[SOLVED] LDO Voltage regulator getting hot - why?

[GH Crash]

I need some help in determining what is causing the voltage regulator to get hot in a simple circuit. Can you give me some ideas of how to find out what is causing the problem? Or, how to fix the problem?

The circuit consists of a super capacitor, acting as the source, a variable output LDO voltage regulator, and a small DC motor. The voltage regulator chip is Semtech's SC4216HSETRT. It is rated for input voltages from 1.4 to 7 volts and current flow up to 3 amps. The super capacitor is rated at 7.5F at 5.4 volts. The motor is a brushed 7mm by 20mm coreless DC motor rated for 3.7 volts. The motor draws less than 1.5 amps at 3.7v under load. Capacitors have been added to the circuit as suggested by the SC4216 datasheet. A diagram of the circuit is attached. (It is almost exactly like the typical application circuit shown in the SC4216H data sheet.)

Prior to a test run, the voltage regulator's output voltage is set to 2.0 volts using a voltmeter and a separate bench-top power supply. The super capacitor is charged, using an external charger, to 4.0 volts prior to start of the test. The problem is that the voltage regulator chip gets too hot to touch within about 2 to 3 seconds of energizing the circuit.

I've added a second heat dissipation pad to the back of the PCB boards. Through board solder paths exists to tie the bottom pad to the surface pad. This didn't have appreciable effect other slightly the time interval before the IC was too hot to touch.

I've tried changing output voltage settings. Although it seemed the the voltage regulator IC took a little longer to heat up at higher output voltage settings, changing the output voltage didn't have an appreciable effects.

I've checked the voltage of the capacitor and it drops less than 0.2 volts under load.

I don't have a scope.

Can you help me to determine what is going on? And how to fix it? What is the cause of the excessive heating? I know that there will be some heat generated during normal operation but I can't understand why the chip gets so hot when it is operation in the middle of it designed operating range?

I'm sure that I have failed to mention something. Please feel free to ask question.

George

 

[GH Crash]

I buy Micro Ready to Fly radio controlled model airplanes at my local hobby store. They are made by E-flite and their main USA distributor is Horizon Hobby.

The coreless brushed motors are cheap and have a very short life. They were originally used to occasionally open and close a CD tray or occasionally operate a vibrator in a phone. The brushes wear out or burn out soon when running continuously on a model airplane or toy helicopter/drone.
The "beginners" RC airplanes use a single-cell Li-Po battery that charges to 4.2V, averages 3.7V during a discharge and the driver circuit disconnects the main motor when the battery voltage drops to 3.0V (leaving the receiver and servos working). The cheap motors need gears.

Brushless motors powered from one or two Li-Po cells are available in model airplanes used by skilled users. They do nor burn out or wear out and "last forever".
PWM is used to control motor speed with barely any losses or heating. The brushless motors produce lots of torque and do not use gears.

I'm familiar with RC. I've been doing it since 1980. The one problem with PWM used in radio control is that it does not automatically adjust the pulse width/frequency as the battery voltage drops.

But lets leave the RC talk for an RC forum.

The photos were posted only to give an idea of the reasoning behind the small and light requirements of the circuit.

 

[Audioguru]

The voltage of a discharging Li-Po battery drops gradually. The voltage of a discharging capacitor drops sharply at the beginning of a discharge. I think the capacitor instead of a battery is your problem and you might not notice the gradual voltage drop from a battery.

 

[GH Crash]

The voltage of a discharging Li-Po battery drops gradually. The voltage of a discharging capacitor drops sharply at the beginning of a discharge. I think the capacitor instead of a battery is your problem and you might not notice the gradual voltage drop from a battery.

True. No argument here on the voltage drop comparison between battery and capacitor.

The device I'm trying to design is specifically for use with super capacitors and hybrid capacitors. The use of batteries is not an option in this case.

 

[KlausST]

Hi,

circuit footprint and circuit weight. I have failed to emphasize these constraints because the original voltage regulator circuit met those requirement.

Now you provided a lot of very useful informations. Application: Your flight model.
I repharse the true goal as "flight model control"
Good information, too: you want the control to be independent of battery/capacitor voltage.
Also extremely good informations: the numbers for weight and size. 1.5g is "solid" information all over the world and for all electronics designers.
Well done.

