How to design power backup system for automotive

[shma103]

Hi,

Currently I'm doing a project to design an air-bag controller system. There are accelerometer, microcontroller, gate driver and power mosfet in the system.

Now, I want to design power backup unit for the system, so if the main supply (from the car battery) is broken, the airbag system still can operate for a short period.

I was thinking on just using one big capacitor to store the charge. Is it sufficient ?
Is there any idea on how to design the unit ?
Instead of using big capacitor, is there any way that this can be implemented in IC (chip) ?

Please help me. Any idea will be much appreciated

 

[aryajur]

How much time do you want to run the circuit. I think one solution could be put lots of capacitors followed by a Buck or Boost Regulator, which will keep the output voltage constant for the time till the capacitor voltage reaches the shutdown voltage of the converter.

 

[shma103]

aryajur,

thanks for your reply. The system only need to supply the voltage for about 1 minute only. Not that long since the airbag activation is done in less than that. The backup system must supply voltage to the microcontroller, gate driver, power mosfet and accelerometer.

can you explain little bit more why I need the buck-boost regulator ? Instead of using a lot of capacitors or big capacitor, can I use supercapacitor ?

 

[rauol]

you can use lithium battery.

 

[aryajur]

1 minute seems like a lot of time. The number of capacitors would depend on the amount of power your circuit consumes to last it for 1 minute. I suggested a Buck/Boost converter to ensure a constant supply to the circuit is maintained even when the voltage of the capacitor starts dying down, which may be pretty quickly, so the Buck(if the capacitors are charged to a voltage more than what the circuit needs)/Boost(if the voltage charged is less) would keep the voltage constant even though the input voltage starts dying down, until the voltage dies below the lower voltage limit of the converter and the converter shuts down. But you may also consider adding a rechargeable battery, rather than capacitors since 1 min can be a long time for capacitors and you may need lots !

 

[codec]

You can estimate the supply current to determine your solution. I*DeltaT=C*DeltaU. However, low capacity rechargeable battery should be feasible.

 

[shma103]

hi all,

thanks for the replies.

aryajur,

do you mean that I need to used capacitor together with buck-boost converter circuit ? So this mean , the capacitor will store the charge from the car battery and then act as the input to the buck-boost converter when the battery is dead. am my understanding correct ?

if I need the supply for just 5 sec, do I still need the buck-boost converter ? because the airbag inflation occurs in a short period (less than 1s) so I guess sustain the power for 5 second is enough.

codec,

what is delta U ? so I can use this equation to estimate what is the capacitor value needed tu sustain the voltage ?

 

[aryajur]

YOu got it correct, shma. If you need it for 5 sec, maybe huge cap may be enough, depends upon circuit current requirement. Although putting a buck/boost will be a safe and more reliable solution still.

 

[IanP]

I think the lithum battery with a step-up converter and automatic power switch build on 2 p-channel mosfets will be much more reliable option than a huge battery of capacitors, which can be quite big in size ..
After all, this back-up circuit, if at all, will be used only once ..
Regards,
IanP

 

[shma103]

aryajur,

I have one question abt your approach. if I'm not mistaken, boost-buck converter will convert to higher voltage. we design the boost-buck converter with a componenet specification to match certain criteria, for example to convert the voltage from +5V to +20V, but for a design where we alrady have 12V supply (stored in the capacitor), the boost-buck converter will up the output voltage higher and may probably damage other components.

so I'm quite confius here, because to design a boost-buck converter, we need to specify the Vin and intended Vout. My inteded Vout is +12V (this is the voltage that will be used for other blocks in the controller) and the input from car battery is +12V.


IanP,

could you explain more about your approach. I'm not familiar with lithium battery. Size also is another criteria. it will be much much better if I can design this in single chip but I think it is quite impossible. So far, what I've seen, people use 2200uF capacitor (quite big) to act as backup system and I dont know how we can design a system to be small in size and preferebly to be implemented in chip.

 

[aryajur]

aryajur,

I have one question abt your approach. if I'm not mistaken, boost-buck converter will convert to higher voltage. we design the boost-buck converter with a componenet specification to match certain criteria, for example to convert the voltage from +5V to +20V, but for a design where we alrady have 12V supply (stored in the capacitor), the boost-buck converter will up the output voltage higher and may probably damage other components.

so I'm quite confius here, because to design a boost-buck converter, we need to specify the Vin and intended Vout. My inteded Vout is +12V (this is the voltage that will be used for other blocks in the controller) and the input from car battery is +12V.

A boost converter can step up a DC voltage, while a Buck converter can step down a DC voltage. When I wrote Boost/Buck I meant that either use Boost or Buck according to your requirement. A Boost doesn't necessarily mean u HAVE to step up the voltage. You may be able to have the output voltage same as that of the input for some converters. A boost converter will be particularly useful since a capacitor voltage will rapidly die down and the boost step up capability will keep the output voltage constant, by stepping up the dying voltage.
Using a battery has again the same issues, since it would make the device bulky with an additional need to have a charging circuit to it, and it would require more maintainence and replacements. Since the activation circuit is a very low power circuit ( I am assuming, since you must be using a low power controller), you may not need huge capacitors to keep the power on to the circuit for 5 sec or so. Again depends on your circuit power requirement.

 

[IanP]

Lithum batteries have quite low internal resistance, good mAh-to-size ratio, and quite extended shelf life (typical storage time is ≈10 years) and therefore will be suitable as a small back-up power source.
Have a look, for example, at this 3V 950mAh battery:
**broken link removed**
You can have almost 1A at 3V for 1hour .. I thing that sould be more than enough ..
Regards,
IanP

 

[shma103]

aryajur,

thanks for your reply, I'll try to design this system and I'll see the outcome. I'll check, how big capacitor I'll need to maintain the power for about 5 sec. My target is to have as small as possible design.

Added after 43 minutes:

IanP,

thanks for your info. but I think, capacitor is more than enough for the airbag applications, since the inflation period is very fast (less than 1s), so we only need small charge storage in case the car battery gone out.

I think battery approach is better for the long run and it will give more backup power but I guess the size and price are the constraints.

I'm trying to find how to implement the backup system in chip. is it impossible ? I've read about supercapacitor with low ESR but I dont know the size of this capacitor.