14 must have the correct ESR to ensure stability.
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Re: Buck Converter: A problem with the output stage of TIP12

Post by matt »

sorry i don't know much about electronics. only know basic thing. i am not populate D220 varistor.
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Re: Buck Converter: A problem with the output stage of TIP12

Post by kb1gtt »

D15 is because we lack quality control in the field, and we often connect the + and - backwards. I agree it's not needed if you have good quality control and don't brain fart.

I agree with the 50V comment, that cap should at minimum be 50V rated.

D16 is because you have devices like the MAX9926 which will dump energy into the 5V rail. This regulator is a push style regulator, it will not pull down to 5V it will only push up to 5V. So if other energy sources add energy to the 5V rail and you don't consume it with the MCU or what ever, then the 5V rail can raise above 5V. D16 is intended to add a load when you start to raise the voltage from alternative methods.
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Re: Buck Converter: A problem with the output stage of TIP12

Post by Rhinoman »

If you have a uni-directional TVS across the front then D15 isn't required as the TVS will blow the fuse, there isn't any harm in having it there but it will raise the minimum voltage at which the ECU will function. D220 looks like its supposed to be a bi-directional TVS which is pointless, with D15 it won't ever conduct in the reverse direction and without D15 it won't protect the circuit against reverse voltages.
If you have an issue with devices feeding back into the rail then you need to deal with that locally rather than at the regulator - D16 will only conduct if the entire 5V rail goes negative, that rail needs to be kept stable if its feeding logic.
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Re: Buck Converter: A problem with the output stage of TIP12

Post by kb1gtt »

D15 would be better off to the left of D220. As well D220 does not need to be uni-directional. D15 should probably have a small 10pF cap across the diodes to prevent EMI sources. As well D15 would be better off as a PMOSFET such that you get a lower drop out voltage. D16 should be at the harness connector, or if possible at the potential over voltage noise source. It should have an indicating LED to let you know that it's powered. The circuit posted by @ will generate 5V, but has several areas which could be made stronger.

I would suggest that it should be something more like the below. If you want a fixed 5V, you can use the LM2940 instead of the LM319. As well if you are OK with the lower front-end voltage surge, you can omit Q1002. I drew that up to be more universal, such that it can handle larger front end voltages and I made it adjustable, such that you could make it 3.3V or 5.0V.
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Source of that found here.
https://github.com/rusefi/rusefi/raw/master/hardware/5V-regulator/

All of these designs should probably have some kind of EMI choke coil upstream from the linear regulator.
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