A short explanation of my way of designing (without saying it's the best one and everyone needs to adopt this)
In the beginning with the given information I was focussed on the linear voltage control. Pure schematics and function.
But now with the given infirmations I think beyond schematics ... I think of a "flight model control system".
The difference is: The schematics does not care about weight, board size and heatsink ...

Now knowing the requiremens: a linear solution is not small nor leight weight anymore, because it needs a lot of board space, maybe even a heatsink.
Additional information: All the dissipated heat needs to be supplied by your battery. So the less heat, the longer the flight time.

SMPS: It's expected that you do simple internet search on your own. Not meant rude. It's just that we don't know what "vocabulary" you are familiar with.
Now in short: Switch Mode Power Supply. Functionally "switching" is the alternative for "linear" regulation. Usually done with regulated PWM. Low power loss.

Again: an SMPS is not the first thing I recommend. I rather recommend a motor control.
If I had to do the design, I'd most probably go with a tiny microcontroller including ADC and PWM.

Klaus

 

[GH Crash]

Hi,

Now you provided a lot of very useful informations. Application: Your flight model.
I repharse the true goal as "flight model control"
Good information, too: you want the control to be independent of battery/capacitor voltage.
Also extremely good informations: the numbers for weight and size. 1.5g is "solid" information all over the world and for all electronics designers.
Well done.

A short explanation of my way of designing (without saying it's the best one and everyone needs to adopt this)
In the beginning with the given information I was focussed on the linear voltage control. Pure schematics and function.
But now with the given infirmations I think beyond schematics ... I think of a "flight model control system".
The difference is: The schematics does not care about weight, board size and heatsink ...

Now knowing the requiremens: a linear solution is not small nor leight weight anymore, because it needs a lot of board space, maybe even a heatsink.
Additional information: All the dissipated heat needs to be supplied by your battery. So the less heat, the longer the flight time.

SMPS: It's expected that you do simple internet search on your own. Not meant rude. It's just that we don't know what "vocabulary" you are familiar with.
Now in short: Switch Mode Power Supply. Functionally "switching" is the alternative for "linear" regulation. Usually done with regulated PWM. Low power loss.

Again: an SMPS is not the first thing I recommend. I rather recommend a motor control.
If I had to do the design, I'd most probably go with a tiny microcontroller including ADC and PWM.

Klaus

--- Updated Aug 29, 2022 ---

"SMPS: It's expected that you do simple internet search on your own. Not meant rude."

Not Rude at all. And I agree completely with the comment.
Here are some of the returns from my simple internet search. Society for Marketing Professional Services,Scanning Mobility Power Supply, School of Mathematical and Physical Sciences, St. Mary Parish School

There are many more SMPSs that are not Switch Motor Power Supply

A plea to all. If you are going to use an acronyms, please define it at least once. The same acronym can mean different things to different people.

I' will get off my soap box now and go study up on SMPS, Switched Mode Power Supply. But a trip to SMPS, St Mary Parish School, would be a lot easier on my brain.

 

[wwfeldman]

have you considered using solar cells instead of super capacitor?

Ultralight solar cells designed to drive drones

Researchers have shown perovskite solar cells just 3μm thick can power miniature model aircraft

www.chemistryworld.com

 

[KlausST]

Hi,

A plea to all. If you are going to use an acronyms, please define it at least once. The same acronym can mean different things to different people.

An objective view...

You used the abbreviations:
LDO:
It means Low Dropout Regulator. But the abbreviation LDO can also mean: "Last day off", "Light diesel oil" and many others.
I guess you used LDO but indeed you meant "linear regulator". An LDO is a special form of a linear regulator. Not every linear regulator is an LDO. In the same category like "LDO" and "linear regulator" there is "SMPS". So if you want others to explain "SMPS", then you have to lead by example and explain LDO. ;-)
As you see - even you don´t explain abbreviations that are familiar to you.

You also used "RC". If you do a search in this forum you will find many discussions using "RC" as abbreviation for "Resistor Capacitor" ...

I could go on..

So, although I understand your concern ... I guess it does not work.
I recommend to do the search for unknown abbreviations on your own.
For sure if you do an random search for "SMPS" you get other hits. But on my first page there are 9 out of 12 "switch mode power supply".

If you refine your search this way: "SMPS electronics", "RC electronics", "LDO electronics" ... you immediatly get almost 100% suitable results. ... but not for "RC electronics"


Klaus

 

[GH Crash]

I know that I'm guilty of using acronyms without explaining their meaning. I try to define the acronym unless I feel that it is a common one within the topic of the discussion. (Do I need to explain what USA means when I say that is where I live? ) The SC4216 data sheet says at the top "Very Low Input /Very Low Dropout." Wouldn't that tend to indicate that the use of LDO would not be out of place when talking about the 44216. I assumed that LDO was a common acronym in electronics.

It was a plea to all and was not directed at you, Klaus, in any way, shape or form.

Yo are right, you have to do your search within the field, or discipline of the topic.

I still believe my plea is still valid. And I will try to do better myself.

George

--- Updated Aug 29, 2022 ---

have you considered using solar cells instead of super capacitor?

Ultralight solar cells designed to drive drones

Researchers have shown perovskite solar cells just 3μm thick can power miniature model aircraft

www.chemistryworld.com

No, I hadn't considered solar cells. There are two reasons for that. The first is that capacitors can be used indoor and I'm not aware of any solar cells that function very well indoors. The second reason is that this forum topic deals with problem solving a voltage regulator for use with capacitors.

Having said that, I would be very interested in learning more about the ultralight solar cells and their possible use in model aircraft. Can you supply more information on the solar cells? Do you have any personal experience or interest in solar cell powered model aircraft.

George

 

[d123]

Hi,

The venerable 555 is a timer IC, very old yet still very much in use, it can output a single pulse of 'any' duration or on-off-on continuously of 'any' duration. There are loads of different timer ICs. And, suggestion was only for you to see how people implement simple PWM circuits.

When you buy components, be healthily skeptical, if it says 1W then assume it is actually probably 1/2 to 1/3 of that rating for an assortment of possible reasons.

You may find SMPS or motor control ICs that are about the size of the regulator you are using, not sure but I would think so. Have a browse on manufacturer's websites, see if any look promising, then get a second opinion here by linking to the datasheet.

Electronics is a bewildering, vast area, isn't it, and often conflicting requirements of squaring circles...

As you are where you are in your project, failing further useful IC recommendations from anyone here, why not look for another LDO that has greater PD capabilities that can fit the PCB real estate you have.

Small but important point: The LDO might not be ideal re PD but it will be far less hot when in use as it will have a forced airflow of cool air when in flight! Unscientific approach: ('my mate in the pub reckons') I doubt it needs a heatsink or you even need to worry about it so long as (Vin - Vout) * Imotor doesn't violate the LDOs junction temperature. Instead of cooling it with a fan, the plane's flight will cool it. Wonderful. You could even check my theory and get a little fan or blower or hairdryer on a cold air setting and see how hot or cold it is then. The speed of flight can be compared to a fan's airflow somehow (numerically speaking) if you want to be sure the theory bears out mathematically.

 

[BradtheRad]

pulse width would have to be adjusted continually as the capacitor voltage drops.

PWM is efficient because the load is switched On-and-Off.

Suppose your control circuit is pulsed DC, and you have a way to increase duty cycle as supply voltage (cap voltage) drops?

The 555 IC is able to operate down to about 3VDC. Voltage applied to pin 5 sets duty cycle.

Say you fix pin 5 at 3V, and start out with the supply capacitor initially charged to 5V. Duty cycle automatically changes as the relationship between these volt levels changes.

Output pulses drive a pass transistor.

 

[GH Crash]

PWM is efficient because the load is switched On-and-Off.

Suppose your control circuit is pulsed DC, and you have a way to increase duty cycle as supply voltage (cap voltage) drops?

The 555 IC is able to operate down to about 3VDC. Voltage applied to pin 5 sets duty cycle.

Say you fix pin 5 at 3V, and start out with the supply capacitor initially charged to 5V. Duty cycle automatically changes as the relationship between these volt levels changes.

Output pulses drive a pass transistor.

View attachment 178288

Brad, I'm a electronics dummy. I didn't understand everything you said.

I think that you said the 555 chip should work. Is that correct?

You mentioned pin 5. Is pin 5 labeled 'ctl' on the circuit diagram?

"Suppose your control circuit is pulsed DC, and you have a way to increase duty cycle as supply voltage (cap voltage) drops?" I'm lost. It appears that this is a question but I'm not sure what it's asking. Or, are you saying that the 555 with a pulsed control input would increase the duty cycle with a dropping capacitor voltage?

What is the source of the 3.1 V in the upper left and the +3V at the ctl pin? I only have the capacitor as a power source. A typical voltage range for a hybrid capacitor is from 2.3v to 4 volts.

George

 

[BradtheRad]

I think that you said the 555 chip should work. Is that correct?

You mentioned pin 5. Is pin 5 labeled 'ctl' on the circuit diagram?

"Suppose your control circuit is pulsed DC, and you have a way to increase duty cycle as supply voltage (cap voltage) drops?" I'm lost. It appears that this is a question but I'm not sure what it's asking. Or, are you saying that the 555 with a pulsed control input would increase the duty cycle with a dropping capacitor voltage?

What is the source of the 3.1 V in the upper left and the +3V at the ctl pin? I only have the capacitor as a power source. A typical voltage range for a hybrid capacitor is from 2.3v to 4 volts.

Yes I was suggesting the 555 has a chance of doing what you need. Or else a lesser-voltage cousin (from replies in your other thread). Configure it as an oscillator (same as pulse generator). Set the frequency to whatever works well with your motor.

Pin 5='ctl' in my simulation.
3V regulated is obtained from a zener diode or LED or string of plain diodes. Raise or lower this volt level so you get desired performance. Or use a TL431 IC which behaves as an adjustable zener diode.

My simulation run starts with the capacitor charged to 5V, discharging to 3.1V (in compressed time). It's your decision which volt levels to begin and end.

My circuit serves as an example of what you might try. There are other oscillator circuits which can operate in this low voltage range, although the chore is to get one that automatically increases duty cycle as supply voltage drops.

 

[Easy peasy]

trying to dissipate 2W in 1 inch square will always result in temp rise, if you get to above 100 deg C, then you will start to get into a difficult area, the start current of the motor may be contributing to the heat.

Perhaps the inherent air flow of a flying craft may help to keep the peak temp down.

 

[Audioguru]

The original NE555 and LM555 are very old and have a minimum supply voltage of 4.5V where they barely work.
Cmos versions LMC555 and TLC555 are available but have a low output current. Their minimum supply voltage is 1.5V for the LMC555 or is 2.0V for the TLC555.

 

[dick_freebird]

Another, potentially much easier scheme is PFM (frequency
mopdulated, fixed pulse width) and hysteretic buck converters
use this for super simple, dead stable converters; scheme
also plays well with resonant power stages where pulse width
wants to be a half sine wave.

Might look for tiny boost converters (which can also do a buck's
job) in the PFM class, they are cheap and plentiful. Diodes Inc,
Central Semi, Microchip, .....

https://ww1.microchip.com/downloads/en/DeviceDoc/21355b.pdf

 

[GH Crash]

Thanks for all the suggestions. It's now up to me to put together a working circuit.

I'm half convinced that with the voltage regulator approach I won't be able to get enough heat sink surface on the PCB and still keep the total area under a square inch. Besides the PWM approach should be more efficient.

 

[d123]

Hi GH,

I don't want to bombard you with yet more information while you are still processing the avalanche received so far, but, inside 'Thermal Derating Curves for Logic Packages' is the coffee cup - it's a short read that is highly palatable (even I understood it first, ...or maybe second, time around) and directly related to the main issue of your thread.

 

[GH Crash]

Hi GH,

I don't want to bombard you with yet more information while you are still processing the avalanche received so far, but, inside 'Thermal Derating Curves for Logic Packages' is the coffee cup - it's a short read that is highly palatable (even I understood it first, ...or maybe second, time around) and directly related to the main issue of your thread.

Thanks much for the link. I hate to admit it but I learned all that stuff in college. The only trouble is/was that was the last time I had used it and I had forgotten most of it.

 

[d123]

Hi,

Sorry about that, then, I thought it might be useful. And, join the club... most projects involve lots of refreshing memory for things done infrequently, inevitable.

 

[GH Crash]

I'm going to close this project. All of your responses have help me realize that using a voltage regulator approach won't work For this project. There is just too much heat to be dispersed with the small size of the circuit board that I want.

Special thanks to Klaus who helped me learn how to better communicate with regarde to electronic projects. It was a long and painful process for him. Thanks Klaus

Future plans are to try some form of PWM, Pulse Width Modulation. I will start another thread for it. I'm sure that I will have lots of questions that you can help with.